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Contains My Article "Automation and Robotics in Construction" on Page 40-46 (www.masterbuilder.co.in)
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The Masterbuilder - March 2012 • www.masterbuilder.co.in54
Computational Tools Appliedto Urban Engineering
Armando Carlos de Pina Filho,Fernando Rodrigues Lima,Renato Dias Calado do AmaralFederal University of Rio de Janeiro (UFRJ), Brazil
The objective of this chapter is to present some ofthe main computational tools applied to urbanengineering, used in diverse tasks, such as:
conception, simulation, analysis, monitoring andmanagement of data.
In relation to the architectural and structural project,computational tools of CAD/CAE are frequently used.One of the most known and first software created toPersonal Computers (PCs), with this purpose, was theAutoCAD by Autodesk. At first, the program offered 2Dtools for design assisted by computer, presentingtechnical and normalisation resources. After that, theprogram started to offer 3D tools, becoming possiblethe conception and design of more detailedenvironments. The program is currently used forconstruction of virtual environments (or virtual scalemodels), being used together with other programs forsimulation of movement and action inside of theseenvironments.
Another software very used currently is the ArcGIS,created to perform the geoprocessing, in which toolsand processes are used to generate derived datasets.Geographic information systems (GIS) include a greatset of tools to process geographic information. Thiscollection of tools is used to operate information, suchas: datasets, attribute fields, and cartographicelements for printed maps. Geoprocessing is used inall phases of a GIS for data automation, compilation,and management, analysis and modelling ofadvanced cartography.
In addition to the programs of CAD and GIS, otherinteresting technology is related to BuildingInformation Modelling (BIM), which represents theprocess of generating and managing building dataduring its life cycle using three-dimensional, real-time,dynamic building modelling software to decreasewasted time and resources in building design andconstruction. Some of the main software used for BIM
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are Autodesk Revit Architecture and Vico Constructor.
Computational tools for monitoring and management arevery important for the urban development. Several urbansystems, such as: transports, water and sewerage system,telecommunications and electric system, make use ofthese tools, controlling the processes related to eachactivity, as well as urban problems, as the pollution.
Therefore, in this paper we will present details about thesetechnologies, its programs and applications, which will tserve as introduction to the use such computational toolsfor study and solution of urban problems.
2. CAD (Computer-Aided Design)
It is a technology largely used in the conception of projectsof Engineering and Architecture. It consists of a softwaredirected to the technical drawing, with severalcomputational tools. Amongst the areas in which the CADis applied, we have the Urban Engineering.
Urban Engineering studies the problems of urbanenvironments, emphasising the creation of plannedenvironments to be sustainable, considering the balanceof economic, territorial, and social factors. Theinfrastructure urban systems are subject of study,searching to optimise the planning of the environment,sanitation sectors, transports, urbanism, etc. It is in thiscontext, that we can begin to understand the use of CADprograms in assisting urban projects.
In respect of development of CAD software, we observethat without the postulates of the Euclidean Mathematics(350 B.C.) it would not be possible to create thiscomputational tool. Thousand of years later, morespecifically at the beginning of the 60th decade of the20th century, Ivan Sutherland developed, as thesis of PhDin the Massachusetts Institute of Technology (MIT), aninnovative system of graphical edition called "Sketchpad".In this system, the interaction of the user with the computerwas perform by "Light pen", a kind of pen that was useddirectly in the screen to carry through the drawing,together with a box of command buttons. It was possibleto create and to edit 2D objects. Such system was alandmark in computer science and graphical modelling,considered the first CAD software.
In the beginning, the use of CAD software was restrictedto companies of the aerospace sector and automobileassembly plants, as General Motors, due to the high costof the computers demanded for the systems. Suchsoftware were not freely commercialised in the market.The Laboratory of Mathematics of MIT, currently calledDepartment of Computer Science, was responsible forthe main research and development of CAD software. Inother places, as Europe, this type of activity was started.
Other prominence developers were: Lockheed, withCADAM system, and McDonnell-Douglas, with CADDsystem.
Fig. 1. Example of virtual scale model: Hospital Metropolitano Norte, Pernambuco,Brazil (http://acertodecontas.blog.br)
From the 70th decade, CAD software had passed to befreely commercialised. The first 3D CAD software, CATIA -Computer Aided Three Dimensional InteractiveApplication, was developed in 1977 by French companyAvions Marcel Dassault, that bought the Lockheed,revolutionising the market. The investments, as well asthe profits, vertiginously grown. In the end of the decade,programs for solid modelling already existed, as, forexample, the SynthaVision of the Mathematics ApplicationGroup, Inc. (MAGI).
From 1980, with the development of the first PersonalComputer (PC), by IBM, the Autodesk released, inNovember 1982, the first program of CAD for PCs, the"AutoCAD Release 1". In 1985, the Avions Marcel Dassaultreleased the second version of CATIA. In this same decade,the workstations (microcomputers of great efficiency andhigh cost, destined to technical applications) weredeveloped, using the operational system UNIX. In the 90th
Fig. 2. Interface of AutoCAD software
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decade, specifically in 1995, the SolidWorks companyreleased the SolidWorks 95 3D CAD, revolutionising themarket for used the operational system Windows NT, whilethe majority of the programs developed was destined toUNIX. In consequence of this, SolidWorks 95demonstrated to be a software with good relation of cost-benefit, when compared with the competitors, excessivelyexpensive.
In the following years to present time, the technologycomes being improved and the software became veryaccessible around the world, with open access versions(freeware). An important application of the 3D CADprograms is the creation of virtual environment, also knownas electronic or virtual scale models (Fig. 1). Suchapplication is largely used in architecture projects.
2.1 Working with CAD
As previously said, we had a great development of CADsoftware in the last decades. Amongst the main programsof CAD, the AutoCAD (http://www.autodesk.com.br) isdistinguished. The software developed by Autodesk hadits first version released in 1982, and recently, theAutodesk released the AutoCAD 2010.
Fig. 3. Project in SolidWorks (http://www.danshope.com)
The AutoCAD (Fig. 2) is a 2D and 3D modelling programwith several applications, such as: mechanical, civil,electric, and urban engineering projects; architecture;industrial manufacture; and HVAC (heating, ventilationand air conditioning). It is important to notice that theAutoCAD is also largely used as tool in academicdisciplines of technical drawing.
AutoCAD have commands inserted by keyboard, makingpossible a practical creation of entities (elements of thedrawing), at the moment of the conception of the desiredmodel, optimising the work of the designer. Such
commands substitute the necessity of navigation with themouse to manipulate the toolbars.
Fig. 4. Example of project of Civil Engineering - a highway (http://usa.autodesk.com)
The program generates diverse types of archive, whichcan be exported to other programs. Some examples:DWG (*.dwg); 3D DWF (*.dwf); Metafile (*wmf);Encapsulated (*.eps); and Bitmap (*.bmp). DWG archiveis an extension shared for several CAD programs.AutoCAD is capable to import archives of the type 3DStudio (*.3ds), from Autodesk 3D Studio Max. User ofAutoCAD is able to associate with your projects, programsmade by programming languages, such as: Visual Basicfor Applications (VBA), Visual LISP e ObjectARX. AnotherCAD software largely known is the SolidWorks (http://www.solidworks.com).
Developed by SolidWorks company, from group DassaultSystèmes, is a 3D CAD program for solid modelling,generally used in the project of mechanical sets (Fig. 3).
SolidWorks can also be used as CAE software (Computer-Aided Engineering), with simulation programs, such as:SolidWorks Simulation, and SolidWorks Flow Simulation.SolidWorks Simulation is an important tool of analysis oftensions in projects. The program uses finite elementmethods (FEM), using virtual application of forces on thepart.
SolidWorks Flow Simulation is a program of analysis ofdraining, based on the numerical method of the finitevolumes. This program allows the professional to getreasonable performance in analysis of the project underreal conditions.
SolidWorks is compatible with DWG files generated byAutoCAD, being able to modify 2D data or to convert into3D data.
Other interesting CAD programs include: CATIA(Computer-Aided Three-dimensional InteractiveApplication), developed by Dassault Systèmes andcommercialised by IBM (http://www.3ds.com), and Pro/
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ENGINEER, developed by Parametric TechnologyCorporation (http://www.ptc.com).
2.2 Application of CAD
CAD software have as main use the aid in projects of CivilEngineering and Architecture for urban environment, suchas: buildings, roads, bridges, etc (Fig. 4). CAD also iswidely used in the project of transmission lines of electricenergy. Such practice consists in optimise the allocation
Fig. 5. Schema extracted from ArcGIS Reference manual showing the three viewsof GIS
of transmission towers and wires, in accordance with thetechnical norms. An important characteristic is thetopography of the land.
Other applications in Urban Engineering include: themaintenance and update of sanitary networks, and theenvironmental recovery in urban areas. In the first case,CAD is used to update the database of the sewer networkof the city, supplying detailed information. In the secondcase, CAD is used for mapping of a region, with the aid ofa GPS system (Global Positioning System), identifyingenvironmental delimitation (sources of rivers, roads,buildings, etc)(Mondardo et al., 2009).
There are several other applications of CAD in urbansystems and areas related to Urban Engineering, and it isimportant to note, in practical terms, that CAD is nearlyalways associate to other technology: GIS (GeographicInformation System), that it will be seen to follow.
3. GIS (Geographic Information System)
Engineering problems were on the last 40 years gradually
directed to employ computerized solving techniques.Precision and increasing speed for calculating multi-variable operations are a good reason to usecomputational resources, but the quite unlimitedpossibilities to organize, simulate and compare dataturned computer sciences on a strong allied for researchand design activities.
The final claim to say that now we are living in aninformation systems age is the large accessibility ofhardware and software, the diffusion of personal systemsand all related facil it ies: servers, networks,telecommunications, etc.
An information system can be defined as an organisedquiver of tools and data that can be used to answer on asystematic way questions structured by specialists. Asthese questions can be classified in patterns, it shouldbe possible to build on artificial intelligence to make thesystem learn and deliberate by itself.
If the answer to a problem employs variables associatedto geographic information, it's recommended the use adataset structure to implement and model graphic objectsthat represents all on earth, natural or artificial. AGeographic Information System (GIS) is a set of tools thatwork with data presenting three basic concepts (Fig. 5):Geodatabase, Geovisualization and Geoprocessing(Harlow, 2005).
Geodatabase represents the set of spatial data that canbe expressed by rasters, vector features, networks, etc.,and every rule to control their creation and management.Geovisualization is an action performed on spatial databy intelligent maps and views, from which we can viewthe database for querying, analysing and editing.Geoprocessing is the term used to designate operationson datasets that obtain outputs of analyses and generatenew information.
Fig. 6. Vector features overlays raster satellite image
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Some engineering projects have territorial themes asindustrial projects, social benefits, general infrastructure,logistics, demography, and other geo/urban/environmental aspects. On those cases the solutioninvolves studying geographic elements and their availableinformation, in order to perform technical analyses. So aninformation system for geographic data organisation,visualisation and processing will be appropriate to thoseproblem solving. To be efficient as a GIS, the system mustperform some general tasks:
acquire, convert, organise and project the geographicelements; import, organise and extract imagery, numericand textual information; process geographic elementsand information with data integrity and operationalefficiency; display appropriately the data and relatedoperations (geoprocess techniques); perform simulationsand comparison of alternatives; present support forprogram language and custom computational routines;generate new data based on selected results; publishmaps and all sort of documents for project discussion;and permit data interchange with other systems.
As we can observe, GIS is designed to manage spatialdata, and the geographic representation of this data canuse many types of elements for plotting the information(Fig. 6): vector based features classes, as points, linesand polygons; raster datasets, as digital elevation modelsand imagery; networks, as roadways, pipelines, hydrologyand other interlaced elements; survey measurements, astopographic annotation; and other kind of information, aspostal codes, address, geographic place names, etc.
These elements can be organised by layers, and couldbe selected by pointing or grouping for edition tasks orcustom display. The selection methods could also beperformed from spatial analyses or statistic classification.Georeferenced co-ordinates and related data tables ofGIS elements help to improve these tasks.
Geographic data representation has integrity rules(Harlow, 2005), performed by spatial relationship patternsbetween elements, as topologies and networks.Topologies are used to manage boundaries behaviour, toapply data integrity rules, to define adjacency andconnectivity properties, to structure creation and editionof new geometry, and to express other topologicaloperations. They are used to represent area contours,parcels, administrative boundaries, etc. Networks areused to represent graphs and their connections,controlling paths, barriers and flows. They are used torepresent behaviour of pipeline, transportation, traffic, etc.
Although organisation and management of spatial datacan be well attempted with modern GIS programs, thereis until an important aspect: how to deal with data quality.
Fig. 7. Example of a workflow model for GIS based research on industrial location
The cartographic databases can be generated from oldcharts or maps digitalisation, or from satellite and aerialimagery treatment. The numeric and textual databasesmust be converted into tables, and quite often comesfrom census and researches output. A great variability ofdata procedures can be observed world-wide whenintegrating data obtained from different fonts, places andscales. The periodicity of data actualisation is anotherdeal to GIS users.
The problems don't result ever from confidence, trustedfonts may have different methodological approaches, andpersonal interpretation can also give different validoutputs. Professional development of GIS operators canhelp them to detect, evaluate and work that variability,and a methodological approach is needed to treat itsuitable to each research task.
3.1 Working with GIS
Many users can be satisfied on using GIS as a datasetmanagement tool for generating maps and classify data,but nowadays GIS is turning on a knowledge approach,where models incorporate advanced behaviour andintegrity rules. The ultimate development on GISprocedures is directed to intelligent use of geoprocessingfor built, explore and share the possibilities of geographicinformation. Users now are able to structure schemas andworkflow models in order to improve their geoprocessingtasks, as import, check, integrate and compose data(Fig. 7).
As GIS is the best way to work data from local to globallevel, an efficient DBMS (Data Base Management System)is needed to perform data integration, actualisation,access and sharing. As result, GIS catalogue portals basedon Web nodes are increasing in number and theirinteroperability is part of a concept called SDI (SpatialData Infrastructure). Servers are used to host enterpriseGIS and their databases, and to provide multi-user access.Geographic Databases are employed to control anddevelop published data, as maps, features and tables.They are known as Geodatabase, have a proper logic to
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work with datasets by applications and tools, and performaccess and management tasks.
But GIS capabilities can also provide single users tocustomise their data. A Personal Geodatabase (PGDB) isan example of option to collect and organise featuresand tables attempting to user needs, using desktopcomputers at low cost and with feasible results. If you area adviser or researcher and are in charge of studyingurban problems, you can go ahead on mounting yourPGDB, however some steps must be observed.
The start point is to structure correctly your problem,identifying the factors and conditions that impacts on, amethodological approach to face it, and a technicalprocedure to get alternatives and produce results.
First, you must study what kind of information you need,identify the sources and think about layer and featuresorganisation. Next, you must acquire geographic datafrom GIS portals or institutional sources. Many researchand administrative institutions provide download of vectorand raster data from their DBMS, or send it by request. Ifthere is no available geographic data, it will be necessaryto digitalis existing map and imagery, but for this task isrecommended a professional with advanced knowledgeof geodesics, cartography and geoprocessing.
After getting the appropriate geographic information isimportant to know that vector data is usually related to atable, which has a column whose contents link the graphicrepresentation to a register. Raster image has pixelposition attached to a co-ordinate value. Vector featuresas point, line or polygon has as code number for the systemlink requirements, but can also have a code for geographiccadastral purposes (Fig. 8). Geocode is a tendency onGIS procedures and has the advantage to make easylater joins and relates of table data with none geographicplot.
In other words: if you get a basic data of shapes withrelated table presenting geocode column you canaggregate new data from other ordinary tables that hasalso this geocode column. GIS also enables visualisationof each element by selecting it from geocode, and permitsediting the tables to insert new columns containing yoursown information. Second, you must organise your features
Fig. 9. Use of GIS in the mapping of water and sewer ducts (http://www.gis.com)
and tables in a dataset, defining co-ordinate systems andimporting independent features and tables to the PGDB.This modality of data organisation provides more securityand flexibility, increasing edition and analysis tasks.
Working with stand alone features can face restrictionsthat are not present on a PGDB structure, as it works moreproperly with layers, overlays, projections and co-ordinates. Third, you must know what to do to improveyour queries on GIS ambience. It is a lost of potential touse a GIS only for data visualisation or map creation, thereis more than this. Both DBMS or PGDB can generate dataperforming spatial analysis or statistic classification. Asyou have the demands of your research well structured,GIS can help you to answer by crossing multi-layerinformation, selecting and editing data from SQL(Structured Query Language) statements and processingnew features containing partial and conclusive results.
Finally, you must obtain a valid output for your problemsolving, and communicate it to others on a suitable way.GIS can help you on producing thematic maps, analyticalgraphs and technical reports. You can also get community,representatives and specialists to work in a participativemode using GIS to generate and validate output ofdecision sessions. Some people have difficulties toidentify and interpret geographic elements, and GIS canhighlight and detach text and visual information formaking it easier.
Fig. 8. Vector features as point, line and polygon with associated table containinggeocode
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Application of GIS
GIS technology is much used in Urban Engineering toanalyse, in a detailed way, characteristics related to urbanplanning. In addition to CAD, GIS presents solutions forseveral problems, and it is applied, in a integrated way, inprojects of Civil Engineering and Architecture, includingthe most diverse urban systems (Fig. 9), making possiblethe maintenance and update of service networks, as wellas the environmental recovery in urban areas.
Nowadays, the accessibility of GIS technology stimulateseducators to work in a new concept, called GeographicalInclusion, which can be performed on basic educationclass in order to provide young students with geographicvisualisation and interpretative capabilities. We are livinga age of saving resources, environmental care andsustainable actions, and GIS with his solving problemdesign and participative net work potential is the moststrong partner in managing data for this purposes.
Concluding the technologies presented in this chapter,we will see to follow the BIM technology (BuildingInformation Modelling), that it represents, in a certain way,an evolution of CAD technology, previously presented.
BIM (Building Information Modelling)
It is a technology that consists in the integration of alltypes of information related to conception and executionof a project of Civil Engineering. Such information, storedin efficient database, not say respect only to design or tomodelling of plants and virtual environments, but also tomanagement of execution time of project, geographicinformation, quantification of material used in all building,detailing of the constructive processes, sustainability, etc.In short, the technology makes possible that the work teamhas an integrated vision of the project. This allows, forexample, that engineers and architects idealise andexecute the project sharing the same base of information.This technology has been spread together with thepractice of Urban Engineering.
In a certain way, BIM is seen as an evolution of 3D CADtechniques. In fact, this technology is defined as 4D CAD,where the fourth dimension is not physical, but the set ofinformation that go beyond the engineering concepts,used in the development of the project.
The use of BIM can mean an effective optimisation of timeand increase of the productivity levels. Other importantcharacteristic is the easiness to perform modifications inthe project, in any phase of execution. BIM makes possiblethe meeting of information, such as: the documentationof licensing for building, the established environmentalconditions, and other legal aspects that are of extremeimportance for execution of the project. Thus, the
technology allows to greater efficiency in the taking ofdecisions during the elaboration of the project, easinessin the emission of building documents, establishment ofdeadlines, estimate of costs, information about theanalysis of risks and management of the operationalconditions of the installations.
Fig. 10. Interface of Autodesk Revit Architecture (http://images.autodesk.com)
Using a CAD software in an engineering project, thedesigner inserts detailed specifications through theheadings, for example: specification of the material usedin the confection of a wall, manufacturer of the material,necessary amount. In the case of BIM technology, suchinformation is directly inserted in the drawing at themoment of the modelling.
Working with BIM
In BIM technology, a set of tools provided by one or moresoftware is used for: modelling of surfaces; modelling andstructural calculation; management of the building;manufacture management; environmental analysis;estimate of costs; and specification.
Fig. 11. Interface of Google SketchUp (http://www.crackvalley.com)
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Autodesk Revit Architecture (usa.autodesk.com)(Fig. 10),is one of the main BIM software, having: tools of 2D and3D drawing; co-ordinated database, in such way thatalterations performed in the information are automaticallyupdate in all model, reducing the possibility of errors and/or omissions; associative sections of divisions table;libraries of details, that can be created and be adaptedto the patterns of the project team; parametriccomponents, that function as an open graphical systemfor design concern and shape creation; inventory ofmaterials, that allows the calculation of detailed amountsof material, updating while the project evolves, on thebasis of parametric alterations; etc.
project; exportation of DWF files (used in CAD programs);and navigation in real time.
Other popular freeware is the Google SketchUp (http://sketchup.google.com)(Fig.11), much used in theacademic area, presenting modelling by means ofsurfaces. Such software presents limitations comparedto the programs already cited. SketchUp works efficientlywith information related to the localisation, size anddesign, reason for which is used in the confection of modelsthat can be exported to programs, as for example, theAutodesk NavisWorks Review.
There are several other programs related to BIMtechnology, as for example, Vico Constructor (http://www.vicosoftware.com), presenting diverse chara-cteristics and resources, such as: the structural analysisof the building; the constant update of the information,correcting possible errors of execution; the estimate ofcosts of the enterprise; etc.
Application of BIM
As well as CAD and GIS technologies, BIM presents aseries of applications in the area of Urban Engineering,and currently it comes substituting CAD, in a effectiveway, because it presents advantages in relation to themanagement of the projects.
A current example of BIM application is the NationalCentre of Swimming of Pequim, China (en.beijing2008.cn/46/39/WaterCube.shtml). Seat of the competitions ofswimming during the Olympic Games of 2008, known asWater Cube, the place have a useful area of 90,000 m²,five Olympic swimming pools and capacity for 17,000spectators, and BIM was used in all phases of the project(Fig. 12 and 13).
Other example of BIM application is the InternationalAirport Maynard Holbrook Jackson Jr., Atlanta, UnitedStates (Fig. 14). This airport is in construction phase with
Fig. 12 and 13. Assembly of the structure of the Water Cube, and aerial photo of theplace (http://comunicacaoexponencial.com.br)
There are other BIM programs by Autodesk, as AutodeskNavisworks (usa.autodesk.com), that it does not presenttools of environment modelling, being destined to therevision of 3D projects or visualisation of models, that isthe case of the freeware NavisWorks Freedom. The maintools include: aggregation of files and 3D data; revisiontools; creation of 4D table; object animation; managementof interference and detention/correction of conflicts in the
Fig. 14. Model of the Airport (http://bim.arch.gatech.edu)
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characteristics and with diverse applications in urbanprojects, providing better results in relation to theplanning, management and maintenance of the systems.
In relation to presented software, it is important to notethat the authors do not have any connections with thecited companies. The programs were shown only ascomputational tools that use the presented technologies,and there are many other commercial software andfreeware that can be used in works involving CAD, GIS orBIM. Therefore, the work presented here does not representany intention of marketing for no one of cited softwareand/or companies.
References
- Autodesk (2006). Orange County Sanitation District - CustomerSuccess Story. Autodesk Infrastructure Solutions
- Autodesk (2009). Langan Engineering & Environmental Services- Customer Success Story. Autodesk Infrastructure Solutions
- Ford, K. (2009). Maynard Holbrook Jackson Jr. InternationalTerminal. Holder Construction Group LLC, Georgia Tech
- Harlow, M. (2005). ArcGIS Reference Documentation. ESRI:Environmental Systems Research Institute Inc., Redlands
- Kymmell, W. (2008). Building Information Modelling - Planningand Managing Construction Projects with 4D CAD andSimulations. The McGraw-Hill Companies, Inc
- Mondardo, D., Bellon, P. P., Santos, L. B., Meinerz, C. C. &Haoui, A. F. (2009). Proposta de Recuperação Ambiental naÁrea Urbana da Microbacia do Rio Ouro Monte. 2nd InternationalWorkshop - Advances in Cleaner Production, São Paulo, Brazil
- Sutherland, I. E. (2003). Sketchpad: A man-machine graphicalcomunication system. Technical Report 574. University ofCambridge, Computer Laboratory, p. 20
- http://acertodecontas.blog.br/ Accessed in August 13, 2009
- http://bim.arch.gatech.edu/ Accessed in December 04, 2009
- http://comunicacaoexponencial.com.br/ Accessed inDecember 04, 2009
- http://en.beijing2008.cn/ Accessed in December 04, 2009
- http://images.autodesk.com/ Accessed in December 01, 2009
- http://usa.autodesk.com/ Accessed in November 27, 2009
- http://www.3ds.com/products/catia/ Accessed in August 24,2009
- http://www.autodesk.com.br/ Accessed in August 21, 2009
- http://www.crackvalley.com/ Accessed December 03, 2009
- http://www.danshope.com/ Accessed in August 22, 2009
- http://www.gis.com/ Accessed in August 14, 2009
- http://www.ptc.com/products/proengineer/ Accessed in August24, 2009
- http://www.solidworks.com/ Accessed in August 22, 2009
- http://www.vicosoftware.com/ Accessed in December 03, 2009
a stipulated deadline for 2011. In this project, of greatmagnitude, BIM is extremely necessary in the optimisationof execution time, since the old airport of Atlanta isoverloaded. The estimated cost of the enterprise isapproximately US$ 1.4 billion (Ford, 2009).
Conclusion
This chapter looked for to present the main details onthree technologies much used in Urban Engineering: CAD(Computer-Aided Design); GIS (Geographic InformationSystem); and BIM (Building Information Modelling). As itcan be seen, each one of them presents specific
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The world is changing, the economy is changing, and
the architectural practice is changing. Designing
energy- and resource-efficient buildings, in many
locations, is no longer optional, but mandatory. While
owners have always sought designs that are cost-effective
to operate and that will command premium lease values,
research shows that green buildings (for example,
LEED®-certified) are more likely to deliver on these
criteria. A 2008 report from McGraw Hill Construction finds
a 13.6 percent decrease in operating costs from green
building and a 10.9 percent increase in building values
as reported by architects, engineering firms, contractors,
and owners over the past three years. (McGraw Hill
Sustainable Design Analysis andBuilding Information Modeling
Manideep Saha
Head, AEC & Geospatial, Autodesk India
Construction September 19, 2008) More pressing is the
growing number of local and national regulations that
mandate targets for energy and resource efficiency as
well as carbon emission reductions in new and renovated
buildings. These government initiatives are certainly put
in place to help reduce greenhouse gas emissions and
slow our impact on climate change, but they are also
instituted to reduce dependence on unpredictable
markets for oil as an energy source and, most recently, to
help stimulate the global economy.
Sustainable Design in Practice
Design decisions made early in the process can deliver
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significant results when it comes to the efficient use of the
vital resources. Employing sustainable analysis tools
helps architects and engineers to make better informed
decisions earlier in the design process and enables them
to have a greater impact on the efficiency and performance
of a building design. Historically, analysis softwares were
complex and required special training-making them
unsuitable for infrequent users such as architects or
designers. Sustainable analysis tools, such as Autodesk
Ecotect Analysis helps users to become proficiently faster
by providing access to immense stores of data and the
ability to more quickly iterate for optimal sustainable
designs.
Designing and delivering more sustainable projects can
be complex. It requires close coordination across different
project stages, from design through construction and
operation. Many firms are looking for the best way to
integrate building information modeling (BIM) technology
with sustainable design and analysis tools. BIM is core to
Autodesk's sustainable design approach for building
performance analysis and simulation.
Analyzing a Building Design
BIM enables architects and engineers to use digital
design information to analyze and understand how their
projects perform before they are built. Developing and
evaluating multiple alternatives at the same time enables
easy comparison and informs better sustainable design
decisions.
