Steel Fibre Corrosion in Craks Durability of Sprayed Concrete

Erik Nordström

Steel Fibre Corrosion in Cracks Durability of Sprayed Concrete

2000:49

LICENTIATE THESIS

Licentiate thesis

Institutionen för Väg- och vattenbyggnadAvdelningen för Konstruktionsteknik

2000:49 • ISSN: 1402-1757 • ISRN: LTU-LIC--00/49--SE

Division of Structural EngineeringDepartment of Civil and Mining Engineering

Luleå University of TechnologySE-971 87 Luleå

Sweden

LICENTIATE THESIS 2000:49

Steel Fibre Corrosion in Cracks

Durability of Sprayed Concrete

ERIK NORDSTRÖM

…on the other hand, it´s all just for fun./ Unknown /

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PREFACEFunding of the literature study and field exposure tests were made by SveBeFo,ELFORSK and the Swedish Road authorities together. All other work wasgenerously financed by ELFORSK.

This work is also part of the research consortium ”Väg/Bro/Tunnel” financed byNUTEK, SBUF, Cementa, Elforsk, LKAB, NCC, PEAB and Skanska. Participationin the consortium made the close cooperation with the department of Civilengineering at Luleå University of Technology possible. It also prepared the groundfor getting interesting contacts with other representatives from the industry as well asprofessors and postgraduate students from other universities.

The type of investigations performed in this study with different type of tests andlong-time experiments also make many persons being connected to the work. Manythanks to:

Adjunct professor Jan Alemo, Vattenfall Utveckling AB and professor LennartElfgren, Luleå University of Technology for good supervision and interestingdiscussions during my work.

Professor Göran Fagerlund, Lund University of Technology for important inputduring the initiation of the project.

All my collegues at Vattenfall Utveckling AB, Concrete Technology. Especiallythose involved in spraying, sawing and cracking all the beams that warm summerdays (and nights) in 1997. I also feel thankfulness to all of you helping me withboth manufacturing of samples and evaluation of field and accelerated exposuretests.

MSc Pär Hansson, Ericsson Radio Access for frutiful discussions and great”colleague-ship” during his time at Vattenfall Utveckling AB.

The always interested reference groups connected to the parts funded by SveBeFo.

Peter Mjörnell, Bekaert for supplying me with cold drawn wire for making thefibres.

Ingemar Andersson, Färdig Betong, Örebro for cutting the fibres.

Georg Danielsson, Luleå University of Technology for help with the uni-axialtests and cracking of samples.

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Last but certainly not least special thanks to:

My family and especially my beloved wife Maria who is as much part of this workas I am, since we together managed to get through both work, studies and extremefamily situations during this work.

Älvkarleby in November 2000

Erik Nordström

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ABSTRACTA combination of the sprayed concrete technique and steel fibre technology givesobvious advantages when saving the work needed to place conventionalreinforcement. In rock strengthening applications this is most accentuated.

Sprayed concrete in general, made by skilled worksmen, will recieve a high qualityand good durability. Durability requirements can also be found in todays regulationswith demands on service-life of more than 100 years. Since the steel fibrereinforcement in wet-mix sprayed concrete has been common practice only since thelate 80:s questions could be raised regarding the resistance to corrosion. It haspreviously been proved that steel fibres show an excellent durability against corrosionin homogenous concrete. At conditions where conventional reinforcement show highrates of corrosion the steel fibres can still be unaffected. Fibres have a smaller sizethan conventional reinforcement and they seem therefore to be better protected by thealkaline environment the concrete give. Smaller cathode area compared to the anodearea is another argument to the better resistance against corrosion.

The high quality combined with relatively thin layers applied in sprayed concretestructures give rise to imposed deformations by shrinkage, which is a common reasonfor cracks. In the design of steel fibre reinforced sprayed concrete (SFRSC) for e.g.rock strengthening purposes the fibres are used both to minimize crack widths fromshrinkage and to obtain a sufficient post-crack behaviour. A system with bolts andSFRSC is depending on a long-term residual strength capacity.

The purpose with the thesis is to investigate the mechanisms ruling initiation andpropagation, or possibly to explain the higher resistance against corrosion of steelfibres in cracked concrete.

Field inspections of old SFRSC show that the amount of corrosion is limited after 5-15 years of exposure. Even with presence of high chloride concentrations the attackseemed small. In all the inspected structures the amount of fibres crossing the crackswas very small.

Two different approaches to study the corrosion of steel fibres in cracks have beentested. Cracked beams of SFRSC have been exposed in field at three different sites.Crack width, fibre length, mix-composition, accelerators and spraying technique(wet-/dry-mix) are parameters beeing tested. After 2.5 years of exposure mainlysamples exposed along a motorway with direct splashing of water containing de-icingsalts show corrosion on fibres crossing the crack. A loss of 15-20% of the fibrediameter in the outer 25 mm is common.

Laboratory studies with accelerated exposure tests have also been performed. Thepurpose is to develop a technique to isolate parameters in a better way than in fieldand to perform exposure tests in a more controllable environment. In addition a

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useful technique combined with a correlation to the field exposures could make itpossible to imitate longer real exposures in a shorter period of time and by thisestimate the long-time behaviour. Mainly the same behaviour as in field, withincreased corrosive attack with increased crack width and fibre length, could be seenin the laboratory exposures. The influence of fibre length accentuate the importanceof the anode- /cathode ratio for the rate of corrosion which also have been stated forconventional reinforcement. A very rough estimation is that the laboratory exposuresaccelerate the exposure with about 10 times compared to the motorway environment.

As mentioned the steel fibres are supposed to be able to carry load during the entireservice-life. A discussion about how knowledge about the rate of corrosion could beused in a design situation is also presented in the thesis. To counteract loss of load-bearing capacity due to fibre corrosion e.g. extra amount of fibres or an increase ofthe layer thickness could be prescribed in the mix-design.

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SAMMANFATTNING (In Swedish)Genom att kombinera sprutbetongtekniken med stålfiberarmering erhålls uppenbarafördelar genom inbesparat armeringsarbete. Detta blir särskilt tydligt ibergförstärkningssammanhang.

Sprutbetong i allmänhet, tillverkad av kunniga hantverkare, får hög kvalitet och godbeständighet. Krav på beständighet finns också i dagens normer med krav på över100 års livslängd. Eftersom stålfiberarmerad, våtsprutad betong bara använts sedanslutet på 80-talet finns frågetecken kring beständigheten mot fiberkorrosion. Det hartidigare bevisats att stålfibrer uppvisar utmärkt beständighet mot korrosion i homogenbetong. Vid förhållanden som ger höga korrosionshastigheter på konventionellarmering kan stålfibrer fortfarande vara opåverkade. Fibrer är små jämfört medkonventionell armering och skyddas därför bättre i betongens alkaliska miljö. Mindrekatodyta i förhållande till anodytan är ett annat argument till varför fibrer uppvisarbättre korrosionsbeständighet.

Den höga kvaliteten kombinerat med att sprutbetong appliceras i relativt tunna skiktger upphov till tvångsdeformationer av t.ex. krympning, som är en vanlig anledningtill uppsprickning. Vid dimensionering av en bergförstärkning med stålfiberarmeradsprutbetong används fibrer både till att minska sprickvidder från krympning och attskapa en acceptabel duktilitet efter uppsprickning. I ett system med bultar ochstålfiberarmerad sprutbetong är man beroende av vidmakthållen residualbärförmågaunder lång tid.

Syftet med föreliggande avhandling är bl.a. att undersöka mekanismerna som styrinitiering och propagering, och möjligen förklara den högre motståndskraften motkorrosion hos stålfibrer i sprucken betong.

I besiktningar av gammal stålfiberarmerad sprutbetong kan endast begränsadkorrosion ses efter 5-15 års epxonering. Även vid närvaro av höga kloridhalter verkarangreppet vara begränsat. I alla de undersökta objekten var dock antalet fibrer somkorsade sprickan mycket litet.

Två olika angreppssätt har använts för att studera korrosion av stålfibrer in sprickor.Spruckna stålfiberarmerade sprutbetongbalkar har exponerats i fält vid tre olikaplatser. Sprickvidd, fiberlängd, blandningstyp, acceleratorer och sprutmetod (våt/torr)är parametrar som testats. Efter 2.5 års exponering uppvisas korrosion, på fibrer somkorsar sprickor, huvudsakligen i prover exponerade längs en motorväg meddirektstänk av vatten innehållande tösalt. Förlust av 15-20% av fiberdiametern i deyttre 25 mm är vanligt där.

Laboratorieförsök med accelererad exponering har också genomförts. Syftet är attutveckla en teknik för att på ett bättre sätt, och snabbare, kunna undersöka olikaparametrars inverkan på korrosionsbeständigheten i ett mer kontrollerbart klimat. De

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accelererade försöken måste genomföras på ett sätt som möjliggör en korrelation medfältförsöken. Därmed kan längre tids verklig exponering efterliknas på kortare tid ochpå så sätt möjliggörs en bedömning av långtidseffekter av korrosion. Huvudsakligenuppvisas samma beteende som i fältexponeringarna med ökat angrepp med ökandesprickvidd och ökad fiberlängd. Inverkan av fiberlängd betonar vikten av anod-/katod-förhållandet för korrosionshastigheten vilket också påvisats för konventionellarmering. En mycket grov uppskattning är att laboratorieexponeringarna ger ca. 10gånger acceleration jämfört med normal exponering i motorvägsmiljö.

Som nämnt tidigare förväntas stålfibrerna kunna bära last under hela konstruktionenslivslängd. En diskussion kring hur kunskap om korrosionshastigheter skulle kunnaanvändas vid dimensionering presenteras också. För att motverka förlust avlastbärande förmåga p.g.a. fiberkorrosion skulle t.ex. extra mängd fibrer eller ökadskikttjocklek kunna föreskrivas vid proportionering.

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TABLE OF CONTENTS PAGE

ABSTRACT VII

SAMMANFATTNING (IN SWEDISH) IX

NOTATIONS XV

1 INTRODUCTION 11.1 Service-life requirements 11.2 Fibres and the sprayed concrete technique 11.3 Steel fibre corrosion 21.4 Research significance 21.5 Disposition of the thesis 2

2 INSPECTION OF OLD SPRAYED CONCRETE STRUCTURES 32.1 Introduction 32.2 Methodology for status control 32.2.1 Collection of object information 32.2.2 General inspection 32.2.3 Detailed inspection 42.2.4 Laboratory investigations 52.3 Selection of structures 52.4 Results of inspections 82.4.1 Structures exposed in mild conditions 82.4.2 Structures exposed in medium conditions 112.4.3 Structures exposed in severe conditions 122.5 Conclusions 13

3 PREVIOUS WORKS ON CORROSION OF STEEL IN CONCRETE 153.1 Corrosion in general 153.2 Reinforcement corrosion in homogeneous concrete 173.2.1 Initiation by carbonation 173.2.2 Initiation by chloride ingress 183.3 Reinforcement corrosion in cracked concrete 203.3.1 Crack width 203.3.2 Anode- / cathode- area ratio 213.3.3 Self-healing of cracks 223.4 Influence of steel quality on reinforcement corrosion 233.5 Previous investigations on corrosion in cracked SFRC 23

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3.5.1 IBAC, Aachen, Germany, Schiessl & Weydert 233.5.2 University of Michigan, USA, Kosa 243.5.3 University of Aberdeen, Great Britain, Mangat & Gurusamy 253.5.4 University of Surrey, Great Britain, Hannant & Edgington 263.6 Discussion 27

4 FIELD EXPOSURE TESTS 294.1 Background 294.2 Scope 294.3 Methodology 294.3.1 Spraying method 304.3.2 Concrete composition 304.3.3 Manufacturing of samples 324.3.4 Exposure environment 334.3.5 Evaluation after exposure 354.4 Results 394.4.1 Climatic conditions 394.4.2 Residual strength 394.4.3 Chloride content 414.4.4 Carbonation 434.4.5 Corrosion on fibres 434.5 Discussion 454.5.1 Residual strength 454.5.2 Chloride content 454.5.3 Extent of corrosion 464.6 Conclusions 47

5 LABORATORY EXPOSURE TESTS 495.1 Scope 495.2 Methodology 495.2.1 Concrete composition 495.2.2 Manufacturing of samples 505.2.3 Exposure environment 525.2.4 Evaluation after exposure 545.3 Results 545.3.1 Fibre corrosion 545.3.2 Exposure environment 565.4 Correlation to the field exposure tests 575.5 Discussion 585.6 Conclusions 59

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6 LOAD-BEARING CAPACITY 616.1 Introduction 616.2 Analytical model 616.3 Service-life modelling 636.3.1 Definition of limit state 636.3.2 Service life 636.4 Discussion 65

7 CONCLUSIONS 67

8 RESEARCH NEEDS 69

9 REFERENCES 71

APPENDICES PAGE

APPENDIX A: Data from spraying of panels to field exposure tests 75

APPENDIX B Residual strengths after 1 year of exposure in field 77

APPENDIX C Residual strengths after 2.5 years of exposure in field 79

APPENDIX D Climatic data from field exposure tests 81

APPENDIX E Paper I : Durability of sprayed concrete – A literature study.Proc. ”Concrete in the service of mankind”, Dundee,Scotland, 1996

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APPENDIX F Paper II : Durability of sprayed concrete repairs. Proc. ”Repairand upgrading of dams”, SwedCOLD, Stockholm,Sweden, 1996.

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XV

NOTATIONSLatin letters

A acceleration factor [-]Ac minimum fibre area [mm]fb average bond strenght [MPa]fs tensile strength of fibre [MPa]Fmean average fibre strenght [N]F load [N]h beam height [mm]Ie corrosion current [A]l fibre length [mm]le embedded fibre length [mm]lec critical embedded fibre length [mm]M1 moment capacity for non corroded fibres [Nmm]M2 moment capacity for corroded fibres [Nmm]N number of fibres in tensile zone [-]r rate of corrosion [mm/year]Rel electrical resistance of concrete [Ω]Rc electrical resistance at cathode [Ω]Ra electrical resistance at anode [Ω]Rs electrical resistance in steel [Ω]RH relative humidity [%]S strength ratio [-]Sc critical strength ratio [-]T1 tension force in non corroded area [N]T2 tension force in corroded area [N]t time for exposure [days]tc service life [years]Ue corrosion potential (V)w crack width [mm]x depth of corroded area [mm]

Greek letters

∆φ loss of fibre diameter [%]δ deflection [mm]φ fibre diameter [mm]φc critical fibre diameter [mm]

Chapter 1 - Introduction

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1 INTRODUCTIONIn the late 80:s the technique to reinforce sprayed concrete with steel fibres becamecommon practice. The construction of an underground deposit for waste materialsfrom the nuclear power plant in Forsmark, Sweden and the railroad tunnels alongGrödingebanan, Sweden were two of the first major projects. Great advantages bydecreased amount of work needed for placing mesh reinforcement was most obviousin rock strengthening applications. Today steel fibre reinforced sprayed concrete iscommon practice for permanent linings in underground construction in Scandinavia.It also occurs in repair and strengthening of concrete structures.

1.1 Service-life requirements

Knowledge of potential or remaining service-life of a structure is essential during allparts of the service-life. From the design process via construction to maintenance ofthe structure this should always be focused on. Therefore todays regulations e.g.TUNNEL 99 (1999) from the Swedish Road Authorities has service life demands.For underground tunnels in rock there is a demand on ”expected technical servicelife” of 120 years (main structural elements in tunnels longer than 1 km). Thedefinition of technical service life is that the prescribed service life is achieved with90 % significance, with ”normal maintenance”. The requirement also gives that theaverage expected service life is 25% higher i.e. 150 years. When the structure doesnot show sufficient perfomance the service life is obtained. A question contractors,designers and purchasers should have is whether steel fibre reinforced sprayedconcrete can fulfill this service life demand. ”Normal maintenance” could notpossibly be reconstruction e.g. every 15-25 years with all the costs and disturbance tothe use of a tunnel that would give (traffic problems, loss of production etc.). Theoriginal structure should therefore withstand a 150 year long exposure. More generalinfo about service-life estimations etc. can be found in Sarja & Vesikari (1996) orFagerlund (1987). Modelling and ideas about how to connect degradation and theinfluence on load-bearing capacity can be found in e.g. Noghabai (1998)

1.2 Fibres and the sprayed concrete technique

Except from advantages during construction, fibres are used in two major purposes.Reduction of crack widths and achieval of a ductile post-crack behaviour. Wherefibres are used in the last mentioned application design criterias can be found inHolmgren (1992). Sprayed concrete is most commonly applied in relatively thinlayers (typically 50-100 mm) and the concrete quality is high. This will give a highdegree of shrinkage due to drying out. High air speeds, in e.g. tunnel applications,due to ventilation, further increase this effect. Some types of accelerators also give anincreased shrinkage (Manns & Neubert, 1992). All this together significantlyincreases the risk for shrinkage cracks (crack width= 0.1-0.5 mm). Loads from

Chapter 1 - Introduction

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movements in the substrate being sprayed on is another possible, but less common,source to cracks. These cracks could be wider. A positive property of sprayedconcrete is that it is always applied on another substrate and due to the limitedthickness the restraint also limits the crack width.

1.3 Steel fibre corrosion

One could suspect that the relatively thin steel fibres would discontinue to carry loadrelatively fast due to decrease of fibre diameter caused by corrosion. Especially incracked concrete. The excellent performance of fibres against corrosion inhomogenous concrete has been shown as early as 1966 by Shroff (1966). Previousinvestigations on cracked concrete are presented in chapter 3. It seems as steel fibrescorrode at a lower rate than conventional reinforcement at the same conditions. Theparameters giving this positive property is not very well known. It is also possiblethat the rate of degradation is not linear. An increase in the rate of corrosion will givea rapid break down of the thin fibres and by this a reduced load bearing capacity.

1.4 Research significance

With knowledge that makes service life estimations possible, the confidence for anduse of steel fibre reinforced concrete can increase. Knowledge of importantparameters ruling initiation and propagation of cracked steel fibre reinforced concretecan also be used in design e.g. when choosing type and amount of fibres or indefinition of an extra cover without fibres. It can further be used to assure a certainservice life of old structures when it concerns acceptable crack widths or chloridecontents in different environments.

1.5 Disposition of the thesis

Chapter 2 contains results from an inventory of existing sprayed concrete structuresthat were examined after different time of exposure in different kinds ofenvironments. In this compilation only structures with steel fibres and informationabout their behaviour is included. In chapter 3 previous investigations found inliterature on cracked concrete with steel fibres are presented. There are other studiesthan the ones presented in chapter 3 but only the major ones are included. Chapter 4deals with ongoing field exposures with cracked steel fibre reinforced concretestarted in 1997. Methodology for and results from evaluations after 1 and 2.5 years ofexposure are presented. In chapter 5 accelerated laboratory exposures with crackedsteel fibre samples are described. Chapter 6 briefly deals with ideas on how theinfluence of corrosion on the load-bearing capacity should be considered. Finally twoconference papers are added in the appendix E & F dealing durability aspects nottreated in the thesis.

Chapter 2 - Inspection of old sprayed concrete structures

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2 INSPECTION OF OLD SPRAYED CONCRETESTRUCTURES

2.1 Introduction

In order to aquire knowledge of the durability, inspections of old sprayed concretestructures were carried out. In the following the results from inspections of a coupleof existing structures are presented. Structures with steel fibre reinforcement areselected from Nordström (1996a). Further information about literature regardingdurability of sprayed concrete in general can be found in Nordström (1995, 1996b).

2.2 Methodology for status control

Inititally a general inspection was performed to control the general status of thestructures. The results from this first inspection then determined whether a detailedinspection with sampling etc. should be made.

2.2.1 Collection of object information

To be able to perform an adequate inspection of a structure there is an obvious needfor knowledge about design, function and construction of the structure. For exampleit is difficult to draw conclusions about visible cracks without information about theintended function of the inspected part the structure. It could also happen, whenarriving on site, that an inspection was not possible to perform due to lowaccessibility. Information about the conditions and progress of construction is anotherand possibly the most important information since many problems with durability canbe connected to this period.

A good way to achieve wanted information is to communicate with the owners and/orcontractors. Some objects have documentation about mix-composition and sprayingmethod and sometimes also test results. Information about other objects requirespersonal communication with people involved, during construction. A combination ofboth has shown to be the best alternative.

2.2.2 General inspection

A general inspection can be a good and relatively cheap method to estimate thecondition of a sprayed concrete structure. Some simple field methods are:

Ocular inspection. Easy to perform but demanding when it comes to interpretingthe results. Requires good knowledge about how concrete responds to differenttypes of mechanical and environmental loads. Cracks and crack patterns giveinformation about the reason to their origin. Leaching and other deposits and their


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location on the structure also give imporant input. Change in colour etc. is anotheruseful observation. An ocular inspection is essential to create a complete pictureof the condition of the structure. Single test results is most commonly not relevantwithout this type of inspection.