A computable Autodesk® Revit® Architecture design
model is devised for sustainability analyses-even during
early conceptual design. As soon as the layout of a
building's walls, windows, roofs, floors, and interior
partitions (elements that define a building's thermal zones)
are established, the information employed to create a
Revit® model can be used to perform analyses.
Performing these analyses in a CAD workflow is a fairly
difficult undertaking as the CAD model has to be exported
and carefully massaged to work with analysis programs.
Using the Autodesk Ecotect Analysis to analyze early
building designs emerging from a Revit-based BIM
process can simplify the analysis process.
Whole Building Energy, Water and Carbon Analysis
The Autodesk® Green Building Studio® web-based
service enables faster, more accurate whole-building
energy, water, and carbon emission analyses and helps
architects-the majority of which are not specially trained
in any of these analyses-to evaluate the carbon footprint
of a Revit-based building design with greater ease.
Built specifically for architects and using green building
extensible markup language for easy data exchange
across the Internet, the web-based service was one of the
first engineering analysis tools to deliver easy-to-use
interoperability between building designs and
sophisticated energy analysis software programs such
as DOE-2.
The link between the Revit platform and the Green
Building Studio web service is facilitated through a plug-
in that enables registered users to access the service
directly from their Revit Architecture design environment.
Inline Energy Analysis
The Autodesk Green Building Studio web-based service
enables architects and other users to perform faster
analyses of a Revit-based building design, from within
their own design environment, directly over the Internet.
This helps streamline the entire analysis process and
enables architects to get faster feedback on their design
alternatives-making green design more efficient and cost-
effective.
Based on the building's size, type, and location (which
drives electricity and water usage costs), the web-based
service determines the appropriate material, construction,
Figure 1: The Autodesk Green Building Studio web-based service enables faster,
whole-building energy, water, and carbon emission analyses of a Revit-based
building design. The building location (being defined here) drives the resulting
electricity and water usage costs.
Figure 2: The link between the Revit platform and the Autodesk Green Building
Studio web-based service is facilitated through a plug-in that enables registered
users to access the service directly from their Autodesk Revit Architecture design
environment.
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The Masterbuilder - March 2012 • www.masterbuilder.co.in78
system, and equipment defaults by using regional
building standards and codes to make intell igent
assumptions. Using simple drop-down menus, architects
can quickly change any of these settings to define specific
aspects of their design; a different building orientation, a
lower U-value window glazing, or a 4-pipe fan coil HVAC
system.
The service uses precise hourly weather data, as well as
historical rain data, that are accurate to within 20
kilometers of the given building site. It also uses emission
data for electric power plants across the United States
and includes the broad range of variables needed to
assess carbon neutrality.
Analysis Results
Usually, within minutes the service calculates a building's
carbon emissions and the user is able to view the output
in a web browser, including the estimated energy and
cost summaries as well as the building's carbon neutral
potential. Users can then explore design alternatives by
updating the settings used by the service and rerunning
the analysis, or by revising the building model itself in the
Revit-based application and then rerunning the analysis.
The output also summarizes the water usage and costs,
and electricity and fuel costs; calculates an ENERGY STAR
score; estimates photovoltaic and wind energy potential;
calculates points toward LEED daylighting credit; and
estimates natural ventilation potential. Unlike most
analysis output, the Autodesk Green Building Studio
report is easier to understand-giving architects and other
users actionable information they need to help make
greener design decisions.
Detailed Environmental Performance
The desktop tools in Autodesk Ecotect Analysis provide
a wide range of functions and simulations, helping
architects and other users to understand how
environmental factors will impact building operation and
performance in the early design phase.
Working with the Environment
To mitigate a building's impact on the environment, it is
important to first understand how the environment will
impact the building. Built specifically by architects and
focused on the building design process, Autodesk Ecotect
Analysis is an environmental analysis tool that enables
Figure 3: Architects and other users can explore design alternatives by updating
the settings used by the Autodesk Green Building Studio web-based service and
rerunning the analysis, or revising the building model itself in the Revit-based
application and then rerunning the analysis.
Figure 4: The Autodesk Revit-based software application user views the output of
the analyses in a web browser, including the estimated energy and carbon
emission summaries (shown left) and a detailed LEED water efficiency guide
(shown below).
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The Masterbuilder - March 2012 • www.masterbuilder.co.in80
designers to simulate the performance of their building
projects right from the earliest stages of conceptual
design. Autodesk Ecotect Analysis combines a wide array
of analysis functions-including shadows, shading, solar,
lighting, thermal, ventilation, and acoustics-with a highly
visual and interactive display that presents analytical
results directly within the context of the building model.
access, and visual impact.
Revit-based design models can be exported to gbXML
format and imported directly into Autodesk Ecotect
Analysis for simulation and analysis throughout the design
process. At the onset of the design process, very early
stage Autodesk Revit Architecture massing models can
be used in combination with site analysis functionality in
Autodesk Ecotect Analysis to help determine the optimal
location, shape, and orientation of a building design-
based on fundamental environmental factors such as
daylight, overshadowing, solar access, and visual impact.
As the conceptual design evolves, whole-building energy,
water and carbon analysis can be conducted using the
integrated access to Autodesk Green Building Studio in
order to benchmark its energy use and recommend areas
of potential savings. Once these fundamental design
parameters have been established, Autodesk Ecotect
Analysis can be used again to rearrange rooms and zones,
to size and shape individual apertures, to design custom
shading devices, or to choose specific materials-based
on environmental factors such as daylight availability,
glare protection, outside views, and acoustic comfort.
Visual Feedback
Perhaps the most unique aspect of the software is its visual
and interactive display of the analysis results. The inability
of the designer to easily interpret the results of analyses is
often the biggest failing of building performance analysis
software. Autodesk Ecotect Analysis provides actionable
feedback to the designer in the form of text-based reports
Figure 5: Early stage Autodesk Revit Architecture models can be analyzed with
Autodesk Ecotect Analysis to help determine the optimal location, shape, and
orientation of a building design-based on basic environmental factors such as the
overshadowing of a particular building (highlighted in red) shown here.
Figure 6: Autodesk Ecotect Analysis can also be used for detailed design analysis. For example, the
visibility analysis displayed here shows the amount and quality of views to the outside mapped over the
floor area of an office.
This visual feedback enables the software to
communicate complex concepts and
extensive datasets more effectively and helps
designers quickly engage with multifaceted
performance issues-at a time when the design
is sufficiently "plastic" and can be easily
changed.
Analyzing a Design in the Context of BIM
Revit-based design models can be exported
to gbXML format and imported directly into
Autodesk Ecotect Analysis for simulation and
analysis throughout the design process. At
the onset of the design process, very early
stage Autodesk Revit Architecture massing
models can be used in combination with site
analysis functionality in Autodesk Ecotect
Analysis to help determine the optimal
location, shape, and orientation of a building
design-based on fundamental environmental
factors such as daylight, overshadowing, solar
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as well as visual displays. These visual displays are more
than just charts and graphs. The analysis results are
presented directly within the context of the model display:
shadow animations resulting from shadow casting
analysis; surface-mapped information such as incident
solar radiation; and spatial volumetric renderings such
as daylight or thermal comfort distribution in a room.
This type of visual feedback lets designers more easily
understand and interact with analysis data, often in real
time. For instance, a designer can rotate a
view of surface-mapped solar radiation
looking for variations over each facade, or
watch an animated sequence of solar rays to
see how sunlight interacts with a specially
designed light shelf at different times of the
year.
Ongoing Building Performance Analysis
During conceptual design, Autodesk Ecotect
Figure 7: Using Autodesk Ecotect Analysis, architects can see the results of their
analysis displayed in the context of a building model, such as the surface-mapped
results of this solar radiation analysis.
Analysis and the Autodesk Revit Architecture model can
be used for a variety of early analysis. For example, the
designer can perform overshadowing, solar access, and
wind-flow analyses to iterate on a form, and orientation
that maximizes building performance without impinging
on the rights-to-light of neighboring structures.
As the design progresses and the elements that define a
building's thermal zones are established (the layout of
the walls, windows, roofs, floors, and interior partitions),
the Revit model can be used for room-based calculations
such as average daylight factors, reverberation times, and
portions of the floor area with direct views outside.
Eventually the Revit model can be used for more detailed
analysis-such as shading, lighting, and acoustic analysis.
For example, the designer can use Autodesk Ecotect
Analysis in conjunction with a shading louver design
modeled in Autodesk Revit Architecture to simulate how
the design will work under different conditions throughout
the year. Or the architect can use Autodesk Ecotect
Analysis to help assess the acoustic comfort of a Revit-
Figure 8: Autodesk Ecotect Analysis software also displays analysis results using spatial volumetric
renderings, such as this analysis of the visual impact of a building within an urban site.
based design, and then adjust the location
of a sound source or adjust the internal wall
layout or the geometry of sound reflectors for
optimal comfort.
Summary
The consistent, computable data that comes
from Autodesk Revit Architecture combined
with the breadth of performance analysis and
meaningful feedback of Autodesk Ecotect
Analysis work in combination to help reduce
the cost and time to perform energy modeling
and analysis. The feedback from these
analyses helps architects and other users to
optimize the energy efficiency of their designs
and work toward carbon neutrality earlier in
the design process-a key ingredient not only
for incorporating energy efficiency into
standard building design practices but also
for mitigating the carbon footprint of our built
environment.
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Mechanisms of Deteriorationof Reinforced Concrete Structures
Dr. Manu Santhanam
Associate Professor, Department of Civil Engineering, IIT Madras
Durability of hydraulic-cement concrete is defined
as its ability to resist weathering action, chemical
attack, abrasion, or any other process of
deterioration (ACI). Durable concrete will retain its original
form, quality, and serviceability when exposed to its
environment.
Concrete is an inherently durable material. Reinforced
concrete structures are expected to be maintenance-free
during their service lives. However, there is evidence of
premature deterioration of modern structures. The
resultant costs to the economy reach 3 - 5% of GNP in
some countries (and up to 50% of construction budgets).
This occurs because existing knowledge not adequately
applied.
As shown in Figure 1, durability of concrete depends on
two primary factors - the concrete system, and the service
environment. The concrete can be further subdivided into
the materials and the process, while deterioration in
service conditions can be through physical or chemical
means.
Concrete has to function in different types of environments,
some of which are aggressive or degrading to the concrete
quality. Typical aggressive environments are: Seawater
(or close to sea), Polluted soils (due to industrial or
agricultural effluents), Freezing conditions, to name a few.
Design of concrete for these environments has to take
into consideration the alterations that cement paste (or
concrete) may undergo upon interaction with the
environment.
The common durability problems in concrete are:
- Corrosion of steel in reinforced concrete
- Sulphate and other chemical attack
- Alkali aggregate reaction (more of a material problem
than environmental)
- Freezing and thawing damage
- Carbonation
Concrete characteristics affecting performance
Porosity and permeability
Durability of concrete is related to its performance in the
service environment. Concrete is subjected to a host of
durability problems, which typically result in:
- Progressive loss of mass from the surface
- Volume changes, which can be of three types: (1) both
paste and aggregate expand, (2) the paste expands,
while the aggregate is inert, or (3) only the aggregate
expands.
Water is common to all the durability problems in
concrete. The presence of water, or its involvement in the
reactions is necessary for the problems to occur. Thus,
the durability of concrete is intrinsically related to its water-
tightness, or permeability.Figure 1. Constituents of concrete durability
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Permeability of concrete is a function of the permeability
of the cement paste, of the aggregate, and of the
interfacial transition zone. The permeability of these
components is in turn related to the porosity. Paste
capillary porosity is typically 30 - 40%, while normal
aggregates have a porosity of 2 - 3% (and rarely greater
than 8 - 10%). The transition zone is highly porous due to
the presence of flaws such as microcracks and bleed-
channels.
Figure 2. Porosity and permeability: A is highly porous compared to B, butprobably less permeable due to the poor interconnectivity of pores
Mix characteristics - w/c and presence of admixtures
Both porosity and permeability increase with an increase
in the water to cement ratio. The permeability also
depends on the degree and nature of curing, and the
presence of mineral admixtures, which can act as fillers
densifying the transition zone. Additionally, the pozzolanic
reaction of mineral admixtures contributes to the
resistance of concrete. Chemical admixtures such as
corrosion inhibitors and air entraining chemicals enhance
the performance of concrete during corrosion and
freezing, respectively.
Type of cement and aggregate
Blended cements perform better (combined benefits of
pozzolanic reaction and reduced permeability) in all
environments. Special cements such as Type V (sulphate
resistant) and Sulpersulphated cement are good for
sulphate resistance.
As far as aggregates are concerned, low density material
is susceptible to freezing damage. The bond with cement
paste will govern the quality of interfacial zone. Some
aggregates have better bond than others.
Presence of cracks
Cracks in concrete could be structural or non-structural
(thermal effects, shrinkage etc.). Most often, the non-
structural cracks occur as a result of poor material
selection, lack of adequate quality control, etc. There is
no use having HPC between cracks, since cracks will serve
as channels for the ingress of water and other chemicals.
Good concreting practice is the only way to minimize
unwanted cracking.
Importance of the cover zone
The importance of a good concrete cover for reinforcing
steel cannot be overemphasized. The cover concrete is
primarily responsible for its response to the service
environment.
Sulphate attack on concrete
Sulphate attack is the deterioration of concrete by means
of reactions between sulphate ions and hydrated cement
products. Generally, sulphate attack is divided into two
categories: External and Internal. External sulphate attack
is when the source of sulphate ions is external to the
concrete, such as when it is from ground water or seawater.
Na2SO
4, MgSO
4, CaSO
4 and (NH
4)
2SO
4 are some
detrimental sulphate sources that are primarily found in
ground water contaminated with industrial effluents and
agricultural products. Internal sulphate attack, on the
other hand, occurs when a late release of sulphates within
concrete takes place. In this case, the formation of
ettringite occurs after the concrete has hardened, and
this results in distress.
Sodium sulfate (N ) and magnesium sulfate (M ) can react
with CH to produce gypsum (C H2), sodium hydroxide
(NH) and magnesium hydroxide (MH, or brucite). It is not
fully understood if gypsum formation causes any
volumetric expansion. The formation of gypsum, however,
is reported to render the structure soft, which leads to a
decrease in strength of the structure.
The formation of gypsum is closely linked to the formation
of other products of sulfate attack, as gypsum can combine
with other hydration products to produce ettringite. This
phenomenon is called ettringite corrosion. The formation
of ettringite is said to be expansive, although the
mechanism of expansion is still debated by researchers.
Numerous theories have been postulated to explain the
expansion due to ettringite formation: (1) crystal growth,
either by topochemical (when the products form at the
reactant sites itself) mechanism, where ettringite crystals
grow on the surface of aluminate particles, or by through-
solution mechanism; (2) swelling due to imbibition of
water, because of the high surface area of ettringite.
The damaging effects on the C-S-H gel are only due to
the action of magnesium sulfates.
The MH and the silica hydrate (SHy) formed in this reaction
further react to produce magnesium silicate hydrate (M-
S-H), which is reported to be non - cementitious, and
leads to complete disinitegration. The phenomenon of
progressive reduction of the C/S ratio within the C-S-H
gel is called decalcification. This process does not
actually begin until the pH drops to very low values (<10).
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The conversion to M-S-H is a very advanced stage of
deterioration. The M-S-H and gypsum formed from the
above reactions are frequently observed to be deposited
in bands parallel to the exposed surface. The
decomposition of the C-S-H gel into a non-cementitious
M-S-H can be achieved only by M . The decomposition
by the M doesn't stop with the formation of M-S-H but
continues further. The action of M renders a low pH to the
Figure 1. Cement mortar in sodium sulphate solution showing deposition ofettringite (E) in the cracked surface zones; the dark region represents decalcifiedC-S-H
pore solution. Hence the C-S-H releases some CH into
the solution in order to stabilize itself at a higher pH. But
since there is M in the surrounding environment, the
deterioration cycle repeats itself beginning with the
gypsum corrosion.
Some scanning electron micrographs that depict the
attack of cement mortars by sodium and magnesium
sulphate solutions are presented below in Figures 1 - 6.
Figure 2. Large deposit of gypsum formed in cement mortar in sodium sulphatesolution
Figure 3. Layer of gypsum surrounding sand grain in cement mortar stored insodium sulphate solution, suggesting a conversion of CH to gypsum; smalldeposits of ettringite are also visible
Figure 4. Layers of M-S-H and gypsum on the surface of concrete subjected tomagnesium sulphate attack
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Figure 5. Formation of a surface double-layer of brucite(B) and gypsum(G) in acement mortar stored in magnesium sulphate solution
Figure 6. M-S-H and gypsum in a highly deteriorated surface zone of a cementmortar stored in magnesium sulphate solution
Protection from sulphate attack
" Use of low C3A cements: The philosophy of prescribing
low C3A cement to improve resistance to sulphate attack
hinges on the need to minimize ettringite formation after
the concrete hardens. When the C3A content is low, most
ettringite will be formed in the plastic state. The use of
very low C3A content, however, is not good in the case of
attack by chlorides. C3A can bind the chlorides that
penetrate into concrete, thus reducing the free chloride
content that can cause corrosion. Thus, a moderate C3A
content should be prescribed in such cases. Lowering of
C3S might also help, since this would reduce the amount
of CH that forms.
- The best protection against sulphate attack is to have
a low w/c in concrete. Blended cements, that lead to a
consumption of CH, need not be good in cases of
magnesium sulphate attack.
- Use of high alumina cement: HAC is good for sulphate
resistance if the conversion of its hydration product
does not occur.
- Supersulphated cement: In this cement, all the
available aluminates are converted to ettringite during
hydration. Thus, there are no excess aluminates
present to react with external sulphate ions.
Sea water attack
Sulphate attack can also take place in seawater. However,
the mechanism may be altered due to the presence of a
high concentration of chlorides. Typically, seawater attack
is characterized by the formation of higher amounts of
brucite compared to groundwater attack. In addition to
the chemical reactions involved in sulphate attack,
physical deterioration of the concrete may also occur due
to cycles of drying and wetting. The tidal zones in
concrete structures are especially susceptible to
alternate drying and wetting, which may lead to the
crystallization of salts in the surface pores, and the
development of expansive pressures that may cause
spalling. The action of waves can further aggravate the
surface concrete.
Acid attack
Attack by sulfuric acid occurs most commonly in sewers,
where a lot of sulphide gases exist owing to the large
degree of microbial action. Sulfuric acid creates an acidic
environment in the concrete, in which the primary cement
phases (C-S-H, ettringite) are extremely unstable. Gypsum
formation occurs when sulfuric acid reacts with CH. The
loss of integrity and softening of the structure occur as a
result of gypsum formation and destabilization of C-S-H.
Carbonation
Carbon dioxide diffuses into the pores of concrete and
reacts with calcium hydroxide; as a result, the alkalinity
(pH) of the concrete is reduced. Reduction of pH causes
the passivity of reinforcing steel (protective layer) to be
destroyed.
Delayed Ettringite Formation (DEF)
Under certain initial storage or curing conditions, such as
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for steam-cured concrete that is subjected to high
temperatures, the ettringite that forms in the process of
cement hydration gets destroyed. The formation of
ettringite in the early stages may also not occur if there is
a late release of sulfates. The reformation of ettringite in
hardened concrete in the presence of moisture leads to
the generation of expansive pressures, and cracking of
concrete. This is called delayed or secondary or late
ettringite formation.
Most researchers believe that three elements are essential
for DEF to occur: presence of microcracks, late sulfate
release, and exposure to water. If cracks are present in
concrete, DEF causes the deposition of ettringite in the
cracks. The late-released sulfates, or the sulfates from the
deteriorated ettringite, go into the structure of C-S-H.
These are released later by moisture, which carries them
to the aluminate phases, resulting in the formation of
ettringite. This ettringite often effectively masquerades
as ASR, since it forms in extremely small crystals.
DEF is more of a problem with modern cements, since
the clinker SO3 levels have increased dramatically over
the years, and the use of sulfur fuels has also grown. Thus,
it is all the more essential nowadays to restrict the
temperature rise in concrete, not only from steam curing
but also from the use of rapid hardening cements. Most
European countries have adopted standards restricting
concrete temperatures.
Alkali aggregate reaction
Many sil iceous igneous (opal, chalcedony) and
sedimentary rocks (chert) possess a glassy or amorphous
texture. In alkaline environments the silica structure can
get dissolved from these aggregates. The resultant
reaction between silica and alkalis results in the formation
of a gel that is expansive. This phenomenon is called the
alkali-sil ica reaction. Strained quartz present in
metamorphic rocks may also be susceptible to damage
by alkali-silica reaction (ASR).
The alkalis may come from the cement, chemical and
mineral admixtures, impurities in aggregate or water. The
reaction itself needs the presence of moisture.
SiO2 + KOH (in the presence of moisture) →Alkali silica
gel (no definite composition)
The first step in this reaction happened on the surface of
the aggregate, where the Si-O bonds are dissolved by
OH-. Thus, the silica becomes available to combine with
the alkalis to form alkali-silica gel.
The alkali silica gel formed from the above reaction could
also contain some Ca2+. The ratio of Ca2+ to the alkalis
(Na+ or K+) in the gel determines its expansive nature.
Usually, the higher the Ca2+, the lesser expansive the gel.
If the alkali hydroxide concentration falls below 0.3N, the
reaction tends to slow down and stop. The rate of reaction
cannot be determined from the amount of gel forming,
since it does not have a distinct composition.
The amount of expansion in ASR depends on the type of
aggregate. For some aggregates, a pessimum type
relation is observed between the % expansion and the %
of reactive aggregate, while for others, the % expansion
increases consistently with an increasing proportion of
reactive aggregate. The decrease of expansion beyond
a certain limit occurs because when there is too much
reactive silica, the gel can form at very early stages when
the concrete is still in the plastic state. At high alkali
contents, the gel that forms has got a low viscosity, and is
thus not able to generate high expansive pressures. In
the case of aggregate size, when the size is too small, the
reaction occurs in the plastic state of concrete, and thus
does not lead to any expansion. On the other hand, for
very large aggregates, the surface area to volume ratio
becomes too small for the reaction surface to be a
significant factor.
Mechanism of expansion
Various theories have been proposed to account for the
expansion that occurs as a result of ASR. These are:
- Absorption (swelling) theory proposed by Vivian: The
imbibition of pore water and the resultant swelling of
the alkali-silica gel causes expansion. The aggregate
grows outward and puts the paste in tension.
- Osmotic pressure theory proposed by Hansen: The
alkali-silica gel acts as a semi-permeable membrane
that allows only an inward diffusion of OH-, Na+, K+,
and Ca2+ from the pores to the aggregate surface.
Thus the aggregate exerts osmotic pressure against
the surrounding paste. Lea modified this theory and
stated that there is actually a preferential diffusion of
some species - Na+, K+ - over others such as Ca2+.
Manifestation of ASR
ASR is a very slow reaction and may take many years to
show up at the surface of the concrete and get detected.
Cracking due to ASR generally shows up as a map pattern
on the surface. Irregular small cracks form at the surface.
These are unsightly, but are rarely the cause of a structural
collapse. However, expansion associated with ASR can
cause misalignments.
Surface aggregates can often pop out of the concrete
due to expansion. The alkali-silica gel can ooze out to the
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surface and get carbonated if wet. The resultant hard
white gel that forms resembles carbonated calcium
hydroxide.
Microstructurally, reaction rims are often visible on the
surface of reactive aggregates due to a slow dissolution
of the silica. However, the rim could also be due to
weathering, Certain ASR reactive aggregates even do
not form rims.
Protection against ASR
Due to circumstances, it is not possible to change an
aggregate source locally even if prior knowledge about
the reactivity of the aggregate is available. Thus, other
methods have to be adopted in order to prevent ASR.
The primary measures for protection are:
- Use of low alkali cement (< 0.6% equivalent Na2O).
- Preventing access of moisture.
- Using coatings (such as silane, which allows water
vapour to go out of concrete, but does not permit
water to come in) or waterproofing agents.
- Use of chemical admixtures such as Lithium salts
(LiNO3, LiOH, etc.) or alkyl alkoxy silanes, which bind
the reactive silica into a non-expansive product.
- Use of mineral admixtures such as silica fume. Mineral
admixtures can act in two ways: (1) by reducing the
penetration of water, and (2) by binding the alkalis
within the unhydrated glass. The alkali silica gel that
forms in mineral admixtures is also high in Ca2+, and is
thus not very expansive.
Freezing and thawing related damage
The damage due to freezing and thawing (F/T) is a physical
problem, unlike the chemical issues that were discussed
earlier. F/T can cause three types of failures:
- Paste failure: This is related to the failure of the paste.
Parallel cracks form in the paste and proceed inward
from the places where concrete first becomes highly
saturated with water. Sometimes, scaling of the top
surface can occur. Scaling is exacerbated when
deicing salts are used.
- Aggregate failure - D-cracking: This relates to the
failure of the paste when it is subjected to expansive
stressed by the aggregate. It shows up as parallel
cracks proceeding inward from the point of saturation.
The pattern of cracking on a jointed concrete
pavement or slab appears like the letter D, as shown
in the figure below.
- Aggregate failure - popout: Popouts are caused when
porous aggregates on the surface of concrete are
Figure 7. D-cracking in a concrete pavement slab
subjected to expansion on freezing. A part or the whole
of the aggregate piece cracks and pops out.
Sometimes, a mortar flake can also pop off as a result
of the expansion of an underlying aggregate.
Mechanism of freezing and thawing
Water expands by 9 - 10% upon freezing. Thus, the critical
saturation of a pore in concrete is about 90%. It must be
understood that freezing point in small pores is depressed
to a large extent. In fact, in some of the small pores in
concrete, freezing does not occur until temperatures as
low as - 40 oC. Also, the presence of other ions in the pore
solution also depresses the freezing point. If the concrete
remains frozen through its lifetime, then not much of a
problem occurs. The deterioration occurs only if there are
successive cycles of freezing and thawing.
The expansion and damage associated with F/T is
explained using various mechanisms. Let us first consider
the case of paste failure (see Figure 8).
Figure 8. Movement of water inside capillaries
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Water in the large pores near the surface of the concrete
is the first to freeze. The freezing of concrete proceeds in
a front parallel to the surface. The expansion of water in
the large pores on freezing drives out the unfrozen water
into the paste. The travel of water through the paste
generates a hydraulic pressure. The longer the path of
flow of water, the higher the pressure generated. If the
flow path is longer than a critical distance (0.2 mm for
concrete) then failure occurs.
The use of air entraining agents is the best remedy for
paste failure. Air bubbles serve as closely spaced
reservoirs into which the unfrozen water can migrate. The
spacing between air bubbles should be smaller than twice
the critical distance (0.2 mm) for the air entrainment to be
really effective. Thus, it is not just the amount of air
entrained, but also the dispersion of air that is important.
The larger the coarse aggregate size, the lower is the air
entrainment required.
When high strength concrete with low w/c is used, the
pore size can be so small that freezing does not even
occur at service temperatures. Thus, air entrainment is
sometimes not necessary in such cases. The use of
mineral admixtures, which act as pore refiners, can thus
be beneficial. However, mineral admixtures tend to
increase the scaling problem in concrete.