Crack width measurment. Can give information about the reason to the cracking.

Hammer tapping. Used to locate areas with loss of bond strength. It can also beused to find areas with sprayed concrete of low quality.

Collection of drilling debris. With a battery operated drilling machine it is easy todrill and collect debris from the sprayed concrete. The debris can be used to makea rough estimation of the chloride content.

Carbonation control. In a freshly drilled bore hole or on other newly exposedsurfaces it is possible to control the carbonation depth with a phenolphthaleinsolution. Concrete not coloured red is carbonated.

Photo and video documentation. A good support for the recollection back at theoffice. This can prevent costly extra visits to the object. Important to take picturesin all scales, both close-ups and overall views.

2.2.3 Detailed inspection

A detailed inspection needs more equipment like a drilling machine for cores, aportable generator and a pump for cooling water to the drilling. On the other hand amore accurate and definite evaluation will be the result. All methods presented belowdepends on the possibility to drill out cores.

Bond strength. With equipment according to Figure 2.1 it is possible to measurethe in-situ bond strength between sprayed concrete and rock or concrete withoutgluing etc.

Ocular inspection of cores. In the envelope surface possible weak zones orlamination can be seen. It is also possible to control corrosion on bars or fibres.

Core drilling across cracks. To study corrosion on reinforcement bars or fibres incracks this can be used. It will also be possible to study leaching in cracks.


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Figure 2.1 Equipment for testing of bond strength according to SS 13 72 43.

2.2.4 Laboratory investigations

With drilling equipment in field it is possible to take samples for further investigationin lab.

Chloride profile. Drilling debris is collected at different depths from the exposedsurface. The debris is then tested regarding the chloride content accordring to e.g.the RCT-method (Rapid Chloride Test) developed by Germann & Hansen (1991).

Compressive strength. From concrete cores samples can be sawn for testing ofcompressive strength. Except from the actual compressive strength of the structurethis will also give an indication of the overall quality (low compressive strength =low quality, more porous and higher permeability).

Fiber content. By crushing of concrete cores the actual fibre content can becontrolled

2.3 Selection of structures

A large number of possible sprayed concrete structures was listed with help fromcontractors and owners. All structures could not be subject to an inspection andtherefore a selection had to be made. The choosen structures should be as old aspossible but constructed with ”techniques relevant for today”. The criterias used forselection of structures are shown below.

Age. The primary criteria used taking into consideration the type of sprayingmethod. The dry-mix spraying method has changed to a limited amount during thedecades. Therefore structures constructed with dry-mix spraying usually are older.Structures with wet-mix spraying combined with steel fibres are most commonlyyounger.


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Structure type. A mix of underground structures and concrete repairs was wanted.

Reinforcement. A classification in the categories steel fibres, conventionalreinforcement and plain sprayed concrete has been used. Primarily steel fibrereinforced structures were selected.

Environment. Structures from as many categories as possible was the goal. Bothrock cavities with relatively constant temperarure and humidity and concretebridge repairs with outdoor conditions and splash from thaw salts are included.

Documentation. Well documented objects were preferred with information aboutthe original structure or construction.

Accessibility. Interesting structures with low accessibility were left out.

A total of 16 different structures where examined during the inspections (see Table2.1-Table 2.3). In Table 2.4 the structures are divided according to exposureenvironment and severeness of freeze-thaw exposure and corrosivity. Class Arepresents the corrosivity and class B the freeze-thaw action. The numbers show thedegree of severeness with a maximum at 4.

Table 2.1 Examined structures with sprayed concrete on rock.

No. Site Structure Method Reinforcement Constructed1 Stornorrfors, Umeå Headrace tunnel, hydro

power plantWet-mix Steel fibres 1985

2 Viskan, Varberg Road tunnel Dry-mix Steel fibres 19803 Skogby, Halmstad Rail road tunnel Dry-mix Steel fibres 19854 Eugenia, Stockholm Road tunnel Dry-mix Bars 19905 Umluspen, Storuman Ventilation tunnel Wet-mix Steel fibres 19906 Graversfors, Norrköping Open cut, railroad Dry-mix Mesh 19607 Öd, Kramfors Rail road tunnel Dry-mix Bars 1957


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Table 2.2 Examined structures with sprayed concrete on concrete.

No. Site Structure Method Reinforcement Constructed8 Nämforsen, Näsåker Spillway Dry-mix Polypropylen fibres 19909 Road E4, Ödeshög Bridge column Dry-mix Bars 199110 Stadsforsen,

BispgårdenSpillway Dry-mix Steel fibres 1989

11 Road E20, Lerum Bridge column Dry-mix Bars 199012 Road E4, Norrköping Viaduct Dry-mix - 199013 Hölleforsen,

BispgårdenSpillway Dry-mix Steel fibres 1990

14 Forshuvud, Borlänge Crane foundation Dry-mix - 1991

Table 2.3 Examined structures with sprayed concrete on corrugated sheets.

No. Site Structure Method Reinforcement Constructed15 Road E20, Lerum Road culvert Dry-mix Steel fibres 199116 Road E20, Lerum Road culvert Dry-mix Bars 1991

Table 2.4 Structures divided according to environmental classification in BBK 94(1994). (Structures in bold text are examined with detailed inspection).

Corrositivity classificationA1 A2 A3 A4

B1- 1. Stornorrfors

15. Road E20, Lerum (steel fibres)

5. Umluspen

- -

B2- 3. Skogby

7. Öd14. Forshuvud

- -

B3

- 6. Graversfors 10. Stadsforsen8. Nämforsen13. Hölleforsen16. Road E20, Lerum

(bars)

4. Eugenia

Free

ze-th

aw c

lass

ifica

tion

B4

- - - 2. Viskan9. Road E4,Ödeshög11. Road E20,Lerum

(bridge column)12. RoadE4,Norrköping


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2.4 Results of inspections

In the following only structures examined with core drilling (detailed inspection) andpresence of steel fibres are presented. Only results regarding steel fibre corrosion arepresented. Detailed information about the other structures and other test results can befound in Nordström (1996a). The presented structures can be found in class A2/B1,A3/B3 and A4/B4. These three combinations of corrosivity and freeze-thawclassification are in the following called exposure in mild, medium and severeconditions.

2.4.1 Structures exposed in mild conditions

Stornorrfors, Umeå (No. 1)The Stornorrfors hydropower plant is situated some 15 km west of Umeå along theUmeå river. Mainly this underground plant was constructed in 1958. Extra space forthe fourth generator was made in 1985 and in connection to this the headrace tunnelswere strengthened with steel fibre reinforced sprayed concrete.

Figure 2.2 Skeleton sketch over headrace tunnel for unit G4, Stornorrfors hydropower plant.

The climatic conditions in the area above water give a relatively constant temperature(8-10°C) and a high level of humidity. Only limited leakage through cracks and otherdefects could be seen.

Generally the sprayed concrete seems to be in good condition with few cracks abovewater level. To examine steel fibres crossing a crack, core drilling was performedfrom a rubber boat in headrace tunnel G3. Cores at levels 0.1m, 0.5 m and 1.25 mabove the mean water level were taken out. Very few fibres were crossing the crackwhen comparing it to the added amount of fibres (70 kg/m3) in the sprayed concrete.No signs of corrosion could be found on fibres crossing the crack, only on fibresexposed to the surface. A thin layer of cement paste (1 mm) seem to be enough toprotect the fibres from corroding in this environment.

Examined


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The sprayed concrete thickness was measured to 70-150 mm. Depth of carbonationwas controlled and the results are presented in Table 2.5.

Table 2.5 Carbonation depth in crack at different heights above water level,Stornorrfors hydro power plant, headrace tunnel G3.

Sample Height abovewater level (m)

Carbonationdepth(mm)

C1 1.25 1-10C2 0.5 1C3 0.1 0

Lime deposits could be found in the crack in sample C1. Close to the surface thecrack was completely healed. In sample C2 lime deposits could be found at a depth of10 mm and downwards in the crack. Sample C3 contains no crack but was split intwo pieces to study the fibres. The amount of fibres was much larger than in the othersamples.

A single test of compressive strength was performed on one core from the bondstrength tests. It was measured to be 109 MPa. The amount of fibres was measuredto 70 kg/m3. The fibres were of type Hörle (low carbon steel and hooked ends, l=30 mm, Ø=0.6 mm). According to available documentation the cement content is 450kg/m3, maximum aggregate size 8 mm and water glass accelerator was used in ”wet”areas.

Discussion:In general the examined part of the structure is in good condition. Steel fibrecorrosion is located to fibres exposed to the surface and after 10 years of use noindications of initiation of corrosion on fibres crossing cracks could be seen.

A question about crack localization could be rised since a low amount of fibres couldbe found crossing the crack, but a sufficient amount was available close to the crack.If cracks propagate in areas with low amount of fibres this focus on the need foradequate distribution of fibres in the sprayed concrete. The load-bearing capacity inthe cracked state is depending on the presence of fibres crossing the crack.

Only one result on compressive strength is too little but the remarkably high strengthis still an indication of very high quality of the sprayed concrete.


10

Road E20, Lerum (No. 15)The existing steel sheet road culvert with 2.5 m diameter was built in the early 60:sand was heavily corroded before sprayed concrete was applied. Especially in thewater line. Access to the road culvert is via a manhole which means that there is nodirect access to outdoor conditions. Even during the winter water is pouring troughand therefore there is a constant humid climate. Above the road culvert there is 10 mof road fill. The sprayed concrete is reinforced with steel fibres of type low carbonsteel and hooked ends (Dramix 40/0.5) according to available documentation. Thefinal layer of sprayed concrete is made without fibres. Tests made during constructionshows a compressive strength of 79 MPa (cubes sawn from sprayed panels).

Figure 2.3 Crack (left) and core drilling (right) in road culvert with steel fiber

reinforcement under road E20, Lerum.

Cracks with various widths (0.5-2 mm) where visible radially all around the cross-section. In several of the cracks red deposits from corrosion of the corrugated steelcould be seen. Core drilling was performed at 0.1, 0.55 and 1.15 m above actualwater level (high water level). The high water level and water speed made drillingdifficult and extra safety precautions had to be taken (see Figure 2.3). At levels 0.1and 1.15 m the core was crossing a crack. In the upper core only one single fibrecrossed the crack, and this was unaffected by corrosion. Slight surface corrosioncould be found on fibres crossing the crack in the lower core. The underlying steelsheet thickness was 5 mm in the upper hole and only 1 mm remaining in the lowerdue to severe corrosion.

Discussion:It is difficult to state the origin of the cracks. Cracks due to shrinkage is mostcommonly not as wide as 1-2 mm and the number of cracks should also be larger.The low content of fibres crossing the crack could however be the reason to largecrack widths. Redistribution of structural load due to damages of corrosion in thesteel sheets is another possible explanation. In spite of the high level of humidity incombination with cracks the corrosion on fibres was limited.


11

2.4.2 Structures exposed in medium conditions

Stadsforsen, Bispgården (No. 10)The Stadsforsen power plant is situated along the river Indal 4 km east of Bispgårdenand it was constructed in 1939 and 1952. The dam has five spillways (see Figure 2.4)and they where repaired in 1989 with sprayed concrete. The repair works wereperformed by removing damaged concrete with hydro demolition. Accurate cleaningand pre-wetting of the surface to a ”humid but slightly absorbing surface” wasprescribed. The surfaces where steel trowelled and they were also vacuum treated inthe lower parts. Water curing five days after spraying was prescribed.

Figure 2.4 View of spillways (left) and detail of spillway C (right) at the Stadsforsen

power plant.

Spillway C was examined. Steel fibres exposed at the surface are corroded. The jointsbetween the different spraying stages can be seen at the surface. In the most left partthere is some map cracking but otherwise there are very few cracks. One single crackcould be found in the center of the spillway and a core was drilled across the crack. InFigure 2.5 a sketch over the spillway and the positions for testing of bond-strengthand core drilling are shown.

20 m

4 m

4 mB3B1

B2

S1S2

Figure 2.5 Sketch over spillway C and positions for testing at the Stadsforsenpower plant, Bispgården.

Core S1 was taken out in the joint between two sections and the joint had been coatedwith bitumen. Parts with inhomogenous concrete could be seen in connection to the


12

joint. In this part partly corroded fibres was found. No signs of fibre corrosion couldbe seen in core S2 taken across the crack. Lime deposits was present outside thecrack and in the crack leached concrete was visible in the upper 2 cm.

The compressive strength of one sample was 129 MPa and compared to measuresduring construction (85 MPa) this is a large increase. The steel fibre content was nowmeasured to 50 kg/m3.

Discussion:In general the spillway is in a good condition without larger damages. The area withlow quality sprayed concrete is most probably due to trapped rebound not beingremoved before spraying. Except from this area the compressive strength isremarkably high which indicates a very high concrete quality. In the examined crack(not joint), no signs of corrosion was visible.

2.4.3 Structures exposed in severe conditions

Viskan, Varberg (No. 2)These 16 year old road tunnels are situated along road E6 approximately 10 km northof Varberg. Sprayed concrete is mainly used in the ceiling but also along drains andin the tunnel mouth down to the ground. The inspections where concentrated to thenorthbound lane.

Figure 2.6 South mouth of the Viskan tunnel along road E6, Varberg.

Core drilling across a crack was made to check corrosion on steel fibres. The corewas taken out from the top of a drain. The thickness of the sprayed concrete was 20-25 mm and farthest out a layer without fibres was applied. In some parts a weldedmesh was visible in the crack but no fibres could be seen across the crack. The meshwas not corroded in the crack. The chloride content in the concrete surrounding thecrack was measured and is presented in Figure 2.7 (In the calculations a cementcontent of 450 kg/m

3 has been assumed).


13

0

0,2

0,4

0,6

0,8

1

Surface 1.5cm 3cmDepth from surface

[Cl-]

/ w

eigh

t of c

emen

t (%

) 0.15m above road0.85m above road1.45m above road

Figure 2.7 Chloride profile for sprayed concrete in the Viskan tunnel along road E6,Varberg.

Discussion:The thin sprayed concrete layer over the drain combined with drying shrinkage ismost probably the reason for cracking. The lack of fibres in the investigated part ofthe crack points to that this is another reason for localization. Measured chloridecontents are quite high in the lowest core and combined with the crack width 0.2 mmone could have expected high rates of corrosion on fibres or welded mesh.

2.5 Conclusions

In all the inspected structures a low amount of fibres crossed the cracks whenchecked after core drilling. A short distance from the crack the amount of fibres wassufficient. This points out that cracking in the inspected structures has ocurred wherethe least amount of fibres are present.

After 5-15 years of exposure no indications of severe corrosion in cracks could beseen. The type of environmental exposure does not matter, even with high amount ofchlorides the amount of corrosion was limited. There was also little influence of thecrack width (0.2 – 2 mm). One should remember that the amount of fibres was verylow in all the inspected structures (no fibres but mesh in the Viskan tunnel).

Generally the concrete quality was very high. More general corrosion only took placein an area with deficient quality due to rebound being sprayed in.

Chapter 3 - Previous works on corrosion of steel in concrete

14

3 PREVIOUS WORKS ON CORROSION OF STEEL INCONCRETE

3.1 Corrosion in general

A more thorough description of concepts regarding corrosion can be found in e.g.Mattsson (1992) or Piron (1991). The word corrosion comes from the Latin word"corrodere" which means ”gnaw apart”. During corrosion the steel changes back tothe more ore-like iron oxide. This condition is also more thermodynamically stable.To obtain corrosion some parameters are essential. Presence of oxygen, humidity(electrolyte) are the two most important ones.

Corrosion is an electrochemical process that is recognised by an exchange ofelectrons. To obtain electrical equilibrium free electrons can not exist to any largerextent. Detachment of electrons by oxidation need a process consuming electrons(reduction). For iron in water the oxidation-reduction reaction is described inequations (3.1-3.3).

−−+−

−−

−+

++→+++

→++

+→

2OH2eFe2eOHO21Fe:sum

2OH2eOHO21:reduction

2eFeFe:oxidation

222

22

2 (3.1)(3.2)

(3.3)

The motive power for corrosion is thermodynamically conditioned. Theelectorchemical reaction occur due to potential differences. Weak parts or defects inthe microstructure of the steel or local variations in the contact between steel andconcrete can be the reason to this. Potential differences gives that an anode-/cathodereaction evolves. The anode- and cathode area must also be in contact via anelectrolyte (e.g. water) for an active reaction. The potential or electrode potential canbe explained as a value of how stabile or noble a material is. A low electrodepotential gives that the metal more easily turns to oxide than a metal with highelectrode potential. In Table 3.1 electrode potentials for different metals in sea wateris shown (galvanic series).

To illustrate how stable different metals are and the current condition for the metalPourbaix (1972) has created potential-pH-diagrams for a large number of metals.Apart from electrode potential and pH the temperature and the redox potential of thewater solution is of great importance. In Figure 3.1 a water temperature of 25°C isused. When iron is exposed in water the common situation is being between thedotted lines. If iron is exposed in concrete with its relatively high pH (13-14)corrosion can slowly occur. Creation of a iron oxide also retards the process.Carbonation of the concrete lowers the pH (<9) and will result in a more active


16

corrosion. Addition of chlorides increase the potential and corrosion can occur at ahigher value of pH.

Table 3.1 Electrode potentials for metals in sea water at 20°C after Mattson (1992)

Metal Elektrode potential. (V)Gold +0,42Silver +0,19Stainless steel (18/8), passive state1 +0,09Cupper +0,02Tin -0,26Stainless steel (18/8), active state1 -0,29Led -0,31Steel -0,46Cadmium -0,49Aluminum -0,51Galvanized steel -0,81Zinc -0,86

Mor

e no

ble

met

als

L

ess

nobl

e m

etal

s

Magnesium -1,361 In the passive state the metal surface has a thin coating which retards the

reaction. In a active state, like in a corrosion pit, there is no coating.

Figure 3.1 Potential-pH-diagram for Fe-H2O at 25 °C; 10-6 M solved Fe (Pourbaix,1972).


17

3.2 Reinforcement corrosion in homogenous concrete

In homogenous concrete steel and e.g. reinforcement bars are well protected againstcorrosion. The high pH and presence of calcium-hydroxide will create a stablepassifying oxide layer on the steel that prevents further corrosion.

The most commonly refered corrosion model is the one by Tuutti (1982) regardingconventional reinforcement in homogenous concrete. In the model the corrosionprocess is divided into two parts (see Figure 3.2). Part one, called time to intitiation,describes the time for the protection given by concrete cover to weaken. During thistime conditions for active corrosion is created. Part two, called propagation, describesongoing corrosion of the reinforcement.

Penetration towardsreinforcement.

Acceptable depth

CO2, Cl-O2

TRH

IInititiation Propagation

Time

Depth of corrosion

Service life or time to repair

Figure 3.2 Corrosion model after Tuutti (1982).

Depassiviation of the reinforcement and break down of the layer can occur due toingress of chlorides and by carbonation of the concrete. Chlorides will also act as acatalyst on the corrosion process by a local acidification at the corrosion pit.Carbonation gives a lowered pH by a reaction between carbon dioxide from the airand the calcium hydroxide.

3.2.1 Initiation by carbonation

Ingress of CO2 from the air will react with the calcium hydroxide in the concreteunder formation of calcium carbonate and a reduction of pH to approximately 9. Thereaction is shown in equation (3.4). The coating making the reinforcement passive isdegraded when when the carbonation front reaches the reinforcement. At this stagecorrosion is initiated, regarded that other parameters are fulfilled (sufficient amountof electrolyte etc.). The rate of carbonation is ruled by a number of parameters and


18

some of them are shown in Table 3.2. A schematic sketch showing the initiation andpropagation due to carbonation is shown in Figure 3.3.

OHCaCOCOCa(OH) 2322 +→+ (3.4)

Table 3.2 Parameters ruling rate of carbonation of concrete from Tuutti (1982) &Fagerlund (1992)

Parameters ruling the rate of carbonation* Amount of material to carbonate * CO2-concentration in the air* Diffusion coefficient for CO2 * Ability to bind CO2

* Relative humidity in the cover * Curing at construction

Unaffected

pH>12.5

pH> 9

CO2 O2 CO2

Short time exposure Long time exposure

Passive Passive Active corrosion

Figure 3.3 Schematic sketch for carbonation induced corrosion after Fagerlund(1992).

3.2.2 Initiation by chloride ingress

In concrete without cracks micro-cell corrosion will develop according to Raupach(1996). This gives that anode- and cathode areas devlop in pairs very close to eachother along a reinforcement bar. The corrosion cells are microscopical and will lookas general corrosion. Micro-cell corrosion occurs usually by carbonation or by aneven and high chloride content along the bars. Iron oxide from the corrosion ofreinforcement bars has a bigger volume than the original steel. This gives an innerpressure that causes tensile stresses in the concrete. Further cracking and increase ofthe rate of corrosion can follow by this. Delamination of the concrete cover is anothercommon type of damage.