In the case of aggregate failure, the type of failure is
dictated by the porosity of the aggregate. If aggregate
porosity if very high, then the expansive stresses
generated by the aggregate are not critical. In other
words, the expansion is accommodated by the aggregate
elastically. Popouts are caused by aggregates that have
a moderately high porosity. Such low-density aggregates
(cherts are especially susceptible) are subjected to high
internal pressures due to expansion. The problem gets
worse when the aggregate size is large. The remedy is to
screen the concrete aggregate for low-density elements.
In the case of D-cracking, the aggregate porosity is not
high. Thus, and expansion of the water in the aggregate
causes the unfrozen water to move into the surrounding
paste, resulting in hydraulic pressures. Air entrainment of
the paste can help in this case to a certain extent.
Corrosion of reinforcing steel
The corrosion of steel in reinforced concrete is a problem
of mammoth proportions. It is estimated that 5% of a
developed nation's GDP is utilized for repair of corrosion-
related damage. The yearly cost of repairs for reinforced
concrete bridge decks in the US alone is estimated to be
$ 50 - 200 million.
Corrosion is an electrochemical problem. The overall
mechanism can be broken up into the anode reaction
and the cathode reaction, as shown in Figure 9. An
electrical current flows through the aqueous medium
Figure 9. Reactions of corrosion
Figure 10. Current flow during corrosion process
opposite to the direction of flow of the electrons (see Figure
10). In addition to the electron current, there is also an
ionic current. The flow of current resembles a battery cell.
This system is thus known as a 'galvanic cell' and the
process is also known as 'galvanic corrosion'.
The ferrous and hydroxyl ions combine to form the rust
products.
2 Fe2+ + 4 OH- → 2 Fe(OH)2 (greenish rust)
The greenish rust, upon further reactions with O2 and OH-
, can form Fe2O
3 (red rust) and Fe
3O
4 (black rust). The rust
often accumulates at places other than the reaction sites.
Likelihood of occurrence of corrosion
Anodic sites can be created on steel due to a multitude of
reasons:
- Compositional variances on the steel surface
- Presence of dust/dirt etc. partially on the steel surface
- Presence of local differences in applied stress
- Microstructural variations in the steel: (1) The ferrite
phase is more active than the cementite phase, (2)
Grain boundary atoms are more active compared to
the bulk
- Strained zones produced during cold working of
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metals may be more active
- Presence of stress concentrations
- Differences in oxygen concentration at different sites
on the steel: generally, the site with a lower oxygen
concentration becomes anodic
Figure 11. Rust formation and delamination of concrete
Figure 12. Relative volumes of corrosion products
Manifestation of corrosion
- Formation of rust causes expansion and cracking
(some forms of rust are 6 - 7 times the volume of
undamaged steel) - see Figures 11 and 12.
- Loss of sectional area of rebar - reduction in load
carrying ability
Factors controlling the rate of corrosion
The following are the principal factors that control the
rate of corrosion:
- Availability of dissolved oxygen and moisture at the
cathode: In order for the cathodic reaction to occur,
both oxygen and moisture are necessary. Due to the
concrete cover, both these elements have to reach
the steel surface by diffusion. This slow diffusion
produces a significant reduction in the potential
difference between the anodic and cathodic areas.
This phenomenon is called 'concentration polarization'.
- Resistivity of the medium (concrete and its pore
solution): The flow of ions has to occur through the
medium of concrete and the pore solution. Thus, the
resistivity of the concrete can have a significant bearing
upon the easy flow of ions.
- Passivation of steel: In an alkaline environment, the
surface atoms of the steel get oxidized to form an thin
oxide layer (thickness of about 10 nm). This film is
stable at the highly alkaline environment of concrete.
The stability of the film is enhanced when the steel
contains a large amount of alloys. This phenomenon
of the formation of a protective layer around the steel
is called 'passivation', and is made possible by the
high concentration of OH- in the concrete pore solution.
The level of OH- required to maintain passivation is
not a constant value, but depends on the presence of
other ions, especially Cl-. The ratio of OH- to Cl- is very
important. Depassivation can occur by a number of
mechanisms: (1) Consumption of OH- by carbonation
and other reactions; when the pH falls below 11.5, the
film is no longer stable; (2) Presence of a high
concentration of Cl-: In addition to lowering the pH
due to ionic balance with OH-, Cl- can react with oxide
films of Fe(OH)2 (that have not been converted to the
Figure 13. Some factors governing the rate of corrosion
stable oxide film because of lack of availability of
oxygen) to form iron chlorides. This results in pitting
corrosion. A threshold concentration of Cl- has to be
exceeded before corrosion can take place, and this
concentration is a function of the OH- concentration or
pH. Limits on Cl- concentration have been stipulated
in various codes.
Figure 14 shows the rate of occurrence of corrosion. As
shown in the figure, the initiation stage lasts until the
depassivation of steel. Beyond this stage, the propagation
of corrosion occurs at an almost constant rate. Finally, the
corrosion process enters an acceleration stage where the
rate is high.
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Protection mechanisms against corrosion
- Galvanization: this process involves the plating or steel
with Zinc. Zn, having a higher electrochemical potential
compared to Fe, becomes the sacrificial anode, and
Fe is protected as the cathode.
- Cathodic protection: An external voltage or current is
Figure 14. Rate of corrosion
supplied to the steel to keep it cathodic and
preventing oxidation from occurring.
- Use of stainless steel (very high Cr): Produces a stable
passivating film.
- Use of epoxy coated steel.
- Use of corrosion inhibitors (see chapter on Chemical
Admixtures).
- Adequate depth of cover.
- Good quality concrete with low permeability.
Summary
Good concrete performance in aggressive environments
can only come about with the combined action of a
number of factors:
- Proper mix design
- Reduction of cracking
- Optimum cover thickness
- Adequate compaction and curing
- Quality of construction
- Correct maintenance
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Corrosion of Steel in Concrete& Assessment Techniques
The corrosion cell
To demonstrate the principles of electrochemical
corrosion, let us consider the simple "Daniel-cell". This
consists of zinc immersed in ZnSO4 solution and copper
immersed in a CuSO4 solution. The two electrodes are
connected to a variable resistor R, voltmeter V, and
ammeter A (Fig, 1). The potential difference (emf) between
the two electrodes when no current is flowing is 1.1 V. If a
small current is allowed to flow through the external resistor
(I1 in Fig. 2), the measured potential difference falls below
1.1 V because both electrodes polarise, Zn to b and Cu to
e (Fig. 2). As the resistance is decreased, the current
increases and the potential difference decreases until,
when the system is short-circuited (the resistance is very
small), maximum current flows and the potential difference
is almost zero (Imax
). The Zn anode polarises along the line
abc and the Cu cathode along the line def. The full
polarisation of Zn in volts is given by c-a and for Cu by f-d.
The anodic reaction in the "Daniel-cell" is:
Zn → Zn2+ + 2e-
where Zn corrodes and goes into solution and the cathodic
reaction is:
Cu2+ + 2e- → Cu
where copper is deposited from the CuSO4 solution.
In the case of steel in oxygenated water, the simplified
anodic reaction is:
Fe→ Fe2+ + 2e-
George Sergi, Ph.DTechnical Director, Vector Corrosion Technologies
Iron atoms undergo oxidation (electron loss) to form F++
ions which pass into solution. The excess free electrons
left in the metal are consumed, converting oxygen and
water to hydroxyl ions in a process of reduction (electron
addition) according to the following cathodic reaction:
½ O2 + H
2O + 2e- → 20H -
Both the anodic and cathodic reactions occur at adjacent
Figure 1 Polarised copper-zinc cell (Daniel-cell)
Concrete is a porous material whose pores contain an electrolyte made up primarily of sodium and potassium hydroxides.1Steelreinforcement is normally protected in such an electrolyte owing to the formation of a dense and uniform passive oxide film.2
Carbonation of the concrete (neutralisation of the alkali constituents by CO2 gas from the atmosphere), or infestation of the
concrete with salt from seawater or from deicing agents leads to the breakdown of the protective oxide film and to corrosion of thesteel.3,4 Corrosion of steel in concrete is an electrochemical process whereby anodic and cathodic reactions occur simultaneouslyon the surface of the steel resulting in the dissolution of the metal at the anodic sites.5
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sites simultaneously on the surface of the steel (Fig. 3). As
was the case with the "Daniel..cell", the nodic and cathodic
reactions of steel in water can be represented in a
polarization diagram (known as an "Evans" diagram), as
shown in Fig. 4. The intersection point of the two lines
corresponds to the electrode potential at which the rates
of the anodic and cathodic reactions are equal and is
termed the corrosion potential, (Ecorr), the value that steel
adopts when corroding freely. The magnitude of the
anodic and cathodic reactions is represented by the
number of electrons flowing per second between the
anodic and cathodic sites on a unit area of the metal
surface and is a measure of the corrosion rate, Icorr. It is
often referred to as the corrosion current density and
expressed in electrical units (amps/m2 of steel area or
more commonly, mA/m2 ). It is the current at the equivalent
Figure 2 Polarisation diagram for copper-zinc cell
Figure 3 Mechanism of corrosion in oxygenated water
point of intersection in Figure 4. Faraday's laws of
electrolysis may be used to obtain corrosion rates in more
familiar terms (average rates of mass loss per unit area)
and these values may be simply converted to average
rates of loss of thickness of metal from knowledge of the
density of the steel.
Figure 4 Evans diagram for iron corroding in oxygenated water
Factors affecting corrosion rates
Both Icorr and Ecorr can vary depending on the degree of
polarization of either the anodic or the cathodic reactions.
When polarisation occurs mostly at the anodes, (i.e. the
anodic process is for some reason hindered) it is said
that the corrosion reaction is anodically controlled.
Similarly, if the cathodic reaction is polarised, the corrosion
reaction is said to be cathodically controlled. Since the
operation of a corrosion cell depends on three processes
occurring in series, viz. the anodic reaction, the cathodic
reaction and the flow of currents through the intervening
electrolyte and metal the overall corrosion rate is not only
governed by the anodic and cathodic reactions but also
by the magnitude of the resistance of the electrolyte which
can hinder ionic conduction. This is represented
graphically in Figure 5.
Corrosion of steel in concrete When steel is immersed in
an alkaline solution such as sodium or potassium
hydroxide, its surface becomes coated with an adherent,
insoluble oxide film (γ-Fe2O3) which is stable over a
range of potentials. Its polarisation curve is modified to
that shown in Figure 6 and shows that the corrosion rate
over intermediate potentials becomes very small.6 Such
a situation is termed passivity, one of the states in which
the metal can thermodynamically exist at variable pH
levels of the electrolyte as shown by a simplified Pourbaix
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diagram for steel (Fig. 7). The pore electrolyte of concrete
is primarily a mixture of sodium and potassium hydroxides
and as such, the embedded steel attains passivity. The
condition is usually characterised by a value of Ecorr
higher than about -250mV (measured relative to a
standard copper/ copper sulphate reference electrode-
CSE scale).
When carbonation of the concrete occurs, whereby the
pH of the pore electrolyte becomes neutral, passivity can
Figure 5 Effect of resistance of the electrolyte on the corrosion rate of the
reinforcement
Figure 6 Evans diagram for passive iron in oxygenated alkaline solution
no longer be thermodynamically stable and the steel
corrodes as it would in an equivalent neutral solution (Fig.
4). The rate of steel corrosion in carbonated concrete is
subject to anodic resistance control.78, It therefore
depends critically on the moisture content of the concrete
since this is primarily what determines the electrolytic
resistivity of the material. The corrosion potential of the
steel in this condition is typically in the range -450 to -
600mV (CSE scale) when corrosion is occurring.
Figure 7 Pourbaix diagram for steel
The other main cause of corrosion of reinforcing steel is
the presence of chloride salts in the concrete. In sufficient
concentrations, they can undermine the passive film of
the steel locally and bring about pitting corrosion. This is
illustrated by a modified Pourbaix diagram (Fig. 8) and
by a series of polarisation scans of steel in concrete
contaminated with chloride (Fig. 9). It is evident from
Figure 9 that the corrosion potential becomes more
negative and the corrosion rate increases as the chloride
concentration increases.
Figure 8 Modified Pourbaix diagram for steel in solutions contaminated with
chloride
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Figure 9 Evans diagram for steel undergoing pitting corrosion in chloride
contaminated concrete
Durability assessment and corrosion detection of steelreinforcement
There are a few obvious and simple techniques that should
be carried out initially in order to obtain an overall picture
of the condition of the reinforced structure. A good visual
survey is probably the most important step in the
assessment. It is essential to visit the site and look for tell-
tale signs such as cracking of the concrete, rust staining,
delamination but perhaps more importantly, signs of water
retention, inadequate drainage and, particularly in flat
surfaces such as car parks, formation of paddles. Edges
and crevices should be studied carefully. An assessment
of possible environmental factors such as exposure of
structure to direct rainfall, sun and wind should be made.
Then, key regions that may require detailed investigation
can be selected.
Chloride and carbonation survey
As was discussed earlier, both carbonation and chloride
infestation can lead to corrosion of the reinforcement. If
chlorides are suspected, dust samples should be
collected from selected nodes on a grid at increasing
depths into the concrete. This can be done simply with
the use of a drill. A concentration gradient of chloride
would suggest that the source was external (deicing salts,
seawater) whereas a fairly constant concentration may
suggest that the chlorides were added to the concrete at
the mixing stage. The depth of carbonation (neutralisation
of concrete alkalinity by acidic gasses such as CO2) can
be determined by spraying phenolphthalein indicator
either on freshly broken concrete or on drilled powder at
increasing depths.
Potential mapping
It was shown above that the potential of steel
reinforcement can give a good indication of its condition.
Potential mapping is used, therefore, as a major
investigative technique for the detection of steel
reinforcement corrosion. The set-up for carrying out a
potential map is simple and it involves a few easy steps.
First of all, electrical continuity of the reinforcement is
checked by measuring the resistance between two points
where the steel has been exposed. A resistance of less
than 1Ω would normally signify continuity of the steel.
The potential on the steel is then measured relative to a
Figure 10 Simple potential mapping set-up
Figure 11 Potential map of concrete section showing several areas of low potential / high corrosion activity
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standard reference electrode (normally copper/copper
sulphate or silver/silver chloride) by resting the reference
electrode on the appropriate position on the surface of
the concrete as shown in Figure 10. A potential map can
then be constructed (Figure 11) which can show
equipotential contours on the surface of the concrete.
Regions of high corrosion activity can normally be
identified as they show characteristically low potentials.
It is generally assumed that the probability of corrosion of
the steel reinforcement increases as the potential
diminishes. There is, however, a lot of uncertainty in the
middle range of potentials (-200 to -350mV). The problem
arises primarily from the way the potential of the steel
reinforcement is determined in practice. Figure 12 shows
diagrammatically the current distribution around a
corroding site (the anode) through the surrounding
concrete to the adjacent cathode regions. These constant
current flux lines, give rise to equipotential lines distributed
through the concrete as shown in Figure 12. In a region
above the anode the "apparent" potential is Ea (Fig. 12)
and at some distance away at the cathode it is Ec. The
value of Ea is normally less (more -ve) than that of Ec so
local corrosion sites can be identified. It can be seen,
however, that the potential measured on the surface is
not necessarily that of the anode. This "surface" potential
is influenced by parameters such as the concrete cover
depth and the resistivity of the concrete which can alter
the constant current flux lines and subsequently the
equipotential lines. Wetting of the concrete will inevitably
reduce the resistivity of the concrete and therefore change
significantly the potential characteristics.
Figure 12 Current and potential distribution in concrete near anodic corrosion
A further complication which gives rise to a common
misunderstanding, is the fact that steel embedded in
submerged or waterlogged structures can exhibit
potentials which are more negative than -0.35 V (CSE
scale) whilst suffering negligible corrosion. This is caused
by the inability of oxygen to penetrate the cover depth in
sufficient quantities which leads to the polarisation of the
cathodic reaction as shown in Figure 13. Provided the
condition of the concrete is taken into account, potential
mapping, when used on its own, can provide a reasonable
assessment of the corrosion activity of the reinforcement
at the time of the survey. It cannot, however, provide
information on the rate or extent of corrosion.
Figure 13 Evans diagram for passive steel in uncontaminated concrete containing
different concentrations of oxygen
Resistivity measurements
As was shown earlier (see for example Fig. 5), the
resistance of the cover concrete can have a significant
effect on the corrosion rate of the reinforcement. The rate
of corrosion at the anode is dependent on the ease with
which ions can pass through the concrete pore electrolyte
between the cathode and anode. Hence a large potential
gradient between the anode and cathode associated with
a low concrete resistivity will normally signify a high
corrosion rate of the reinforcement.
The 4-point Wenner technique, developed from its use in
geotechnical surveying, is a technique that can measure
the resistivity of concrete. It involves passing an
alternating current between the outer pair of four
equispaced probes in contact with the concrete surface,
as shown in Figure 14. The resistivity of the concrete can
be calculated from the measured voltage between the
inner probes from:
Where, a, is the spacing between the probes.
The resistivity of the concrete will normally be affected by
many factors including moisture and salt content of the
concrete, mix proportions, water /.cement ratio, type of
cement replacement etc. As a general rule, if the resistivity
of the concrete is lower than 10,000 Ωcm, the corrosion
rate of the reinforcement which is suspected from a
potential mapping survey to be corroding, is likely to be
high. Higher resistivity values would normally signify a
lower corrosion risk.
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In theory, it should be possible to estimate fairly accurately
with the use of computer modelling the corrosion current
of steel reinforcement from a combination of potential
mapping and resistivity measurements. In practice, there
are, however, limitations as to the accuracy of the obtained
potential values, as was shown earlier, and of the resistivity
values owing to the inhomogeneity of concrete. Increasing
the spacing between the probes will diminish the effects
of localised non homogeneity but there is a l imit
determined by the concrete cover depth as the highly
conductive steel provides an easy path for the input
current. Furthermore, resistivity on the surface layer of the
concrete is different to that of the bulk owing to curing
effects, differential wetting or drying and particularly
carbonation of the surface layer. It is common practice to
drill shallow holes at the positions where the probes will
be located so that these surface effects are minimised.
Linear polarisation techniques
The actual corrosion rate of a section of steel reinforcement
can be determined by linear polarisation. The technique
essentially involves shifting the corrosion potential of a
known area of steel reinforcement in either the positive or
negative direction with the use of a potentiostat and
measuring the current that flows between the steel and
an external auxiliary electrode placed in close proximity
to the steel and in contact with the concrete via the
potentiostat. This can be done in single steps of potential
shifts or by scanning the potential over a range of typically
± 20mV with respect to the corrosion potential.8 The
increase in current at such a small shift in potential is
essentially linear and the gradient of the current- potential
plot gives the polarisation resistance, Rp, which can be
related to the corrosion rate by the Stern and Geary
equation:
where βa and βc are the anodic and cathodic Tafel
slopes respectively.The slopes can be determined
experimentally but they are normally assumed to be about
120 mV/decade so that the above equation is simplified
to: Rp = 26 / Icorr
As the corrosion rate can vary by several orders of
magnitude such an assumption is acceptable and will
only introduce comparatively small errors in the calculated
corrosion rates. Laboratory work has shown that corrosion
rates of steel in concrete calculated in this way are in
good agreement with weight loss determinations of the
same steel.9 The biggest limitation of this technique is
the inherent difficulty of applying it to large structures
where the steel reinforcement cage is electrically
connected and the true area of the steel being polarised
during the test is diff icult to establish. Recent
developments10 have attempted to concentrate the
polarisation over a small calculated area of the steel
reinforcement with the use of a special anode
arrangement (Figure 15) but problems are always likely
to exist because of variations in the concrete cover depths,
the geometry of the structure and the distribution of the
steel reinforcement bars.
Figure 15 Arrangement of insitu linear polarisation device
Final assessment
A combination of the assessment techniques described
above, should give a fair indication of the present
condition of the reinforcement. They do not, however, show
how the corrosion varies with the change of environmental
conditions. That information can be obtained by frequent
visits to the site or by continuous monitoring with the use
of embeddable reference electrodes and other probes.
Neither do they reveal the extent of the corrosion and the
length of time that corrosion had been occurring. Figure
16 describes the different stages of the corrosion process.
There is a virtually corrosion-free period before initiation
occurs (a), followed by a period of corrosion propagation
(a-b) before the reinforced structure totally loses its
serviceability. A survey of the kind described earlier can
Figure 16 Stages of structural element deterioration related to reinforcement
corrosion
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easily detect whether you lie either before or after point a
of the graph. Determining exactly where you are on the
propagation stage is much more difficult and would require
exposure of the reinforcement and a detailed structural
assessment.
If the correct choice of assessment techniques is made,
the limitations of the techniques employed are well
understood and the results are interpreted wisely, it is
possible to determine whether the structure is sound or
whether some remedial measures are required. The
assessment should also assist in the choice of
rehabilitation techniques but that is practically a different
discipline.
References
1. Barneyback R. S. & Diamond S, "Expression and analysis of
pore fluid from hardened cement pastes and mortars", Cem.
&Concr. Res. 11, 1981, 279-285.
2. Arup H "The mechanisms of the protection of steel by concrete",
Crane A. P. (ed), 'Corrosion of Reinforcement in Concrete
Construction', Ellis Horwood, Chichester, 151-157, 1983.
3. Gonzalez A., Algaba S. & Andrade C. 'Corrosion of reinforcing
bars in carbonated concrete" Br. Corros. J.,15, 1980, 135-139.
4. Hausmann D. A "Steel corrosion in concrete" Mater. Prot. 6,
1967, 19-23.
5. Burstein Q.T. "Passivity and localised corrosion" Shreir L . L.,
Jarman R. A. & Burstein G.T, (eds), 'Corrosion', 3rd edn., 1994,
1.118-1.150.
6. Page C. L. & Tteadaway K. W. J. "Aspects of the electrochemistry
of steel in concrete, Nature, 297, 1982, 109-116.
7. Glass G. K., Page C. L. & Short N. R. "Factors affecting the
corrosion rate of steel in carbonated mortars" Corros. Sci., 32,
1991, 1283-1294.
8. Sergi G., Lattey S. & Page C.L. "Influence of surface treatments
on corrosion rates of steel in carbonated concrete" Page C. L.,
Treadaway K. W. J. & Bamforth P. B. (eds), 'Corrosion of
Reinforcement in Concrete Construction', SCI / Elsevier, London,
1990, 409-419.
9. Andrade C. & Gonzalez J. A. "Quantitative measurements of
corrosion rate of reinforcing steels embedded in concrete using
polarization resistance measurements", Werkstoffe und Korrosion
29, 1978, 515-519.
10. Rodriguez J., Ortega L. M., Garcia A. M., Johansson L. & Petterson
K. "On-site corrosion measurements in concrete structures"
Construction Repair, Nov./Dec, 1995, 27-30
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Recent Developments in Mitigationof Rebar Corrosion in Concrete
M N Ramesh, CEO
Savcor India Private Limited
Corrosion of reinforcing steel in concrete is a
widespread and enormously costly problem
worldwide. Numerous concrete structures
including bridge decks and substructures, parking
garages, balconies and others are deteriorating as a result
of reinforcing steel corrosion. Virtually any reinforced
concrete structure is susceptible to the ravages of
corrosion if subjected to a conducive environment. The
corrosion process that takes place in concrete is
electrochemical in nature, very similar to a battery.
Electrochemical corrosion is a phenomenon accompanied
by a flow of electrons between cathodic and anodic areas
on a metal surface. In concrete the electro-chemical
corrosion reactions are most often triggered when three
factors- chloride, oxygen and moisture-meet at the
reinforcing steel surface. A sort of natural battery develops
within the reinforced concrete structure, generating a low-
Fig.1 Cracking due to corrosion of reinforcement
level internal electrical current. The points where this
current leaves the metal surface and enters the concrete
electrolyte are called anodes. The current leaving the
concrete and returning to the steel does so at the cathodes.
Corrosion or oxidation (rust) occurs only at anodes. When
corrosion of reinforcing steel occurs, the rust products
occupy more volume than the original steel, causing
tensile forces in the concrete.
Since concrete is relatively weak in tension, cracks soon
develop as shown in Figure 1, exposing the steel to even
more chlorides, oxygen and moisture-and the corrosion
process accelerates. As corrosion continues, delamination-
separations within the concrete and parallel to the surface
of the concrete occur. Delamination is usually located at,
or near, the level of reinforcing steel. Eventually concrete
chunks break away or spall off. Visual signs of corrosion-
induced damage on many types of reinforced concrete
structures are becoming more and more prevalent. In many
parts of the country one can hardly drive across a bridge
or enter a building that doesn't have some degree of
corrosion damage!
The rate of concrete deterioration at any given time is
dependent on many factors including corrosion rate,
reinforcing steel concentration, concrete properties, cover
and the environment, to name a few. Once corrosion has
begun there is one thing for certain-it will only get worse
and it will do so at an ever-increasing rate (see fig.2) .
Ultimately, if corrosion is allowed to continue, structural
integrity can be compromised due to loss of section of the
reinforcing steel and/or loss of bond between the steel
and the concrete, and replacement may be the only
solution. In order to mitigate or control a corrosion problem
(provide low future maintenance and long term protection)
specific information is needed for any given structure.
Fortunately, proven technology and scientific methods are
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Fig 2. Time dependent rebar corrosion based on various factors
available to evaluate corrosion of reinforcing steel (and
other embedded metals) and associated damage on
reinforced concrete structures. These techniques are
designed to determine the extent of damage, define the
corrosion state of steel in undamaged areas, evaluate the
cause, or causes, of corrosion, and determine the potential
for the steel to corrode in the future resulting in further
damage. It is only after this information is obtained through
a detailed corrosion condition evaluation that a suitable
repair and protection specification can be developed for
a corrosion-plagued structure. It is important to point out
that concrete; itself can deteriorate, regardless of the
condition of embedded reinforcement. Examples of this
include freeze/thaw damage, sulphate attack and alkali-
silica reactions. Various concrete tests are therefore often
conducted as part of an overall evaluation. Although there
are similarities between corrosion of conventionally
reinforced concrete structures and prestressed concrete
structures, this paper deals with conventionally reinforced
concrete structures only, particularly with respect to the
applicability of Electro-chemical protection techniques
that are developed recently in the area of corrosion
engineering.
Electro chemical method of rectifying carbonation:
This method is a recent development in concrete
restoration. This addresses the root cause of carbonation
and not its symptoms. Hence, this is a permanent solution.