Chlorides can have their origin from e.g. de-icing-salts, sea water or admixtures.Chlorides diffuse from the surface into the concrete and the concentration mostcommonly increases with time. Depending on the source of chlorides and howprotected the structure is from precipitation seasonal variations can be obtainedaccording to Wirje & Offrell (1996). A period without addition of chlorides


19

combined with the structure not being sheltered from rain can make the chlorideconcentration decrease in the outer parts.

When the chloride concentration reaches a critical level, called the threshold value,corrosion will be initiated. The chloride ingress and its influence on propagationdepends on a number of factors and some of them are presented in Table 3.3.

Table 3.3 Parameters ruling the rate of initiation by chloride ingress after Byfors(1990) and Fagerlund (1992)

Parameters ruling the rate of initiation by chloride ingress* Chloride concentration of the exposure * Chloride binding capacity* Transport rate for chlorides * Threshold values

As can be seen in Table 3.3 chlorides can bind chemically to the cement paste.Chemical composition (mainly C3A, C4AF, alkali content) of the cement is the mainfactor affecting the ability to bind chlorides according to Byfors (1990). Other cations like OH-, SO4

2-, CO32- also compete about spots where chlorides can bind. An

increased C3A content and decreased alkali content both give an increased chloridebinding. Addition of mineral additive materials like e.g. silika and flyash increasesthe chloride binding. It is only the content of free chlorides in the pore solution (notbound) that influences the corrosive process.

As mentioned above the attack of free chlorides on the reinforcement is connected tothe amount of other cat ions (mainly hydroxide ions) in the pore solution. The criticalratio between these two when corrosion is initiated is described by Hausmann (1967)as the threshold value (see equation 3.5) Relevant figures on threshold values varybetween authors according to Sandberg (1998). In previous investigations thedifference between total amount and the content of free chlorides in the pore solutionis not always made. The choice of method for determination of chloride content canalso influence the result. Sandberg (1998) means that there is a particulary largedifference in determining the content of free chlorides in concretes with a w/c-ratiobelow 0.45. If the total amount of chlorides is used, the thresholdvalue can varybetween 0.17-2.2 % according to Sandberg (1998).

[ ][ ] (ekv/l) OH

(mol/l) Cl−

−

(3.5)

More general recommendations about acceptable levels of chloride content inconcrete can be found in BRO 94 (1994). A maximum level at 0.3 % (per weight ofcement) of free chlorides is accepted at reinforcement level. This can be used as a”rule of thumb” in normal concrete.

Chlorides also give a catalytic effect to the corrosive reaction. The chlorides not onlydegrade the passive layer on the reinforcement. Chlorides also form solvable complex


20

with the iron ions. Sandberg (1998) furthermore means that the complex formed willbe degraded with access to oxygen and this will result in a local decrease of pHwhich further increase the corrosion process. Another effect of chlorides in concreteis that the electrical conductivity in the concrete increases and this facilitates theanode-/cathode-reaction especially in macro-cell corrosion (see part 3.3.2).

The rate of corrosion after initiation depends on available amount of free chlorides,the humidity conditions and access to oxygen. A schematic sketch of the process canbe seen in Figure 3.4.

Short time exposure Long time exposureUnaffected

pH>12.5

Passive Passive Active corrosion

O2Carbonation front

Cl-

Conc. free Cl-

"Safe" concentration

Cl-

Conc. free Cl-

"Harmful" concentration

Figure 3.4 Schematic sketch for chloride induced corrosion after Fagerlund (1992).

3.3 Reinforcement corrosion in cracked concrete

Cracks can occur in concrete structures according to several different reasons. Nomatter the reason, cracks can give rise to corrosion on reinforcement. Cracksfacilitate ingress of chlorides and give rise to a fast local corrosion attack on thereinforcement.

For conventional reinforcement it is common to distinguish between cracksperpendicular to or parallell to the reinforcement. Cracks perpendicular to thereinforcement most commonly give rise to a very local attack on the reinforcement.This type of cracks in concrete give rise to macro-cell corrosion (see part 3.3.2).Parallell cracks give a more general corrosion and spalling of the cover.

3.3.1 Crack width

There are many suggestions of critical crack widths with regard to corrosion. Thebackground material can sometimes be questioned since the effect of the concretecomposition and the exact exposure conditions can vary very much. According toRaupach (1996) cracks mainly affect the initiation of corrosion. As soon as corrosionhas been activated other parameters are more important (micro-chlimate, chloridebinding capacity, anode-/cathode-area etc).


21

The rate of corrosion in cracks can also be decreased by the formation of corrosionproducts according to Tuutti (1982) since corrosion products prevent access to e.g.oxygen. Alonso & al. (1998) support this hypothesis by showing results from long-time exposure where a dry environment can make the anode passive again. This as aneffect of accumulated iron-oxides on the anode, the electrical conductivity in the iron-oxides, a redox process of iron-oxide (Fe2+ / Fe3+) and access to oxygen.

3.3.2 Anode-/catode-area ratio

Cracks give rise to so-called macro-cell corrosion which means distinctly separatedanode- and cathode-areas up to several centimeters according to Raupach (1996). Theprinciples for the reaction is shown in Figure 3.5. Raupach (1996) also presents asimplified model of an electrical circuit to illustrate the reaction (see Figure 3.6). Itcan be seen that important factors are the electrical resistance from the oxide layer inthe transition zone between bar and concrete as well as in the concrete and in thecorrosion products.

Figure 3.5 Macro-cell corrosion in cracked reinforced concrete according toRaupach (1996).

Figure 3.6 Simplified electrical model for macro-cell corrosion according toRaupach (1996).


22

The anode reaction is visible due to the formation of corrosion products, while thecathode reaction is more complex to demonstrate. By measuring the current betweenanode and cathode the importance of the ratio between them can be shown. In Figure3.7 measured out anode- and cathode-currents are shown for a specimen with a crackand with separated cathode areas.

Figure 3.7 Current between anode och cathode as function of the distance (fromcathode to crack) according to Raupach (1996)

3.3.3 Self-healing of cracks

At limited crack widths it is also important to take into consideration the ability forself-healing. Under humid condition with limited flow of water through a crackcombined with small crack widths, self-healing can occur. Self-healing is a reactionwhen carbon dioxide in the water dissolve calcium ions out from the calciumhydroxide content in the concrete. At low flow of water, the water becomessupersaturated with calcium- and bicarbonate ions. Calcite crystals deposit along thecrack walls and the crack can slowly be sealed. Concrete quality, crack movements(static-/dynamic), type of water (pH etc.) are other parameters of importance for self-healing to occur or not. In Table 3.4 some figures are given on acceptable crackwidths in relation to water pressure and structural dimensions.

The influence of self-healing has been tested by Schiessl and Brauer (1996). Theauthors mean that if the water flowing through a crack has a high chloride content(>0,5 M-% /kg cement) the corrosion process continues in spite of self-healing of thecrack. Otherwise the corrosion process stops.


23

Table 3.4 Acceptable crack width for self-healing (h= column of water, t=thickness of structure) after Lohmeyer (1984)

Fall of pressure, h/t(m/m)

Acceptable crack width(mm)

< 2,5 < 0,2< 5 < 0,15

< 10 < 0,10< 20 < 0,05

3.4 Influence of steel quality on reinforcement corrosion

Normally hot-rolled ribbed steel bars are made of steel qualities SIS 2164, 2165,2167 or 2168. The yield stress for these types of steels are in the region of 600 MPa.In a normal low carbon steel the carbon content is approximately 1.3%. According toMattson (1993) these low levels does not affect the corrosion process.

Stainless steels have alloys with mainly chromium at a content of more than 12%according to Mattsson (1993). Other alloying materials can be molybdenum, nickel,cuper and manganese. Even stainless steels can be attacked by corrosion atunfavourable conditions. Stainless steels are not dealt with in this thesis. A commonmaterials composition for normal low carbon steels is shown in Table 3.5

Table 3.5 Materials composition of normal low carbon steel according to SIS 1421 68.

Elements C(%)

Si(%)

Mn(%)

P(%)

S(%)

Max 0.28 0.6 1.6 0.06 0.05

3.5 Previous investigations on corrosion in cracked SFRC

Most of the previous work presented in the area deals with SFRC (Steel FibreReinforced Concrete) exposed to a marine environment. One of these is a compilationby Hoff (1987). Mainly corrosion of steel fibres in uncracked concrete has beeninvestigated, but there are a few dealing with cracked SFRC. A short summary of thefour most important ones is presented below. In Table 3.6 a compilation of the datafrom these investigations is presented.

3.5.1 IBAC, Aachen, Germany, Schiessl & Weydert (1998)

The scope of the investigations was to study corrosion mechanisms mainly in crackedSFRC. A few sprayed concrete samples are also included. Both initiation by ingressof chlorides and by carbonation was dealt with. The specimens were beams sawn out


24

of larger slabs. The crack widths ranged from 0.05-0.4 mm divided in two testrounds. After exposure the beams were evaluated by dismembering into small platesaround the cracked area. Chloride profiles were created both from the exposedsurface and perpendicular to the crack surface.

In this investigation no significant correlation between crack width and chloridecontent could be found. The results of the chloride measurements showed adecreasing chloride content with increasing distance from the crack opening. Theextent of corrosion also decreased in the same way. Schiessl & Weydert (1998)suggest that no critical crack width can be stated under which corrosion does notoccur. The reason for this should be that chlorides still can penetrate very thin cracks.A requirement for corrosion is access to oxygen that can be limited in thinner cracks,and the positive effect of the alkaline environment in the crack should also beemphasized in thinner cracks.

The authors also claim that chloride initiated corrosion never will be repassivated in acrack. Therefore no service life, in the sense that fibres carry load by crossing cracks,above 10 years for cracked SFRC can be expected.

In specimen exposed to accelerated carbonation the crack walls were carbonated andthe fibres therefore not fully protected against corrosion. In spite of this the fibres didnot corrode to a great extent. The authors suggest that this would be due to lack ofhumidity. Therefore carbonation in cracks is of minor importance, but the state ofhumidity is the ruling factor. The tests performed also included galvanised fibres thatgave a delay in initiation of corrosion but no full protection. After some longerexposure these fibres also corroded.

3.5.2 University of Michigan, USA, Kosa (1988)

Kosa (1988) tried to determine the rate of corrosion in SFRC and how corrosionaffects load carrying capacity and ductility. Only a small number of the specimen wascracked prior to exposure. Apart from the cracked specimen pre-carbonated, highpermeability and pre-corroded fibres cast in concrete specimen were tested.

Three types of flexural load setups were tested:

1. Small specimen (seeTable 3.6) loaded to 3 mm deflection.2. Larger specimen loaded to 3 mm deflection.3. Larger specimen loaded over the ultimate load before exposure.

When testing properties in flexural load, the specimen size used in Kosas (1988)investigation has to be questioned. The relatively ”thin” beams used should give alarge scatter in the results. Average crack widths were 0.12; 0.20 and 0.27 mm for thethree different specimen types.


25

The investigation only gives limited amount of information about the rate ofcorrosion but shows instead clearly that the decrease of fibre diameter is vital for thechange of load carrying capacity. The increased bond between a corroded fibre andthe concrete is of minor importance. A decreased fibre diameter especially affects theresidual strength but also the ultimate load carrying capacity. A rough estimationshows that a decrease of average fibre diameter with 20% gives a 10% decrease ofthe load carrying capacity and a 25% decrease of toughness (I5) (I5 is a measure of theductility or ”work” the cracked sample perform up to a deflection of 3.5 times thedeflection when the first crack appears). The fracture type turning from being a bondfailure between fibre and concrete to a tensile failure of the fibre explains thephenomenon.

When it concerns the influence of crack width on corrosion and the following changeof load carrying capacity, Kosa (1988) suggests a critical crack width of 0.15 mm.Below 0.15 mm no change of load carrying capacity can be seen with the exposureconditions (see Table 3.6) used in the investigation. Kosa (1988) also presents ananalytical model to determine the influence of fibre corrosion in cracked concrete onresidual strength.

3.5.3 University of Aberdeen, Great Britain, Mangat & Gurusamy (1985,1987a,b)

The first investigation by Mangat & Gurusamy (1985) aimed at defining the chloridediffusion in SFRC exposed to a marine environment. The specimens were exposed toa simulated tidal marine situation by spraying with seawater in the laboratory.Evaluations of the tests were performed by measuring the chloride penetration bothfrom the exposed surface as well as 10 mm perpendicular to the crack surface. Nosignificant difference between SFRC and plain concrete specimen could be found. Itcould also be stated that the chloride diffusion coefficient decreases with time bothdue to continued hydration and by deposit of a brucit-like material formed from ionsin the sea water. The chloride concentrations increased with increasing crack width.At crack widths below 0.2 mm there was no difference from uncracked specimen. Atcrack width above 0.5 mm the effect is significant. The authors also found that mostof the ingress of chlorides took place during the first 3 months.

In another article of Mangat (1987a) results of tests on residual strength arepresented. At crack widths below 0.2 mm there was an increase of residual strengthcompared to uncracked samples. Probable explanations given by the author areautogenous healing and/or increased anchorage of the fibres.

Mangat (1987a) also tested the effect of different steel qualities. Low carbon fibresdid not show any corrosion below crack widths of 0.24 mm. Melt extract(ME)(stainless steel) fibres showed corrosion above crack widths of 0.94 mm.Mangat (1987a) points out that the resistance for corrosion on stainless steel fibres is


26

due to how stable the passive oxide layer on the fibres are. The stability of the oxidelayer depends on e.g. the chromium content (above 12 % is good).

With the exposure method that Mangat (1987a) used autogenous healing seemedpossible for crack widths below 0.5 mm. For larger crack widths the material waswashed away from the crack walls. Taking both corrosion and effect on residualstrength into consideration Mangat (1987a) recommends a maximum crack width forME fibres of 0.2 mm in marine environment. Corresponding crack width for lowcarbon fibres is 0.15 mm.

In a paper Mangat (1987b) presents results from pore squeezing aiming to determinethe content of free chlorides in the exposed samples. No difference in the amount ofbound chlorides between the concrete with or without fibres could be seen. It couldalso be stated that the amount of free chlorides was higher in the samples exposed inlaboratory environment (actual sea water was used). Mangat (1987b) explains thiswith increased concentration due to evaporation. Mangat (1987b) also noted that thechloride concentration decrease close to the surface and explains this with eithercarbonation or a reaction with the hydroxide ions.

3.5.4 University of Surrey, Great Britain, Hannant & Edgington (1975, 1976)

The aim of the investigation was to determine the effect of steel fibre corrosion incracks on the remaining load carrying capacity. Experiments were not accelerated.Samples without cracks are also exposed but not dealt with here. Cracking of thebeams were performed 8 days after concreting. Some of the samples had their cracksealed before exposure. The author’s point out that those beams showing mostdamage at pre-cracking was chosen to be sealed. This must be regarded whenevaluating the results. Evaluation of the load carrying capacity was made beforeexposure and after different exposure lengths.

A relatively large scatter in ultimate load is explained with uneven fibre distribution.The residual strength generally increased after exposure though corrosion wasinitiated. Carbonation of the crack surfaces occurred and the depth of the area withcorroded fibres increased with increased time of exposure. The sealed specimenshowed unchanged or slightly increased load carrying capacity. Hannant et.al (1975)state that residual strength is no efficient tool to determine the rate of corrosion. Thisdue to the increased load carrying capacity in spite of different degree of corrosionattack.

In a second article Hannant (1976) presents results after another 8 months ofexposure. The trend is the same as earlier. In addition a model to estimate the bondstrength between the fibre and the concrete with the results from flexural loading asbase is presented. Hannant (1976) suggests that the increased bond possibly can bederived from shrinkage around the fibres. The hypothesis must be questioned sinceshrinkage should occur evenly in the cement paste and therefore should give lowered


27

bond strength instead. From the model it could be stated that the fibre diameter has tobe reduced with 77% before turning from bond failure to tensile fibre failure.

3.6 Discussion

This thesis has a focus on steel fibres in cracked sprayed concrete. This differs fromthe presented previous works where most of the applications are of ordinary concretein marine environment.

In commonly reinforced concrete structures the cracked state with initiated corrosionis considered as the limit for service-life. For steel fibres this is especiallyunfavourable since steel fibre reinforced sprayed concrete structures use the crackedstate in serviceability limit state. Design of rock strengthening structures is one suchexample of application. A further refinement is therefore needed in a service lifemodel for steel fibre reinforced sprayed concrete.

The usage of stainless steel fibres as a solution to ascertain a long time resistanceagainst corrosion is not motivated due to the extra high material costs. Especially aslong as the resistance against corrosion in cracks has not been proved to be adurability problem.

One possible influence from the sprayed concrete technique on the corrosion processcould be the usage of accelerators. Alkali silicate based ones were most commonlyused before and addition of alkali to the concrete lowers the possibility for chloridebinding which is a negative effect. More commonly today is the usage of so called”alkali-free” accelerators. Many of these accelerators have a very low pH (2.5-3.5)which also could influence the resistance to corrosion in a negative manner. On theother hand some of them have a calcium-aluminate-part which on the other hand willincrease the chloride binding. Common for both type of accelerators are that theamounts added is in the range of 5-8% of the weight of cement which is very low.

Silica fume is also commonly used in sprayed concrete mix-design. As previouslymentioned the silica fume increases the chloride binding.

Splashing from de-icing salts gives a type of exposure which should be more severethan the marine exposure. This is so because periods with no addition of chloridescombined with precipitation will significantly decrease the possibilities for self-healing in comparison to the marine applications.

Chapter 3 - Previous w

orks on corrosion of steel in concrete

28

Table 3.6Experim

ental data from investigations w

ith exposure of cracked SFRC

EVALUATION

parameters

flexural load

geometrical

chloride cont.

carb. depth

electrode

potential

flexural load

geometrical

flexural load

energy absorb.

ocular

flexural load

geometrical

length

(months)

14-17

18

2,6 & 9

2; 30

1, 11, 19

cycle

(days)

7;7;14

28;90

3;3

2 x

(0,25;0,25)

145,145-

load

1%NaCl;

water;

air

3%CO2;

air

3,5%NaCl

&

50°C;air

spray with

sea water,

air

air,

marine

EXPOSURE

type

acc1

acc

acc

norm2

norm

BEAM SIZE

(mm)

150x100x700

75x37,5x300

/

75x12,5x450

100x100x500

100x100x500

CRACK

WIDTH

(mm)

0,2-0,4

0,05-0,4

0,5 / 1

0,07-1,08

0,1-0,3

content

(kg/m3)

60

(30-120)

156

225

187

165

110

ø / L

(mm)

0,5/30

0,51/25

0,48/28

0,6/40

0,25/25

0,5/50

FIBRE

types

(no.)

9

1

3

2

cement

(kg/m3)

350

?

590

435

(+155

PFA3)

480

CONCRETE

w/c

0,5

(0,4)

0,42

0,4

IBAC,

Schiessl &

Weydert

(1998)

University of

Michigan,

Kosa

(1988)

University of

Aberdeen,

Mangat &

Gurusamy

(1985;1987a,

1987b)

University of

Surrey,

Hannant &

Edington

(1975,1976)

1 Accelerated exposure cycle2 N

ormal tim

e exposure3 Pulverised Fuel Ash

Chapter 4 - Field exposure tests

29

4 FIELD EXPOSURE TESTS4.1 Background

Relevant experimental data (or long-time experience) is needed in order to be able topredict service life due to degradation by corrosion of steel fibres in cracks. Todaythere is not enough data available from long time experience. Two experimentalapproaches are possible to use: normal or accelerated rate of exposure tests. Normalrate exposure tests take time and the evaluation of the actual climatic conditions ismore difficult. Accelerated rate exposure tests give results in a shorter period of time,but the correlation to real conditions is more complicated. Therefore a combination ofthe both types are preferred and also used in this thesis. Normal rate exposure testsgive the correlation to actual conditions and by changing the same parameters avalidation of the accelerated rate exposure tests in laboratory can be obtained.

4.2 Scope

The purpose with the field exposure tests is to study steel fibre corrosion in realenvironments with controlled material properties during exposure with normal rate(not accelerated). The goal with the field exposure tests are:

• Define the time to initiation and the rate of propagation for corrosion in crackedsteel fibre reinforced sprayed conrete exposed in field.

• Investigate the influence of some relevant material parameters on time to initiationand/or rate of corrosion.

• Collect reference data for definition of critical crack width for initiation.• Create a reference for accelerated exposures in laboratory.

4.3 Methodology

The field exposure tests were started in September 1997 and evaluations have beenmade in the autumn of 1998 and the winter of 2000. The purpose with the evaluationsis to examine the status of fibres crossing cracks after different time of exposure.