The principle of the solution is driving an alkaline solution
into the carbonated concrete by an electro chemical
process. It comprises of installing an external anode kept
inside an alkaline electrolyte solution on the surface of the
concrete. This solution can either be sodium carbonate or
sodium bicarbonate. A direct current rectifier with its
positive terminal connected to external anode and its
negative terminal is connected to the reinforcing steel
making it cathode. Upon switching on the system the
alkaline solution will move inside concrete cover and will
reach the reinforcing steel making the entire section and
around the reinforcing steel alkaline. Thus, this process
corrects the carbonation by realkalining the concrete, and
forming hydroxyl ions around the steel. The schematic
details of the process are illustrated in the sketch below:
Electro-chemical remedial measures for ChlorideContaminated Concrete:
The problem associated with the chloride contaminated
concrete is pitting corrosion of rebars. The traditional
methods, to remedy the problem, has been isolating the
rebars from the concrete and replacing the removed
concrete by a new material free from chloride such as
polymer-modified mortar or micro concrete. This method
however suffers the impracticability of removing concrete
behind the bars and reinstating the same with new
material, if a very large extent of concrete is affected by
Fig 3. Realkalisation setup for correcting corbonation of concrete Fig.2. Setup for desalination or chloride extraction of concrete
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chlorides. In order to overcome the above problem the
recent development in the area of remediation to the
chloride attacked concrete is "desalination" and "cathodic
protection". These methods being electro chemical in
nature address the root cause of the problem and not the
symptoms. Electrochemical Chloride Extraction The
Electro Chemical Chloride Extraction (ECE), extract
chloride ions from contaminated concrete and is used to
reduce chloride concentration near steel reinforcement to
a level below the corrosion threshold. Hydroxyl ions are
also generated at the reinforcement, which allows the
passive oxide layer on the steel to be reformed after
treatment. An externally placed anode in the electrolytic
solution is connected to the positive pole and the
reinforcing steel is connected to the negative pole of the
rectifier. When the system is powered, all the negatively
charged chloride ions will be repelled by the cathode, i.e.
steel, and are forced migrate towards the anode which is
place outside the concrete. These ions will be neutralized
by the anode thereby extracting all the chloride ions from
the concrete. This method is also known as Desalination
Process. The schematic of the process setup is illustrated
in fig 2 This method is recommended for the concrete
structures which are located in the chloride laden
atmosphere. After the process is completed, further
diffusion of chloride ions in to the concrete should be
prevented by providing appropriately selected protective
barrier concrete surface coatings.
Impressed Current Cathodic Protection (ICCP)
Impressed current cathodic protection is achieved by
driving a low voltage direct current from a relatively inert
anode material, through the concrete to the reinforcing
steel. Figure 3 shows the basic layout required for
impressed current cathodic protection systems. Direct
current of sufficient magnitude and polarity is applied, so
as to oppose the natural flow of current resulting from the
electrochemical corrosion process. The direct current is
supplied by an external power source, most often a CP
rectifier. Recently, the use of solar power has received
attention and further research is going on for its adaptation
in actual structures. Impressed Current Cathodic
protection is a widely used and effective method of
corrosion control. Many people, engineers included, think
cathodic protection is so complicated and expensive that
it has no practical use in the concrete rehabilitation
industry. It is also believed that CP doesn't work or that it is
unreliable in the long term. The fact is ICCP is not so
complicated, is often the most cost-effective technique,
and has practical application on reinforced concrete
structures and that it most definitely works. Of course,
performance of ICCP systems, like all other corrosion
protection systems, is directly dependent on sound
specifications, proper installation, and monitoring and
maintenance. With ICCP, one cannot simply install it and
forget it. Good long term performance of all ICCP systems
requires good monitoring and maintenance procedures,
often due to this fact is discounted as a corrosion protection
system. Every corrosion protection system requires
Fig 3. Schematic of impressed current cathodic protection of reinforcement in
concrete
periodic inspection and maintenance. Impressed Current
Cathodic protection has been successfully used to protect
pipelines, ship hulls, off shore oil platforms, heat
exchangers, underground tanks, and many other facilities
exposed to a corrosive environment for many decades. Its
first application to steel in concrete was only in 1973. Since
then, many r c structures are protected with ICCP.
Recognizing that the corrosion process generates electric
currents, Impressed Current Cathodic protection supplies
a source of external current to counteract the corrosion
current. Hence, corrosion stops, or at least is greatly
minimized.
After an intensive research in the areas of corrosion of steel
in concrete, cathodic protection evolved as the only
technique which could positively arrest corrosion of steel
in existing concrete structures. In fact, cathodic protection
is the only rehabilitation technique that had proven to stop
corrosion in chloride affected concrete regardless of its
concentration in concrete. It should be noted, however,
that ICCP is not always needed nor is suitable on all
structures.
To select and design a proper repair and protection
scheme it is imperative that the causes of the distress are
properly diagnosed, fully understood, and the extent of
damage is determined Hence the first step is to have a
concrete and corrosion condition survey conducted in
order to define the cause and extent of the problem.
Electrical continuity of the reinforcing steel to be protected
is also a primary factor in considering ICCP. A closed
electrical circuit (unbroken electrical path) between all
Concrete Corrosion Reinforcing Steel
The Masterbuilder - March 2012 • www.masterbuilder.co.in122
reinforcing steel is required in order for the ICCP system to
function properly. Electrical continuity testing can be done
during the condition survey. The chloride concentration in
the concrete throughout the structure is also important. If
sufficient chlorides are present at the reinforcing steel depth
in many areas of the structure, ICCP may be the
economically viable alternative Based on the results of the
condition survey determination is made on the type of
repair and protection method to use. One advantage of
ICCP is that removal of sound concrete is not required,
thus a considerable cost savings may be realized. It may
be a viable alternative to removing two or three inches of
concrete over a large area in order to prevent future
corrosion. Cathodic protection is usually most cost effective
when long term performance is desired.
Galvanic, or sacrificial anode, cathodic protection is based
on the principles of dissimilar metal corrosion and the
relative position of as shown in Table.1. No external power
source is needed with this type of system and much less
maintenance is required. Such systems also provide
protective current primarily to areas on the steel surface
which need it the most. However, the relatively high
resistivity of concrete results in driving low voltage
provided by such systems would be inadequate for
cathodic protection of steel in concrete especially in the
splash zones of marines rc structures subjected to drying
and wetting cycles. Sacrificial anodes can be used to
mitigate corrosion in certain circumstances especially for
patch repair, specifically, if corrosion activity is low. In case
that pitting corrosion has not initiated or propagated but
in a situation prone to active corrosion, such as splash
zone, a full active impressed current system is the only
technically sound and cost effective option of corrosion
mitigation However, if the chloride content is relatively low,
or if the chlorides are generally located only in isolated
areas of the structure, sacrificial anode system may be
most appropriate. Basically sacrificial anode may enhance
the repair work but in most cases does not deliver the
protection in accordance to CP standards. So in theory it
can't be called as CP. In any real corrosion problem
situation where pitting corrosion has initiated and
propagated, high current density, may be around 20mA/
m2 of steel surface, is required to bring the potential of
rebars to certain level at which corrosion is arrested. This
cannot be delivered by sacrificial anodes.
Galvanized rebars
The zinc on galvanized reinforcing steel functions as a
sacrificial anode much the same way as zinc in a sacrificial
anode system does. In this case, the steel is protected by
the zinc from the day the rebar is galvanized. However,
once all the zinc is consumed, the base steel will be
susceptible to corrosion in the same way as plain
reinforcing steel.
Summary
Reinforcing steel corrosion causes extensive damage to
concrete structures. Various NDT methods are successfully
employed to carryout condition survey to evaluate
corrosion of reinforcing steel and the associated damage
on reinforced concrete structures. These tools help in
determining the extent of damage, define the corrosion
state of steel in undamaged areas, evaluate the causes of
corrosion and determine the potential for the steel to
corrode in the future resulting in further damage.
The recent development in the electro-chemical methods
are aimed at addressing the cause of the problem there
by ensuring a long-lasting solution to the corrosion
problem. Cathodic protection is a widely used and effective
method of corrosion control for reinforced concrete
structures. Cathodic protection supplies a source of
external current to counteract the corrosion current. Hence,
corrosion stops or minimizes to a greatly low level.
Almost any atmospherically exposed reinforced concrete
structure or structural members of almost any geometry
can be catholically protected. However, existing structures
must be considered individually with regard to the need
for the applicability of CP. Not all structures are good
candidates for CP, but ICCP is the only system that can
truly retard or mitigate corrosion in the harshest of the
environment.
References:
- Atef Cheaitani, M N Ramesh- Corrosion prevention
considerations to achieve a 100 year design life for reinforced
concrete structures in marine environments, NCCI
Seminar,2010
- Atef Cheaitani, M N Ramesh- Maintaining infrastructure - the
latest development in the repair and maintenance of reinforced
concrete structures, Asian Conference on ecstasy in concrete
2010, Indian Institute of Technology Madras, Chennai
- Ali Sohanghpurwala and William T. Scannell -' Repair and
protection of Concrete Exposed to Seawater
Element
Electrode Potential
Magnesium
Electrode P-2.38
Aluminium
-1.67
Zinc
-0.78
Chromuim
-0.58
Iron/Steel
-0.44
Nickel
-0.25
Tin
-0.14
Hydrogen
0.00
Platinum
+1.2
Gold
+1.80
Table 1: specific metals in the galvanic series
Concrete Corrosion Reinforcing Steel
The Masterbuilder - March 2012 • www.masterbuilder.co.in124
Repair Principles for Corrosion DamagedReinforced Concrete Structures
MG Alexander and JR Mackechnie
Department of Civil Engineering, University of Cape Town
Corrosion is the inevitable process that occurs when
refined metals return to their more stable combined
forms as oxides, carbonates and sulphides. The
corrosion process may be defined as the surface wastage
that occurs when metals are exposed to reactive
environments. Costs associated with corrosion damage
and control can be substantial, being as much as 3.5% of
the GNP of some industrial countries.
Reinforced concrete structures have not been immune to
the ravages of corrosion despite the protection that
concrete provides to embedded steel. Reasons for the
increasing incidence of corrosion damage to reinforced
concrete structures include the use of deicing salts and
calcium chloride set-accelerators, increased construction
in aggressive environments, fast-track construction
practices, changing cement composition resulting in finer
grinding and lower cement contents, lower cover depths
and poor construction practice including inadequate
supervision.
Reinforcement corrosion is particularly pernicious in that
damage may occur rapidly and repairs are invariably
expensive. Furthermore by the time visible corrosion
damage is noticed, structural integrity may already be
compromised. There is currently considerable debate
about the merits of the various systems for the repair of
reinforcement corrosion. This monograph attempts to
clarify some of the important issues by drawing on
international experience as well as local findings.
Ultimately the effectiveness of repair systems should be
measured in terms of cost, risk of failure and long-term
performance. As such no single system is appropriate for
all repairs but will depend on the type of structure, service
conditions, level of deterioration and financial constraints
of the project.
This monograph focuses on repair principles rather than
dealing with issues of detail that have been competently
published by others. Repair options can only be rationally
compared when the corrosion process and its influence
on concrete are fully understood. The document also
focuses on South African conditions and experiences,
derived from almost ten years of research on concrete
durability and repairs at the University of Cape Town.
Corrosion Fundamentals
Steel reinforcing bars will corrode freely when exposed to
moisture and oxygen under ambient conditions. When
steel is embedded in concrete however the high alkalinity
(pH of 12.5 or higher) stifles corrosion by the formation of
a passive ferric oxide film on the steel surface. The ferric
oxide layer forms a dense, impenetrable film that
suppresses further corrosion by limiting the movement of
cations and anions near the steel surface. This passive
ferric oxide film on embedded reinforcement may be
disrupted by a reduction in the alkalinity of the concrete
(principally by carbonation) or by the presence of
aggressive ions such as chlorides and sulphates.
Depassivation of the steel occurs as follows:
- in carbonated concrete, insufficient hydroxyl ions are
available to repair pits in the passive film
- in salt contaminated concrete, chloride ions break
down the passive layer at localized pits and encourage
metallic dissolution
Once depassivating conditions exist in concrete either
by a reduction in alkalinity (pH <10.5) or by the presence
of sufficient chloride ions (termed the corrosion threshold
value), corrosion may occur. For corrosion to occur at a
significant rate the following conditions are required:
- a reactive metal that will oxidise anodically to form
soluble ions
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- a reducible metal that provides the cathodic reactant
(typically hydroxyl ions)
- an electrolyte that allows ionic movement between
the material and environment
It is important to note that the establishment of
depassivating conditions at the steel (i.e. carbonation or
chlorides) is not necessarily indicative of a high probability
of corrosion damage since other factors (e.g. oxygen
availability, moisture content) will largely determine the
rate of corrosion. A schematic diagram of the corrosion
process of steel in con crete is shown in Figure 1.
Figure 1: Schematic of corrosion of steel in concrete
Four states of corrosion may be defined for reinforced
concrete depending on environmental conditions1:
- Passive state where minute levels of corrosion are
needed to sustain the ferric oxide film (typical of
embedded reinforcement in sound, alkaline and
uncontaminated concrete).
- Pitting corrosion causing local breakdown of the
passive film, usually due to the presence of chloride
ions. Adjacent steel acts as the cathode, being
considerably larger in area than the anode (typical of
steel embedded in chloride contaminated concrete).
- General corrosion due to an overall loss of passivity
that results in multiple pits along the steel surface
(typical of steel in carbonated concrete or concrete
containing high chloride concentrations).
- Active, low potential corrosion that occurs slowly when
insufficient oxygen is available to sustain the passive
film despite the high alkalinity of the concrete (typical
of reinforcement embedded in concrete underwater).
Clearly only pitting and general corrosion represent a
threat to the reinforcement and their severity will depend
on a number of internal and external factors which need
to be assessed when doing a corrosion survey. Internal
factors include concrete microstructure, cover depth and
moisture condition. External influences such as stray
currents and microbial activity may introduce a new
dimension into the corrosion system, but are not
considered here.
The nature of steel corrosion in concrete depends on local
conditions at the surface of the bar. High resistivity
concrete with relatively deep covers tends to favour micro-
cell corrosion where anode and cathode are close together
and cause localized pitting. Conductive concrete
contaminated with salt is often able to sustain more widely
spaced anode and cathode sites, termed macro-cell
corrosion.
Corrosion Damage
Once the passive layer on the reinforcing steel has been
disrupted and corrosion is activated, the chemical
reactions are similar whether the corrosion was initiated
by chloride attack or by carbonation. Steel dissolves into
solution and gives up electrons at the anode.
Anodic reaction: Fe ↑Fe2++ 2e- . . . . . . . . . . . . . . . . . . . . . (1)
The excess electrons are used up at the cathodic site
where water and oxygen are reduced to hydroxyl ions.
Cathodic reaction: 2e- + H2O + ½O
2 ↑2(OH)- . . . . . . . . (2)
These two reactions are necessary for electrochemical
corrosion to proceed. Little distress would be caused to
the surrounding concrete however if steel merely dissolved
into the pore water without further oxidation. Several more
oxidation stages occur which form expansive corrosion
products or rust capable of causing cracking and spalling
of the surrounding concrete. The oxidation stages may
be described as follows:
Fe2+ + 2(OH)↑Fe(OH)2Ferrous hydroxide . . . . . . . . . . . . (3)
4Fe(OH)2 + O
2 + 2H
2O ↑4Fe(OH)
3 Ferric hydroxide . . .
................... . (4)
2Fe(OH)3 ↑Fe
2O
3.H
2O + 2H
2O Hydrated ferric oxide . .
................... . (5)
The expansion associated with rust is mostly due to
hydrated oxides that may swell up to ten times the original
volume of the steel. The type of corrosion product formed
at the steel depends on environmental con ditions:
- red or brown rust forms under high oxygen
concentrations, forming flakey rust which is relatively
soft and easy to dislodge from the rebar
- black rust forms under low oxygen concentrations,
forming a relatively dense and hard layer that may be
difficult to remove from the parent steel
Two major consequences of reinforcement corrosion are
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commonly observed, cracking and spalling of the cover
concrete as a result of expansion of the corrosion product,
and a reduction of cross-sectional area of the rebar by
pitting (usually only a problem in prestressed con crete
structures). Manifestations of corrosion depend on a
number of influences that include:
- geometry of the element (large diameter bars at low
covers allow easy spalling)
- cover depths (deep cover may prevent full oxidation
of corrosion product)
- moisture condition (conductive electrolytes
encourage well-defined macro-cells)
- age of structure (rust stains progress to cracking and
spalling)
- rebar spacing (closely spaced bars in walls and slabs
encourage delaminations)
- crack distribution (cracks may provide low resistance
paths to the reinforcement)
- service stresses (corrosion may be accelerated in
highly stressed zones)
The loss of serviceability of corroded reinforced concrete
structures may be described by a three phase damage
Figure 2: Three-phase corrosion damage model
model shown in Figure 2 2.
The different phases are defined as follows:
- An initiation period, before corrosion is activated by
either carbonation or chloride attack, during which
negligible concrete deterioration occurs.
- A propagation period in which active corrosion
commences and cracking of the cover concrete
occurs due to the formation of expansive corrosion
products at the steel surface.
- An acceleration period of damage where corrosion
increases due to easy access of oxygen and water
through cracks in the cover concrete, resulting in
spalling of concrete.
Unfortunately most reinforced concrete structures that
exhibit cracking and spalling have gone beyond the point
where simple, cost-effective measures can be taken to
restore durability. Condition surveys are therefore an
important strategy to identify and quantify the state of
corrosion of a structure timeously.
Condition Surveys
A detailed corrosion or condition survey is vital in order to
identify the exact cause and extent of deterioration, before
repair options are considered. Various diagnostic sheets
are given in the Appendix for guidance during condition
surveys.
a) Visual assessment
Corrosion damage may be identified and defined using a
systematic visual survey. Classification of visual evidence
of deterioration must be done objectively, following clear
guidelines that define damage in terms of appearance,
location and cause. Defects may be defined in terms of
cracks (caused by corrosion, temperature, shrinkage or
fatigue), joint deficiencies (joint spalls, upward movement,
lateral movement, seal damage) surface damage
(abrasion, rust stains, delaminations, popouts, spalls),
changes in member shape (curling, deflection, settlement,
deformation) and textural features (blow holes,
honeycombing, sand pockets, segregation).
Visual assessment of deterioration can provide useful
information when done in a rational, systematic manner
but the data may come too late for cost-effective repairs.
Rebar corrosion damage is often only fully manifest at the
surface after significant deterioration has occurred. Early
evidence of distress can sometimes be detected by an
experienced engineer before major distress takes place.
b) Delamination survey
A hammer survey or chain drag is a simple method of
locating areas of delamination in concrete. Hollow
sounding areas can be marked up on the concrete or
recorded directly in a survey form. Delamination surveys
often under-estimate the full extent of internal cracking
and should not be considered as definitive. Radar and
ultrasonic instruments may provide a more sophisticated
approach to locating areas of delamination, particularly
at greater depths.
c) Cover surveys
Cover surveys are routinely done to locate the position
and depth of reinforcement within a concrete structure.
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Covermeters use an alternating magnetic field to locate
steel and any other magnetic material in concrete (note
that austenitic stainless steels are non-magnetic). Cover
measurements may be unreliable when:
- rebar is at deep covers (e.g. covers greater than 80
mm)
- measuring regions of closely spaced bars
- measuring differing bar types and sizes (unless
specifically calibrated) o other magnetic material is
nearby (e.g. window frames, wire ties, bolts) To ensure
reliable cover depths from a survey, direct
measurements of rebar depths should be made by
exposing a limited number of bars. Calibration can
then be made for site specific conditions such as rebar
type, concrete and environmental influences.
d) Chloride testing
The presence of sufficient chloride at the surface of
reinforcement is able to depassivate steel and allow
corrosion to occur. Chlorides exist in concrete as both
bound and free ions but only free chlorides directly affect
corrosion. Measuring free chlorides accurately is
extremely difficult and water-soluble chloride tests are
unreliable, being strongly affected by the method of
sample preparation. Further, bound chlorides may be
released into solution under carbonating conditions or
by dissolution, making all chlorides in concrete potentially
corrosive. Chlorides are therefore most commonly
determined as acid soluble or total chlorides in
accordance with BS 1881 3.
Chloride sampling and determination in concrete is
illustrated in Figure 3 and is usually done in the following
manner:
- concrete samples are extracted as either core or
drilled powder samples
Figure 3: Chloride content determination and typical chloride profile
- depth increments are chosen depending on the cover
to steel and the likely level of chloride contamination
(increments are typically between 5 and 25 mm)
- dry powder samples are digested in concentrated
nitric acid to release all chlorides
- chlorides are analysed using a colorimetric or
potentiometric titration
- chloride contents are generally expressed as a
percentage by mass of cement
- chloride profiles may be drawn such that chloride
concentrations may be interpolated or extrapolated
for any depth (see Figure 3)
- future chloride levels can be estimated from Fick's
second law of diffusion
Table 1: Qualitative risk of corrosion based on chloride levels
Chloride content by mass of
cement (%)
< 0.4
0.4 - 1.0
> 1.0
Probability of corrosion
Low
Moderate
High
The corrosion threshold depends on several factors
including concrete quality, cover depth, and saturation
level of the concrete. The probability of corrosion may be
assessed from the following qualitative rating shown in
Table 1 for acid-soluble chloride contents.
Limitations of chloride testing of concrete are asfollows:
- presence of chlorides in aggregates may give
misleading results
- chloride contents in cracks and defects cannot be
accurately determined
Figure 4: Schematic of the carbonation process
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- slag concretes may be difficult to analyse with
colorimetric titration methods
- relatively large samples are required to allow for the
presence of aggregates
e) Carbonation depth
Carbonation depth is measured by spraying fresh
concrete with a phenolphthalein indicator solution (1%
by mass in ethanol/water solution). Phenolphthalein
remains clear where concrete is carbonated but turns
pink/purple where concrete is still strongly alkaline (pH
> 9.0). Carbonation moves through concrete as a distinct
front and reduces the natural alkalinity of concrete from a
pH in excess of 12.5 to approximately 8.3, with a pH level
of 10.5 being sufficiently low to depassivate steel. The
progress of the carbonation front is shown in Figure 4.
Environmental conditions most favourable for carbonation
(i.e. 50 - 65 % R.H.) are usually too dry to allow rapid steel
corrosion that normally requires humidity levels above
80% R.H. Structures exposed to fluctuations in moisture
conditions of the cover concrete, such as may occur during
rainy spells, are however vulnerable to carbonation-
induced corrosion.
Limitations to carbonation testing are as follows:
- phenolphthalein changes colour at pH 9.0 whereas
steel depassivation occurs at a pH of approximately
10.5, hence the corrosion risk is slightly under-
estimated
- some concretes are dark (e.g. slag concretes) and a
distinct colour change is difficult to discern visually
- phenophthalein may bleach at very high pH levels
(e.g. after electrochemical realkalization)
- testing must be done on freshly exposed concrete
surfaces before atmospheric carbonation occurs
f) Rebar potentials
Chloride-induced corrosion of steel is associated with
anodic and cathodic areas along the rebar with
consequent changes in electropoten tial of the steel. It is
possible to measure these rebar potentials at different
points and plot the results in the form of a 'potential map'.
Measurement of rebar potentials may determine the
thermodynamic risk of corrosion but cannot evaluate the
Rebar potential (-mV Cu/CuSO4)
< 200
200-350
>350
Qualitative risk of corrosion
Low
Uncertain
High
Table 2: Qualitative risk of chloride-induced corrosion 4
kinetics of the reaction. Rebar poten tials are normally
determined in accordance with ASTM C876 using a
copper/copper sulphate reference electrode connected
to a handheld voltmeter 4. The qualitative risk of corrosion
based on rebar potentials is shown in Table 2. Note that
the technique is not recommended for car-bonation-
induced corrosion where clearly defined anodic regions
are absent.
The procedure for undertaking a rebar potential survey is
as follows:
- mark up a grid pattern in the area of measurement
(not more than 500 mm centres)
- make an electrical connection to clean steel by coring
or breaking out concrete
- check the steel is electrically continuous over the
survey area using a multimeter
- wet the concrete surface with tap water if the concrete
appears to be dry
- take and record readings either manually or using a
data logger
- check data on site to correlate with visual signs of
corrosion
- Rebar potential measurements are relatively quick to
perform but have the following limitations:
- interpretation of results must be done with caution
(preferably by a specialist)
- rebar potentials from carbonated concrete are difficult
to interpret (the reading is a mixed potential of anodic
and cathodic sites)
- delaminations may disrupt the potential f ield
producing false readings
- environmental effects will influence potentials (e.g.
temperature and humidity)
- rebar potentials cannot be directly correlated with
corrosion rates
- stray currents may affect measured potentials
Absolute values are often of lesser importance than
differences in rebar potential measured on a structure. A
shift of several hundred millivolts over a short distance of
300-500 mm often indicates a high risk of corrosion.
g) Resistivity
Concrete resistivity controls the rate at which steel
corrodes in concrete once favourable conditions for
corrosion exist. Resistivity is dependent on the moisture
condition of the concrete, on the permeability and
interconnectivity of the pore structure, and on the
concentration of ionic species in the pore water of concrete
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such that:
- poor quality, saturated concrete has low resistivity (e.g.
less than 10 kOhm.cm)
- high quality, dry concrete has high resistivity (e.g.
greater than 25 kOhm.cm)
Measurement of resistivity is done with a simple in situ
Wenner probe connected to a portable resistivity meter.
The outer two probes send an alternating current through
the concrete while the inner two probes measure the
potential difference in the concrete. Once the concrete
resistivity is known a rough assessment of likely corrosion
rates can be made as shown in Table 3. This assessment
assumes conditions are favourable for corrosion.
Resistivity measurements are simple to perform on site
but have several limitations:
- measurements are affected by carbonation and
wetting fronts
- surface conductive layers and rebar directly below
the probe should be avoided
- readings may be unstable in concretes with high
contact resistance at the surface
Table 4: Qualitative assessment of site corrosion rates
Corrosion rate
(? A/cm2)
> 10
1.0 - 10
0.2 - 1.0
< 0.2
Qualitative assessment of corrosion rate
High
Moderate
Low
Passive
h) Corrosion rate measurements
Corrosion rate measurements are the only reliable method
of measuring actual corrosion activity in reinforced
concrete. A number of sophisticated corrosion monitoring
systems are available, based primarily on l inear
polarization resistance (LPR) principles. These
techniques require considerable expertise to operate
reliably. Corrosion rate measurements on field structures
are most commonly done using galvanostatic LPR
techniques with a guard-ring type sensor to confine the
area of steel under test. Experience indicates that
corrosion rates fluctuate significantly in response to
environmental and material influences and single readings
are generally unreliable. Table 4 shows a qualitative guide
for the assessment of corrosion rates of site structures 5.
Epair Strategies
Numerous repair options are available and new
technologies continue to make an impact in the field of
concrete repairs. The suitability and cost-effectiveness of
repairs depends on the level of deterioration and specific
conditions of the structure.
a) Patch repairs
Before patch repairs are considered it is important that
the distinction between chloride- and carbonation-
induced corrosion is appreciated. As a general rule
chloride-induced corrosion is far more pernicious and
difficult to treat than carbonation-induced corrosion. This
often dictates a completely different approach to
repairing damage due to the two types of corrosion.
Carbonation-induced corrosion causes general corrosion
with multiple pitting along the reinforcement. Carbonated
concrete tends to have fairly high resistivity that
discourages macro-cell formation and allows moderate
corrosion rates. Steel exposed to corrosive conditions will
therefore show signs of corrosion that can be easily
identified (e.g. surface stains, cracking or spalling of
concrete). Repairs are generally successful provided all
of the corroded reinforcement is treated.
Chloride-induced corrosion is characterized by pitting
corrosion with distinct anode and cathode sites. The
presence of high salt concentrations in the cover concrete
means that macro-cell corrosion is possible with relatively
large cathodic areas driving localized intense anodes.
High corrosion rates can be sustained under such
Figure 5: Formation of incipient anodes after patch repairs
Resistivity (kOhmcm)
< 12
12-20
>20
Likely corrosion rate given corrosive
conditions
High
Moderate
Low
Table 3: Likely corrosion rate based on concrete resistivity
Corrosion Repair & Rehabilitation
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conditions resulting in severe pitting of the reinforcement
and damage of the surrounding con crete. Much of the
reinforcement may be exposed to corrosive conditions
without showing any signs of corrosion, this is particularly
noticeable when corroded structures are demolished.