A great number of parameters could be of interest for testing in the field exposuretests. In Table 4.1 some parameters are listed. Due to the methodology withdestructive evaluation of the samples after exposure only a limited number ofparameters could be choosen (see Figure 4.1). Since the goal is to register corrosiondata after different time of exposure the number of samples increase greatly for eachparameter added to the matrix. After validation of the laboratory exposure tests moreparameters could be included there for faster evaluation than in the field exposuretests.


30

Table 4.1 Potential parameters influencing steel fibre corrosion in cracks

METHOD SPECIMEN CONCRETE EXTERNALMix Fibers Accelerator Additions

Wet-mix Crack width w/c-ratio Length ”water glass” Silica fume Humidity

Dry-mix Crack depth Cement(amount,

type)

Diameter ”alkali free” Limestonefiller

Temp.

Size Dosage Dosage Dosage Chlorides

Steel grade Load (static)

Productiontype (colddrawn, cut,chopped)

Air pollution(NOx, SOx)

Coating

4.3.1 Spraying method

Fibre technology is commonly used in Scandinavia, mainly in rock strengtheningpurposes since the usage of fibres make it possible to achieve large capacity and goodwork environment. In a majority of the applications the wet-mix technique is useddue to i.a. lower fibre rebound than with dry-mix spraying. Fibre rebound for wet-mix applications is according to Kobayashi (1983) approximately 10 to 40 %. Fordry-mix spraying much higher amount of fibre rebound is typical which makes it lesscost effective where large quantities are to be sprayed. Fibre rebound between 50 to80 % has been reported by Kobayashi (1983). Dry-mix spraying in combination withsteel fibre reinforcement is therefore not commonly used in Scandinavia today. Insome repair applications it can however occur where the flexibility and the lowercapacity dry-mix spraying gives is required. Nevertheless dry-mix spraying has beenincluded in the exposures to investigate if there is any influence of the sprayingtechnique. The dry-mix method could give lower permeability of the concrete andthereby lower chloride penetration and lower rate of corrosion. The main sprayingmethod in the field exposure tests is still wet-mix.

4.3.2 Concrete composition

Four different concrete mix-types are used. The wet-mix sprayed concrete with30 mm fibres and usage of accelerator (WA30) is the main mix used in allcombinations of exposure type and crack widths. The mixes and the abbrevations arepresented in Table 4.2.


31

Table 4.2 Mix types used in the field exposure tests

Wet-mix Dry-mix Accelerator Dramix 30/0.5 Dramix 40/0.5WA30 X - X X -W30 X - - X -WA40 X - X - XD30 - X - X -

A Swedish Standard Portland cement (called Degerhamn Std P in Sweden) is used inall mixes. All data for the different mix designs used are shown in Table 4.3.

Table 4.3 Mix design used in field exposure tests.

WA30 WA40 W30 D30w/c 0.42 0.42 0.42 0.31

Cement (kg/m3) 510 510 510 500Aggregate 0-8 mm (kg/m3) 1202 1202 1202 815Aggregate 4-8 mm (kg/m3) - - - 286Aggregate 2-5 mm (kg/m3) - - - 260Aggregate 0-1 mm (kg/m3) 298 298 298 138Plasticiser (melamine) (%/kg C) 1.4 1.4 1.4 -Accelerator (%/kg C) 3.5 3.5 - -Fibre (kg/m3) 70 70 70 65

1) Approximated by measuring the amount of water required during spraying (see appendix A)

The longer fibres (Dramix 40/0.5) are tested to study the influence of theanode/cathode area on the rate of corrosion. Research results by e.g. Raupach (1992)and Okada & al (1980) show that this is an important parameter for conventionalreinforcement. This is valid under conditions where the electrical resistance of theconcrete matrix surrounding a reinforcement bar are similar.

A sodium silicate based accelerator was used in production of mixes WA30 &WA40. For the time of producing the samples and also previously this was the mostcommon type in Sweden. The effect of it and the influence on other concreteproperties is well documented. The target amount added to the mixes was 3-5 % byweight of cement. According to Burge (1984) this reduces the compressive strengthwith 12-20 %. Too high addition would give a coarser matrix and a further decreasein compressive strength. The increased coarseness of the concrete matrix should givean increased permeability and larger ingress of eg. chlorides. An increase in rate ofcorrosion rate is therefore suspected. This could possibly be counteracted by the highpH (10.5-11.5) since alkali silicates is commonly used for corrosion protection inwater plants. The influence on the total concrete pore water pH is not expected to behigh and therefore this effect should not be dominating.

A sketch over the test program is shown in Figure 4.1.


32

Method Concrete Fibre length

Crack width Exposure Time Analyse

method

0.5 mm Lab

0.0 mm 1 yearRv 40

0.1 mm 2.5 yearresidual strength

30 mm River Dal0.5 mm ? geometrical

Eugenia tunnel

Normal 1.0 mm ?

Rv 40Wet-mix 40 mm 0.5 mm

River Dal

0.1 mm Rv 40Normal - acc. 30 mm

0.5 mm River Dal

0.1 mm Rv 40Dry-mix Normal 30 mm

0.5 mm River Dal

Figure 4.1 Field exposure test program.

4.3.3 Manufacturing of samples

The concrete was firstly sprayed as large slabs (2*1.2*0.15 m). The purpose ofspraying the large slabs was to reduce the amount of rebound and recieve a morehomogenous composition. At the end 11 large slabs were sprayed. From the slabsbeams were sawn with the dimension 75*125*500 mm. All concrete was sprayedusing a rotor machine of type ALIVA 262 for both wet- and dry-mix spraying (seeFigure 4.2). When adding accelerators a pump of type ALIVA 403 was used. InAppendix A all data from mixing and spraying are shown. Some problems withaddition of accelerator occured during spraying due to stoppage in the nozzle anddifferences in coefficient of fullness in the rotor (Average 56 %, standard deviation11 %). The average addition of accelerator was 4.4 % by weight of cement with1.7 % standard deviation.

After storing under humid conditions for approximately 56 days the beams werecracked to the desired crack width. Cracking was obtained by performing a four-pointload set up to create a flexural crack. In general the flexural test is in accordance withthe ASTM C1018 test (other beam dimensions and rate of deflection 0.25 mm/min).To reach the desired crack width after unloading the elastic deformations had to beconsidered by exceeding the crack width before unloading. To obtain a crack width(w) of 0.1 mm after unloading the beam was subjected to a deflection (δ) of 0.4 mm.


33

Corresponding values for 0.5 and 1.0 mm crack width are 1.0 and 1.7 mm. For everycombination of parameters and occasion for evaluation there are three samplesavailable. This gives a total of 348 beams exposed in the tests. To minimize the riskfor systematic errors the choice of beams from every type of mixture, different crackwidth and exposures site was made by random selection.

Figure 4.2 Spraying equipment ALIVA 262 (left). Setup for flexural load test (right).

4.3.4 Exposure environment

Two major climatic factors ruling the rate of corrosion are relative humidity andpresence of chlorides. In giving the field exposure tests relevance to actual situationswhere steel fibre reinforced sprayed concrete is commonly used the choice ofexposure environment had to be made carefully. In the field exposure tests threedifferent sites were choosen together with the control site in the laboratory (20°C och65% RH).

RV40. National Road 40 close to Borås, outdoor along motor highway.DAL. River Dal at Älvkarleby, outdoor with specimen partly immersed.EUG. Eugenia tunnel, road tunnel in Stockholm.

Details and examples are shown in Table 4.4 and Figure 4.3.

δ

150 150 150

75

w

F F


34

Table 4.4. Exposure environments.

Location Type of exposure Typical structureEugenia tunnel,

StockholmHumid

ChloridesSheltered from rain

Acidifying gases

Rock strengthening in tunnels

Main road Rv40,Borås

HumidChlorides (direct splashing)

Rain

Rock strengthening of open cutsConcrete repairs

River Dal,Älvkarleby

HumidRain

Intake channelIntake tunnel

Figure 4.3 Frame for field exposure tests at Rv40 (left) and map over exposure

sites (right).

To make it possible to use the results from the field exposure tests in a service lifemodel and for correlation to accelerated exposures a follow up of the actual exposureenvironment is needed. At the Rv40-site a number of other tests in other Swedishresearch projects are also running. In Table 4.5 the instrumentation and type ofclimatic follow up for all the sites are presented.


35

Table 4.5 Climatic and environmental measurements.

Rv 40 River Dal Eugenia tunnelair temperature x x xwater temperature - x -relative humidity x x xamount of deicing salts x - -

4.3.5 Evaluation after exposure

Originally there were supposed to be four occasions for evaluation of the fieldexposure tests. Three beams of every combination of parameters were supposed to beexaminated. During the first year of exposure the technique to evaluate changed andonly two beams of every combination were used. The major purpose is to extend thetime for exposure and give further opportunities for evaluation. In the following thetechniques for evaluation are described. One of the beams is used to measure theresidual strength after exposure, the depth of area with corroded fibres and thechloride contents. The other beam is used for evaluation of the corrosion on singlefibres.

Residual strengthSteel fibres are used in the design process for e.g. permanent linings in undergroundconstruction to achieve sufficient post-crack behaviour of the sprayed concrete. Thefunction of e.g. a system with steel fibre reinforced sprayed concrete combined withbolts is fully dependent on obtained residual strength. Therefore the influence ofcorrosion on the long-term residual strength is of interest and also studied here.

Ductility in concrete with steel fibres is obtained by the pull-out resistance occuringbetween fibre and concrete matrix. Active forces are bond strength and for mostfibres also friction due to hooked ends or corrugated shapes of the fibre. Steel fibresmost commonly have high tensile strength (typically >1000 MPa) to utilize theductility from interaction between steel and concrete. If corrosion has been initiatedthe fibre cross section decreases locally in the crack region. This should give achange from a ductile pull-out from the matrix to a brittle fibre failure.

By comparing the residual strength levels at initital flexural cracking and afterdifferent time of exposure this effect can be investigated. By re-loading the beamswith flexural load up to 5 mm deflection after exposure it was possible to check thechange in residual strength. The residual strength levels in the re-load are comparedwith the levels when the initial flexural load was interrupted.


36

Removal of fibresTo check if corrosion has been initiated (or possibly the degree of corrosion on fibrescrossing cracks) the fibres have to be removed from the concrete matrix. Crushing ofthe concrete plates would give a risk for steel fibre failure and ruined possibilities forevaluation. This is valid especially for fibres showing great extent of corrosion.Therefore the small plates (see Figure 4.4) where subjected to repeated freezing andthawing. The plates where sawn on different levels from the crack mouth from thebeams not subjected to continued flexural load. To secure a failure during freezingand thawing the plates where first dried out completely for 24 h in 200 C. Afterdrying they were put in a vessel for vacuum treatment. For 3 days they were exposedto 98 % vacuum. Before normal pressure was restored the vessel was filled with tapwater. When normal pressure was obtained in the vessel the water will be sucked intothe plates due to the gradient in air pressure in the plates. The gradient will lead tothat the plates recieve a very high degree of saturation which gives a total degradationwhen exposed to freezing (Fagerlund, 1994) (see Figure 4.5).

80

20

500

75

125

Figure 4.4 Dismembering of beams (measurements in mm) (5 mm is lost betweenevery plate due to sawing).

Time0

-

+0

Deg

ree

ofsa

tura

tion,

STe

mp.

°CFu

nctio

n

A B Scrit

Sact

Figure 4.5 Mechanism for degradation by freezing at high levels of saturation

according to Fagerlund (1994) (left). Degraded plate after saturation andexposure to freeze-thaw cycle (right).


37

After saturation the specimen were exposed to a freeze thaw cycle from +20 C to –30C to +20 C in 24 hours. The exposure continued for approximately 3 weeks. From thecompletely degraded concrete plates the fibres are collected by using a magneto.Only whole fibers where picked out i.e. only fibres crossing cracks is included in theevaluation. Unfortunately this also make fibers crossing two plates on different levelsfrom the crack mouth useless.

One might suspect that there will be a risk for corrosion during the drying or freezingprocess. High temperature (200°C) makes the plate dry very fast and the time at acondition when the humidity is sufficient to give rise to corrosion is very short. Sincethe plates completely degrades during freezing the water surrounding the plate willrecieve a very high pH (average pH=12.2 from 6 measurements). The plates are alsosubjected to sub-zero temperatures more than half the time during freezing, whichwill make a potential corrosion process very slow. Due to this there will be no risk forcontinued corrosion of the fibres during theese two processes.

Corrosion attackOnly fibres crossing the crack are of interest and they are therefore selected by ocularexamination. The fibres selected show corrosion in any part except from the ends. Allother fibres are discarded. To make it possible to measure the loss of fibre diameter,the fibres are pickled to remove all corrosion products by using “Clark´s”-solution.The solution consists of antimony trioxide (Sb2O3) (20 g/l) and tin chloride (SnCl2)(50 g/ml) dissolved in concentrated hydrochloric acid. The solution will make onlythe corroded part of the steel fibres to be “washed” away. After treatment in“Clark´s”-solution the fibre diameter was measured with a micrometer in the partcrossing the crack. The diameter in the corroded area was compared with theunaffected part beside the crack. The loss is presented as percent of the originaldiameter. Inititally weighing of the fibres to estimate the fibre loss was an alternative.

Measuring the weight of 60 fibres shows however that it is not a realistic alternativesince there is over 1 % variance in the original weight. Loosing e.g. 10 % of thediameter on 3 mm length would not be detectable (gives a loss of 0.07 % of totalweight). The actual original weight is also not controllable afterwards for a singlefiber.

Measuring the fibre diameter of 50 fibres gives a variance of 0.8 % in originaldiameter which is acceptable i.e. a 10 % loss of diameter is detectable with availableequipment.


38

0

2

4

6

8

10

12

14

16

18

0.5 0.502 0.505 0.507 0.51 0.512 0.515 0.517 0.52 0.522 0.525Fibre diameter (mm)

Amou

nt o

f fib

res

Average = 0.507 mmStd. deviation = 0.004 mm

Figure 4.6 Frequency diagram on fibre diameter of 50 Dramix 40/0.5 fibers.

Chloride content and depth of carbonationTo find out the amount of chlorides penetrating the concrete and crack, measurementswere made as a chloride profile from the exposed surface. The chloride content wasalso measured at different distances from the crack mouth along the crack surface(Figure 4.7). Drilling was performed with a 5 mm drill to a depth of 2-4 mmperpendicular to the crack surface. This will make the measurements in the crackbeing an average for 2-4 mm from the crack surface. Drilling for the chloride profilefrom the exposed surface was made to a depth of 5-10 mm perpendicular to a newcrack. During drilling the debris was collected and solved in hydrochloric acid beforemeasuring with the RCT-method by Germann and Hansen (1991). The RCT-methodis based on measuring the chloride content in a solution with a chloride selectivemembrane electrode.

Carbonation depth was measured by splitting the concrete and detecting thecarbonation depth with a phenophtalein-solution (areas not coloured red arecarbonated).

1530

45

Existing crackplaneNew crackplane

Exposed surface

Figure 4.7 Plan for collection of drilling debris.


39

4.4 Results

4.4.1 Climatic conditions

A continous measurement according to 4.3.4 was made and an example of the resultsis presented in Figure 4.8 below. A full set of data can be found in Appendix D. Theslightly higher temperatures in the Eugenia tunnel could be explained by heat fromthe intence motor traffic. The higher humidity at the River Dal depends on thenearness to the water. Slightly lower values on RH in the Eugenia tunnel is dependingon the samples being sheltered from rain, but fluctuations still occur since rain affectsthe RH in the tunnel by the motor traffic (pulling humid air into the tunnel and roadsurface being wet).

-10

-5

0

5

10

15

20

25

1997-08-23 1998-03-11 1998-09-27Date (YYYY:MM:DD)

Tem

pera

ture

(°C

)

RV40

DAL

EUG

0102030405060708090

100

1997-08-23 1998-03-11 1998-09-27Date (YYYY:MM:DD)

RH

(%)

RV40

EUG

DAL

Figure 4.8 Measurements of air temperature (left) and RH (right) during 1997-1998(week average).

4.4.2 Residual strength

A typical test result for a thin crack (w= 0.1 mm) is shown in Figure 4.9 where it canbe stated that the residual strength has increased with 15 % for the sample exposed 1year and 29 % for the sample exposed 2.5 years. Complete results can be found inAppendix B and C.

The change of residual strength after 1 & 2.5 years of exposure is shown in Figure4.10. A slightly higher increase of the residual strength can be seen for crack width0.1 mm. For crack widths 0.5 and 1.0 the increase seem to have stopped or started todecline after 2.5 years. The influence of type of exposure can also be studied inFigure 4.10 where it can be seen that the increase of residual strength is lower or evendeclines (w= 0.1 mm & w= 1.0 mm) for samples exposed at the River Dal site.


40

0

2000

4000

6000

8000

10000

12000

0 0,5 1 1,5 2 2,5 3Deflection (mm)

Load

(N)

Year 1

Year 2.5

+15 %

+29 %

Figure 4.9 Example of residual strength before and after 1 and 2.5 years ofexposure (mix WA30, w= 0.1 mm, RV40). (The dottet lines indicate theprobable behaviour if the initial load had not been interrupted)

All sites

-20

-10

0

10

20

30

40

50

60

0 0.5 1 1.5 2 2.5 3Time of exposure (years)

Cha

nge

of re

sidu

al s

treng

th (%

)

w= 0.1 mm

w= 0.5 mm

w= 1.0 mm

w= 1.0 mm

-20

-10

0

10

20

30

40

50

60


Cha

nge

of re

sidu

al s

treng

th (%

) RV40

EUG

DAL

w= 0.5 mm

-20

-10

0

10

20

30

40

50

60


Cha

nge

of re

sidu

al s

treng

th (%

) RV40EUGDALLAB

w= 0.1 mm

-20

-10

0

10

20

30

40

50

60


Cha

nge

of re

sidu

al s

treng

th (%

) RV40

EUG

DAL

Figure 4.10 Development of residual strength for samples from mix WA30.Influence of crack width, all sites (Top left). Influence of exposure site,w= 1.0 mm (Top right), w= 0.5 mm (Low left), w= 0.1 mm (Low right).


41

In Figure 4.11 the influence of mix type on the residual strength can be studied. Thewet-mix samples with 40 mm fibres and accelerator (WA40) did not recieve anyincrease of the residual strength and the wet-mix samples with 30 mm fibres andaccelerator (WA30) seem to increase the residual strength more than other mix types.No significant influence of spraying method can be seen from the results of residualstrength.

River Dal, w= 0.1 mm

-20

-10

0

10

20

30

40

50

60


Cha

nge

of re

sidu

al s

treng

th (%

) WA30

W30

D30

Rv40, w= 0.1 mm

-20

-10

0

10

20

30

40

50

60


Cha

nge

of re

sidua

l stre

ngth

(%)

WA30

W30

D30

River Dal, w= 0.5 mm

-20

-10

0

10

20

30

40

50

60


Cha

nge

of re

sidu

al s

treng

th (%

) WA30W30D30WA40

Rv40, w= 0.5 mm

-20

-10

0

10

20

30

40

50

60


Cha

nge

of re

sidu

al s

treng

th (%

)

WA30W30D30WA40

Figure 4.11 Development of residual strength for all mix types. River Dal, w=0.1 mm(Top left). Rv40, w=0.1 mm (Top right), River Dal, w=0.5 mm (Low left),Rv40, w=0.5 mm (Low right).

4.4.3 Chloride content

Generally the chloride contents were low after year 1 but with a few values at higherlevels. After 2.5 years of exposure the chloride contents were higher than at year 1.


42

From Figure 4.12 and Figure 4.13 it can be seen that the trend for chloride contentafter 2.5 years shows a higher concentration closer to the crack mouth.

The chloride concentration is slightly lower for crack width 0.1 mm but no significantdifference between mix types can be seen. No difference in behaviour can be seenbetween Eugenia and Rv 40.

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

5 15 25 35 45

Depth from crack opening (mm)

Chl

orid

e co

nten

t (%

Cl /

kg

cem

ent)

w= 0.1 mm w= 0.5 mm w= 1.0 mm

Data year 1 Trend year 2.5 Trend year 1

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

5 15 25 35 45

Depth from crack opening (mm)

Chl

orid

e co

nten

t (%

Cl /

kg

cem

ent)

w= 0.1 mm w= 0.5 mm w= 1.0 mm

Data year 1 Trend year 2.5 Trend year 1

Figure 4.12 Infuence of crack width on chloride content at crack surface. Rv40 (left)and Eugenia tunnel (Right).

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

5 15 25 35 45Depth from crack opening (mm)

Chl

orid

e co

nten

t (%

Cl /

kg

cem

ent)

WA40 WA30 W30

D30 Data year 1 Trend year 2.5

Trend year 1

Figure 4.13 Influence of mix type on chloride content at crack surface for samplesexposed at Rv40. Crack width = 0.5 mm.


43

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0 10 20 30 40 50Depth from exposed surface (mm)

Chl

orid

e co

nten

t (%

Cl /

kg

cem

ent)

WA40 WA30W30 D30Data year 1 Trend year 1Trend year 2.5

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0 10 20 30 40 50Depth from exposed surface (mm)

Chl

orid

e co

nten

t (%

Cl /

kg

cem

ent)

WA30 Data year 1Trend year 1 Trend year 2.5

Figure 4.14 Chloride profile at a distance from the exposed surface for samplesafter 1 and 2.5 years of exposure. Samples from Rv40 (left). Samplesfrom Eugenia tunnel (right).

4.4.4 Carbonation

It could be stated that all concrete types were carbonated only a few millimetres. Theoutdoor conditions and the short time of exposure is the reason to this. High qualityof the sprayed concrete giving a low permeability to carbondioxied is anotherimportant factor.

4.4.5 Corrosion on fibres

From examination of the crack surface at the beams subjected to continued flexuralload additional information about the amount of corrosion could be found. In Table4.6 the depth from the crack mouth of the area where corroded fibres could be foundis shown.

The depth of corrosion on single fibres was measured by using a micrometer. InFigure 4.15 and Figure 4.16 some of the results can be seen.

Table 4.6 Depth of area with corroded fibres in samples WA30 (distance in mmfrom crack mouth).

Crack width(mm)

Rv40 Eugenia River Dal Lab

-98 -00 -98 -00 -98 -00 -98 -000.1 15 15 0 3 0 2 0 00.5 30 40 0 3 0 2 - -1.0 5 60 0 0 0 8 - -


44

0-2525-50 50-

75

w=0

.1 /

1998

w=0

.1 /

2000

w=0

.5 /

1998

w=0

.5 /

2000

w=1

.0 /

1998

w=1

.0 /

2000

0

5

10

15

20

25

30D

ecre

ase

of fi

bre

diam

eter

(%)

Depth from crack mouth

(mm)

WA30

0-25 25-

50 50-75

W30

/ 19

98

W30

/ 20

00

WA3

0 / 1

998

WA3

0 / 2

000

WA4

0 / 1

998

WA4

0 / 2

000

0

5

10

15

20

25

30

Dec

reas

e of

fibr

e di

amet

er (%

)


(mm)

w= 0.5 mm

0-2525-50

50-75

D30

/ 19

98

D30

/ 20

00

WA3

0 / 1

998

WA3

0 / 2

0000

5

10

15

20

25

30

Dec

reas

e of

fibr

e di

amet

er (%

)

Depth from crack mouth (mm)

w= 0.5 mm

0-25 25-

50 50-75

D30

, 199

8

D30

, 200

0W

30, 1

998

W30

, 200

0W

A30,

199

8W

A30,

200

0

05

10152025

30

Dec

reas

e of

fibr

e di

amet

er (%

)

Depth from crack mouth (mm)

w= 0.1 mm

Figure 4.15 Decrease of fibre diameter for samples exposed at Rv40. (Top left) Mixtype WA30, different crack widths. (Top right) Crack width w= 0.5 & mixtype WA40, WA30 & W30. (Low left) Crack width w= 0.5 mm and mixtype D30 & WA30. (Low right) Crack width w= 0.1 mm and mix typeWA30, W30 & D30.

0-25 25-

50 50-75

w=

0.1

mm

/19

98

w=

0.5

mm

/19

98

w=

1.0

mm

/19

98

0

5

10

15

20

25

30

Dec

reas

e of

fibr

e di

amet

er (%

)


EUG

0-25 25-

50 50-75

w=

0.1

mm

/19

98 w=

1.0

mm

/19

98

05

1015202530

Dec

reas

e of

fibr

e di

amet

er (%

)


(mm)

DAL

Figure 4.16 Decrease of fibre diameter for mix type WA30 with different crackwidths. Eugenia tunnel (left) and The River Dal (right).


45

4.5 Discussion

The expectations after the first year of exposure was solely to verify that no specimenstarted to corrode and to test the methods for evaluation. The number of samplesbrought back to the lab was also small to make additional future occasions forevaluation possible. To make accurate statistical treatment of the results the amountof samples was too small. Another factor not taken into consideration in theevaluations is that the samples are places on slightly different levels (+/- 250 mm)above ground (except the River Dal). This will influence the micro climate byslightly different humidity conditions (lower level samples= higher humidity andhigher degree of exposure to de-icing salts).

4.5.1 Residual strength

The initial increase of the residual strength after one year of exposure is probablycaused by continued hydration of the concrete. Continued hydration results inincreased anchorage strength between fibre and concrete which gives an increasedresidual strength. The increase seems to be higher for smaller crack widths and theeffect of self-healing could be an additional reason. Some of the thinner cracks werecompletely filled with deposits from self-healing. For samples with ongoingcorrosion of fibres the increase of residual strength must not be expected to continue.The fibres should instead turn to show a brittle tensile failure instead of a ductilepullout failure at some critical degree of corrosion. The trend after 2.5 years is thatthe increase of residual strength already has slowed down or started to decrease forall samples except for 0.1 mm crack width. The influence of self-healing seemstherefore to be important. It also seems like that the continued increase for 0.1 mm issmaller at the River Dal. This could be an effect of conditions with more constanthumidity and without splashing or a lower ability to self-heal due to other watercharateristics. Continous flow of water could also flush the cracks. Ice formation inthe cracks or pressure from ice between the beams could make the crack width varyover the year and therefore diminish the possibilities for self-healing.

The influence of larger loss on fibre diameter for the 40 mm fibres is also visible onthe residual stregth. The loss in residual strength is larger for theese samples.

4.5.2 Chloride content

The evaluation after year 1 was performed in the autumn. This is the reason formeasuring lower chloride contents closer to the crack mouth after year 1. It is a resultof the samples being exposed to rain and splashing from the road during the summerperiod. Chlorides close to the crack mouth are then washed away. This is validated bythe evaluation during winter conditions after 2.5 years of exposure where the chlorideconcentration instead is higher closer to the crack mouth. Samples exposed in theEugenia tunnel are sheltered from rain but also show this behaviour. One possibleexplanation is that the tunnel surface is washed with water applied at high pressure


46

once a year (late summer). It could also be an effect of splashing of rain water fromthe road during the summer. High pressure water cleaning is more likely if Figure4.14 is studied since very high chloride concentrations (higher than at Rv40) can beseen. The reason should therefore be that no wash-out from rain occurs during thewinter-time in the tunnel.

4.5.3 Extent of corrosion

Surprisingly corrosion had initiated after only one year of exposure at Rv40. No orvery small amounts of corrosion could be seen in samples exposed in the Eugeniatunnel or at the river Dal. After 2.5 years the corrosion continues to propagate atRv40 and is starting in samples with larger crack width in the Eugenia tunnel. Theinfluence of crack width (0.5 mm and 1.0 mm) seems to be less important after 2.5years provided corrosion is ongoing. This supports the theory that crack width mainlyaffect the time to initiation of corrosion and not the rate of corrosion in the samemanner. Samples with crack width 0.1 mm still show a very low amount of corrosiondue to self-healing.

It can be seen that the corrosive attack on fibres decreases with increased depth downin the crack. The reason should be decreased crack width and therefore lower accessfor oxygen due to higher RH longer periods than in the parts close to the crackmouth.

From the limited amount of samples it seems like the 40 mm fibres show a moresevere damage from corrosion than the 30 mm fibres. The loss of fibre diameter onthe 40 mm fibres is twice the loss on the 30 mm fibres. This supports the hypothesisthat the effect of the anode to cathode area to be an important factor. No significantinfluence of mix-type can be seen.

Uncertanties in the evaluation of loss of fibre diameter could be the identification offibres crossing the crack. The choosen methodology makes it impossible to detectnon-corroded fibres crossing the crack region. Therefore the number of fibres behindthe figures on loss of fibre diameter sometimes is very small (2-10 fibres). Animprovement of the methodology could be to grout the cracks with epoxy beforesawing the plates from the beams.

In general the rate of corrosion is much higher than expected in the Rv40environment. If the rate of corrosion in this environment is constant there will be agreat loss of residual strength in a couple of years for cracks larger than 0.1 mm. Nolarger cracks can therefore be accepted in a structure placed in this type ofenvironment if fibres are used as structural reinforcement.


47

4.6 Conclusions

After 2.5 years of field exposure tests the following preliminary conclusions can bedrawn:

- Possible continued hydration and therefore increased anchorage strength for thesteel fibres gives an initial increase in residual strength after started exposure. Theincrease is temporary for crack widths above 0.1 mm since loss of fibre diameterby corrosion will lower the ductility and reduce this effect. Self-healing of thincracks (w=0.1 mm) makes the increase continue or stabilize at the same level.

- The chloride content along the crack surface depends on seasonal variations. Highlevels can be measured during the winter (deicing period) and lower after asummer. The effect of seasonal variations depends on the degree of exposure torain. The effect is also most obvious in parts close to the crack mouth.

- Generally the degree of corrosion is limited. A high degree of exposure fordeicing-salts during the winter time gives a larger corrosive attack than the otherexposure environments.The importance of crack width seems to decrease withtime after initiation. Longer fibres corrode more than shorter ones at the samecrack width. Fibre length is therefore more important than crack width.

- The loss of fibre diameter has decreased the residual strength. After 2.5 years ofexposure the only mix-type showing a net loss (from start of exposures) inresidual strength is the wet-mix samples with 40 mm fibres. The other types stillhave an increased, or returned to the original, residual strength.

Chapter 5 - Laboratory exposure tests

48

5 LABORATORY EXPOSURE TESTS5.1 Scope

The main objective with the laboratory exposure tests was to make a relativecomparison between different parameters tested under controlled climatic conditions.In general there are two major techniques to predict long-time behaviour duringdegradation. Either by more accurate measurements of the degrading process or byincreasing the severeness of the exposure. In the following exposure tests the lastmentioned technique has been used. When designing accelerated exposure tests greatcare has to be taken to minimize the risk for changing the process to be studied.

The previously described field exposure tests can be used to estimate the degree ofacceleration in the laboratory tests. When establishing a correlation between field andlaboratory conditions other, and more, parameters can easily be tested in laboratoryexposures. The accelerated laboratory exposure tests can then also be used to estimatelong-term behaviour. In this first set of parameters tested, the purpose is solely tocontrol if the technique choosen in the tests is possible to use for adequateacceleration of corrosion.

5.2 Methodology

In the following section the methodology for how samples where poured, cured,cracked, exposed and evaluated is presented.

5.2.1 Concrete composition

This first set of parameters to be tested are choosen mainly to coincide with the testedparameters in the field exposure tests. Three fibre lengths (35, 70 and 105 mm) andthree crack widths (0.1 , 0.5 , 1.0 mm) are tested. The combination of the differentparameters can be seen in Table 5.1.

Table 5.1 Combination of parameters in the accelerated laboratory exposure tests.

Fibre length (mm)35 70 105

0.1 - x -

0.5 x x x

Cra

ck w

idth

(mm

)

1.0 - x -

In the field exposure test the maximum aggregate size used was 8 mm. To make itpossible to pour with a small distance between the fibres in the laboratory exposures


49

the maximum aggregate size used was 4 mm. The water-cement-aggregate ratio isstill the same. Usage of 4 mm instead of 8 mm leads to a relatively larger number ofsmaller aggregate than in the field exposures. On the other hand the relation betweenaggregate and cement paste area exposed on the crack surface is the same in bothcases. This should give approximately the same permeability for e.g. chlorides. InTable 5.2 the mix-composition can be seen. To make pouring possible with remainedlocation of fibres the lower part was poured with plastic consistency and mechanicalcompaction. The upper part was poured with a more fluid consistency and only aslight compaction by tapping the mould (see also Figure 5.1). The cement used is thesame type as in the field exposure tests (Swedish standard portland cement,Degerhamn).

Table 5.2 Mix composition for samples in laboratory exposure tests

Mix compositionw/c-ratio 0.45Cement (kg/m3) 5100-4 (kg/m3) 12020-1 (kg/m3) 292Plasticiser (co-polymer) (%) 2.51

1) increased to 4.5% in mix for upper part

5.2.2 Manufacturing of samples

In the first stage three samples of every combination of fibre-length, crack width,exposure type and exposure time were poured. The samples are cylindrical with adiameter of 57 mm and a length of 170 mm with a nocth in the middle. 37 fibres wereplaced in every sample with a 8 mm gap between every fibre. A total of 113 samples(3 samples of every combination of parameters, exposure envirionment and occasionfor evaluation) where poured. The fibres where made from cutting cold-drawn wireinto desired lengths. Cutting was made by using an equipment for direct cutting offibres into concrete mixers. The wire used is the type beeing used in manufacturing ofthe Dramix-fibre from Bekaert.

PouringBy pouring the lower part of the samples first and then placing the fibres manuallyfull control over the location was obtained. Especially for the 35 mm fibres it wascrusial to place the fibres on the ”notch level” (see Figure 5.1) to secure them to crossthe crack. A notch was created by gluing a rubber ring on the inside of the mould.Demoulding after one day was followed by three days of water curing.


50

Step I Step II Step III

Figure 5.1 Moulds and sequence for pouring (left). Placing of fibres (right).

CrackingCracking was performed 5 days after pouring and to create a crack of desired widththe samples where subjected to uniaxial stress and displacement controlled loading.In the beginning the samples where cracked using closed-loop equipment with COD-transducers (Crack-Opening-Displacement). This method was too time consuming touse for the large number of specimen to be cracked. Therefore a simpler method withcontrolled stroke displacement speed was used instead. The load was applied with arate of deformation of 0.02 mm/s until the desired crack width was obtained. Tomake the cracking of samples rational a device with friction grips was alsoconstructed. The friction grips and the equipment used for cracking is shown inFigure 5.2.

Figure 5.2 Device with friction grip for uniaxial testing (left). Equipment fordisplacement controlled cracking of samples (right).

Since the friction grips have quite small dimensions some rotation occured duringcracking. The crack width was therefore adjusted by hand to the desired value. Sinceno full fracture mechanical evaluation was needed this methodology is considered tobe acceptable. The major purpose is solely to create a crack of desired width. Afurther development of the equipment could be made to increase the stiffness andminimize the effect of rotation (see Figure 5.3). From Figure 5.3 it can be seen that


51

the concrete sample is clamped inside a notched steel tube which is welded to a steelplate. Another effect to take into consideration is the effect of applying a radialpressure on the sample and the contribution this has on the results. If it works thetime consuming technique to glue concrete on steel plates could be eliminated.

Clamping device

Notchedconcrete sample

Steel plate

Notched steel tube

Figure 5.3 Suggested design of friction grip for uniaxial testing.


The accelerated exposure test where started approximately 30 days after pouring ofthe samples. Two types of exposure environments are used in the tests and the cycleis similar for both types.

CHLORIDE - Submerged in water with 3.5% chlorides (3 days) + Air with RH 50%(4 days).

TAP WATER - Submerged in water (3 days) + Air with RH 50% (4 days).

Control specimen stored continously in air with RH 50% where also made. Ambienttemperature was 20°C (+/- 2°C) for all environments.

The choice of climate and cycle in the two chambers are made to accelerate thecorrosion process sufficiently. The CHLORIDE environment can be related to a realsituation with splashing of water from de-icing salts and the structure being shelteredfrom rain. When it concerns chloride concentration the target is to create as highconcentration as possible inside the samples. On the other hand a too highconcentration in the water will give rise to a risk for formation of salt crystals insidethe cracks which would make the process deviate too much from real conditions. TheTAP WATER environment simulates a situation where the structure is not shelteredfrom rain or in connection to water with large fluctuations. A limitation in the tests isthat the automatized control of climate is depending on a 7 day cycle timerequipment.

In choosing the RH of the air faned down into the storage tank there where twoobvious options depending on the access to climate chambers with 50 or 65% RH.


52

The lower level was selected since the period of drying the samples from completelysoaked to a state where corrosion propagates at maxium speed (RH 80-95%) wasonly a couple of days (7-days cycle). There are large uncertanties regarding thehumidity conditions inside the crack. Capillary action, evaporation and supply ofwater from the surrounding concrete makes an estimation difficult.

A continous follow up of the climatic conditions inside the storage tank and in theclimate chamber was made with loggers that registered temperature and RH. Inaddition the pH and the redox-potential in the water were measured approximatelyevery two weeks. Addition of tap water was made monthly in both systems (chloride& tap water) since the continous evaporation by fanning otherwise would increase thechloride content in the remaining water. The only loss of chlorides is by removal ofsamples containing chlorides, but this is considered being insignificant on the totalconcentration.

TimerWatertank

Storagetank

Fan

Magnetvalve

Figure 5.4 Equipment for accelerated laboratory exposure tests.

In Figure 5.4 the equipment for acceleration of corrosion is shown. The principle isthat the samples are stored in the storage tank all the time. The temperature isconstantly kept at +20°C. The sequence to create the climate described above is:

1. Submerged in water (chloride or tap water)– 3 days2. Evaquation of water by pumping to water tank3. Air (RH 50%) from the climatic chamber is faned into the storage tank –

4 days.4. Water is let back into the storage tank by opening the magnet valve.


53

5.2.4 Evaluation after exposure

The evaluation was made in the same manner as in the field exposure tests. Drying,vacuum treatment, water saturation and freezing made the fibres easy to free from theconcrete matrix. For details see description in part 4.3.5. Samples where taken out at70 and 120 days of exposure. Examination of the samples were also made as in thefield exposures with pickling of iron oxide and measurement of fibre diameter in thecorroded area with a micrometer. Samples for further evaluations after longer time ofexposure are available.

5.3 Results

5.3.1 Fibre corrosion

In general, corrosion have been initiated on all samples already after 70 days. In thefollowing a compilation of the results is described. In Figure 5.6 and Figure 5.7 theloss of fibre diameter in the crack region is showed. From the figures it can be seenthat samples stored in the CHLORIDE environment corrode more than samplesstored in TAP WATER environment. Another difference is that some of the fibres(mainly 105 mm:s) stored in the CHLORIDE environment show an oxide layer onother parts of the fibre length than exposed in the crack region (see Figure 5.5). Thisimplies formation of micro-cell corrosion on other places along the fibre. On theother hand the oxide layer on parts not exposed in the crack was very thin and nosevere attack on the fibre could be seen. This effect could be possibly be explainedwith the pouring technique. Inhomogenous enclosure by the concrete surrounding thefibres could make chlorides penetrate the interface between fibre and concrete. Itcould also possibly be explained by the small size of the samples which make fibreswith little cover corrode in a similar way as conventional reinforcement. Aspreivously mentioned this is also only accentuated in samples with long fibres.

As can be seen from Figure 5.6 the influence of both crack width and fibre length isobvious. The difference between crack width 0.5 and 1.0 mm is however very small.The influence of crack width and fibre length is not that accentuated for TAPWATER samples. An important aspect is to take into consideration the deviation ofresults. In Table 5.3 calculated standard deviations are presented. To keep in mindstudying the standard deviations is that the distanced to the crack mouth has not beenconsidered.


54

Figure 5.5 Corroded area in crack from accelerated exposure tests in CHLORIDE

(left) and TAP WATER (right) environment. (L= 70 mm, w= 1.0 mm,70 days of exposure).

0

20

40

60

80

100

0 70 120Time of exposure (days)

Loss

of d

iam

eter

(%)

0.2 mm0.5 mm1.0 mm

0

20

40

60

80

100


Loss

of d

iam

eter

(%) 35 mm

70 mm

105 mm

Figure 5.6 Average loss of fibre diameter in CHLORIDE environment from onesample (37 fibres). Influence of crack width, L=70 mm (left) and fibrelength, w=0.5 mm (right).

Table 5.3 Standard deviation (%) on loss of fibre diameter from acceleratedexposure tests in chloride environment

Fibre length: days of exposure35:70 35:120 70:70 70:120 105:70 105:120

0.2 - - 9,1 17,2 - -

0.5 6,1 10,6 11,9 14,8 13,2 ?

Cra

ck w

idth

(mm

)

1.0 - - 10,3 15,7 - -


55

0

20

40

60

80

100


Loss

of d

iam

eter

(%)

0.2 mm0.5 mm1.0 mm

0

20

40

60

80

100


Loss

of d

iam

eter

(%)

35 mm70 mm105 mm

Figure 5.7 Average loss of fibre diameter in TAP WATER environment from onesample (37 fibres). Influence of crack width, L=70 mm (left) and fibrelength, w=0.5 mm (right).

Table 5.4 Standard deviation (%) on loss of fibre diameter from acceleratedexposure tests in tap water environment.

Fibre length: days of exposure35:70 35:120 70:70 70:120 105:70 105:120

0.2 - - 6,4 14,3 - -

0.5 12,4 22,9 11,7 13,0 24,3 ?