Localized patch repairs of areas of corrosion damage are
popular due to their low cost and temporary aesthetic
relief. This form of repair has limited success against
chloride-induced corrosion as the surrounding concrete
may be chloride-contaminated and the reinforcement is
therefore still susceptible to corrosion. The patched area
of new repair material often causes the formation of
incipient anodes adjacent to the repairs as shown in
Figure 5. These new corrosion sites not only affect the
structure but often also undermine the repair leading to
accelerated patch failures in as little as two years.
Consequently, it is necessary to remove all chloride-
contaminated concrete from the vicinity of the
reinforcement.
Complete removal of chloride-contaminated concrete,
where it is possible should successfully halt corrosion by
restoring passivating conditions to the reinforcement.
Mechanical removal of cover concrete is usually done
with pneumatic hammer, hydrojetting or milling machines.
This form of repair is most successful when treating areas
of localized low cover, before significant chloride
penetration has occurred. If repairs are only considered
once corrosion damage is fairly widespread it will be
expensive to mechanically remove chloride-
contaminated concrete from depths well beyond the
reinforcement.
Patch repairs consist of the following activities that are
briefly described below:-
- removal of cracked and delaminated concrete to fully
expose the corroded reinforcement
- cleaning of corroded reinforcement and the
application of a protective coating to the steel surface
(e.g. anti-corrosion epoxy coating or zinc-rich primer
coat)
- application of repair mortar or micro-concrete to
replace the damaged concrete
- possible coating or sealant applied to the entire
concrete surface to reduce moisture levels in the
concrete
b) Coating systems
A variety of coating and penetrant systems are available
that are claimed to be beneficial in repairs of concrete
structures. Barrier systems attempt to seal the surface
thereby stifling corrosion by restricting oxygen flow to the
cathode. In large concrete structures, corrosion control is
theoretically unlikely due to the presence of oxygen
already in the system. In practice barrier systems are
generally ineffective due to the presence of defects in the
new coating during application and further damage
during service. Such an approach is more likely to promote
the formation of differential aeration cells further
exacerbating the potential for corrosion.
The application of a hydrophobic coating (sometimes
referred to as penetrant pore-liners) may be used to reduce
the moisture content of concrete and thereby
electrolytically stifle the corrosion reaction. The drying
action works on the principle that surface capillaries
become lined with a hydrophobic coating that repels water
molecules during wetting but allows water vapour
movement out of the concrete, to facilitate drying.
Hydrophobic coatings using silanes and siloxanes are
gen erally most effective on uncontaminated concrete,
free from cracks and surface defects. The feasibility of
such an approach is questionable for marine structures
where high ambient humidity, capillary suction effects and
presence of high salt concentrations all interfere with
drying.
Figure 6: Sorptivity results from bridge cores
The long-term effectiveness of hydrophobic systems
applied to new construction is not known but local studies
suggest reasonable performance over 10-15 years service.
The Storms River bridge was coated with a silane system
in 1985 and concrete cores were extracted from several
parts of the structure in 1996 for analysis 6. The effect of
the hydrophobic coating on absorption was determined
by sorptivity testing at increasing depth increments into
the concrete. Sorptivity results are shown in Figure 6 for
arch and column concrete. The sharp increase in sorptivity
at depths between 0.5 and 3 mm may be ascribed to the
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Table 5: Likely performance of migrating corrosion inhibitors in concrete
Likely
inhibition
Good
Moderate
Poor
Corrosive
conditions
Mildly corrosive,
low chlorides or
carbonation
Moderate levels of
chloride at rebar
(i.e. <1%)
High chloride levels
at rebar (i.e. > 1%)
Concrete
conditions
Dense concrete
with good cover
depths (> 50 mm)
Moderate quality
concrete, some
cracking
Cracked, damaged
concrete, low
cover to rebar
Severity of
corrosion
Limited corrosion
with minor pitting
of steel
Moderate
corrosion with
some pitting
Entrenched
corrosion with
deep pitting
presence of the silane in the concrete near-surface zone.
c) Migrating corrosion inhibitors
A corrosion inhibitor is defined as a chemical substance
that reduces the corrosion of metals without a reduction
in the concentration of corrosive agents. Corrosion
inhibitors work by reducing the rate of the anodic and/or
cathodic reactions thereby suppressing the overall
corrosion rate. The effectiveness of migrating corrosion
inhibitors is generally controlled by environmental,
material and structural factors, shown in Table 57.
Migrating corrosion inhibitors are generally organic-based
materials that move through unsaturated concrete by
vapour diffusion. Organic corrosion inhibitors such as
amino-alcohols are believed to suppress corrosion by
primarily being adsorbed onto the steel surface thereby
displacing corrosive ions such as chlorides. The adsorbed
organic layer inhibits corrosion by interfering with anodic
dissolution of iron while simultaneously disrupting the
reduction of oxygen at the cathode.
When assessing the suitability of repairs with migrating
corrosion inhibitors, two important issues must first be
considered:
- the likely penetration of the material into the concrete
needs to be determined
- the severity of the corrosive environment at the
reinforcement must be quantified
Migrating corrosion inhibitors are designed to move fairly
rapidly through partially saturated concretes that allow
vapour diffusion. Penetration has however been found to
be poor in near-saturated concretes typically found in
partially submerged marine structures. This poor
penetration performance may be ascribed to high
moisture and salt levels that prevent significant vapour
diffusion through the concrete. It is critical therefore that
satisfactory penetration of corrosion inhibitors is checked
before undertaking full-scale repairs.
The performance of migrating corrosion inhibitors in
controll ing chloride-induced corrosion is largely
dependent on chloride levels at the reinforcement. Work
done by Rylands indicates that effective inhibition is not
possible at chloride levels above 1.0% at the
reinforcement 8. This can be seen in Figure 7 where ribbed
steel bars embedded at 25 mm in a grade 40 portland
cement concrete were subjected to wetting and drying
cycles with a salt solution for a period of 18 months.
Concrete blocks were either controls (CON) or contained
organic corrosion inhibitor, either admixed during casting
(ADM) or coated after 30 cycles (CTG). The chloride
content at the level of the reinforcement was approaching
2% at the time of application of the migrating corrosion
inhibitor and resulted in poor inhibition. Better inhibition
is possible if treatment is done earlier when chloride
contents are lower.
The effectiveness of migrating corrosion inhibitors appears
to be enhanced when used in combination with
hydrophobic coatings to reduce moisture levels in
concrete. This has been noted in both laboratory trials
and field monitoring of repairs. Such an approach has
also been found to be effective in the repair of carbonation-
induced corrosion damage.
d) Electrochemical techniques
Corrosion of reinforcement in concrete is an
electrochemical process that occurs when embedded
steel is depassivated by a reduction in concrete alkalinity
or the presence of corrosive ions such as chlorides. Two
repair techniques, electrochemical chloride removal and
realkalization, attempt to restore passivating conditions
by the temporary application of a strong electric field to
the cover concrete region.
Realkalization is the process of restoring the original
alkalinity of carbonated concrete in a non-destructive
manner. The electrochemical treatment consists ofFigure 7: Corrosion rate measurements with time for grade 40 concrete
Corrosion Repair & Rehabilitation
www.masterbuilder.co.in • The Masterbuilder - March 2012 133
placing an anode system and sodium carbonate
electrolyte on the concrete surface and applying a high
current density (typically 1 A/m2). The electrical field
generates hydroxyl ions at the reinforcement and draws
alkalis into the concrete. Alkaline conditions may be
restored in the concrete in as little as one to two weeks
using the system.
Electrochemical chloride removal (ECR) is a more time-
consuming and complex technique and its suitability
needs to be carefully assessed. Chloride removal is
induced by applying a direct current between the
reinforcement and an electrode that is placed temporarily
onto the outside of the concrete. The impressed current
creates an electric field in the concrete that causes
negatively charged ions to migrate from the reinforcement
to the external anode. The technique decreases the
potential of the reinforcement, increases the hydroxyl ion
concentration and decreases the chloride concentration
around the steel thereby restoring passivating conditions.
Figure 8 shows the basic principles of ECR.
The effectiveness of ECR depends on several factors that
include the following:-
- extent of chloride contamination in concrete
- structural configuration including depth and spacing
of reinforcement
- applied current density and time of application
- pore solution conductivity and resistance of cover
concrete
- presence of cracks, delaminations and defects
causing uneven chloride removal
Figure 8: Schematic illustration of electrochemical chloride removal technique
Figure 9: Chloride profiles before and after ECR treatment for 8 weeks
ECR typically takes 4-12 weeks to run at current densities
within the normal range of 1-2 A/m2. Results from ECR
trials performed in the laboratory are shown in Figure 9
and indicate that complete extraction may take longer
than 8 weeks at a current density of 1 A/m2 9. In some cir
cumstances chlorides beyond the reinforcement may be
forced deeper into the concrete during the process. There
is a risk that chlorides left in the concrete may diffuse
back to the reinforcement and cause further corrosion
with time.
The feasibility of using ECR depends on several factors
such as:-
- the presence of major cracking, delaminations and
defects that will require repair before ECR
- large variations in reinforcement cover that will cause
differential chloride extraction and possible short-
circuiting
- reactive aggregates requires special precautions to
avoid possible alkali silica reaction; lithium salts
should be used in these cases
- prestressed concrete structures may be susceptible
to hydrogen embritt lement after ECR; special
precautions are needed to eliminate this risk
- temporary power supplies of significant capacity are
required during application of ECR
e) Cathodic protection systems
Cathodic protection systems (CP) have an excellent track
record in corrosion control of steel and reinforced concrete
structures. The principle of CP is that the electrical
potential of the steel reinforcement is artif icially
decreased by providing an additional anode system at
the concrete surface. An external current is required
between anode and cathode that diminishes the corrosion
rate along embedded reinforcement. The current may be
produced either by a sacrificial anode system or using an
impressed current from an external power source.
Sacrificial anode systems consist of metals higher than
steel in the electrochemical series (e.g. zinc). The external
anode corrodes preferentially to the steel and supplies
electrons to the cathodic steel surface. Sacrificial anode
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The Masterbuilder - March 2012 • www.masterbuilder.co.in134
systems are most effective in submerged structures where
the concrete is wet and resistivity is low. Warm
temperatures are also generally required for sacrificial
CP systems (i.e. above 200C).
CP systems more commonly use an external electrical
power source to supply electrons from anode to cathode.
The anode is placed near the surface and is connected to
the reinforcement through a transformer rectifier that
supplies the impressed current (see Figure 10). Anodes
may be conductive overlays, titanium mesh within a
sprayed concrete overlay, discrete anodes or conductive
paint systems. Anode systems are usually designed for a
minimum service life of 20 years but may last in excess of
50 years.
Before CP repairs are undertaken several factors needto be considered:
- reinforcement must be electrically continuous
- concrete cover must be uniformly conductive and free
of delaminations
- alkali reactive aggregates and prestressing steel need
special treatment
- power must be available to drive the impressed
current in the structure
CP repair of concrete structures requires a thorough
corrosion survey by a specialist and the design needs to
be undertaken by a corrosion expert. Reliable CP systems
are fully controlled and monitored by a series of
embedded sensors in order to ensure optimum
performance. This is essential since under or over-
protection of the reinforcement may be potentially harmful
to the structure or the CP system. Continuous monitoring
of CP systems is usually done remotely by modem and
the power consumption during operation is extremely
small.
The first major CP repair of a reinforced concrete structure
in South Africa was done at the Simonstown Jetty in 1996
10. The structure was almost 80 years old and in an
extremely poor condition with widespread chloride-
corrosion damage. Several previous patch repairs had
failed and the concrete was contaminated with chlorides
making conventional repairs unfeasible. An impressed
current CP system was installed with metallic ribbon
anodes protected within a sprayed concrete overlay. The
structure has been restored to full serviceability and
should require no further repairs for at least 40-50 years.
f) Demolition/reconstruction
Deterioration of reinforced concrete structures is often so
advanced that demolition and reconstruction becomes
viable. This option should only be considered as a last
resort since the total cost (capital costs plus loss of service
and temporary works) is usually well in excess of repairs
costs. Corrosion damage is also generally confined to
near-surface regions and engineers often over-estimate
the extent of damage to corrosion-dam-aged structures.
Recent demolition of several bridge-decks along the Cape
coast revealed that actual corrosion damage was less
than anticipated.
Demolition and reconstruction is often preferred by
engineers who have limited repair experience or lack
confidence in new repair systems. It is crucial nevertheless
that lessons are learnt from the old structure when
designing the replacement. Guidance about ensuring
durable reinforced concrete structures is given in
Monographs 1 and 2.
Economics of Repairs
Repairs of reinforced concrete structures damaged by
corrosion have often proved to be unsuccessful with further
damage occurring after repair. Reasons for the poor
performance of repairs include:-
- lack of understanding of deterioration processes
- inadequate investigation and testing prior to repairs
- inadequate funds to undertake satisfactory repairs
- ineffective or inappropriate repairs being specified
- poor supervision and implementation of repairs on site
Repairs are not generally anticipated by owners and funds
for repairs are nearly always extremely limited. Economics
largely dictate the timing and scale of repairs but
unfortunately only short-term costs are often considered.
Whilst corrosion damage is to some degree unique to
each structure some basic tenets hold for most cases.
- Performance of the concrete structure prior to
Figure 10: Typical cathodic protectipon layout
Corrosion Repair & Rehabilitation
The Masterbuilder - March 2012 • www.masterbuilder.co.in136
treatment often dictates the likely performance after
repair. Structures with high levels of damage and rapid
rates of deterioration require more substantial repair
than those less seriously affected.
- The timing of treatment is crucial since corrosion rates
and damage increase with time. A structure that has
been neglected and allowed to reach an advanced
level of damage will not respond to 'quick-fix' solutions.
Conversely a structure that is repaired early enough
may be restored to full serviceability relatively cheaply.
- The effectiveness of treatments in retarding corrosion
is not equal and may range from highly effective to
detrimental (e.g. cathodic protec tion versus patch
repairs)
considered, a practical example is given in the Appendix.
Closure
The notion that reinforced concrete structures require no
maintenance or repair during their service life is gradually
being dispelled. It has been said that owners will have to
pay for durability at some point in the life of a structure.
Inadequate designs with excessive cost-cutting will
merely transfer the savings in capital costs to much more
expensive repairs at a later stage. While accountants may
encourage some deferment of capital costs into
maintenance, experience suggests that investments in
the form of design and construction for durability bring
better rewards than allowing for maintenance. Despite
this evidence, economic imperatives that attempt to
maximise short-term profits, often impact detrimentally
on the durability and service life of infrastructural
developments.
Repair of reinforced concrete structures needs to be
undertaken in a rational manner to guarantee success.
An increasing number of repair options are available that
must be considered in terms of cost, technical feasibility
and reliability. Engineers need to understand all the
relevant material, structural and environmental issues
associated with concrete repairs in order to make
intelligent choices.
High quality repairs require a thorough investigation into
the causes of deterioration, appropriate repair
specifications and competent execution of the repair work.
This can only be done when structural investigations are
carried out by independent experts, specifications are
drawn up by engineers with specialist repair expertise
and repairs are undertaken by competent contractors.
Appendix 1:Repair example
A 60-year old bridge structure is in need of major repairs
arising from widespread corrosion damage. The bridge
spans a tidal estuary with direct exposure to seawater
splash and spray action. Concrete is heavily contaminated
with salt and chloride levels at the reinforcement are
around 1.0% by mass of cement. Damage in the form of
cracking, spalling and delaminations are widespread
over much of the structure and are the result of chloride-
induced corrosion. Urgent repairs are essential to restore
full serviceability to the bridge.
Rough estimates of service life of the various options are
based on recent experience in South Africa and specialist
publications13,14,15. Whilst the projected performance
of the various repairs is a subjective assessment, the
figures serve to illustrate the many issues that need to be
considered when costing repairs.
Importantly, repairs costs need to be compared in a
rational way by comparing life-cycle costs of the structure.
Scott showed that when life-cycle costs are compared, a
maintenance-free structural design is cheaper than
cutting initial costs and deferring some money for repair
and maintenance at a later date (data shown in
Table 6) 11.
Strohmeier showed that repair costs escalate dramatically
as deterioration proceeds and that repairs should be
done as soon as distress is noted 12. This research helped
quantify what many engineers had long realized; that
durability-based designs are cost-effective in the long-
term and that delays in repairs cause an exponential
increase in costs.
Engineers considering repair of concrete structures do
not have the freedom to change either the original design
or the timing of the repairs. Repairs therefore need to be
considered on the merits, logistics, costs and risks of the
many options that are available to rehabilitate the
structure. To illustrate some of the issues that need to be
Option
Original
design
Repairs/
main-
tenance
Relative
costs
1
60 MPa 30%
fly ash 55
mm cover
None
1.0
2
60 MPa 30%
fly ash 30
mm cover
Surface
treatment at
10-year
intervals
2.0
3
60 MPa 30%
fly ash 40
mm cover
Patch repairs
after 20 and
35 years
2.3
4
60 MPa 30%
fly ash 40
mm cover
Cathodic
protection
after 20 after
20
3.0
5
60 MPa
100%PC 75
mm cover
Patch repairs
after 15, 25
and 35 years
3.5
Table 6: Total life cycle costs of typical beam members exposed to marine
environment
Notes on repair options:-
Option 1. Durability design for maintenance free 40 year service life
Option2. Based on anticipated life of surface treatment
Options 3-5. Based on the likely stage at which spalling damage becomes
excessive
Option 5. Design required by SABS 0100:1992
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www.masterbuilder.co.in • The Masterbuilder - March 2012 137
For the purposes of costing the repair options, the
following assumptions are made:-
- unescalated 2001 costs are used due to uncertainties
about future discount, inflation and tax rates
- site establishment costs are fixed at R 250 000 for
each repair option
- total area of concrete under repair is 2000 m2
- unit rates for repair include allowance for labour,
materials, access and supervision
- repairs are focused on chloride-induced corrosion
damage only
A The following repair options are considered for the
bridge. A Localized repairs of corrosion-damaged
areas with only cosmetic con sequences. Assuming
15% of the structure requires patching and that
concrete is only broken back to the reinforcement, a
unit rate of R250/m2 is used. Given the limited nature
of the repairs and the likelihood of incipient anode
formation an effective life of 8 years is con sidered
possible.
B More extensive mechanical break-outs and patching
are done with all corroded reinforcement being
exposed, cleaned and a good quality repair material
used for patching. Approximately 30% of the structure
is treated at a unit rate of R280/m2. Despite the effort
made to repair the structure, corrosive conditions still
exist at the reinforcement and further corrosion
damage limits the effective life to 12 years before more
repairs must be considered.
C Conventional corrosion repairs are done but a
migrating corrosion inhibitor is applied to the repaired
concrete surface together with a hydrophobic coating
(silane/siloxane). Mechanical breakout is limited to
damaged areas of concrete and not all corrosion on
reinforcement is removed resulting in a unit rate of
R300/m2. This includes the cost of the migrating
corrosion inhibitor and coating at R40/m2. The chloride
level at the reinforcement (1.0%) is at the upper level
for corrosion inhibitor performance resulting in an
effective service life of only 15 years.
D Electrochemical chloride extraction is applied to the
concrete to remove chloride from around the steel.
The cost of the system is approximately R750/m2 for a
six week application and includes repair to damaged
concrete. Unfortunately not all the chloride is removed
from the concrete resulting in an effective service life
of 25 years.
E Cathodic protection is applied to the structure to
protect the embedded reinforcement. The cost of the
system is R900/m2 at installation and a nominal
maintenance and monitoring fee of R5000 per year.
The anode system is designed to last 50 years thereby
dictating the effective life of the system.
Present value costs for the various options are shown in
Table A1. From these findings it is clear that initial repair
costs and total repair costs over 40 years vary significantly.
Timing
Initial
20 years
40 years
Option A
0.75
2.25
3.75
Option B
0.81
2.43
3.24
Option C
0.85
1.70
2.49
Option D
1.75
1.75
3.50
Option E
2.05
2.15
2.25
Table A1: Total present value costs (million rands)
Item
Structure
name
Location
Environment
History
Date
inspected
Surface
condition
Early
cracking
Concrete
quality
Rebar cover
Structural
effects
Surface
damage
Staining
Cracking
Rebar
Condition
Carbonation
Delamination
Previous
repairs
Example
Background data
Identification, reference number
Physical address or location
Severity and type of exposure
Age, design data, repairs
Date
Original condition
Honeycombing, bleeding, voids, popouts
Plastic settlement or plastic shrinkage
Surface hardness, density, voids, colour
Covermeter survey, mechanical breakout
Overloading, dynamic effects, structural cracking
Present Condition
Abrasion, chemical attack, spalling, leaching
Rebar corrosion, AAR gel, effloresence, salts
Width, pattern, location, causes of cracking
Visual examination of bar, rust and pitting damage
Indicator test on cores or mechanical breakouts
Size, frequency, severity of delamination
Integrity of repairs, signs of damage near repair
locations
Observa-
tion
APPENDIX 2: Diagnostic sheets
Table A2: Checklist for investigation of structural deterioration
Option A is most cost-effective when only short-term costs
are considered but most expensive in the longer-term.
For a structure that only has to last another 20 years, option
C may be preferable whereas for 40 years further service,
option E is most economical for the hypothetical example.
Corrosion Repair & Rehabilitation
The Masterbuilder - March 2012 • www.masterbuilder.co.in138
References
- Arup, H., 'The mechanisms of the protection of steel by concrete',
Corrosion of reinforcement in concrete construction, SCI, 1985.
- Miyagawa, T., 'Durability design and repair of concrete structures:
chloride corrosion of reinforcing and alkali aggregate reaction',
Magazine of Concrete Research, 43(156), 1991, pp 155-170.
- British Standards Institute, 'Chloride content determination for
concrete', BS 1881 Part 124, 1988.
- American Society for Testing and Materials, 'Standard test
method for half-cell potential measurement of reinforcement in
concrete', ASTM C876, Philadelphia, 1991.
- Broomfield, J.P., 'Corrosion of steel in concrete: appraisal and
repair', Chapman and Hall, 1997.
- Hoppe, G.E. and Varkevisser, J., 'Long term monitoring of the
effectiveness of the silane impregnation of a concrete arch bridge
to inhibit further effects of alkali aggregate reaction', FIP
Symposium: The Concrete Way to Development, CSSA, 1997,
pp 777-786.
- Mackechnie, J.R., Alexander, M.G. and Rylands, T., 'Performance
of Ferrogard corrosion inhibitor in chloride environments',
Unpublished report, University of Cape Town, 2000.
Type of
corrosion
Chloride-
induced
Carbonation-
induced
Stray current
Chemical
induced
Secondary
forms
Artificially
induced
Environment or
causative conditions
Marine environments Industrial
chemicals Admixed chlorides
(older structures)
Unsaturated concrete Polluted
environments Low cover depths
to steel
DC power supplies Railway
systems Heavy industries,
smelters
High sulphate groundwaters
Fertilizer factories Industrial
plants Sewage treatment works
Primary cracking due to alkali
aggregate reaction, delayed
ettringite formation, structural
cracking
Bimetallic corrosion Partial
sealing of concrete High
temperatures (>2000 C) Patch
repairs of corrosion
Significant features of
deterioration
Rapid and severe corrosion
Distinct anode & cathode
regions Corrosion damage
may affect structural integrity
General corrosion along rebar
Moderate corrosion rates
except when wet & dry faces
are close Corrosion damage
generally only affects
aesthetics
General corrosion of rebar
exposed to moist conditions
Corrosion not confined to
low cover depths Large crack
widths possible
Corrosion generally
associated with near saturated
conditions Concrete
deterioration occurring
together with corrosion
Corrosion localized in regions
where cracks intersect rebar
Other forms of distress
evident in concrete (i.e. AAR
gel deposits)
Generally very localized
intense corrosion due to well
defined anode/cathode
regions
Table A3: Conditions and features of different forms of reinforcement corrosion
Type of
deteriora-
tion
Reinforcement
corrosion
Alkali
aggregate
reaction
Shrinkage/
creep
Chemical
attack
Softwater
attack
Fire damage
Structural
overload
Visual evidence /
associated factors
Rust stains, cracking along
reinforcement, spalling of
cover concrete, delamination
of cover concrete
Expansive map cracking,
restrained cracking following
reinforcement, white silica
gel at cracks
Characteristic cracking,
excessive displacements,
time dependent movements,
exposure to drying conditions
Surface attack, salt deposits
on surface, expansive internal
reactions causing cracking,
exposure to aggressive waters
Surface leaching of
concrete, exposed
aggregate, exposure to
moving waters in conduits
Surface discolouration,
concrete spalling, thermal
cracking, buckling, loss of
strength, microcracking
Major cracking in areas of
high stress, localized
crushing, excessive
deformations and deflections
Confirmatory testing
Cover depth of rebar
Carbonation & chloride
testing Exploratory coring
Electrochemical testing
Core analysis for gel and
rimming of aggregates
Petrographic analysis
Aggregate testing
Concrete core analysis
Loading and structural
analysis Aggregate and binder
analysis
Chemical analysis of
concrete Core examination
for depth of attack and
internal distress
Chemical analysis of water
Core examination for
leaching damage
Core examination for colour
variations, steel condition
Petrographic analysis
Specialist techniques
Loading and structural
analysis Core testing for
compressive strength and
elastic modulus
Table A4: Diagnostic sheet for concrete deterioration (all forms)
- Mackechnie, J.R., Alexander, M.G. and Rylands, T., 'Laboratory
trials with an organic corrosion inhibitor', 14th Int. Corrosion
Congress, Cape Town, 1999, CD-ROM.
- Mackechnie, J.R. and Le Maire, H.R.A., 'Electrochemical
extraction of chlorides from OPC and fly ash concrete', Concrete
Beton, 82, 1996, pp 9-17.
- Stevenson, C.E., Unpublished MSc thesis in progress, University
of Cape Town, 2001.
- Alexander, M.G. and Scott, A., 'Designing reinforced concrete
structures for durability and economy in marine environments',
SAICE Journal, 41(4), 1999, pp 15-21.
- Strohmeier, J.H. and Alexander, M.G., 'Deterioration, repair and
maintenance of reinforced concrete structures in the Cape
Peninsula', Concrete Beton, 81, 1996, pp 14-21.
- Mackechnie, J.R., 'Observations from case studies of marine
concrete structures', SAICE Journal, 40(4), 1998, pp 29-32.
- Addis, B.J. and Basson, J.J., 'Diagnosing and repairing the
surface of reinforced concrete damaged by corrosion of
reinforcement', Portland Cement Institute, Midrand, 1989.
- Standards Australia, 'Guide to concrete repair and protection',
ACRA, Homebush, 1996.
Corrosion Repair & Rehabilitation
The Masterbuilder - March 2012 • www.masterbuilder.co.in140
Concrete Repair & Rehabilitation
Repair and Rehabilitation of CorrosionDamaged Concrete Structures
P. Srinivasan
Principal Scientist, ACTEL, CSIR- Structural Engineering
Research Centre , CSIR Campus, Taramani
Corrosion of steel reinforcement in concrete
structures is a techno-economic problem for
several reasons. Technically (i) it poses challenges
in research and development to discover methods and
materials either to control or prevent corrosion (ii) inspite
of considerable research work world-wide, it is now well
recognised that corrosion in plain carbon steel can only
be controlled and a total prevention is nearly an impossible
task (iii) corrosion of reinforcing steel in concrete is peculiar
in the sense that the corrosion product, because of its
volume growth, causes cracking to the concrete. This
physical effect together with sectional loss of reinforcing
bars affect the load carrying capacity, serviceability, and
the service life of a structure (iv) rehabilitation of corrosion
damaged structures is often cumbersome requiring high
technical expertise and competence. This paper
highlights the materials and techniques for the repair and
rehabilitation of corrosion damaged concrete structures.