Cra

ck w

idth

(mm

)

1.0 - - 12,4 16,9 - -


An example of the measured values on RH can be seen in Figure 5.8. It can be seenthat the conditions in the room seems to be varying a lot. In general the RH-levelduring the ”dry” period of the cycle is roughly 65-70% inside the storage tank. Adecrease can be seen as the 50% RH air from the climate chamber successivelylowers the RH in the storage tank. The air surrounding the storage tank seems to beaffected by the humid air being pressed out from the tank.

To control the water quality and possible changes during the exposures measurementsof the pH and redox-potential was made once every two weeks. In Figure 5.9 resultsare presented. In the CHLORIDE environment there is a slow acidification where pHhas decrased from 9.7 to 9 during the exposure tests. This can also be seen on theredox-potential which has been increasing.


56

10

15

20

25

30

00-05-31 00-06-10 00-06-20Date (YY:MM:DD)

Tem

pera

ture

(°C

)

Storage tank

Climate chamber

0

20

40

60

80

100

00-05-31 00-06-10 00-06-20Date (YY-MM-DD)

RH

(%)

Storage tank

Climate chamber

Figure 5.8 Measurments of temperature (left) and RH (right) in storage tank andclimate chamber

6

7

8

9

10

11

12

00-04-09 00-05-29 00-07-18 00-09-06 00-10-26Date (YY:MM:DD)

pH

TAP WATERCHLORIDE

-160

-120

-80

-40

0

00-04-09 00-05-29 00-07-18 00-09-06 00-10-26Date (YY:MM:DD)

Red

ox-p

oten

tial (

mV)

TAP WATERCHLORIDE

Figure 5.9 Measurments of pH (left) and redox-potential (right) in water tank.

5.4 Correlation to the field exposure tests

There is only a small amount of data available at present but nevertheless an attemptto correlate the laboratory exposure tests to the field exposure tests (presented inchapter 4) is made below. With additional data from future evaluations thecorrelations could be developed further. At present there are large uncertainties in theestimation.

There are a couple of differences in the parameters included in the field andlaboratory exposure tests. The shortest fibre length used in the laboratory is 35 mmand 30 mm in field. Studying the right part of Figure 5.6 it can be seen that anincrease of fibres length from 35 to 70 mm gives an increase in the loss of fibrediameter from 19 to 33 % after 70 days of exposure. In a rough approximation thiscan be used to estimate the loss of fibre diameter a 30 mm fibre would have recievedin the laboratory exposures. Using linear extrapolation from the results a 30 mm fiberwould have corroded according to equation (5.1).


57

Studying the field exposure tests for 30 mm fibres and w= 0.5 mm exposed at Rv40give that the plate nearest the crack mouth show a loss of 16% of fibre diameter after2.5 years of exposure which is fairly similar. Therefore the acceleration factor can becalculated as in equation 5.2.

( ) %175*3570193319*

)70()70()70()70( 3035

3570

,35,70,35,30 =

−−−=−

−∆−∆

−∆=∆ llll

ClClClCl

φφφφ (5.1)

1370

5.2*36540 ===

lab

fieldRV t

tA (5.2)

The situation for the Eugenia tunnel is that a fibre loss of 5% is present after 2.5 yearsand this gives an acceleration factor roughly 3 times the one compared to Rv40.

393*133* === EUGEUG AA (5.3)

Unfortunately there are no results available from the River Dal at present but acomparison in a similar way as the chloride environment but for tap waterenvironment with the results from 1998 can be made. The influence of fibre length isnot as obvious in the tap water experiments. Therefore the estimation of how the30 mm fibres would have corroded in laboratory conditions is choosen to be similaras the 35 mm ones which is 15% (from Figure 5.7). Since the loss of fibre diameter isonly 3% in the field exposures which is 1/5 of the estimated loss in laboratory theacceleration factor will be:

265*70

1*3655* ===lab

fieldDAL t

tA (5.3)

5.5 Discussion

A question possibly rised regarding the pouring technique is the risk for fibres to getin direct electrical contact with each other. This a possible scenario especially forlonger fibres. Since macro-cell corrosion will be dominant in cracked concrete thissituation is not necessary a problem. The sum of cathode area with e.g. two fibresbeing connected in relation to the combined anode area will still be the same andshould therefore not affect the corrosion process.

In this first study of the technique to accelerate the exposures no measurements of theamount chlorides penetrated the samples has been made. During further evaluationsthis should also be controlled at the time for evaluation of corrosion attack.


58

To take into consideration when studying the standard deviations are that all fibresfrom the cross sample cross section are included. That leads to that both fibresexposed close to the crack mouth and in the center of the sample as well as bothfibres with or without corrosion are included in the deviation. This explains the quitelarge scatter. Some of the fibres were broken by corrosion or by the process ofremoving them from the degraded concrete. In the calculations all broken fibres aretreated as having lost 90% of the fibre diameter.

It is difficult to estimate the humidity conditions in a concrete crack. The humidityconditions are also very important in trying to understand the mechanisms rulinginitiation and propagation of corrosion in cracks. On the other hand short timevariations should be of little interest when estimating the service life in a 100 yearperspective. Some kind of average environment is possibly more interesting increating a model for prediction of durability.

The decrease of pH in the chloride environment is difficult to explain and the generallevel is also higher than in a real situation. Measurements by e.g. Paulsson-Tralla(1999) show pH in the region of 7 in water from a road surface subjected to de-icingsalts.

If the rough estimations on correlation between field exposure tests and laboratoryexposure tests is used to extrapolate the coming behaviour, the samples with 30 mm fibres and 0.5 mm crack width will have lost about 20% of the fibre diameter(according to the results after 120 days) after 4 years in the Rv40 environment.

5.6 Conclusions

Keeping the large uncertanties, depending on limited number of test data, in mind thefollowing conclusion can be drawn from the first two evaluations of the acceleratedlaboratory exposure tests:

- In general the methodology choosen can be used in accelerated exposure ofcracked steel fibres samples. The acceleration factor is roughly estimated as 10times the environment at Rv40, 40 times the environment at Eugenia tunnel and25 times the environment at the exposure site River Dal.

- In the chloride environment longer fibres corrode faster than shorter ones. Anapproximation is that a change of fibre length with 10 mm should give a change inloss of diameter in a crack of roughly 5%. In the tap water environment the resultsare more contradictory.

- An increased crack width give a larger loss of fibre diameter.

Chapter 6 - Load-bearing capacity

59

6 LOAD-BEARING CAPACITY6.1 Introduction

In this part the effect of corrosion on the loadbearing capacity will be studied.

6.2 Analytical model

The corrosion of steel fibres will give a reduced cross section and a change in failuremode. Commonly the unaffected steel fibres have sufficient tensile strength to bepulled out from a concrete matrix giving a ductile behaviour of a structure. A reducedcross section gives a tensile failure of the fibre instead and a more brittle failure.

Kosa (1988) has suggested a simplified model for calculation of the effect of reducedcross section for fibres. The model takes into consideration the change from bondingfailure of the fibers to tensile failure of the fibers with reduced diameter. Input neededin the model is the number of fibers in the tensile zone, bond strength, fiber tensilestrength, embedded length of fibers etc. The parameters needed are listed below.

h

0,2h

0,8h T

C

T1

C

T2x

cracked concrete uncorroded state corroded statecorroded fibers

Figure 6.1 Analytical model for flexural load test on a cracked specimen, Kosa(1988)

In the model it is assumed that ¼ of the fibre length is contributing to the bond-strength.

The strength ratio (S) represents the residual moment capacity in percent of theoriginal, uncorroded state. To be taken into consideration when using the abovementioned model is that only straight fibres can be simulated with this model. Usageof hooked end fibres makes the model more complex since friction between the fibreand the concrete has to be taken into consideration.


60

1

2

2

0

2/

2

1

212

1

4)(

2/

8,0

48,0)8,0(

28,0

2)8,0(

4,04

MMS

ffl

ldlAfdllfF

fNh

xT

flNh

xhT

xhTxhTM

hlfNM

b

scec

l l

l csebmean

mean

b

b

ec

ec

=

⋅⋅⋅=

⋅+⋅⋅⋅=

⋅⋅⋅

=

⋅⋅⋅⋅⋅⋅−⋅=

−⋅⋅+

−⋅⋅=

⋅⋅⋅⋅⋅⋅=

φφ

φπ

φπ

φπ (6.1)

(6.2)

(6.3)

(6.4)

(6.5)

(6.6)

(6.7)

x = depth of the corroded area N = number of fibres in tensile zone

M1,M2 = moment capacity for non corroded / corroded fibres

φ,φc = original / minimum fibre diameter

T1,T2 = tension force in non-corroded / corroded area

l, le, lec = fibre / embedded / critical embedded length

Fmean = average strength per fibre h = height of concrete

fb = average bond strength (fiber/matrix)

Ac = minimum area of the fibre

fs = tensile strength of the fibre S = strength ratio

tc = service life (years) Sc = critical strength ratio


63

6.3 Service-life modelling

6.3.1 Definition of limit state

For conventionally reinforced concrete the limit state when the service life ends isdefined by Sarja & Vesikari (1996) as:

1 The steel is depassivated2 Corrosion of rebars leads to cracking and spalling.

For steel fibres these two criteria are very unfavourable. The first criterion is notsuitable since the propagation of corrosion is much slower than for conventionalreinforcement, even with chlorides present. Therefore the propagation rate should betaken into consideration. Fibres are also distributed all over the crack plane.Therefore the effect of reduced fibre diameter in a part of the crack is not as dramaticas corrosion on rebars all in one level. Criteria no. 2 is not valid at all since theamount of corrosion products is too small to exceed the tensile strength of theconcrete.

Taking the propagation period into consideration a suitable limit state is to define anacceptable reduction in load bearing capacity (i.e. an acceptable/critical strengthratio Sc).

6.3.2 Service life

As mentioned earlier the initiation of corrosion is more or less instantaneous forcracked steel fibre reinforced concrete. The service life (time to reach limit state)should therefore be a function of the corrosion rate and a critical strength ratio.

With an assumed rate of corrosion and a critical strength ratio the service life can becalculated as follows:

rt cc

φφ −= (6.7)

Insertion of equation (6.1)-(6.6) in (6.7) will give:

⋅⋅⋅⋅⋅

−⋅−⋅⋅⋅−+⋅

⋅⋅⋅⋅

⋅−=

b

c

s

bc flNx

xhTSMh

ffl

rrt

φπφπ

πφ 2

8,0(6,111

24 11

2

(6.8)


64

0

0,2

0,4

0,6

0,8

1

0 2 4 6 8 10 12 14Time (year)

Sc

r= 0.03 mm/year

r= 0.09 mm/year

r= 0.06 mm/year

0.7

4.23

Figure 6.2 Propagation scenario for corrosion of steel fibres in cracked concrete.

In Figure 6.2 a graphical presentation is given of the expected service life withdifferent corrosion rates and critical strength ratios. If it e.g. is accepted that thestrength ratio is reduced to 70 % this would give a service life of only 4.23 years witha corrosion rate of 0.06 mm/year. The corrosion rate is taken from the field exposureswhere the WA30-mix with a crack width of 1 mm showed an average corrosion of15 % in the upper 25 mm of the crack. at Rv40. Other assumptions in the calculationare:

Table 6.1 Parameters used in calculation with analytical model

Parameter Value Unitφ 0.5 mmfb 7.8 MPal 30 mm

fs 1250 MPax 25 mmh 75 mmN 20

When designing a conventionally reinforced structure durability is taken intoconsideration by defining a minimum concrete cover over the reinforcement. In asteel fibre reinforced concrete structure the designer has to estimate the resultingcrack widths from the design load. With knowledge of crack width, expectedenvironmental exposure and designed service life the designer can compensate theloss in loadbearing capacity due to corrosion by increasing the amount of fibers. Thisto keep up the wanted capacity during the service life.


65

6.4 Discussion

The use of a model as the one presented is not an attempt to make accurateestimations of the service-life. It has been presented just to illuminate the need fortaking corrosion in cracks into consideration already in the design stage.

In the presented model only a linear elastic approach is used. Bond-slip behaviourand/or friction is not considered at all in the model. Since hooked end fibres are mostcommonly used in Scandinavian sprayed concrete applications, an analytical modelrequires that friction between fibre and concrete during pull-out has to be taken intoconsideration. The loss of fibre diameter is also depending on the actual depth downin the crack and this effect must be included in a refined model. The model does noteither take into consideration the continued hydration and therefore increasedstrength or the effect of self-healing.

Chapter 7 - Conclusions

66

7 CONCLUSIONSBoth from the literature study and the inspections of old sprayed concrete structures itcould be stated that steel fibres are relatively resistant to corrosion even withchlorides present. The experiences from exposure to de-icing salts is however limited.The lack of really long time experiences with steel fibre reinforcement also makeestimations in a 100 year perspective uncertain. The usage of stainless steel fibres asa solution to ensure a long time resistance against corrosion is not motivated due tothe extra high material costs. Especially as long as the resistance against corrosion incracks has not been proved to be a durability problem.

To start with, the samples in the field exposures increased their residual strength afterexposure. For the 0.1 mm cracks this effect is still accentuated, most probably due toself-healing. A connection between the detected corrosion on fibres and a trendturning from increasing residual strength to a decrease is obvious for crack widths 0.5and 1.0 mm. The majority of the samples still does not show any loss in residualstrength compared to the estimated behaviour without an exposure. A corrosion rateof approximately 0.02-0.03 mm/year was measured along the Rv40 for samples withcrack width 0.5 and 1.0 mm (30 mm fibres). The samples with 40 mm fibres havecorrode at a rate almost twice the one for 30 mm fibres. The fibre length andtherefore the anode-/cathode-ratio is of great importance in estimating the rate ofcorrosion. The influence of crack width is more unsecure in the field exposures. Theonly unambiguous fact is that the samples with crack width 0.1 mm show a smalleramount of corrosion than samples with larger crack widths. To take intoconsideration is that the recieved amount of data still is small and the scatter is quitelarge. Future evaluations will reduce the uncertainties. At this stage a preliminaryrecommendation could be that cracks wider than 0.1 mm should not be accepted inthe motorway environment.

The concept choosen in the laboratory exposure tests seem to be useful in estimatingthe long term behaviour. The rough estimations show that the laboratory exposuresaccelerate the motorway environment with 10 times. Studying the influence of fibrelength and crack width it is most obvious in the chloride environment in the samemanner as in the field exposures that longer fibres corrode at a higher rate and withcracks <0.2 mm the fibres corrode at a lower rate. In the tap water environment theresults are contradictory. To keep in mind is that there is almost no data availablefrom the River Dal since the amount corrosion is still very limited. The behaviourcould be the same there. If using the suggested correlation to extrapolate linearly,future problems with corrosion could be the result in the motorway environment.

The simple analytical model used to illuminate the basics of a future service-lifemodel is probably underestimating the residual strength capacity and will therefore befurther developed. No consideration to friction or bond-slip behaviour is taken. Noteither the effect of self-healing.

Chapter 7 - Conclusions

67

Chapter 8 – Research needs

68

8 RESEARCH NEEDSThe suggested correlation between the laboratory exposure tests and the fieldexposure tests should be updated with more future data from the ongoing fieldexposures. The prediction that 120 days of laboratory exposures is similar to 4 yearsof exposure in the motorway environment would be an interesting follow-up by anevaluation at that time. With a strengthened correlation extrapolations to predict long-term behaviour will be possible.

Cracks can arise from imposed deformations by e.g. shrinkage. Sprayed concrete isapplied in relatively thin layers and therefore subjected to higher shrinkage thanconventional concrete. The actual crack distribution from shrinkage with or withoutfibres should be further investigated. Influence on crack distribution from restraintsby the substrate being sprayed on could be of great importance.

The suggested analytical model is still only an embryo to a service-life model andneed further refinement. Non elastic behaviour like a bond-slip and/or friction shouldbe considered to estimate the behaviour hooked end fibres show. The loss of fibrediameter is also depending on the actual depth down in the crack and this effect mustbe included in a refined model. The model does not either take into consideration thecontinued hydration and therefore increased strength or the effect of self-healing. Ananalytical model could also be combined with FE-modelling where the non-linearbehaviour could be included more easily in a service-life prediction.

In all the inspected structures a low amount of fibres crossed the cracks whenchecked after core drilling. A short distance from the crack the amount of fibres wassufficient. This points out that cracking in the inspected structures has ocurred wherethe least amount of fibres are present. It also accentuate the need for adequate fibredistribution when spraying concrete. If cracks anyway occur where there are leastfibres available the function is lost. This area need further investigations anddevelopments to achieve expected properties of a SFRSC.

Sprayed conrete differs from conventional concrete in e.g. specific materialproperties (high quality concrete, usage of accelerators) and production technique(compaction by spraying) and the possible influences from this on fracturemechanical properties would be important input in understanding the crackmechanisms.

Chapter 8 – Research needs

69

Chapter 9 - References

70

9. REFERENCESAlonso, C.& al. (1998). Effect of protective oxide scales in the macrogalvanicbehaviour of concrete reinforcements. Corrosion science, vol 40, no 8, pp. 1379-1389.

BBK 94. (1994). Boverkets handbok om betongkonstruktioner. (In Swedish). TheSwedish National Board of Housing, Building and Planning.

BRO 94. (1994). Allmän teknisk beskrivning för broar. (In Swedish). Swedish roadauthorities. publ. 1994: , p. 179.

Byfors, K. (1990). Chloride-initiated reinforcement corrosion - chloride binding.Swedish Cement and Concrete Institute, report no 1:90.

Fagerlund, G. (1992). Betongkonstruktioners beständighet-en översikt. (in Swedish)Cementa, 3:e upplagan.

Fagerlund, G. (1994) Betonghandbok material (in Swedish) AB Svensk Byggtjänstand Cementa AB, second edition, pp 711-726 & 727-783.

Germann, C. & Hansen, A.J (1991). Rapid chloride test, RCT-metoden – Målningaf betons chloridindhold på byggepladsen. (In Danish) Dansk Beton, nr.1.

Hannant, D.J. & Edgington, J. (1975). Durability of steel fibre concrete. Proc.Fibre reinforced cement and concrete, RILEM, pp. 159-169.

Hannant, D.J. (1976). Additional data on fibre corrosion in cracked beams andtheoretical treatment of the effect of fibre corrosion on beam load capacity. Proc.Fibre reinforced cement and concrete, RILEM, vol 2, pp.533-538.

Hoff, G. (1987). Durability of fibre reinforced concrete in a severe marineenvironment. Proc. Concrete durability-Katherine and Bryant Mather int. Conf SP-100, ACI, pp. 21-38.

Holmgren, J. (1992). Bergförstärkning med sprutbetong. Vattenfall.

Kobayashi, I.K. (1983) Developments of fibre reinforced concrete in Japan. Cementand Concrete Composites, 5(1), pp. 27-40.

Kosa, K. (1988). Corrosion of fibre reinforced concrete. PhD thesis, University ofMichigan, Ann Arbor.


71

Lohmeyer, G. (1984). Wasserundurchlässige Betonbauwerke – Gegenmassnahmenbei Durchfeuchtungen. Beton, nr. 2, vol. 34.

Mangat, P.S. & Gurusamy, K. (1985). Steel fibre reinforced concrete for marineapplications. Proc. 4th International conference on Behaviour of Offshore Structures,Delft, Nederlands, pp. 867-879.

Mangat, P.S. & Gurusamy, K. (1987a). Permissible crack widths in steel fibrereinforced marine concrete. Materials and structures, pp. 338-347, vol. 20.

Mangat, P.S. & Gurusamy, K. (1987b). Pore fluid composition under marineexposure of steel fibre reinforced concrete. Cement and concrete research, pp. 734-742, vol. 17, no. 5.

Manns, W. & Neubert, B. (1992). Mechanical-technological properties of shotcretewith accelerating admixtures. Otto Graf Journal, pp. 115-136.

Mattsson, E. (1987). Elektrokemi och korrosionslära. (In Swedish) Swedish CorrosionInstitute, bulletin 100, revised edition.

Noghabai, K. (1998). Effect of tension softening on the performance of concretestructures. Experimental, computational and analytical studies. Ph.D. thesis, LuleåUniversity of Technology, report 1998:21.

Nordström, E. (1995). Beständighet hos sprutbetong – En litteraturstudie. (InSwedish) Elforsk, 95:11.

Nordström, E. (1996a). Sprutbetongs beständighet - Inventering. (In Swedish)SveBeFo, report no.26, Elforsk, 96:18.

Nordström, E. (1996b). Durability of sprayed concrete. Proc. Int. Conf. Concrete inthe service of mankind, Dundee, 27-28 June, pp. 607-617.

Okada, K. and Miyagawa, T. (1980) Chloride corrosion of reinforcing steel incracked concrete. Proc. Conf. Performance of concrete in marine environment,Detroit. pp. 237-254.

Paulsson-Tralla, J. (1999) Service life of repaired concrete bridge decks. PhD-thesis, Royal University of Technology, TRITA-BKN, Bulletin 50.

Pourbaix, M. (1972). Atlas of electrochemical equilibria in aqueous solutions.Pergamon press, Oxford, NACE, Houston.


72

Raupach, M. (1996). Chloride-induced macrocell corrosion of steel in concrete –theoretical background and practical consequences. Construction and buildingmaterials, vol. 10, no 5, pp. 329-338.

Raupach, M. (1992) Zur chloridinduzierten Makroelementkorrosion von Stahl inBeton. PhD thesis, Rheinisch-Westfälische Technische Hochschule Aachen, Heft433, p.106.

Sandberg, P. (1998). Chloride initiated reinforcement corrosion in marine concrete.PhD-thesis, Lund University of Technology, report no. TVBM-1015.

Sarja, A. & Vesikari, E. (1996). Durability design of concrete structures. RILEM,Technical Committé 130-CSL.

Schiessl, P. & Brauer, N. (1996). Influence of autogeneous healing of cracks oncorrosion of reinforcement. Proc., Durability of building materials and components 7,Stockholm, Sweden 19-23 maj.

Schiessl, P. & Weydert, R. (1998). Korrosion von Stahlfasern in gerissenem undungerissenem Stahlfaserbeton - Abschlussbericht. Institut für Bauforschung Aachen,rapport nr. F516.

Shroff, J.K. (1966). The effect of a corrosive environment on the properties of steelfiber reinforced portland cement mortar. M.S. Thesis, Clarkson College ofTechnology, Potsdam, NY. p. 130.

TUNNEL 95. (1995). Allmän teknisk beskrivning för tunnlar. (In Swedish). Swedishroad authorities. publ. 1995:32, p. 179.

Tuutti, K. (1982). Corrosion of steel in concrete. Swedish cement and concreteresearch institute, report no. Fo 4.

Wirje, A. & Offrell, P. (1996). Kartering av miljölast, kloridpenetration vid Rv40.(in Swedish) MSc-thesis, Lunds University of Technology, report no. TVBM-7106.

Appendix A - Data from spraying of panels to field exposure tests

73

Appendix A – Data from spraying of panels to field exposuretests

Mix type Slab Batch Moisture Mix water w/c-ratio Slump Temp. Accelerator Comments(kg) (kg) (mm) (°C) (% of kg cement)

WA30 1 1 7,35 28,93 0,44 95 23,3 3,742 7,35 27,43 0,43 90 24,2 5,94

2 3.1 7,35 28,75 0,44 105 23,1 3,3 Clogging in acc.nozzle3.2 7,35 28,75 0,44 105 23,1 3,14 7,35 28,75 0,44 100 22,7 4,53

3 5.1 7,35 30,33 0,46 100 21,1 8,24 Clogging in acc.nozzle5.2 7,35 30,33 0,46 100 21,1 2,566.1 7,35 28,75 0,44 100 21,3 7,1 Clogging in acc.nozzle6.2 7,35 28,75 0,44 100 21,3 8,217.1 7,35 29,25 0,45 110 22 3,61

4 7.2 7,35 29,25 0,45 110 22 0,52 Clogging in acc.nozzle7.3 7,35 29,25 0,45 110 22 5,128 7,35 29,25 0,45 110 21,8 4,469 7,35 29,25 0,45 115 22 4,17

10.1 7,35 29,25 0,45 115 22,1 3,755 10.2 7,35 29,25 0,45 115 22,1 2,69

11 7,35 29,25 0,45 100 22,6 4,3512 7,35 29,25 0,45 100 22,4 4,02

6 13.1 10,21 27,78 0,47 85 23,6 3,5 Clogging in acc.nozzle13.2 10,21 27,78 0,47 85 23,6 3,8714.1 10,21 29,4 0,49 100 23,5 3,65 Clogging in acc.nozzle14.2 10,21 29,4 0,49 100 23,5 3,7915.1 10,21 28,3 0,47 110 23,2 3,8815.2 10,21 28,3 0,47 110 23,2 -

W30 7 15.3 10,21 28,3 0,47 110 23,2 -16 10,21 29,44 0,49 95 -17 10,21 29,4 0,49 90 -18 10,21 29,94 0,49 70 -

8 19 6,54 28,00 0,42 100 19,6 -20 6,54 26,09 0,40 80 20,5 -21 6,38 27,7 0,42 80 20,9 -22 6,38 29 0,43 65 21,8 -

WA40 9 23.1 6,38 28,5 0,43 85 21,0 4,66 Clogging in acc.nozzle23.2 6,38 28,5 0,43 85 21,0 5,4724 6,38 28 0,42 80 21,2 7,05

25.1 6,38 ? ? ? ? 6,76 Clogging in acc.nozzle25.2 6,38 ? ? ? ? 6,3525.3 6,38 ? ? ? ? 7,09 Clogging in acc.nozzle25.4 6,38 ? ? ? ? -

D30 10 26 3,5 12,35 0,20 - - -27 3,5 20,3 0,30 - - -28 3,5 17,89 0,27 - - -29 3,5 16,26 0,25 - - -30 3,5 17,89 0,27 - - -

11 31 3,5 21,95 0,32 - - -32 3,5 19,51 0,29 - - -

32.1 3,5 21,95 0,32 - - - Clogging due to fibre ball32.2 3,5 21,95 0,32 - - -

Wet-mix Average 0,45 97 22,2 4,4Std.dev. 0,02 13 1,1 1,7

Dry-mix Average 0,28Std.dev. 0,04

Appendix B - Residual strength after 1 year of exposure in field

74

Appendix B – Residual strengths after 1 year of exposure infield

WA30

WA30; w= 0 mm

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5Deflection (mm)

Load

(kg)

RV40 15BDAL 16AEUG 19A

WA30; w= 0,1 mm

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5

Deflection (mm)Kr

aft (

kN)

DAL 61ARV40 38AEUG 14B

WA30; w= 0,5 mm

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5

Deflection (mm)

Kraf

t (kg

)

DAL 35BRV40 21BEUG 22A

WA30; w= 1 mm

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5

Deflection (mm)

Kraf

t (kg

)

RV40 110BEUG 313ADAL 44B

W30

W30; w= 0,1 mm

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5

Deflection (mm)

Load

(kg)

DAL 71ARV40 73A

W30; w= 0,5 mm

0

200

400

600

800

1000

1200

1400

0 2 4 6 8 10

Deflection (mm)

Load

(kg)

DAL A2ARV40 76A

Appendix B - Residual strength after 1 year of exposure in field

75

WA40

WA40; w= 0,5 mm

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5

Deflection (mm)

Load

(kg) DAL C7A

RV40 C2A

D30

D30; w= 0,1 mm

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5

Deflection (mm)

Load

(kg)

RV40 D6ADAL E1B

D30; w=0,5 mm

0

200

400

600

800

1000

1200

1400

0 2 4 6 8 10

Deflection (mm)

Load

(kg)

DAL D4ARV40 D4B

Appendix C - Residual strength after 2.5 years of exposure in field

76

Appendix C – Residual strengths after 2.5 years of exposurein field

WA30

WA30, w= 0mm

02000

40006000

800010000

1200014000


Load

(N)

DAL 41ARV40 413AEUG 311A

WA30, w= 0.1mm

0

2000

4000

6000

8000

10000

12000

14000


Load

(N)

DAL 413BEUG 314BRV40 48B

WA30, w= 0.5mm

0

2000

4000

6000

8000

10000

12000

14000


Load

(N)

DAL 63AEUG 36ARV40 511A

WA30, w= 1mm

02000400060008000

100001200014000

0 1 2 3 4 5

Deflection (mm)

Load

(N)

DAL 414BRV40 412BEUG 26A

W30

W30, w= 0.1mm

02000400060008000

100001200014000

0 1 2 3 4 5

Deflection (mm)

Load

(N)

DAL A3ARV40 A9A

W30, w= 0.5 mm

02000400060008000

100001200014000

0 1 2 3 4 5

Deflection (mm)

Load

(N)

RV40 A4ADAL A10A

Appendix C - Residual strength after 2.5 years of exposure in field

77

WA40

WA40, w= 0.5mm

02000400060008000

100001200014000


Load

(N)

DAL C13ARV40 C6B

D30

D30, w= 0.1 mm

02000400060008000

100001200014000

0 1 2 3 4 5

Deflection (mm)

Load

(N)

RV40 E4BDAL E9B

D30, w= 0.5 mm

02000400060008000

100001200014000

0 1 2 3 4 5

Deflection (mm)

Load

(N)

RV40 E10BDAL D10B

Appendix D – Climatic data from field exposure tests

78

Appendix D – Climatic data from field exposure testsEugenia tunnel

-15

-10

-5

0

5

10

15

20

25

1997-08-08 1998-09-12 1999-10-17

Date (YYYY:MM:DD)

Tem

p (°

C)

0102030405060708090

100

1997-08-08 1998-09-12 1999-10-17

Date (YYYY:MM:DD)

RH

(%)

Figure D1 Temperature (left) and RH (right) in the Eugenia tunnel (sliding average7d).

River Dal

-15

-10

-5

0

5

10

15

20

25

1997-08-08 1998-09-12 1999-10-17Date (YYYY:MM:DD)

Tem

p. (°

C)

0102030405060708090

100

1997-08-08 1998-09-12 1999-10-17Date (YYYY:MM:DD)

RH

(%)

Figure D2 Temperature (left) and RH (right) at the River Dal site (sliding average7d).

Appendix D – Climatic data from field exposure tests

79

RV40

-15

-10

-5

0

5

10

15

20

25

1997-08-08 1998-09-12 1999-10-17Date (YYYY:MM:DD)

Tem

p (°

C)

0102030405060708090

100

1997-08-08 1998-09-12 1999-10-17Date (YYYY:MM:DD)

RF

(%)

Figure D3 Temperature (left) and RH (right) in the Eugenia tunnel (sliding average7 d).

Appendix E – Paper I

81

DURABILITY OF SPRAYED CONCRETE- A LITERATURE STUDY

Erik NordströmVattenfall Utveckling AB

Sweden

Presented at the congress "Concrete In The Service Of Mankind" in Dundee,Scotland 24-28/6 1996.

ABSTRACT. This study is aiming to compile the knowledge about durability ofsprayed concrete and to elucidate the areas were further research is needed. The studyis treating materials, additions, additives and their effect on durability. Further arefactors like bond, cracking, freeze-thaw action, corrosion and leaching and theireffect also included. In areas where little or no information specific for sprayedconcrete is available, parallels with conventional concrete are drawn. The processesfor durability should be fairly similar. Except from performance are the differencesmainly the w/c-ratio (affects permeability) and the usage of accelerators for concretesprayed with the wet-process.

The conclusion in this paper is that sprayed concrete with correct composition andgood performance is a durable material. The durability in combination withadvantages in performance makes sprayed concrete a very useful material.

Further research is needed about corrosion of steelfiber reinforcement in sprayedconcrete, freeze-thaw action in combination with chlorides and methods to determinepermeability in situ.

Keywords: Sprayed concrete, Durability, Cement, Aggregate, Reinforcement,Additions, Additives, Performance, Bond, Cracking, Freeze-thaw action, Corrosion,Leaching, Chemical attack

Mr Erik Nordström received his MSc in Civil Engineering from Luleå Universityof Technology in Sweden 1993. He is now working as a Research Engineer at thedivision of Concrete Technology, Vattenfall Utveckling AB, Sweden. His mainresearch area is sprayed concrete durability with emphasis on corrosion of steelfiberreinforcement and freeze-thaw action. Other research areas are early thermal crackingin concrete and lightweight concrete with addition of flyash.


82

MATERIALS

Cement

The chemical composition of the cement is the main factor affecting durability ofsprayed concrete. Low C3A-content increases the sulphate resistance and low contentof alkali (K+ & Na+) reduces the risk for alkali aggregate reactions. The resistanceagainst damage due to freeze-thaw action is also increased since the airpore structureis more fine and dense (Fagerlund [1]). Some authors claim that low content of C3Awould give a reduced chemical bonding of chlorides and therefore increased rate ofcorrosion. This is though refuted by Byfors & Tuutti [2]. Increased cement contentreduces the rebound which gives an increased durability, but at the same time the riskfor cracking due to shrinkage increases when more water is normally needed.

Aggregate

The aggregate should have sufficient strength, be resistant to freeze-thaw action andnot be reactive to alkali. The gradation and the maximum aggregate size also affectsthe durability by affecting compaction, permeability, mechanical properties andrebound. Alkali-aggregate reactions arise from aggregate dissolving in the stronglyalkaline environment the cement paste gives. Figure 1 shows the principle for thereaction, and if any of the components is missing the reaction will not take place.

Calcium hydroxide

AlkaliReactiveSilica

Sometimesgives expansion

Alkali-silicagel

Water

Figure 1. Basic sketch for alkali aggregate reactions (Lagerblad & Trägårdh [3])

Reinforcement

Fibres are gradually replacing the conventional net reinforcement due to economyand performance criteria's. The fibres give effects like reduced plastic shrinkage,increased toughness at failure and limited crack widths. Glassfibres or other types ofplastic fibres mainly affects the properties of the fresh sprayed concrete whereas steelfibres more affects the properties of the hardened concrete. According to Opsahl [4]the risk for debonding caused by shrinkage is reduced with addition of fibres. Other


83

authors claims this is not the case (Holmgren [5]). Concerning durabilityperformance is an advantage with fibres compared to conventional net reinforcementsince it is easier to make a homogeneous concrete with fibres.

Additions

Commonly used additions are silica fume, fly ash and ground granulated blastfurnace slag. They are used to achieve good workability, reduced permeability, higherresistance to leaching and chemical attack and reduced risk for alkali aggregatereactions. As the additions react with the free content of calciumhydroxide this couldreduce the possibility of "self-repairing" small cracks (Fagerlund [6]). Materialproperties are shown in table 1.

Density ofparticles [kg/m³]

Bulk dens.[kg/m³]

Spec. surface[m²/kg]

SiO2 content[%]

Portlandcem. 3.12-3.15 1.4 250-500 17-25Silica 2.16 0.20-0.22 18 000-22 000 88-98Fly ash 2.35 1.00 300-500 40-55

Table 1. Material data for Portland cement, silica and fly ash (Burge [7])

Silica

Silica is added either with the dry materials or as a slurry. Addition of silica incombination with a superplasticizer gives a cohesive mix which will give reducedrebound and better bonding to the sprayed surface (Durand, Mirza & Nguyen [8]).This also enables increased layer thickness and reduced usage of accelerators(Wolsiefer & Morgan [9], Fidjestøl [10]).

The hardened concrete will receive reduced permeability and therefore increasedurability against freeze-thaw action (Burge [11], Glassgold [12], Mailvaganam &Samson [13], Morgan & al. [14], Wolsiefer & Morgan [9]). Addition of silica alsogives increased durability against breaking down due to chemical attack and higherelectrical resistivity which reduces the risk for corrosion (Wolsiefer & Morgan [9]).

If damages occur due to freeze-thaw action, in a concrete with silica fume butwithout entrained air, the breaking down is very fast compared to ordinary concrete(figure 2.) (Fagerlund [1]). Addition of silica also gives increased plastic shrinkage atearly age and therefore higher demands on proper curing. Pettersson [15] points outthe risk for a reaction between undispersed silica-gel and the alkaline cement paste.At normal dosage of well-dispersed silica is the risk for alkali aggregate reactionsinstead reduced (Lagerblad & Trägårdh [3]).


84

Weig

th los

s (%)

A: W/C= 0.35; S/C= 0B: W/(C+S)= 0.54; S/C= 0.11C: W/(C+S)= 0.35; S/C= 0.19

100

50

0200 300

No of freeze-thaw cycles0

100

Figure 2. Freeze-thaw test with salt solution of concrete with and without silica fume and with noair entrainment. W= water, C= Portland cement, S= silica fume (Fagerlund [1]).

Fly ash

In the fresh sprayed concrete fly ash has the effect of increased cohesion and reducedrisk for separation. The early strength growth is slower and this behaviour isaccentuated at low temperatures. Fly ash gives increased durability against sulphateattack, reduced permeability (Mailvaganam & Samson [13]), increased freeze-thawresistance and reduced risk for alkali aggregate reactions. High content of fly ash alsogives reduced diffusivity for chloride ions. (Tuutti [16], Johansson, Sundbom &Woltze [17]).

Variations in the remaining coal content can affect the water demand for the sprayedconcrete and possibly be the reason for the reduction of the air content of approx. 1%(Fagerlund [1]). To high addition of fly ash gives an increased tixotropic effect,reduced strength, increased shrinkage at desiccation (Mailvaganam & Samson [13])and increased carbonation rate (Ljungkrantz, Möller & Petersons [18]).

Slag

GGBS makes the concrete less water demanding and reduces the risk for separation.Other effects are increased resistance to sulphate attack and reduced chloridepermeability. A temporarily increased resistance to freeze-thaw attack is, in the longrun, replaced with reduced resistance. The reason is the increased water absorption inthe air-pore system. With freeze-thaw load in combination with water containingchlorides the resistance is due to the amount of added GGBS. Luther, Mikols,DeMaio & Whitlinger [19] claim that amounts of added GGBS between 35-65%gives a reduction of the freeze-thaw resistance.


85

Additives

There are several various additives for sprayed concrete, both dry- and wet-mix.Their effect could be both on the fresh and the hardened concrete. The mostcommonly used additive for wet-mix sprayed concrete is the binder accelerator.Other types of additives are retarders, air-entraining agents, plasticizers, waterreducers and polymers.

Accelerators

Accelerators are used to achieve early support in rock stabilisation and to make itpossible to spray thicker layers. This gives reduced amount of discontinuities and amore homogeneous concrete with increased durability. The most commonly usedbinding accelerators consists water soluble salts of alkali metals or alkaline-earthmetals. Most known are chlorides, silicates, carbonates and aluminates. Chlorides areused only to a limited extent because of the increased risk for corrosion. Manyinvestigations (Burge [11], Manns & Neubert [20]) points out the effect ofaccelerators on durability. Alkaline accelerators give a reduction of the compressivestrength at high dosages. Gebler & al. [21] also discovered a connection betweenreduced compressive strength and reduced resistance to freeze-thaw load. Probably isthe phenomenon due to increased porosity and increased micro-cracking caused bythe accelerated hydration. This is the reason to why Opsahl [22] observed increasedpermeability with the usage of accelerators. Other effects caused by alkalineaccelerators are increased shrinkage with 20-50% both with wet- and dry-mixsprayed concrete (Manns & Neubert [20]). They also discovered doubled creep withnormal addition of accelerators based on potassium carbonate/aluminate (wet-mix)and sodium carbonate/aluminate (dry-mix).

Air entraining agents

The purpose with air entraining agents is to create a concrete with small, well-distributed air pores. The air pores will act as expansion tanks for water pressed awayby freezing. In dry-mix sprayed concrete the air entraining agent is added to the drymaterials or in to the mixing water. This leads to difficulties in predicting the air porecontent before spraying. There are many different opinions about the effect of airentraining agents in dry-mix sprayed concrete. According to ACI 506 R-90 there isno effect while Durand & Mirza [8] claims there is with reference to their tests.When using wet-mix sprayed concrete the air entraining agent is added in advanceand the air pore content could therefore be measured before spraying. Generallyknown is that the air pore content is reduced with 50% at spraying (Durand & Mirza[8], Seegebrecht & al. [23], Morgan & al. [14], Schrader & Kaden [24]). To highaddition of air entrainers gives reduced strength.


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PERFORMANCE

The performance is very important to achieve high durability and the skill of thepersonnel is essential. An optimally composed concrete could easily be wasted witherroneous performance.

Wet-mix sprayed concrete

High addition of accelerators at wet-mix spraying or variations of added water at dry-mix spraying reduces the possibility to achieve durable sprayed concrete. Nonhomogeneous concrete surrounding reinforcement could cause corrosion. Preparationand curing should also be correctly performed. To obtain an unobjectionableperformance the staff should be competent and well experienced. The conditionswhen it concerns equipment, materials, planning of the site should also be satisfying.

For wet-mix sprayed concrete the advantage is that the concrete is ready mixed andcan be controlled before spraying. Other advantages compared to dry-mix spraying ishigh capacity, less dust and reduced wastage. It is difficult to keep the dosage ofaccelerator at a constant level and uneven dosage can give differences in strength anddiscontinuities. An other disadvantage is that the equipment normally requires morewater-rich concrete, but this can be solved with usage of water reducers orplasticizers.

Dry-mix sprayed concrete

The main advantage of dry-mix sprayed concrete is the possibility of making sprayedconcrete with low w/c -ratio and that accelerators are not needed. It is also possible touse pre-bagged material and equipment with relatively low capacity, which can be anadvantage at smaller repair jobs. When using dry-mix sprayed concrete the worker isadjusting the water content manually and this could make the properties varyconsiderably. Uneven distribution of the dry materials will give a variation of thewater content in the sprayed concrete. The wastage is often greater with dry-mixsprayed concrete.