Few case studies are also presented.
Steel is passive under high alkalinity environment, and
therefore corrosion will not occur. Then, there are two ways
to initiate the onset of corrosion. One is by the penetration
of carbon dioxide from the environment into the cover
concrete and the other is the penetration of water
containing dissolved salts through the concrete cover or
through a concrete crack. In the first case, the alkalinity of
the concrete surrounding the steel could be reduced by
carbon dioxide which reacts with calcium hydroxide in
cement paste to form calcium carbonate (called
carbonation of concrete) and further to form carbonic acid
with the pore solution. The reduction of calcium hydroxide
leads to a low pH value. This creates an environment for
the corrosion of steel to take place. In the case of chloride
diffusion, the alkalinity of concrete is not reduced, but when
the chloride ion concentration is high enough, reaching a
certain ratio with the hydroxyl ions (Cl/OH), the
deapssivation of steel takes place and corrosion of steel
may start.
The required treatments for restoring the protective
environment for steel depend on the extent and cause of
the corrosion damage:
Carbonation-induced corrosion damage. Under such
conditions, carbonated concrete should be removed and
new concrete should be installed, re-passivation is
provided by the new repair mortar or concrete.
Chloride-induced corrosion damage. Under such
conditions, if chloride has penetrated to the level beyond
the steel reinforcements, removal of chloride around steel
bars does not guarantee re-passivation as chloride ions
may diffuse back from the deeper part of the concrete to
the new concrete cover. This is the so-called redistribution
of chloride after the repair. In this case, the repaired
concrete will become cathodic and the rebar will be the
anode. The corrosion will occur in the bars immediately.
Other factors may influence the re-passivation of steel, for
instance, coating of the steel reinforcements, and the
application of membranes or sealers to limit the moisture
content. In most cases, the strategy of repair is either a
comprehensive or a partial repair of the concrete member.
These strategies are common in the rehabilitation of
concrete and they depend on the structural system,
external environmental factors, and the degree of structural
degradation.
Steps in executing repair
There are several regular steps in the repair of all structures
exposed to corrosion.
- The first step is to strengthen the structure by
performing structural analysis and designing a suitable
www.masterbuilder.co.in • The Masterbuilder - March 2012 141
location for the temporary support.
- The second step is to remove the cracked and
delaminated concrete. It is important to clean the
concrete surface and also the steel bars by removing
rust. After rust is removed by brush or sand blasting,
the steel bars should be painted with epoxy coaling or
replaced; additional steel rods shall be added if
necessary. Then new concrete can be poured.
- The final step is to coat the concrete member with
concrete surface coating as external protection. These
steps will be explained in detail in the following
sections.
Materials
The materials adopted for repair of corrosion damaged
structures are described below:
Polymer Concrete
Three to four fold increase in strength up to 140 N/sq mm
has been obtained using polymer in concrete.
Corresponding increase in tensile strength of concrete is
also achieved by polymer impregnation. The durability of
polymer impregnated concrete is substantially increased
when exposed to freeze-thaw cycles. This has been the
strong reason for its application in cold regions, and against
corrosive salts and acids. These properties can be fully
utilised in repairs and renovation of old structures
damaged due to heavy wear and tear and by corrosion
due to marine atmosphere. The development of
techniques for such applications is in progress. Materials
(monomers and polymers) used for the impregnation are
Styrene, Polypropeleyne, Methyl Methacrylate (MMA) and
Poly Methyl Methacrylate (PMMA).
Epoxy Grouts, Mortars and Coatings
Epoxy resin is a product of Epichlorohydrin and Bisphenol
with or without additives such as plasticisers and dilutants.
To get a cured epoxy resin product, a hardener (usually an
amine) is blended with the epoxy resin at ambient
temperature. The resin mortar may be obtained by adding
fillers, such as, coarse sand or calcined bauxite grit. They
develop excellent strength and adhesive properties, and
are resistant to many chemicals. They have good chemical
and physical stability; they harden rapidly and resist water
penetration. In all, they provide a toughness that couples
durability with crack resistance.
Latex Modified Concrete
The third group of materials which can be used for repair
Materials
Portland Cement Mortar
Portland Cement Concrete
Microsilica Modified
Portland Cement Concrete
Latex Modified Portland
Cement Concrete
Polymer Modified Portland
Cement Mortar with
Non-sag Filler
Magnesium Phosphate
Cement Concrete
Preplaced- Aggregate
Concrete
Epoxy Mortar
Methylmethacrylate (MMA)
Concrete
Shotcrete
Ingredients
Binder
Portland cement
Portland cement
Portland cement
Portland cement
Portland cement
MagnesiumPhosphate
cement
Portland cement
Epoxy resin
Acrylic resin
Portland cement
Additive
Micro- silica
Non-Sag fillers
Pozzolans
Pozzolans
Admixture
Water reducer
Air-entr
Water reducer
Air-entr
HRWR Air-entr
Latex SBR
Acrylic latex
Fluidifier
Water reducer
acceler latex
Application Requirements
Thickness Limitation
in/cm
Curing
Wet 7 days
Wet 7 days
Wet 7 days
Wet 3 days
Sheet 45 min-
2 days
Wet 7 days
-4 hrs.- 2 days
1 hr.- 6 hr.
Wet 7 days
Installation Tempera-
ture 0F/0C
Table- 1 Repair and Overlay Materials
Concrete Repair & Rehabilitation
The Masterbuilder - March 2012 • www.masterbuilder.co.in142
purposes are the latex or acrylic-modified mortars. These
are conventional patching mixes to which is added a
synthetic latex. These additives actually give a mortar
greater internal strength. For this reason they are usually
preferred where strength or heavy loading is an important
factor, for example, on bridge decks or for factory floors
subject to heavy wheel loads. Both compressive and
tensile strength are improved, while flexibility of the patch,
a major factor influencing its durability, is increased
substantially. Resistance to alkalies and dilute acids is
good; the concrete has low water absorption properties
and freeze-thaw stability is improved over a conventional
patch. Bond strength of the latex modified mortar is said
to be greater than the shear strength of the old concrete.
Table- 1 gives the summary of the repair and overlay
materials with the properties.
Structure Strengthening
One of the most dangerous and important first steps
necessary for the repair is selecting the temporary support,
which depends on the following:
- evaluating the state of the whole structure
- determining how to transfer loads in the building and
its distribution
- determining the volume of repair that will be done
- determining the type of concrete member that will be
repaired
- the repair process must be carried out by a structural
engineer with a high degree of experience who has the
capability to perform the structure
Removal of damaged concrete
There are several ways to remove the part of the concrete
that has cracks on its surface and shows the effects of
steel corrosion. These methods of removing the
delaminated concrete depend on the ability of the
contractor, specifications, the cost of breaking, and the
whole state of the structure. The selection of the breaker
methods is based on the cause of corrosion; if it is due to
carbonation or chlorides, then one must also consider
whether cathodic protection will be performed in the future.
In this situation, the breaking work would take place on the
falling concrete cover; it would be cleaned and all the
delaminated concrete and cracked concrete parts
removed. Then, high-strength, nonshrinking mortar would
be poured.
If the corrosion in steel reinforcement is a result of chloride
propagation into concrete, most specifications
Drying
Shrink - age
Moderate
Low
Low
Low
Moderate
Moderate
Very low
Low
Moderate
Moderate
Compressive StrengthCoeff. of thermal
Expansion
Equal to substrate
Equal to substrate
Equal to substrate
Compat w/substrate
Compat w/substrate
Equal to substrate
Equal to substrate
(1.5-5) *concr.
(1.5-5) *concr.
Equal to substrate
1 HR
0
0
0
0
0
1 Day 3 Day 28 DayElastic Modulus
psi/Mpa
Permeability
(Con-
crete=10)
9
9
6
5
5
9
10
1
1
6
Freeze Thaw
Resis- Tance
Good
Good
Good
Excellent
Excellent
Excellent
Good
Excellent
Excellent
Good
Non-Sag
Quality
Moderate
NA
Good
NA
Excellent
Low
NA
Moderate
NA
NA
Exo-
Therm
Low
Low
Low
Low
Moderate
High
Low
High
High
Low
Com-
ments
ACI 30
4R - 23
ACI 503.4
Vapor mayCause
ProblemsinConfined
areas
ACI 506
R - 90
Table- 1 ( Contd.,)
Concrete Repair & Rehabilitation
www.masterbuilder.co.in • The Masterbuilder - March 2012 143
recommend removing about 25 mm behind the steel and
making sure that the concrete on the steel has no traces of
chlorides after the repair process. The difference between
good and bad repair procedures is shown in Fig. 1.
The difference in the procedure of breaking the
delaminated concrete is due to the difference in the causes
of corrosion. Therefore, a careful study to assess the state
of the structure and the causes of corrosion is very important
to get high quality after the repair process. The evaluation
process is the same as illness diagnosis. It is necessary
and important to remove concrete for a distance greater
than the volume required for removal of defective concrete
so that proper steel can be reached. This will be important
later in the repair process. Several methods are commonly
used for breaking and removing the defective concrete
and these are explained next.
Manual Method
One of the simplest and easiest methods is to use a
hammer and chisel to remove defective concrete. This is
considered one of the most inexpensive ways, but it is too
slow compared with mechanical methods. However,
mechanical methods produce high noise and vibration,
have special requirements, and need trained labor. Using
the manual method makes it difficult to spare concrete
behind the steel. Any worker can manually break the
concrete, but it is necessary to choose workers who have
done repair work before as they must be sensitive in
breaking the concrete to avoid causing cracks to the
adjacent concrete members.
Pneumatic Hammer Methods
These hammers work using compressed air; they weigh
between 10 and 45 kg. If they are used on the roof or walls,
their weight will be about 20 kg. They need an attached
small power unit to do the job, but in large areas may
require a separate, bigger air compressor. This machine
requires proper training for the worker that uses it. The use
of pneumatic hammers is more economical when a small,
rather than large, area is to be removed.
Performance rates are about 0.025-0.25 m3 per hour using
hammers weighing 10-45 kg, respectively.
Water Jet
This method has been commonly used since it was
introduced to the market in the 1970s. It relies on the
existence of water at the work site and on the removal of a
suitable depth of concrete in a large area. It removes
fragmented concrete, cleans steel bars, and removes part
of the concrete behind the steel bars, as shown in Fig. 2
The water jet is used manually by an experienced worker
who has previously dealt with the hose, which is pushing
water under high pressure Very high safety precautions
need to be applied to the worker who uses it and the site
around it.
Grinding Machine
This is used to remove concrete cover in the case of large,
flat surfaces. The grinding machine is usually used after
the water gun or the pneumatic hammer to obtain final
concrete breakdown around and under the steel
reinforcement. Therefore, one must take into account
whether the thickness of the concrete cover is equal. The
rate of removal of the concrete by this machine is very fast.
Clean concrete surfaces and steel reinforcements
This phase removes any remaining broken concrete with
a process of cleaning. At the same time, the process of
assessing the steel and cleaning up and removing
corrosion takes place.
Concrete
The stage of preparing a surface by pouring the new
concrete is one of the most important stages of the repair
process. Before application of the primer coating, which
provides the bond between the existing old concrete and
the new concrete for repair, the concrete surface must be
well prepared, and this takes place according to the
materials used. The concrete surface must be clean and
not contain any oils, broken concrete, soil, or lubricants.
The surface must be cleaned completely through sand
blasting, water, or manually using brushes. This stage is
very important and very necessary, regardless of the type
of material used to bond the new concrete with the old.
Clean Steel Reinforcement Bars
After removal of the concrete covers and cleaning the
surface, the next step is to evaluate the steel reinforcement
by measuring steel diameter. If the cross-sectional area of
the steel bars is found to have a reduction equal to or more
than 20%, additional reinforcing steel bars must be added.
Before pouring new concrete, one must be sure that the
Good Repair Wrong Repair
Fig.1
Concrete Repair & Rehabilitation
The Masterbuilder - March 2012 • www.masterbuilder.co.in144
development length between the new bars and the old
steel bars is enough, as shown in Fig. 3 It is usually
preferable to link the steel by drilling new holes in the
concrete and connecting the additional steel on concrete
by putting the steel bars in the drilled hole filled with epoxy.
However, in most cases the steel bars are completely
corroded and need to be replaced.In the case of beams
and slabs that need to add additional steel reinforcement
bars, it is preferable to connect the steel bars with concrete
by drilling new holes in the concrete and making the bond
of the steel bars in the holes by using adhesive epoxies.
For beam repairs, the additional steel bars are fixed in a
column that supports this beam. In the case of slabs, the
steel bars are fixed in the sides of the beam that is
supporting the slab, as shown in Fig. 3
Dry Pack
Dry pack is a mortar mixture with a very low water-cement
ratio, applicable for small area of repair. It is normally placed
by hand. Materials commonly used in drypack are Portland
Cement, sand, and water. Other types of Portland cement
can also be used.
Pre-placed Aggregate Concrete
This essentially involves first placing the coarse aggregate
in the forms, and thereafter filling the voids by pumping in
cement grout (sanded or unsanded). This has been found
to be suitable on areas where accessibility is a problem.
Joint Sealers
Joint sealers are very important in concrete structures as
every concrete structure has joints (or cracks). Joint sealers
should ensure the structural integrity and serviceability. In
addition, they should serve as protection against ingress
of harmful liquids, gases, or other undesirable substances
which would impair the quality of concrete.
Jacketing
Jacketing is the process of fastening a durable material
over concrete and filling the caving with a grout that
provides needed performance characteristics. The
materials used for jacket are metals, rubber, plastics, and
concrete. This restores structural values, protects the
reinforcement from exposure to the harmful elements and
improves the appearance of the original concrete.
Jacketing materials may also be secured to concrete by
means of bolts, screws, nails, or adhesives; by bond with
the existing concrete; or by gravity. The method of securing
employed, will depend upon the exposure, the material
Fig. 2 Concrete Surface after removing with water jet
Fig. 3 Installing additional steel
The techniques for the replacement of the cover concrete/
damaged concrete are given below.
Shortcrete or Gunite
Shotcrete or Gunite is mortar or concrete conveyed
through a pressure hose and applied pneumatically at
high velocity onto a surface. This material has found wide
applications in several major repair works as it can be
applied on vertical, horizontal or overhead surfaces, with
the area to be repaired being either reinforced or
unreinforced. For the purpose of design, gunite may be
considered as good quality concrete of grade M.20.
Shotcrete mixed with steel fibres can also be used.
Fig. 5 Use of concrete collars for strengthening concrete compression member
Concrete Repair & Rehabilitation
The Masterbuilder - March 2012 • www.masterbuilder.co.in148
used and the positioning of the jacketing material. Fibre
glass reinforced plastics, ferrocement, and other hard
materials such as polypropylene can be used for jacketing.
A few examples of guniting, jacketing, and strengthening
are schematically shown in Figs. 4-7 respectively.
Cathodic Protection
As discussed, the corrosion of reinforcement in concrete
is an electrochemical process. Cathodic protection is a
technique by which the electrical potential of the steel is
increased to a level at which corrosion can not take place.
It is widely used for both steel and concrete offshore
structures, while on land it has been used for the protection
of pipelines and similar structures. It has been used on a
limited scale, for concrete structures as discussed below.
Two different methods are employed, an impressed current
and the use of sacrificial anodes. In the first the structure
is connected to the negative terminal of a DC power source,
ideally using an anode which does not corrode. In the
second the reinforcement is connected to anodes with a
more negative corrosion potential than steel, such as zinc
or aluminium. The current is reversed and corrosion now
takes place at the anode, which is gradually used up. In
both cases, electrical continuity of the reinforcement is
required. Fig. 8 shows the schematic setup for the cathodic
protection.
Use of FRP Wrapping
The growing interest in FRP systems for strengthening and
retrofitting can be attributed to many factors. Although
the fibers and resins used in such systems are relatively
expensive compared with traditional strengthening
materials like concrete and steel, labor and equipment
costs to install FRP systems are often lower. Fiber-
reinforced polymer systems can also be used in areas with
limited access, where traditional techniques would be very
impractical-for example, a slab shielded by pipe and
conduit. These systems can have lower life-cycle costs
than conventional strengthening techniques because the
FRP system is less prone to corrosion.
Fiber-reinforced polymer (FRP) can serve as an alternative
to the use of steel sheets. The use of FRP has a wide range
of advantages and offers an alternative to the steel used in
the strengthening process. There are different types of
FRPs; the famous type is carbon fiber-reinforced polymer
(CFRP), which is most commonly used as appropriate in
practical applications and because of its unique properties
in terms of resistance and the resistance with time, as well
as resistance to stress.
Corrosion Inhibitors
It has been shown that certain admixtures can be used to
inhibit corrosion of the reinforcement in the presence of
chlorides8. One that shows promise is calcium nitrite. When
corrosion takes place in untreated concrete, the ferrous
ions at the anode pass into solution and, in a secondary
reaction, are converted to rust. With the calcium nitrite,
Fig. 6 Beam strengthening with steel plates
Fig. 7 Replacement of concrete using pressurized forms
Other Methods
Other techniques employed on repair of corroded
concrete structures include removal of chloride ions,
cathodic protection, use of Fibre reinforced Polymer
Wraps, corrosion inhibitors and concrete coatings. The
details are given below
Concrete Repair & Rehabilitation
www.masterbuilder.co.in • The Masterbuilder - March 2012 149
ferric ions are formed which are insoluble and hence stay
on the surface of the reinforcement, preventing further
corrosion. The addition of calcium nitrite extends the time
to corrosion initiation The corrosion rate, once corrosion is
initiated, is less with calcium nitrite.
Corrosion inhibitors can be classified based on their action
and their chemistry and function:
Different types of coatings are available such as
chlorinated rubber coating, vinyl coatings, epoxy coatings,
acrylic based, polyurethane, etc. Sealers are an
intermediate application between penetrants and
coatings. They protect concrete by blocking the pores.
Sealers are more viscous than penetrants and generally
form a thin film on the surface of concrete.The effectiveness
of surface treatment materials in preventing the ingress of
aggressive ions depends on the penetrability of the
material to provide protection of the concrete matrix.
Various organic polymers are used as sealers and coatings.
The most widely used penetrating materials tend to be
siliceous, which line the pores of concrete forming silicone
resins and thus provide protection through water repellent
properties. Silane/siloxane primer with aliphatic- acrylic
top coat gives good protection. CECRI also have
developed and implemented concrete coatings.
Case-studies
Many corrosion-affected structures were investigated by
the author at SERC for the its condition assessment through
NDT & PDT and repair measures were formulated to
increase its service life. One of the structure is the prill
tower for the manufacture of urea and the age more than
30 years ( Fig.9) The structure is a RCC shaft and the
thickness is 230 mm. Since the structure was constructed
in marine environment, a coating was provided on the
surface right from its construction. After the detailed
investigation it was found that the carbonation depth was
only 12 mm and chloride content in the concrete was within
allowable limits. The reinforcements are found to be in
very good condition. The test results have proved the
efficiency of concrete coating.
The other structure is a 30 year old RCC water tank
constructed in Bangalore (Fig. 10) and was affected by
Fig. 8 Schematic diagram of CP including modern link and remote monitoring
Anodic inhibitors
Cathodic inhibitors
Ambiodic inhibitors
Suppressing the anodic corrosion reaction
Suppressing the cathodic reaction
Suppressing both anodes and cathodes
Inorganic inhibitors
Organic inhibitors
Vapour phase or
volatile inhibitors
nitrites, phosphates and other inorganic
chemicals
amines and other organic chemicals
a subgroup of the organic
inhibitors (generally Amino alcohols) that
have a high vapour pressure
By their action:
By their chemistry and function:
Coating to Concrete
Surface treatment materials are often used to protect
concrete from deterioration due to reinforcement corrosion.
These materials are classified as Penetrants, Coatings,
and Sealers.
Penetrants are low viscosity liquids designed to penetrate
into concrete and line its pores. They protect concrete by
forming a hydrophobic layer and thus repel moisture, but
they facilitate the evaporation of water vapor and other
gases from the interior of the concrete mass. Coatings
provide protection to concrete by forming a thick,
protective film on the surface. However, due to minimal
breathability, these materials may contribute to concrete
deterioration.Fig. 9 Prill Tower ( RCC Shaft- 30
Years Old)
Fig. 10 Water Tank repaired and coated
with Concrete coating
Concrete Repair & Rehabilitation
The Masterbuilder - March 2012 • www.masterbuilder.co.in150
corrosion. After detailed investigation, remedial measures
were formulated. The repair measures consist of
strengthening and finally a concrete surface coating was
provided. It was found that after seven years of exposure,
the carbonation depth was almost nil.
Conclusions
Since corrosion is a complicated problem, the cause has
to be diagnosed and proper material and technique has
to be adopted. The cost of repair will vary with the type of
technique being adopted. Repairing of a corroded
damaged structure requires skilled personnel. The
repaired structure has to be monitored periodically for their
performance.
References
- Page C.L, Treadway, KWJ and Bamforth PB (Editors) - Corrosion
of reinforcement in concrete; Society of Chemical Industry;
Elsevier Applied Science, May 1990.
- IS 13620 -1993, "Specification for fusion bonded epoxy coated
reinforcing bars".
- British Standards institution, BS6744: 1986, "Austenitic Stainless
Steel Bars for the Reinforcement of Concrete".
- British Standards institution, BSEN-10088-1:1995, "Stainless
Steels, Part 1 - List of Stainless Steels".
- Mani, K., and Srinivasan, P., "Service life of structures in corrosive
environment : A comparison of carbon steel and SS bars", The
Indian Concrete Journal, July 2001, pp 452-456.
- John L. Clarke (Editor),(1993), "Alternative Materials for the
Reinforcement and Prestressing of Concrete", Blackie Academic
& Professional, First edition,
- Jones et al., (1995), "Concrete surface treatment : Effect of
exposure temperature on chloride diffusion resistance", American
Concrete Institute, Materials Journal, pp.197-208.
- Srinivasan,P., Firdows M.Z.M., Prabakar J., and Chellappan, A., A
simple accelerated test method for rapid assessment of chloride
penetration of concrete with and without surface coating, The
Indian Concrete Journal, January 2007, pp 43-47.
- Srinivasan, P., Mohd. Firdows, M.Z., & Mani, K., "Surface coatings
for protection of concrete in marine environment - performance
evaluation through laboratory experiments", National Seminar on
Harbour Structures, (NASHAR- 2003), IIT Madras, Chennai, 21-
22, February 2003, pp 341-350
- Alonso, C. and Andrade C. (1990), "Effect of nitrate as a corrosion
inhibitor in contaminated and chloride - free carbonated mortars",
American concrete Institute, Materials Journal, pp.130-137.
- John Broomfield, (1999) "Corrosion inhibitors for steel in concrete",
pp.45-47.
- Berkeley, K.G.C. and S. Pathmanabhan, "Cathodic Protection of
reinforcement steel in concrete", Bulter works, London.
- Mohamed A. El-Reedy, "Assessment and Repair of
Corrosion",CRC Press, London, New York, 2007
Concrete Repair & Rehabilitation
The Masterbuilder - March 2012 • www.masterbuilder.co.in168
Formwork Failure: Cases & causesSpecial Correspondent
Collapse of concrete structures during construction
has been happening since concrete has been
placed in formwork. Cases and causes of these
type of failures have been documented and recorded in
many texts, articles and journals. This article will try and
focus on a few of them from the available reports, starting
with The New York Coliseum on May 9, 1955, 2000
Commonwealth Ave. on January 5,1971, Skyline Plaza in
Bailey's Crossroads on March 2, 1973, The Harbour Cay
Condominium in Cocoa Beach, Florida in March 1981 and
ending with The Tropicana in Atlantic City on October 30,
2003.The focus will be on what has been learned over time
from these failures and what has been done to keep these
type of tradgedies from occurring in the future.
Although there were many cases of concrete failures during
construction prior to the New York Coliseum collapse as
illustrated in (McKaig 13-27, 1962), only a few will be
looked at after this point because of the changes and
progressions being made in the construction industry at
this time in history.
(A) New York Coliseum on May 9, 1955
Pic source: http://www.ppconstructionsafety.com
Formwork Failure
www.masterbuilder.co.in • The Masterbuilder - March 2012 169
The construction method was a flat plate waffle slab with
solid slabs at the column caps. It was one of the first times
the use of motorized buggies had been used in the pouring
of this type of structure. The floor that collapsed was the
first floor above grade supported on two tiers of shores at
a total of 22' high. It can be seen from Figure 1 how collapse
happened. The buggies weighed about 3000 lb loaded,
ran at about 12 mph, and there were eight of them at the
time of the failure with about 500 cubic yards of concrete
already placed. The investigation that followed put the
blame solely on inadequate provisions in the formwork to
resist lateral forces, it even went on to say that "if there had
been sufficient diagonal, horizontal, and end bacing of
the temporary supporting structure, the collapse could
have been prevented entirely,...", (McKaig 15-16, 1962).
After the collapse the district attorney called attention to
the lack of inspections and made recommendations to
revising the building code with respect to formwork
because of the new advances.
(B) 2000 Commonwealth Avenue: January 5, 1971
This was a progressive collapse of a cast-in-place
reinforced concrete flat-slab structure. Punching shear was
determined to have been the triggering mechanism but
the real problem was in the numerous errors and omissions
by every party involved in the project (Delatte 133-143).
The investigating committee determined that if the
construction had had a proper building permit and had
followed codes, then the failure could have been avoided
(Delatte 142) (See Figure 2 and 3 how failure occurred).
Some of the problems leading to the collapse are
- Not following the structural engineers specifications
for shoring and formwork
- Lack of concrete design strength
- Lack of shoring or removed too soon
- Improper placement of reinforcement
- Little construction control on site
- Owner changed hands many times
- Almost all jobs were sub contracted
- No architectural opr engineering inspection done
- Inadequate inspection by the city of Boston
- The general contractors representative was not a
licensed builder
- Construction was based on arrangements done by the
subcontractors
- No direct supervision of subcontractors
Figure 2: Typical flatplate with uniform distributed loading
Figure 3: Punching shear failure diagram
Figure 4: Skyline Plaza at Bailey's Crossroads, National Archives
Figure 1: N.Y. Coliseum Collapse, National Archives
Formwork Failure
The Masterbuilder - March 2012 • www.masterbuilder.co.in170
(C) Skyline Plaza: March 2,1973
Skyline Plaza (See Figure 4) in Bailey's Crossroads is an
example of a catastrophic collapse of a 30 story cast-in-
place reinforced concrete structure. This was also a flat-
plate design structure that failed due to punching shear
on the 23rd floor and resulted in a progressive collapse.
Some of the reasons for this failure again were 1) premature
removal of shores and reshores, 2) insufficient concrete
stength, 3) no preconstruction plans of concrete casting,
formwork plans, removal of formwork schedules, or
reshoring program (Kaminetzky 66-67).
(D) Harbour Cay Condominium: March 1981
Built just 10 years after 2000 Commonwealth Ave. and 8
years after Skyline Plaza, was another cast-in-place
reinforced concrete structure that collapsed during
construction. It was determined that the most important
factor towards its failure was a design error coupled with a
construction error of the wrong size rebar and chair height.
The designer never performed any calculations to check
for punching shear, the most common form of failure in
these type of structures (Feld & Carper 18).
Figure 5: Tropicana Casino; Parking Garage Picture taken fromwww.CTLGroup.com
(E) The Tropicana Casino parking garage in AtlanticCity, N.J.: October 30,2003
The structure collapsed during construction killing another
four construction workers and and leaving more than 30
others injured. Larry Bendesky, Mongeluzzi's partner of the
Philadelphia law firm Saltz, Moongeluzzi, Barrett &
Bendesky, P.C, the lead counsel for the litigation with Paul
D'Amato of the D'Amato Law Office and a member of the
trial team, said that "the simple explanation of the cause of
the collapse is that the floors were not connected to the
walls with the required reinforcing steel. Built without the
necessary steel, it is no wonder it collapsed like a house of
cards." (pr newswire) The vertical columns left standing
and the fact that the floors were not connected implies
that this was another punching. Refer Figure 5 for the
collapse picture.
Codes & Regulations
Codes in Place
ACI, The American Concrete Institute's origins started in
1905 with its first building code published in 1910 and
changing its name to the current designation in 1913. ACI's
first design handbook came out in1939 and the first
building code titled ACI 318 came out in 1941. The
beginning volumes of ACI were less tha fifty pages with
the current code specification being nearly 470 pages of
design specifications and commentaries (ACI 318). This
clearly shows the history of ACI is closely tied to the ever
changing demands of concrete construction and
technology. The ACI sees itelf as an expanding, alert,and
informed organization prepared to stimulate imaginative
applications of concrete and better knowledge of its
properties and uses, and will take an increasingly active
part in solving problems affecting the public welfare
(History of ACI).
Lessons Learned
(A) New York Coliseum on May 9, 1955
From this failure the construction industry learned that
shoring systems should be well braced to resist lateral
loads and to consider the effect of power or motorized
buggies/carts on the formwork (Auburn University).
(B) 2000 Commonwealth Avenue: January 5, 1971
From 2000 Commonwealth Ave. the industry learned that
this type of failure is a critical failure mechanism for flat-
plate-slab concrete construction. Structural safety
depends on adequate slab thickness, proper placement
of reinforcement, and adequate concrete strength (Delatte
144).
(C) Skyline Plaza: March 2,1973
Six lessons learned from the colloapse of Skyline Plaza at
Bailey's Crossroads are listed in (Kaminetzky 67)
- the contractor should prepare formwork drawings
showing details of the formwork, shores, and reshores.
- The contractor should prepare a detailed concrete
testing program, to include cylinder testing, before
stripping forms.
Formwork Failure
www.masterbuilder.co.in • The Masterbuilder - March 2012 171
- The engineer of record should ascertain that the
contractor has all the pertinent design data (such as
live loads, superimposed dead loads, and any other
information which is unique to the project).
- Inspectors and other quality control agencies should
verify that items 1 and 2 above are being adhered to.
- Uncontrolled acceleration of formwork removal may
lead to serious consquences. 6) Top and bottom rebars
running continuously within the column periphery must
be incorporated in the design.
(D) Harbour Cay Condominium: March 1981
The Harbour Cay Condominiums presented the industry
with six more lessons learned in this type of construction
also listed in (Kaminetzky 77-78). This tradgedy happened
only eight years after the Skyline Plaza tradgedy and yet
some of the same lessons are listed again, they are
- A punching shear strength check s critical to the
success of a flat-slab, since punching shear is the most
common failure mode of concrete slabs.
- Minimum depth of a flat-slab must be checked to
assure proper strength and acceptable deflections.
- Reinforcing bars, both at the top and at the bottom of
the slab, should be placed directly within the column
periphery to avoid progressive collapse. This can easily
be accomplished routinely in all flat-slab jobs at no
additional cost at all.
- Proper construction control must be provided in the
field, including design of formwork by professionals.
This must include shoring and reshoring plans,
procedures, and schedules, with data on minimum
allowable stripping strength of the concrete.
- When there are failure warning signs of any type on a
construction site, work must stop. All aspects of the
project must be carefully evaluated by experienced
professional help. Immediate evacuation of the
structure must be considered.
- Special care must be taken during cold weather to
evaluate the actual in place strength of the concrete. It
is also a fact that the level of construction carelessness
increases in the winter months.
(E) The Tropicana Casino parking garage in Atlantic City,
N.J.: October 30,2003
The Tropicana lessons learned have not yet been
published in any documented form, but from articles such
as the one from ASQ Newsletter published in the summer
of 2004, one can reasonably determine that all of the above
lessons learned will be revisited. The article states that all
of the errors were remarkably simple engineering error.
Contractor failed to tie rebar in the frames floor beams to
the columns and shear walls in several places was only
one reason as listed in (ASQ Newswire 11-12).
Statistics
Statistics released in 1984 by the National Safety council
reported over 2200 deaths were reported for the
construction industry for that year, and 220,000 disabling
injuries, the largest total for the eight major industries
surveyed (Carper 312).
Over $1.6 billion is lost annually in the U.S. due to
construction accidents (Carper 312). Forty-nine percent
of falsework collapse happens during concrete placing
(Hadipriono & Wang 115).Untimely removal of falsework is
the second most significant event related toconcrete failure
(Hadipriono & Wang 116). Investigations prove that many
accidents causing thousands of dollars worth of damage
could have been prevented if only a few hundred dollars
had been spent on diagonal bracing for the formwork
structure (University of Washington).
Conclusions
OSHA, ASCE, and ACI have all responded to these as wellas many other accidents and issues with activities,publications and codes aimed at improving constructionsafety and the welfare of our construction workforce;
however, these organizations alone cannot be responsiblefor all construction related activities and failures.
The safety record in the construction industry can be and
must be improved in all phases. As C. Roy Vince has stated,many construction accidents are the result of ignorance,carelessness, and greed (Carper 133). The lessons learned
from above being repeated over and over again can onlypoint to the fact that this statement is precisely true. "Aslong as structures are constructed by humans, using
imperfect materials and procedures, failures are likely tocontinue. Many of these failures will occur during theprocess of construction, endangering the lives of
construction workers." (Carper 143) There is no way to breakeveryone of their bad habits but awareness has to be raised
and the consequences have to be sharply increased.
More focus has to be placed on required education of all
construction personel beyond certain levels of
responsibility, this is to include the workers themselves
who are actually assembling these structures. Better
licensure requirements, more stringent inspections, and
increased factors of safety during construction (because
it is at this time when the structure will be likely to see its
most significant loading), should also be considered to
help prevent these tragedies from reoccurring. From the
initial design phase to maintenance of the structure after
Formwork Failure
The Masterbuilder - March 2012 • www.masterbuilder.co.in172
completion everyone involved needs to pay strict attention
to all details and warning signs of impending failures. There
can be NO SHORTCUTS if we are to protect the safety and
lives of the individuals who provide us with all of the
essential structures in our lives.
Most often it is not their mistake that cost them their life
and the misery of the families who lost them too soon.
References
- American Concrete Institute. "History of ACI" <http://
www.concrete.org/members/mem_info_history.htm> (October
10, 2009)
- ACI Committee 318, (2008). ACI 318-08 "Building Code
Requirements for Structural Concrete and Commentary"
pp. 81-82
- ACI Committee 318, (1963). ACI 318-63 "Building Code
Requirements for Structural Concrete and Commentary" pp. .
- Arthur H. Nilson, David Darwin and Charles W. Dolan, (2004).
"Design of Concrete Structures" pp. 12-17
- The ASQ Newsletter. "Extracts from Engineering News Record"
OSHA Report Claims that Atlantic City Garage Contractors Failed
to Tie Rebar and Properly Shore <http://www.library.illinois.edu/
archives/e-records/ASQ%20Archives/1182001_Division_General/
DesignDiv/Design-News-Summer2004.pdf> (summer 2004),
(October 10, 2009)
- Auburn University. "Lateral Stability of Structures" New York
Coliseum <https://fp.auburn.edu/heinmic/StructuralStability/
newyork%20coliseum.htm> (2009), (Sept. 18, 2009)
- Charles D. Reese and James Vernon Eidson, (2006). "Handbook
of OSHA Construction Safety and Health" pp. 181-183
- Fabian C. Hadipriono,1 M. ASCE and Hana-Kwang Wang2,
(March/April 1986). "Analysis of Falsework Failures in Concrete
Structures" J. Constr. Engrg. Mgmt. 112(1), pp. 112-121.
- Jacob Feld and Kenneth L. Carper, ((1997) "Construction Failure"
pp. 242-274 Kaminetzky D. (1991). "Design and Construction
Failures" Lessons In Forensic Investigations pp. 67-78
- M. ASCE, (August 1987). "Structural Failures During Construction"
J. Perf. Constr. Fac., ASCE, 1(3), pp. 132-144.
- McKaig T. (1962). "Building Failures" Case Studies in Construction
and Design Norbert J. Delatte Jr., Ph.D., P.E. (2009). "Beyond
Failure" Forensic Case Studies For Civil Engineers pp. 129-155
- PR Newswire. "$101 Million Settlement in Deadly 2003 Tropicana
Parking Garage Collapse That Killed Five Workers" < http://
www.prnewswire.com/news-releases/101-million-settlement-in-
deadly-2003-tropicana-parking-garage-collapse-that-killed-five-
workers-58264282.html> (October 10, 2009)
- University of Washington. "CM 420 Course Lecture 1" Temporary
Structures <http://www.courses.washington.edu/cm420/lec1/
lec1.ppt> (Spring Quarter 2002), (Sept. 18, 2009)
- Zallen Engineering. "Collapse of Flying Formwork During Concrete
Placement" <http://www.zallenengineering.com/On-Line_Issues/
OL-8.pdf> (July 2002), (Sept. 18, 2009)
- http://failures.wikispaces.com/2000+Commonwealth+Avenue+-
+Boston
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The Masterbuilder - March 2012 • www.masterbuilder.co.in180
Fabric Formwork: Sky’s the LimitSpecial Correspondent
Picture Source: www.matsysdesign.com
Fabric formworks for reinforced concrete construction
and architecture is an emerging technology with
the capacity to transform concrete architecture and
reinforced concrete structures. The natural tension
geometries given by flexible fabric membranes provide
extraordinarily light and inexpensive formworks, some
using hundreds of times less material than conventional
formworks, and some providing zero-waste formwork
systems. The flexibility of a fabric formwork makes it
possible to produce a multitude of architectural and
structural designs from a single, reusable mold. The use of
a permeable formwork fabric produces improved surface
finishes and higher strength concrete as a result of a filtering
action that allows air bubbles and excess mix water to
bleed through the formwork membrane.
A brief history
According to the International Society of Fabric Forming,
the first practical applications for fabric formwork were
introduced in the mid-1960s for erosion control and to line
ponds, although there are several patents for 19th- and
early 20th-century fabric forms. In the 1970s, the Spanish
architect Miguel Fisac used thin plastic sheets as formwork
for textured wall panels. In the late 1980s and early 1990s,
three men, each on his own, invented a variety of
techniques for fabric-forming aboveground structures.
Kenzo Unno, a Japanese architect in Tokyo, invented a
fabric formwork system for in situ cast concrete walls. Rick
Fearn, a builder and businessman in Canada, invented a
number of fabric formwork techniques. This led him to
develop a series of foundation footing and column
products now manufactured and sold by Fab-Form
Industries in Surrey, British Columbia, Canada. He is
president of the company. Mark West - an artist,
architectural educator and builder who is now the director
of the Centre for Architectural Structures and Technology
Focus Fabric Formwork
The Masterbuilder - March 2012 • www.masterbuilder.co.in184
(CAST) at the University of anitoba's Faculty of Architecture
in Winnipeg - invented a series of techniques for
constructing fabric-formed walls, beams, columns, slabs
and panels. CAST is the first research center dedicated to
fabric formwork technology and education.
Visualizing the end result
"Fabric is so much more efficient than plywood (for forms),
but the industry is slow to change," says Rick Fearn. "It's
staggering how long it takes to get new ideas into the
marketplace." He thinks the biggest stumbling block to
fabric formwork's acceptance is that many contractors
cannot picture the end result before they start. "(Unlike
rigid formwork), it's just a loose piece of fabric. What you
get is not what you see." To help contractors visualize an
end product, Fearn has a computer program that predicts
the shapes fabric forms will produce. He's hoping that as
more contractors accept computer-generated virtual-
reality scenarios, fabric formwork will grow in use. "Fabric
is a tension membrane," Fearn says. "If you use a different
fabric, it will give you a different texture, but the shape will
be the same." Also, some fabrics aren't coated, so they let
excess water bleed out, he notes. This can make fabric-
formed concrete products stronger than those made with
traditional lumber forms. In a world where resources are
dwindling, he notes, fabric forms, like the ones he sells for
columns, just make good sense. Fast-Tubes, made from
high-strength polyethylene, come in 120-foot rolls that
easily fit behind the seat of a truck and can be cut to any
length with minimal waste. Fabric formworks are such a
green product and so efficient. They take up 1% of the
space cardboard does and they are 1/10 the weight. Also
unlike cardboard, there is no waste to be hauled to the
landfill after the column forms are trimmed to size or when
the forms are stripped. "Fast-Tubes can be put under a
slab after they are stripped. They act as a moisture
protector." Besides allowing contractors to form sturdy
columns of varying lengths - Lawton used Fast-Tubes to
make 29-foot columns for a treehouse he built in Vermont
- Fearn's fabric-formed columns can be easily decorated
by simply tying ropes or putting bands around the forms
while the concrete is still wet.
Flexible fabric vs. hardened forms
The primary differences between both the formwork is ease
of errection.While rigid formwork needs more time to errect.
Also lot of staging and design work is needed for rigid
formwork which Is not required for flexible formwork. One
more striking advantage with flexible formwork is that any
shape can be designed and made using fabric formwork.
The same fact is supported by the all the Figures in the
entire storey. Use of fabric formwork saves lot of manpower
cost and saves lot of energy for preparation of the rigid
formwork. Morover where space is a concern, stocking and
keeping of rigid formwork will be a major concern. Since
most of the fabric formworks are made for one time use
only, they can be kept after concreting which will facilitate
in curing of the concrete. If any kind of aesthetic treatment
is required to be given in the structure fabric formwork is
the only option as it is very tough with rigid formwok and in
some cases it is impossible also.
End product using fabric formwork
A flexible fabric mold awakens concrete to its original wet,
plastic nature by naturally producing concrete members
with complex sensual curvatures. The sculptural and
architectural freedom offered by this method of
construction is matched by new possibilities for efficiently
curved structures. Research at CAST has produced simple
methods for forming beautiful and efficient beams, trusses,
panels, vaults, slabs, and columns.
The Centre for Architectural Structures and Technology
(C.A.S.T.) is fundamentally interested in finding simple ways
to reduce the amount of material consumed in
construction, while at the same time, making these
constructions more beautiful. C.A.S.T. is also committed
to making these methods accessible to as many people
as possible.
The end product is divided here into two parts viz. (A)
Architectural application, (B) Strutural Application.
(A)Architectural application
Fabric formwork can be used to give tough architectural
shapes to the structural member very easily. Figure 1 shows
typical surface of a fabric cast panel and Figure 2 shows a
branched column made with fabric formwork.
Fabric forms can be used to produce complex concrete
Figure 1:Surface detail of a fabric-cast
panel
Figure 2:Branching column formed in
a geotextile form-liner
Focus Fabric Formwork
www.masterbuilder.co.in • The Masterbuilder - March 2012 185
shapes that would be extremely costly or nearly impossible
to create with traditional rigid formwork. Anne-Mette
Manelius, an architect and doctoral student in
Copenhagen, Denmark, made this chair as part of her thesis
work on fabric formwork for concrete. She wanted the soft-
looking chair to fool sitters (Figure 3).
Figure 3: Chair produced with fabric formwork
Green, clean, relatively inexpensive and incredibly
practical, fabric formwork can be used with concrete to
produce structurally efficient and architecturally
compelling components in all shapes and sizes, ranging
from footings, columns and beams to walls, sinks, furniture
and an array of accessories
"It's allowed us to create masonry architecture using very
simple skills," says Sandy Lawton, owner of ArroDesign, a
design/build construction company in Waitsfield. With a
background in carpentry, Lawton says, he found rigid
formwork complicated and labor intensive. "Fabric
formwork has given us the freedom to do complicated
structural work in a very different way that's not complicated
at all. That's the bigger advantage. There's a lot more
flexibility with this system." Fabric formwork also has
benefits from a sustainable viewpoint, Lawton says. "Fabric
formwork basically reduces the amount of everything
required to construct something - placement, storage and
even building the forms. There are huge savings every
step of the way."
Also, he points out, depending on the type of fabric you
use for the formwork, you can get a really nice finish. "You
don't have to go behind and refinish." Instead of using rigid
forms made from lumber, plywood, cardboard, steel or
aluminum, fabric forms use a flexible textile membrane to
form concrete in place. Wet concrete is poured into a
tensile membrane, which produces efficient structural
curves and extraordinary surface finishes. The shape is
determined by how the material is restricted. This can
happen in a number of ways, from creatively using form
ties to make "buttonholes" to placing a brick under a fabric
form to make a relief.
Kenzo Unno, a Japanese architect in Tokyo, devised
methods to cast beautifully shaped walls with thin, flexible
textile sheets. These methods are collectively called "Unno
Reinforced Concrete (Shown in Figure 4)."
Figure 4: Walls casted with Fabric formwork
Figure 5 shows a thin GFRC stingray sink created by
students of Brandon Gore of Gore Design Co.
Figure 5: The 1-inch-thick GFRC Stingray Sink
(B) Strutural Application
Here the use of fabric formworks in various structural
members is shown. Figure 6 and Figure 7 shows casting of
a isolated footing and slab footing using fabric formwork.
The fabric comes in rolls of certain widths and it is simply
cut on site to suit the size needed. Apart from normal tools
for cutting and fixing the braces and perimeter frame, the
only extra items are a Stanley knife and a staple gun. The
fabric is cut neatly with the knife and staple to the timber.
There is a very simple method of cutting the fabric at the
Focus Fabric Formwork
The Masterbuilder - March 2012 • www.masterbuilder.co.in186
corners, and when it is simply stapled in position that it, in
effect, holds the corners together just as strongly as normal
methods. Before the pour, a sheet of standard plastic
vapour barrier is laid on top of the fabric to stop the footing
absorbing moisture if it is required.
It can be noticed from Figure 6 and Figure 7, that no
movement at the top and a slight bulging at the bottom is
there in the freshly concreted isolated footing.
Figure 6: Fabric formwork used for casting isolated footing in a construction site
Figure 7: Fabric formwork used for casting isolated slab footing in a construction
site
A system for forming round concrete columns usingfabric formwork
Figure 8 and Figure 9 shows round various stages of casting
of round concrete column. It can be very easily seen the
end product finish in Figure 9 and also the ease of casting
from the other Figures ( from Figure 8 (a) to (c) ).
Figure 8: (a) Column ready to be poured, (b) Column pouring in progess, (c)
column pouring completed
This method of casting column is beneficial because offollowing reasons
- The fabric come ready made up in tube sections to
form the desired diameter of the column.
- The fabric tube is
simply cut to length
with a Stanley knife.
- In the manufacture,
tabs are made
vertically along a
center line.
- The loose sleeve of
fabric is fitted over
the rebar.
- The tabs are then
nailed to a straight
length of 4" x 2"
timber.
- The 4" x 2" timber is
then positioned, and
braced to hold it
plumb.
- For the first foot or so a guy hold the base of the tube in
the correction position with a boot on either side.
- During the pour, it is possible for a guy to feel and guide
the rebar cage, to make sure that it is in the correct
position.
- Unlike conventional formwork, because this is a throw
away, one off system there is never any reason for undue
haste to strip the formwork.
- Therefore the fabric can be left in position to act as a
perfect curing membrane
However if there is a doubt that whether this system can
work for higher columns, then Figure 10 shows the 20ft tall
column ready to be poured in one hit. The project for a
church in Nicaragua in Central America.
The concrete was mixed by hand on the site and lifted up
by hand. In itself, this was probably a good thing as the
slowness of the pour would mean that the concrete at the
bottom would be stiffening up nicely as the height
increased, reducing the theoretical hydrostatic pressure.
Conclusion
It is very essential to use fabric forms and rebar in an area
where wood is scantily available. Fabric is a very forgiving
material.However one should remember that fabric
formwork is not as uniform as standard formwork. Engineers
had to create some structure to give the appearance of
what they wanted, but in the same breath it gives us a lot
of design freedom. It is really an exciting medium. As for
fabric formwork's limitations, "It's wide open. No one have
tested its limits yet.
Figure 9: Fabric Formwork - Stripped
column
Focus Fabric Formwork
The Masterbuilder - March 2012 • www.masterbuilder.co.in188
The Influence of ConstructionChemicals on Tunnel Durability
Willie Kay
Managing Director of WAK Consultants Pte Ltd /
WAK Technologies Pte Ltd , MC Bauchemie Muller GmbH & Co
Injection Systems
Injection systems in tunnels and underground
constructions are now often considered in the planning
and design stage. They can be a means to simplify
construction, enhance safety, and control potential leaks
or many other applications.
The reason for this change is due to advances in materials
in terms of set times in resin to particle sizes in cement
suspension. Equipment technology in mixing has
improved and pumps are now capable of handling just
about any material even at tropical ambient temperatures
around 35oC.
Engineers and clients need documentary proof of materials
consumed and at what pressure to ensure correct grouting
and this equipment is now readily available.
Injection resins based on polyurethane have been around
for more than thirty years. In general these were a single
component with an accelerator and reacted with water.
There were and are many manufacturers with varying
quality and properties. Figure 1 show a high quality water
reactive resin foamed to approximately 35 times its original
volume.
Newer technologies have two part polyurethane bases and
have properties from highly elastic to highly rigid elastic.
New technologies in gels allow swelling of up to 30% with
This paper looks at the role construction chemicals in the Tunnelling Industry. Advances in both Tunnel boring machine technology(TBM) and ground conditions have accelerated the need and growth of specialised material.
Specialised additive and admixtures have revolutionised the durability and production of precast segments. The advancement ofAlkali free shotcrete accelerators has enabled much safer working conditions. The uses of supplementary cementitious additiveshave allowed high build high strength concrete tunnels by robotic spraying. This paper however will look at the role of injectionresins in tunnels with case histories.
Figure 1
negligible pressure on the substrate. Many of these
products have both CE and REAch compliance. Table 1
shows some typical properties of a gel material.
Thixotropic Gels
Swell up to 30%
Excellent adhesion to most substrates
Ductile up to 300% (see figure 2)
High tear resistance
Variable set times from less than 10 second to minutes
Table 1. Typical Properties
Certification
REACh is the uniform chemical legislation with a strong
Tunnelling Construction Chemicals
www.masterbuilder.co.in • The Masterbuilder - March 2012 189
focus on the protection of human health. Companies
registered can be checked on the internet by contacting
Helsinki. All the injection products we have been
discussing all have REACh certification.
Polyurethane Injection Resins (Elastomer)
Polyurethane and Gel Technology have made major
advances due to understanding the critical nature of mix
ratio, mixing technology and advancement of twin line
pump technology.
The term polyurethane is very generic and does not reflect
the technical changes that have taken place over the last
twenty years. The term elastomer is adopted to describe
the material as it technically describes the material
function. To many people, polyurethane is a brown liquid
that foams and stops leak. This statement is simplistic, as
it does not reveal some of the key properties of a water
reactive resin. In order to fill a void and stop water ingress,
of the following properties are needed.
- Expansion of the material in contact with water
- A stable dense foam
- Non Shrinkage after foaming
- Closed cell structure to prevent water permeation
To achieve all these properties with a single component
water reactive resin is impossible under all conditions. The
foam density will depend on the amount of water and
reaction time. The expansion will vary with the specific
environmental conditions at each project. Due to these
constraints, Europe and specifically Germany have
adopted a two-stage process of injection to ensure
Figure 2. - Example of deformation
permanent leak sealing. In applications of high water inflow
a water reactive open cell foaming resin is first injected as
initial seal. This is ten followed by a second injection using
a two part elastomer resin, which will penetrate the open
cell and give a permanent watertight seal. This method is
adopted from the German Training Council and German
Concrete and Construction Association Deutscher Beton
UndBautechnik Verein e.V. (DBV) for injection of water leaks.
Two part elastomer resins have customisable stiffness
properties and can be engineered from elastic and flexible,
to strong and semi-rigid.
The ability to adjust the setting time is of great importance
to ensure complete penetration of the crack as void
viscosity is another critical factor and this will be discussed
later in his paper. Table 2 shows some basic properties
achievable in the market today.
Std
30 secs
60
Differing Properties of Elastomer Resin
Pot Life
Elongation
Strength (N/mm2)
Viscosity (mPas)
Long Life
45 mins
60
UW
43 secs
80
NV
35 mins
100%
Compressible
Rigid force transmitting
Table 2
Hydro-Structure Resins
The name hydro-structure is used to dissociate these resins
from the toxic acryl gels, which has caused major
environmental problems in Europe. All the resins discussed
and described in this paper comply with the highest
standards of non-toxicity in contact with potable or
drinking water. These resins cross-link and depend on
water migration for long-term performance. The latest
generation has "thixo" or skinning effect which makes them
an ideal solution for buried leaking joints in car parks,
stations and other underground structures. The ability to
be pumped into very specific locations and then set, gives
an ideal method of repairing joints and damaged
membranes. The viscosity of these materials is very low
thus making penetration into tiny voids and fissures very
quick, which is impossible to achieve with a high viscosity
resin. Table 3 lists some key properties.
Solidification
-
Sealing flexible
++
Sealing swelling
+++Hydro-Structure
Resins
+ dry ++ wet +++ water pressure
Table 3
These properties have simplified the repair of leaky
segment joints. "Steps" often occurs when building tunnel
rings in precast concrete and this can lead to failure of the
gasket with subsequent leakage. The hydro structure resins
Tunnelling Construction Chemicals
The Masterbuilder - March 2012 • www.masterbuilder.co.in190
with the thixo agents will be able to rebuild a membrane
behind the joint and effectively waterproof the ring. Skill is
needed in packer selection, gel time of the resin and pump
pressure. The use of Twin Line pumps with the correct mix
head technology is essential.
Equipment
Advances in equipment technology in the last twenty years
have enable resin injection to provide a long-term durable
repair where previously demolition and rebuilt would have
been the only answer. Twin Line pumps with varying
pressure and volume outputs allow correctly trained
applicators to repair almost all leak problems in tunnels.
The reason why Twin Line pumps are so important and
especially in tropical climates are as shown in Figure 3.
Figure 3. - Pump pressure versus injection duration
From this table one can see that the resin penetration is
dependent on three factors; viscosity, time and pressure.
Too high a pressure often causes more damage to the
structure by re-cracking or worse. Time is something we
cannot keep extending as the viscosity is increasing and
the injection costs keep rising. Imagine a situation where
each injection port requires a 15 minutes injection. Spacing
of the injection ports could be at 250 mm centre so each
linear metre of crack would take one hour to inject. The
duration is also dependant on the thickness of the concrete
structure.
The answer is the Twin Line equipment where the resin is
mixed only at the point of discharge and this enables the
lowest possible injection viscosity at the packer. This allows
filling of the crack in the shortest possible time and to the
finer parts of the cracks.
Twin Line pumps are only part of much bigger technical
break through as both mix head technology and online
monitoring have become available. Resins which have
different viscosities or mix ratios require different degrees
of mixing. Some resins can be mixed in 60 seconds with a
shear mixer while others require 3 minutes for complete
mixing. Each resin type has a specific mixer length and
this is critical if the mixed resin is to achieve the designed
property.
On many projects the Engineer would like to predetermine
the pressures at which injection is taking place, others
would like to restrict the volume of resin pumped into each
packer. Other sites require a list of packers used and record
of the volume, pressure and duration when the resin was
pumped. All this information can be made available by
using the German made control device.
This equipment pictured below Figure 4 comprehensively
monitors the injection process. It ensures that the machine
is calibrated and should the mixing ratio be out of margin
it will stop and sound an alarm. Given that the machine is
in good working order it will start pumping and record
pressure volume and time. At the end of a shift the tagged
packers are photographed and the information down
loaded. This is then transferred to a computer and a report
is generated automatically. This can be co-related to the
site by grid reference and crack mapping showing an as
built and as repaired document.
The equipment can also be used with water to carry out
void surveys in structures with very heavy reinforcement
when other techniques may not be suitable.
Figure 4
Applicators
With the sophistication of materials and equipment
technology, a new approach to applicator training has
evolved. Companies licensed to use the materials and
equipments are required to have a government backed
independent certification. This requires attending a two
weeks residential course in Europe taking and passing an
exam supervised by impartial and independent bodies.
Manufacturers are not allowed to give this independent
overview in a training course. The course is operated by
the BZB Akemie and the course topics include Basics of
concrete and steel, repair of concrete construction parts,
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www.masterbuilder.co.in • The Masterbuilder - March 2012 191
polymer and spray polymer repair mortars, and injection
of cracks cavities, joint repair, surface protection systems
and strengthening using carbon fibre laminates. An
examination occurs at the end of the course and if
successful a certification is given. After which, these
licensed operators then attend specific product and
machine training to ensure the total system Man, Materials
and Machinery works.
Case Histories
Brisbane Road Tunnel - Case Histories
North South Bypass Tunnel - NSBT
The SMART Project provides a storm water diversion
scheme including floodwater storage and a 10 km, 11.8 m
diameter bypass tunnel, sufficient to save the city from
flooding in the foreseeable future. With no major flood event
most of the year the tunnel a dual use was engineered,
with double road decks built into the central three kilometre
section, relieving traffic congestion by providing 2 x 2 traffic
lanes for cars connecting the city centre to the southern
gateway, the KL - Seremban Highway.
Suspended slab / Segment detail Application
TBM Segment Installer
The flood water is diverted at the confluence of the Klang
and Ampang rivers into a Holding Pond. From there the
water passes through the tunnel into the Taman Desa
Attenuation Pond and via a box culvert discharges into
the Kerayong River.
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MC was involved in supplying admixtures for both the
backfill grout and the road deck concrete. We were also
involved with all grouting to stop water ingress from within
the tunnel.
Figure 5 and Figure 6 show two specialised injection
systems. Figure 5 shows how we repaired damaged
gaskets using specially developed packers and Figure 6
shows a specially developed packer for resealing leaking
grout sockets.
Area of Application
Application Preparation
Shaft & Joint Sealed
Full Depth Penetration
SMART Tunnel Malaysia - Case Histories
Summary
As tunnel technology advances new materials have been
developed to keep up with these advances and no doubt
will continue in the future.
Figure 5
Figure 6
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Tunnel Repair and RehabilitationUsing ShotcreteSpecial Correspondent
The art of rehabilitation of tunnels has flourished and
developed significantly over the last couple of
decades. Several hundred railroad, highway, and
conveyance tunnels have been successfully rehabilitated,
converted, and/or enlarged. Much of this development
can be attributed to the successful use of steel fiber
reinforced shotcrete. One of the major attributes of
shotcrete is excellent bond to the substrate, usually
superior to the bond achieved with cast-in-place concrete.
This has made shotcrete particularly well-suited for repair
and or rehabilitation work of vertical and overhead
surfaces. The flexibility and adaptable nature of steel-fiber
microsilica shotcrete is ideal for rehabilitation of tunnels.
Many developments in shotcrete technology during the
1980s have enhanced shotcreting capabilities. These
include advances in shotcreting materials technology and
improved methods for batching, mixing, supply, and
application. The developments have stemmed largely
from the desire of engineers and contractors to improve
the quality and durability of inplace shotcrete, increase
View of a shotcrete operation. Look closely and you can see the stream of wet concrete being
blasted onto the rock surface
Pic courtesy: http://thelaunchbox.blogspot.in
Tunnel Engineering Repair & Rehabilitation
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shotcreting productivity and economy, and expand the
range of shotcrete applications. It is because of these
developments in shotcrete technology, enlargement and
rehabilitation of tunnels without fully taking the tunnel out
of service is not only technically but also economically
feasible considering the cost of other alternatives
Enlargement was usually accomplished by raising the
crown but some have been enlarged by lowering the invert,
which is much more difficult and time-consuming.
Harvey Parker & Associates, Inc., in Bellevue, WA have
rehabilitated several tunnels that were over a century old,
allowing these tunnels to begin their second century of
service. This long life represents a huge life-cycle benefit
for the tunnel owner, and this cost advantage can be
maintained by conducting an occasional rehabilitation
from time to time. The increase in the number and type of
tunnels being rehabilitated over the last few decades was
largely made possible by the continued development of
ground support methods using rock bolts and steel fiber-
reinforced shotcrete. Repair and Rehabilitation is done for
several reasons. Sometimes rehabilitation work is done
simply to extend life or to improve future performance,
such as reduction of maintenance or to improve safety.
Generally, highway tunnels, such as the one illustrated in
Fig. 1, fall into this category. Other reasons for rehabilitation
include: 1) enlarge the tunnel to increase clearances or
capacity or 2) change the type of tunnel from one use to
another. On the other hand tunnel gets damaged because
of following reasons needing urgent repair to bring back
the traffic into operation through it again, these are: 1)
Damage due to lack of maintenance, 2) Damage due to
fire, 3) Damage due to natural calamity such as earth quake,
4) Damages due to unexpected operational problems.
Examples of damage occurring due to reason 4 are shown
in Fig. 2.
The introduction of double-stack container cars and other
special or extra-large cars (for example, tri-level auto racks)
created a need for enlargement of most of the tunnels in
the United States and Canada. This is an ideal example of
Figure 1: Shotcreting for rehabilitation of highway tunnel.
tunnel rehabilitation to satisfy a need for larger tunnels
and better service rather than just to extend their lives.
Many of the railroad tunnels in the west and several on the
east coast have been enlarged by increasing clearance in
the crown. Clearances were improved mostly by crown
mining, which consisted of either cutting a notch in the
existing lining or rock walls, as shown in Fig. 3, or by
Fig. 2. Some failed tunnels at Jiulongkou Coal Mine.
Figure 3: Tunnel clearance notch in a railroad tunnel.
complete or substantial removal of the brick or concrete
lining.
Importance of fast recovery of the Tunnel foruninterrupted service:-
Tunnels are vital to keeping our transportation systems
going, and interruptions of service are rarely permitted.
Rehabilitation that requires invert work usually shuts the
entire tunnel down. It is better to concentrate tunnel
rehabilitation on the crown and sidewalls if at all possible.
Typically, there are no alternate routes so tunnel work must
be done with the least disruptive effect on paying traffic.
This is done by either temporarily shutting down one lane
or one track in multiple lane/track tunnels or by managing
traffic to permit work windows that might last from 1 to 8 h.
Yes, work can be accomplished in windows of 1 or 2 h; it is
not very efficient but sometimes that is all the time one can
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get in any one work window. Rehabilitation work while
keeping the tunnel in service requires enormous planning,
coordination, and selection of proper construction methods.
Advances in Shotcrete Materials for Tunnel Repair/Rehabilitation job
Before the 1980s, most shotcrete used for repair and
rehabilitation in North America was made of conventional
portland cement and sand mixtures applied by the drymix
shotcrete process. Some polymer- modified shotcrete was
used for remedial work in aggressive exposure conditions.
There also was limited use of wet-mix shotcrete, primarily
for large- volume projects. Today, both dry- and wet- mix
shotcretes often contain supplementary cementing
materials, such as fly ash and silica fume, as additions or
partial cement replacements. These materials improve
shotcrete workability and performance. In the early 1970s,
a major advance in shotcrete technology was the
development of steel-fiber- reinforced shotcrete (SFRS).
SFRS is particularly useful for remedial applications in
aggressive chemical or marine environments because it
resists corrosion better than shotcrete with conventional
steel reinforcement. As long as the shotcrete matrix retains
its inherent alkalinity and remains uncracked, deterioration
of SFRS is unlikely. Corrosion of the discreet steel fibers
occurs only to the depth of surface carbonation in the
shotcrete. If corrosion of the surface fibers is aesthetically
objectionable, a flash coat of plain, unreinforced shotcrete
can be applied. SFRS has another advantage: It's more
user friendly and less prone to problems caused by
inadequate workmanship. For example, it eliminates the
shadowing and voiding problems sometimes encountered
in conventionally reinforced shotcrete repairs
(Refer Fig. 4).
Fig. 4. An extreme example of shotcrete improperly applied
to mesh reinforcement shows build-up of shotcrete on the
face of the mesh and shadowing and voids behind.
Steel-fiber reinforcement addition rates vary from about
60 to 140 pounds per cubic yard, depending on job
requirements and fiber type and size. Generally, higher
fiber addition rates are used in structures subject to severe
stresses and strains imposed by:
- Impact or explosive forces.
- Heavy, repeated, dynamic cyclic loading.
- Large exposed surfaces, which are more susceptible
to shrinkage cracking
Advantages of Steel Fiber Reinforced Shotcrete
Steel fiber-reinforced shotcrete offers the flexibility needed
to adapt to rapidly changing ground conditions and
uncertain work window schedules. In some projects, due
to the remote location, a concrete batching plant is not
available. Shotcrete dry mix including steel fibers and
microsilica can be purchased in prepackaged 1 y3 (0.75
m3) bags (sling bags) and conveniently stored at the site
until needed (refer to Fig. 5,6,7). Usually the dry mix is
batched at a centrally located plant where the quality of
the shotcrete mixture can be controlled before shipping
to the site. Shotcrete from sling bags can be placed by
the dry or wet method. When placing steel fiber-reinforced
shotcrete in tunnels, costly steel or wood arch forms, and
even rebar or mesh, are not required. Time is not wasted
while erecting, curing, and removing forms or hassling with
mesh. Shotcrete will conform to the rock surface and
smooth out the irregularities caused by blasting. In cases
where the tunnel rock is locally unstable, the design ground
support can be increased to carry the unbalanced load.
Additional shotcrete and rock bolts are placed as
necessary to stop movements as documented by
monitoring. Shotcrete can be finished with a trowel to a
smooth surface equivalent to a form finish. In a pedestrian
tunnel, shotcrete was placed in the steel reinforced arch
of the horseshoe-shaped tunnel and elegantly finished to
a smooth surface. In tunnel sidewalls, the presence of steel
Fig. 5: Shipment of prepackaged Shotcrete in 2205 lb (1 metric tonne) bags to an
underground mine
Tunnel Engineering Repair & Rehabilitation
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Fig. 6: Shotcrete storage area at Falconbridge Raglan property
Fig. 7: Sling bags of shotcrete mixture and work train at a railroad siding
fibers on the surface could cause scratches on the arms of
pedestrians. In these situations, the last 2 in. (5 cm) of
shotcrete are placed without steel fibers. Typical shotcrete
specifications for mixture proportioning indicate that each
cubic yard contains a minimum of seven and a half sacks
of cement (420 kg/m3), 80 to 100 lb (50 to 60 kg/m3) of
steel fiber, 80 lb (50 kg/m3) of microsilica, and a coarse
aggregate/total aggregate ratio of 0.4. The compressive
strength of these mixtures exceeds 5000 psi (34.5 MPa) in
28 days. The fiber content can be adjusted higher or lower
as necessary to accommodate the ground conditions.
How Rehabilitation is done Keeping Tunnel in Service
Rehabilitation work while keeping the tunnel in service
requires enormous planning, coordination, and selection
of proper construction methods. The flexibility of shotcrete,
Fig.8. Typical Railroad Work Train (Schematic Diagram)
Figure 9: Railroad tunnel clearance excavation: single to double track
especially with volumetric mixing, is extremely valuable to
tunnel rehabilitation. Usually, all work is done from work
platforms designed specifically to make all the work
(including handling muck and rebound) done as efficiently
as possible. A schematic of a special work train that is
used for railroad tunnel rehabilitation is shown in Fig. 8.
Examples of Tunnel Rehabiltation Using Shotcrete
- A railroad tunnel in the eastern United States was
enlarged from a single-track tunnel to a twin-track
tunnel. Originally lined with brick, the tunnel was taken
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out of service and enlarged to obtain the double-track
clearance. The new liner consisted of steel fiber-
reinforced shotcrete and rock bolts. Shotcrete
thickness varied from 4 to 12 in. (10 to 30 cm). Typical
rock bolt lengths were 12 ft (3.7 m) but in places ranged
up to 18 ft (5.5 m) long. Figure 9 shows the excavation
process and the shotcrete liner. The tunnel
encountered open abandoned coal mine works, and
the flexibility of utilizing shotcrete and tensioned rock
bolts was invaluable in advancing the work through
difficult ground.
- A highway tunnel on the west coast was rehabilitated
because of the limited clearance and continued
deterioration of the timber lining. The liner consisting
of timber sets and lagging was replaced with 4 to 5 in.
(10 to 13 cm) of steel fiber reinforced shotcrete in the
arch and 2 to 4 in. (5 to 10 cm) of concrete on the
sidewalls. Rock dowels anchored with epoxy resin
cartridges were installed after an initial layer of shotcrete
was installed.
- Repair of a deep-mine permanent access tunnel using
bolt, mesh and shotcrete Jiulongkou Coal Mine, China).
Shotcrete prevents the failed rock mass from falling
and further weathering. The total thickness of shotcrete
applied was 120 mm on average and was sprayed as
three layers. The first and second layers together were
70 mm in thickness. This allows the dilatancy
deformation to be released. Sometimes there were local
failures in the first and second layers. The final layer
was 50 mm in thickness and was sprayed after the
surrounding rock mass deformation became stable.
Steel mesh was used together with shotcrete to
increase the tensile and bending strengths of the
shotcrete. Steel wire with a diameter of 6.5 mm was
selected to form a 125x125-mm2 mesh. Application of
rock bolt, steel mesh and shotcrete to repair seriously
deformed tunnels of the Jiulongkou deep coal mine
shows that the support approach and techniques
based on the loosening zone concept were very much
successful.
- Restoration of a Tunnel Damaged by Noto Offshore
Earthquake in coast of Suzu city in Japan in 1993 was
carried out using steel-fiber-reinforced shotcrete
(SFRS). Spray of steel-fiber-reinforced shotcrete was
adopted because it was considered to increase
bending tensile strength and ductility under uncertain
additional loads from the ground loosened under the
influence of the earthquake. The SFRS design thickness
was 150 mm and mean extra thickness provided was
50 mm. The restoration procedure is shown in Fig. 10.
Conclusion
In the 80 years since the shotcrete process was developed,
shotcrete has played a valuable role in repair and
rehabilitation projects. One of its major attributes is
excellent bond to the substrate, usually superior to the
bond achieved with cast-in-place concrete. This has made
shotcrete particularly well-suited for repair of vertical and
overhead surfaces. The use of steel fiber-reinforced
shotcrete made the rehabilitation of railroad and highway
tunnels practical and economically viable. The strength
and durability of steel fiber microsilica shotcrete in
combination with tensioned or untensioned anchor bolts
can handle almost any type of tunnel ground loading.
Shotcrete can be installed utilizing the wet or dry methods
and can be installed to sculpt any tunnel shape without
the use of costly forms or the need for rebars or mesh.
However advanced research is still going with with other
varieties of shotcretes with polypropylene fibers and other
polymers.
Reference
- H.W. Song, S.M. Lu, Tunnelling and Underground Space
Technology 16 (2001), pg. 235-240.
- M. Kunita, R. Takemata and Y. Lai, Tunneling and Underground
Space Technology, Vol. 9, No. 4, pp. 439-448, 1994.
- The are of Tunnel rehabilitation with shotcrete, Harvey Parker
et al.
Fig. 10. Restoration Procedure
Tunnel Engineering Repair & Rehabilitation
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Durable Concrete forTunneling Applications
Eugenkleen
MC - Bauchemie Mueller GmbH and Co. KG
Over the past decade, the use of concrete
admixtures, especially plasticizers and
superplasticizers, is showing upward trend in
India. The advent of concrete pumps and transit mixers
has also contributed to this, as the use of superplasticizers
enables trouble-free pumping operations and minimizes
pipe blockages. With the advent of Major Metro Projects
across India, durability of concrete used especially for
tunneling segments is of prime importance. The earlier
attitude of taking recourse to the use of admixtures only
after facing problems is changing fast, and now, in most
tuneling projects, high performing admixtures are already
included in the specifications and the mix is designed to
achieve the necessary properties.
The concrete for tunnel segments necessitates the
concrete to have the following properties:
- Compressive strength
- Workability
- Surface Finish
- Durability
As part of the durability requirements, concrete is or should
be generally tested for the following properties:
- Chloride migration
- Sulfate resistance
- Water absorption
- Acid resistance
- Porosity
- Freeze Thaw Resistance
This can be achieved using the latest technologies
available for concrete. Concrete is now no longer a material
consisting of cement, aggregates, water and admixtures
but it is an engineered material with several new
constituents like PFA, GGBSF, Microsilica, Metakaolin,
Colloidal Silica and several other Binders, Fillers and
Pozzolanic materials. The concrete today can take care of
any specific requirements under most exposure
conditions.
The mix designs are getting relatively complex on account
of interaction of several materials and mix design calls for
expertise in concrete technology and materials. High
Performance Concretes will have to be adopted for
tunneling segments, considering special properties as well
as low cost maintenance strategies.
What type of Concrete do we use?
Concrete used in tunneling applications need the following
outstanding properties viz. Compressive Strength, High
Workability, Enhanced Resistances to Chemical or
Mechanical Stresses, Lower Permeability, Durability
etc.This will necessitate the useof High Performance
Concrete. SomeHPC types which will hold the key for
tunneling applications, can be classified into:
- Self Compacting Concrete / High workability concrete
- Concretes resistant against aggressive media
1. Self-Compacting Concrete (SCC)
Self-Compacting or Consolidating Concrete (SCC) as the
name signifies should be able to compact itself by its self-
weight under gravity without any additional vibrations or
compaction. Self Compacting Concrete should be able
to assume any complicated formwork shapes without
cavities and entrapment of air. The reinforcement should
be effectively covered and the aggregates should be fully
soaked in the concrete matrix. In addition, the concrete
should be self-leveling type and self-defoaming without
any external compaction. Figure 1shows SCC.
The formulation of Self Compacting Concrete has the latest
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concrete technology and it requires in-depth knowledge
of materials and meticulous testing procedures before the
concrete is designated as Self Compacting Concrete
(SCC).Self Compacting Concrete has the following special
advantages.
- Saving of costs on machinery, energy and personnel
for vibrating the concrete
- Considerable improvements to exposed surfaces (Fair
Faced Concrete), less efflorescence.
- Marked improvements in durability on account of better
compaction
- Extremely suitable for slim and complicated moulds
- Covers reinforcement effectively.
- Better adhesion between cement binder and
aggregates.
- Reduction in demoulding time
- Advantage with respect to sound pollution
Figure 1: Flow of Self Compacting Concrete around reinforcement
Therefore while calculating the costing and economics of
Self Compacting Concrete all the above mentioned
advantages should be converted to cost parameters. This
kind of concrete can give advantage of good Compressive
Strength, workability and finish to the tunnel segments
and may prove suitable.
2. Durable Concrete resistant against aggressivemedia
One major application of HPC is to increase the durability
of concrete where aggressive underground conditions are
anticipated. This can be achieved physically by resorting
to very dense aggregate packing. The packing curve is
shown in Figures 2a and 2b. This is practically possible by
selecting a very smooth sieve line from largest aggregate
to the smallest grain of Mineral Additives like Microsilica
or New Generation Aluminosilicate slurries. Chemically,
cement by itself is not acid resistant. The acid resistant
binder is formed by combination of cement, microsilica /
aluminosilicate and flyash.To control permeability very low
water cement ratio has to be adopted. So as to provide
the essential concrete properties a high-performance PCE
(polycarboxylate ether) needs to be incorporated in the
mix. By adjusting the particle size distribution on a micro
scale the permeability of the concrete is reduced which
minimizes the penetration of aggressive substances.
Depending on the degree of dispersion these material
particles more or less completely fill the spaces between
the cement particles. During hydration the pozzolanic silica
reacts with the free calcium hydroxide to form calcium
silicate hydrates. This gives a denser concrete structure.
Figure 2a: Densest packing grading curve
The main materials, which can help change normal
concrete to durable aggressive media resistant concrete,
are:
- New Generation PCE Based Admixtures
- Condensed Silica Fume or Microsilica Slurry or
- Latest Generation Aluminosilicates
a. PCE Based Admixtures: Most of the new generation
superplasticizers are from the Acrylic Polymer (AP) family.
Polycarboxylate is a common term for the substances that
are specifically used as Polyacrylate or Polycarboxylate
ethers (PCE). The PCE based Super Plasticizers are by far
superior to the conventional ones with respect to initial
slump as well as slump retention with time. The efficient
working of these plasticizers is due to the new type of
Figure 2b: Pictorial representation of Densest packing of aggregates in Concrete
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molecule designs. PCE based superplasticizers produce
excellent properties when used with cementitious
materials. The disadvantages associated with longer
setting times of conventional superplasticizers is offset by
PCE based super plasticizer and therefore its use in
concrete can also attain high early strengths.Figure 3
shows the structure of PCE molecule and its working
mechanism - steric hindrance. The development of highly
effective superplasticizers with long and consistent
duration of action is therefore an important precondition
for the production durable concrete, due to low water
contents and high early strength requirements.
Figure 3: Structure of PCE Molecule and its Mechanism of action
Concrete additives based on PCE offer advantages like:
- Significant reduction of the water demand of the mix
- Little loss of consistency
- Short setting times
- High early strengths
- Low tendency to segregation
The advantages of these New Generation polymers are
very clear, not only in terms of performance but also in
terms of the dosages used for similar conditions and this
factor balances the disadvantages in economy, as New
Generation Superplasticizers are relatively expensive per
unit price.Figure 4shows workability comparisons of MSF/
SNF against PCE. Figure 5 shows comparative
development of compressive strengths and the dosages
required are very low.
b. Condensed Silica Fume / Microsilica:The term
"Microsilica" is adopted to characterize the silica fume,
which is used for the production of concrete. Microsilica or
Condensed Silica Fume (CSF) is a by-product resulting
from reduction of high purity quartz with coal in the Electric
Figure 4: Workability Comparison of MSF/NSF against PCE at lower dosage
Arc Furnaces used in manufacture of Silicon, Ferrosilicon
and other alloys of silicon.
There are three main reasons for the incorporation of Silica
Fume as an additive for HPC. Microsilica has a filler effect
i.e very fine particle distribute itself in the space between
the materials in the concrete in a homogenous way to give
rise to more dense concrete. Silica Fume improves the
strength of the transition zone between cement paste and
aggregates. CSF is highly pozzolanic in combination with
Portland cement. Figure 6 shows structure and effect of
Microsilica.
During cement hydration there is surplus of Calcium
Hydroxide. The Added Condensed Silica Fume's SiO2
reacts with surplus of Calcium Hydroxide. This results in
greater amounts of Calcium Silicate Hydrate, which are
denser and stronger than Calcium Hydroxide. The
pozzolanic reaction and the fil ler-effect lead to a
compaction of the cement paste and the conversion of
CH crystals into CSH gel leads to a homogeneous paste.
This phenomenon of dense packing in the interface zone
of aggregates also contributes to increase in strength of
the concrete on account of aggregates fully contributing
their strength to the set concrete. Therefore the high
strength of concrete with silica fume is greater than those
of the matrix, indicating the contribution of the aggregate
to the total strength.Experience shows that slurry forms of
Microsilica (50:50 with water) have all the benefits in
transportation, dispensing methods, mixing times and
dispersions to get the desired effect in durable concrete
for tunneling segments.
3. New Generation Aluminosilicates:New generation
aluminosilicates based on special nano-crystallizers have
been developed. These new materials improve the
properties that are crucial for the durability of high-
performance concrete. In addition to reducing chloride
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migration, an exceptional chemical and resistance to
aggressive media of the concrete can be achieved with
Aluminosilicates. The concrete structure is simultaneously
reinforced right down to nanoscale, density is improved
and compressive and flexural strength as well as abrasion
resistance of the high-performance concrete is increased.
There is also a significant reduction of micro-crack
formation,which makes it particularly suitable for the
production of tunneling concrete. Aluminosilicates reduce
the proportion of portlandite by way of a pozzolanic
reaction that changes it into the aluminosilicate crystals
into calcium silicate hydrate. In addition to the unique
resistance against acids a crystalline micro-reinforcement
within the concrete structure is achieved. This reduces
the risk of micro-crack formation, rendering concrete
impermeable.
Due to high homogeneity and reduced tackiness
compared with microsilica-basedconcrete, workability is
improved significantly. In many instances this enables the
production of high-performance concrete that can be
pumped. In addition, a distinct improvement of the building
structure's aesthetics is gained due to the fair appearance
Figure 5: Strength Comparison of PCE versus MFS/NFS at lower dosage
of the concrete surface.Aluminosilicates performs over the
some of the disadvantages of Microsilica:
- Graded for dispersion in concrete
- Graded particle size
- Optimizes mixing time within concrete
- Good dispersion reduces unreacted material in the
mix and increases passivation by C-S-H gel on
aggregate surface
Figure 6: Structure and Microfiller effect of Microsilica in Concrete
- Material if agglomerated improve strength of the mix
- Reduces risk of Alkali Silica Reaction by Agglomeration
of aluminosilicate particles
Table 2 shows some of the key differences between
Microsilica and Aluminosilicate slurries. Figure 7 shows
the comparison of strength development between
Microsilica and Aluminosilicates.
All in all the use of PCE Admixtures and Microsilica or
Aluminosilicate Slurries in addition to the standard
ingredients in concrete, plus excellent mix-design
practices can facilitate the production of high performance
concretes resistant to aggressive media, suitable for use
in tunneling applications.
Microsilica
By-product of the Ferrosilicium- &
Silicium production, not specifically
produced for concrete
Quantities are depending on the
metal industry and the economic
development
Quality of the product has a higher
deviation because it is only a by
product
Aluminosilicates
Manufactured product, it is only
produced for use as concrete additive
Quantities are not depending on other
industries and are unlimited, the
reforereliable availability
High quality standards for end
product because every step in
production is controlled
Table 2: Key Difference Between Microsilica and Aluminosilicates
Figure 7: Comparison of Strength Development between Microsilica and
Aluminosilicates
Tunnel Engineering Concrete Admixtures