Preparation & Curing

Sufficient preparation of the surface that is to be sprayed is important. Anything thatcould reduce the bond has to be removed. When repairing concrete constructions alldamaged concrete must be removed. Morgan & Neill [25] presents an investigationof bridges in Canada repaired with sprayed concrete. A common reason to failurewas continued breaking down of the underlying repaired concrete while the repair itself was unaffected. Other factors reducing durability is reduced bond due toinsufficient cleaning of rock surfaces with eg a film of oil from diesel engine exhaustor water containing soil or clay. The surface should be damp but with no free wateron the surface for concrete and as dry as possible for rock. For rock surfacestrengthening it is therefore important with sufficient drainage of water from thesurface to be sprayed.


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When applying new layers of sprayed concrete it is important to remove old curingagents from the surface (should not be applied between layers) since some agentsreduce bond effectively.

After spraying the young concrete must be protected against drying out to avoidcracking. Either by water or membrane curing. Some authors question the effect ofsome membrane curing agents. Particularly important is the curing of sprayedconcrete containing silica while this gives increased plastic shrinkage (Opsahl [4]).

FACTORS AFFECTING DURABILITY

Bond

Interaction between the concrete/rock and the sprayed concrete is essential for thefunction of a thin sprayed concrete layer and it can be obtained by bond. Insufficientinteraction can lead to failure or stability problems. Debonding can occur eg if thesurface cleaning is insufficient or if damaged concrete is not removed sufficiently. Ifthe reason for repair is an alkali aggregate reaction in concrete the reaction couldproceed, if not removed sufficiently, and cause debonding. An underlying concretewith chlorides could cause a concentration of chlorides in the boundary between oldand new concrete and by this cause debonding. The same phenomena with moist cancause debonding by water freezing in the boundary.

Shrinkage is another reason for debonding if the sprayed concrete does not crack. Therisk for this type of damage is most common for thicker layers (Schrader [26]).

Cracking

Cracking can lead to reduced durability against eg corrosion, freeze-thaw action andleaching. Cracking is normally caused by movements in the underlying surface or inthe sprayed concrete. The movements can be due to plastic shrinkage, desiccationshrinkage or load caused by temperature differentials.

Corrosion

Corrosion in sprayed concrete is activated by chlorides or by carbonisation. Chloridescould come from eg sea water in rock or from splashing of thaw salts from roads.Already a low chloride content can cause problems, since water that evaporates causea chloride concentration on the surface. The penetration of chlorides can be both dueto diffuse leaching and through cracks. The penetration is decreased with usage of flyash (Johansson, Sundbom & Woltze [17]) but a negative effect is that the tresholdvalue for corrosion is reached more quickly since the content of OH- is reduced.Carbonisation is a reaction between carbondioxide from the air and the calciumhydroxide in the sprayed concrete. By the decreasing pH the corrosion is initiatedsince the layer making the reinforcement passive is disappearing. The speed of


88

carbonisation is ruled by relative humidity, carbon dioxide concentration, concretepermeability, amount of material to carbonise and the time after pouring. Resistanceto carbonisation should be good for sprayed concrete since eg the permeability is low.Corroding fibres at a surface cause an unwanted aesthetical effect. Fibres inside thesprayed concrete are well protected and the corrosion of the fibre is usually limited.Mangat & Gurusamy [27] claim that this is due to the lime-rich coating that isprotecting the fibre well since it has a large specific area. Fibres in small cracks alsoare generally well protected of the environment in the sprayed concrete, and the "self-repairing" effect is of great importance (Malmberg [28]). A field study by Malmberg[28] with investigations of structures in different environmental and climaticconditions show that in cases when corrosion should have appeared according toaccepted theories, it did not always do.

Freeze-thaw action

Compared to conventional concrete sprayed concrete normally has lowerpermeability by usage of low w/c-ratios (mainly dry-mix), high binder content andlow maximum aggregate size. This should lead to better freeze-thaw durability. Themain difficulties are to maintain the air-pore system after spraying with the wet-mixmethod, and to predict the air-pore content with the dry-mix method (Morgan & al.[29]&[14], Schrader & Kaden [24], Seegebrecht, Litvin & Geblier [23]). There alsoare investigations indicating durability against freeze-thaw action without airentraining agents (Burge [11]). Fagerlund [30] points out the possibility of goodresistance to freeze-thaw action due to enclosed air especially in dry-mix sprayedconcrete. At standardised testing there is a possibility that the time for making thespecimen saturated with water is not long enough for sprayed concrete. This wouldmake the level for critical degree of saturation not being obtained. To be kept in mindis that already limited cracking will increase the permeability dramatically and bythat the risk for freeze-thaw damages. An illustration of the theory with criticaldegree of saturation is shown in figure 3.

Time0

-

+0

Deg

ree

ofsa

tura

tion,

STe

mp.

°CFu

nctio

n

A B Scrit

Sact

Figure 3. Illustration of relations between degree of saturation, temperature and function. At pointB damage occurs when the degree of saturation reaches the critical one at the same timeas a sufficiently low temperature is ruling. (Fagerlund [30]).


89

Testing of freeze-thaw durability is divided in to two tests, one with water containingchlorides and one without, usually the resistance is lower to water with chlorides.The phenomena is probably due to osmotic pressures (pressure obtained bydifferences in chloride concentration) occurring between the surface and the porewater in the concrete. The damages are most commonly surface damages. Accordingto the Norwegian Concrete Association [31] the most common damage on sprayedconcrete due to freeze-thaw action is debonding from water-bearing rock. Thispoints-out the importance of sufficient drainage behind sprayed concrete in zoneswith low temperatures.

Leaching

The low permeability of sprayed concrete normally makes it resistant to diffuseleaching of water. The ability of "self-repairing" thin cracks is reduced if one-sidedwater pressure is causing leaching by the running through of water. Especially if thewater is soft the risk for leaching is great (Bodén [32]). Leaching in cracks will causea local reduction of strength. Greater leakage of water will reduce the risk forleaching compared to conventional concrete since sprayed concrete has a lowerpermeability and a higher binder content. On the other hand the risk for leaching willincrease since the cracks are more well distributed (especially with fibrereinforcement), the layers are thinner and the aggregate is less protecting.

Chemical attack

Chemical attack could be of eg acids, sulphates or salts. Acids cause surface damagesin the sprayed concrete and reinforcement corrosion. The phenomena can occur whenstoring acids or in waste water pipes. Normally the sulphate contents in Swedish soilsare too low to cause a reaction. A concentration at the surface of the sprayed concretewhen water containing sulphate evaporates is though possible. Different kinds ofsalts could concentrate at the surface or at the boundary in the sprayed concrete and agrowth of crystals could cause cracks and spalling.

FURTHER RESEARCH

From this investigation it can be said that more knowledge about corrosion of steel-fibre-reinforcement in sprayed concrete is needed. Especially the theories about fibrecorrosion in cracked sprayed concrete, but also the effect on load-bearing capacitywith corroded fibres. Not many investigations about freeze-thaw resistance forsprayed concrete with presence of water containing chlorides are found in this studyand this can be an area for further research. This should be a common situation inroad-tunnels with splashing of water containing thaw salts. To predict or measure thedurability of a structure with sprayed concrete it could be convenient to be able tomeasure the permeability in situ. Available methods should be tested.


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REFERENCES

1. FAGERLUND, G. "Betongkonstruktioners beständighet - en översikt",Cementa AB, 3:e upplagan, 1992 (in Swedish).

2. BYFORS, K. & TUUTTI, K. " Betonghandbok material ", AB SvenskByggtjänst och Cementa AB, 2:a upplagan, 1994, pp 785-808 (in Swedish).

3. LAGERBLAD, B & TRÄGÅRDH, J. " Alkalisilikareaktioner i svensk betong", CBI rapport, 4:92, 1992 (in Swedish).

4. OPSAHL, O.A. " Bruk av silika i sprøytebetong ", Norske SivilingeniørersForeningen, Fagernes Hotel, 21-23 april, Fagernes, 1982 (in Norwegian).

5. HOLMGREN, J. " Bergförstärkning med sprutbetong ", Vattenfall - handbok ,1992 (in Swedish).

6. FAGERLUND, G. " Vattenbyggnadsbetong ", Cementa AB, 1989 (in Swedish).

7. BURGE, T A. "Additives and mixtures for shotcrete" Tunnels & tunneling, Jan,1993.

8. DURAND, B.; MIRZA, J. & NGUYEN, P. " ASTM C666 (A) Freeze-thawdurability of air-entrained wet- and dry-mix shotcrete ", Shotcrete forunderground support VI, 1993, pp 188-196.

9. WOLSIEFER, J Sr. & MORGAN, D R. " Silica fume in shotcrete ", ConcreteInt., pp 34-39, April, 1993.

10. FIDJESTØL, P. "Applied silica fume concrete", Concrete Int., Nov, 1993, pp33-36.

11. BURGE, T A. "Fiber reinforced high-strength shotcrete with condensed silicafume" ACI report SP 91-57, 1991, pp 1153-1170.

12. GLASSGOLD, L I. "Shotcrete durability: an evaluation", Concrete Int., Aug.,1989, pp 79-85.

13. MAILVAGANAM, N P. & SAMSON, D. " The role of admixtures in theeffective use of fly ash and silica fume in concrete mixes ", Proceedings from "Ash - A valuable resource ", 1987, pp 1-18.

14. MORGAN, D R.; KIRKNESS, A J.; McASKILL, N & DUKE, N. " Freeze-thaw durability of wet-mix and dry-mix shotcretes with silika fume and steelfibres ", Cement, concrete & aggregates, 1988, pp 96-102.

15. PETTERSSON, K. " Effects of silica fume on alkali-silica expansion in mortarspecimens ", Cement & concrete research, vol 22, no 1, 1992, pp 15-22.

16. TUUTTI, K. " Korrosion på armering ", Marina betongkonstruktionerslivslängd, seminariehandling, Uppsala, 1993, pp 85-101 (in Swedish).

17. JOHANSSON, L.; SUNDBOM, S. & WOLTZE, K. " Permeabilitet - provningoch inverkan på betongs beständighet ", CBI rapport, 2:89, 1989 (in Swedish).


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18. LJUNGKRANTZ, C. ; MÖLLER, G. & PETERSONS, N. " Betonghandbokmaterial ", AB Svensk Byggtjänst och Cementa AB, 2:a upplagan, 1994 (inSwedish).

19. LUTHER, M.D.; MIKOLS, W.J.; DeMAIO, A J. & WHITLINGER, J E. "Scaling resistance of ground granulated blast furnace slag concretes ",Durability of Concrete, Third International Conference, Nice, France, 1994, pp47-64 .

20. MANNS, W. & NEUBERT, B. " Mechanical-technological properties ofshotcrete with accelerating admixtures ", Otto Graf Journal, 1992, pp 115-136.

21. GEBLER, S H.; LITVIN, A.; McLEAN, WILLIAM J. & SCHUTZ, R. "Durability of dry-mix shotcrete containing rapid -set accelerators " , ACI -Materials Journal, May-June, 1992, pp 259-262.

22. OPSAHL, O A. " A study of a wet-process shotcreting method-vol. I ",Norwegian Institute of Technology, Trondheim, BML report 85.101, 1985.

23. SEEGEBRECHT, G.W., LITVIN, A. & GEBLIER, S.H. " Durability of dry-mix shotcrete ", Concrete Int., Oct. 1989, pp 47-50.

24. SCHRADER, E. & KADEN, R. " Durability of shotcrete ", ACI report SP 100-57, 1987, pp 1071-1101.

25. MORGAN, D R. & NEILL, J. "Durability of shotcrete rehabilitation treatmentsof bridges in Canada", Paper TAC Annual conference, Winnipeg, Canada,1991, pp c13-c51.

26. SCHRADER, E. " ´Misconceptions about durability and bond in conventionaland latex shotcrete ", ACI Fall Convention, Chicago, Oct., 1985.

27. MANGAT,P S. & GURUSAMY, K. "Corrosion resistance of steel fibres inconcrete under marine exposure", Cement and concrete research, vol 18, 1988,pp 444-54.

28. MALMBERG, B. " Beständighet hos fibersprutbetong ", Föredrag vidBergmekanikdagen 1994, SveBeFo, Stockholm, 1994, pp 79-92 (in Swedish).

29. MORGAN, D R.; McASKILL, N.; CARETTE, G G. & MALHOTRA, V.M. "Evaluation of polypropylene fiber reinforced high-volume fly ash shotcrete ",ACI - Materials Journal, March-April, 1992, pp 169-177.

30. FAGERLUND, G. " Betonghandbok material ", AB Svensk Byggtjänst ochCementa AB, 2:a upplagan, 1994, pp 711-726 & 727-783 (in Swedish).

31. NORSK BETONGFØRENING. "Sprøytebetong til fjellsikring", NorskBetongforenings komite for sprøytebetong, Publikasjon nr.7, 1993 (inNorwegian).

32. BODÉN, ANDERS. "SFR Kontrollprogram, Bergkontroll - Sprutbetongensbeständighet", Vattenfall Energisystem AB rapp. BEG PM 29/91, Oktober1991(in Swedish).

Appendix F – Paper II

93

Durability of Sprayed Concrete Repairs

LONGÉVITÉ DES RÉPARATIONS DE BÉTON PROJETÉ

E. Nordström, Vattenfall Utveckling ABS-814 26 Älvkarleby, Phone: +46 26 83500 Fax: +46 26 83630

Summary

As a part of ongoing research in the area of sprayed concrete durability, VattenfallUtveckling has accomplished a survey of the status on sprayed concrete structures inSweden. The survey is financed by Elforsk (Swedish Electrical Utilities Research andDevelopment Company) and SveBeFo (Swedish Rock Engineering Research). Included inthe survey are both strengthening of rock and concrete repair, but the paper will putemphasis on concrete repair of dams. Three objects of spillways repaired with sprayedconcrete will be presented in this paper. Two objects where damages appeared after a fewyears of use and one showing no damages. It seems like the difference in durability can bederived from erroneous design of joints, thin layers of sprayed concrete and difference insurface treatment after spraying.

Resumé

Dans le cadre de la recherche en cours dans le domaine de la longévité du béton projeté.Vattenfall Utveckling a réalisé une étude de l’état des structures en béton projeté en Suède.L’étude est financée par Elforsk (Société Suédoise de Recherche et de Développement desUtilités Electriques) et SveBeFo (Société Suédoise de Recherche de l’Ingénierie desRoches). Le renforcement des roches et la réparation du béton sont compris dans l’étudemais le compte-rendu soulignera la réparation des barrages. Trois objets de déversoirsréparés à l’aide de béton projeté sont présentés dans le compterendu. Deux où desdommages se sont manifestés après quelques années de service et un ne démontrant aucunsdommages. Il semble que la différence de longévité soit causée par des erreurs deconception des raccords, de fines couches de béton projeté et des différences dans letraitement de surface après pulvérisation.


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1. INTRODUCTION

Previous research at Vattenfall Utveckling AB on durability of sprayed concrete has consisted of acompilation of available literature in the area [1]. It could be established the fact that many parallelsto conventional concrete can be drawn but some main differences will affect the durability. Thedifferences are mainly the performance, the water/cement-ratio and the usage of accelerators. Thedifference in performance when it concerns e.g. compaction affects the air pore system whichshould give a reduced frost resistance. There are however examples with sprayed concrete that showup a good frost resistance with low air content and this can probably be derived to low permeability.The source of low permeability is the general usage of a low w/c-ratio. Most accelerators usedmainly in wet-mix sprayed concrete can on the other hand increase the porosity and therefore thepermeability and as a consequence of this reduce the durability.

The ongoing research in the area of durability of sprayed concrete at Vattenfall Utveckling AB isaiming to make a survey of the condition of sprayed concrete structures in Sweden. The study isalso aiming to find out, if structures show up damages, the reason to the reduced durability. Thestudy includes both sprayed concrete used for rock strengthening and repair of concrete structures.This paper will emphasise on concrete repair with sprayed concrete.

2. SELECTION OF OBJECTS

After making inquiries for constructions with sprayed concrete a selection of suitable objects had tobe done. The inquiry resulted in several answers and it would not be practicable to study all of them.The selection was done with the following criteria.

Age. The primary criterion which also have been governed by the used spraying method. Structuresperformed with the dry-mix method could be relatively old since the method it self has beendeveloped to a limited extent. Whereas structures performed with the wet-mix method was chosenwith an age less then 15 years, since the method and the admixtures are under constantdevelopment.

Type of reinforcement. The objects were divided in the three groups steel fibre reinforcement,conventional reinforcement and no reinforcement. The study has emphasis on objects with steelfibre reinforcement.

Climate and environment. The study is aiming to include as many different types as possible. E.g.repeated freeze-thaw action, one sided water pressure and wear due to running water.

Documentation. Objects with more information (both by personal communication and writtendocumentation) about preparation and performance is selected in front of others.

Accessibility. Interesting objects with limited or no accessibility has been left out.


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3. METHODOLOGY FOR INSPECTION

For a more thorough inspection of each object a great amount of equipment is needed for e.g. takingout of test specimen. Therefore it was decided at an early stage that an overarching inspectionshould be carried out firstly. For objects showing up something interesting it would then be possibleto go back for a more thorough inspection.

When making an overarching inspection the available test methods are limited but in the followingsection some of the methods are described.

3.1 Object documentation

To make an accurate inspection it is of great importance to know something about the history of thestructure. When, how and why the sprayed concrete is applied and preferably something about thematerial composition. Anyone that have tried to find out all this information about sprayed concreteobjects know that it is no simple task. The available information is often limited to why andsometimes when the sprayed concrete is applied. If there are previous test results and reports fromformer inspections this can give a great deal of information about the object and a possible timepoint for damage appearance.

3.2 Overarching field inspection

When making this type of inspection there are a couple of methods that can be used and these aree.g. the following.

3.2.1 Ocular inspection

The easiest method to perform but the most difficult to evaluate the results from. Ocular inspectionrequires thorough knowledge about the concrete behaviour under different kinds of load fromconstructive forces as well as climate. At an ocular inspection cracks and patterns of cracks can bestudied and from this it might be possible to conclude a source for cracking. Deposits from waterleaching through cracks can also be studied. Areas of the sprayed concrete surface diverging in e.g.colour can give important information.

The ocular inspection together with other types of investigations can give a general picture of thestatus of the structure and the reason to this. Separate test results not completed with an ocularinspection has often little or no relevance.

3.2.2 Crack width

In its simplest form it is carried out with a crack width gauge. The crack width can give a hint aboutthe reason for appearance.


96

3.2.3 Delamination control

By stroking the sprayed concrete with a hammer it is easy to discover areas where little or nobonding is present between the sprayed concrete and the concrete. It is also possible to identify areaswith sprayed concrete of low quality.

3.2.4 Collection of drilling debris

With a battery supplied drilling machine it is easy to drill and collect drilling debris. The drillingdebris can be used to control the presence of e.g. chlorides in a laboratory. It will only give a hintabout the presence and no total chloride profile analysis.

3.2.5 Carbonisation control

In a drill hole or at other newly, to air, exposed surfaces it is possible to control the depth ofcarbonisation. At the newly exposed surface a solution of phenolphthalein can be sprayed to controlthe depth. Concrete not coloured red is carbonised.

3.2.6 Photography and video documentation

When evaluating the results of an inspection it is a good aid for the memory to have plenty ofphotos and video recordings from site. This can also prevent new visits on site if other questionswill arise during the evaluation. It is also an excellent way of presenting the results from aninspection to others.

4. OBJECTS

Three objects on dam constructions have been subject of investigation and two of them wereinspected on site. All objects are spillways repaired with sprayed concrete. In all cases the reason forrepair has been erosion of concrete and corrosion of reinforcement followed by spalling of concrete.The two inspected dams are located at the hydroelectric power stations of Nämforsen in the riverÅngermanälven and Hölleforsen in the river Indalsälven. Both these objects have concrete damagesto a different extent today. The third object is the dam at the hydroelectric power station ofStadsforsen which also is situated in the river Indalsälven. This object does not show any visualdamages today.

4.1 Nämforsen

4.1.1. Background and repair

The construction of the Nämforsen power station was completed in 1947 and the repair of spillwayswas performed in February and March 1990. The repairs were performed during the wintertime andto keep a suitable climate in the spillways a tent was built over the site. The tent was heated. The olddamaged concrete was removed with hydrodemolition before spraying with the dry-mix method.The composition of the sprayed concrete is unknown but it could be seen on site that it containedplastic fibres to reduce plastic shrinkage. The thickness of the sprayed concrete is varying between


97

15 and 50 mm according to an investigation from October 1990 [2]. The concrete is applied fromthe gate and down the spillway in a section of 3-4m width (se figure 1). The sections were dividedwith a bulkhead and the space between the sections was filled afterwards.

1 2 3

Sprayed Concrete Old Concrete

Joint

20.5m

9.0m

Figure 1. Order of execution for spraying in spillways, Nämforsen.

In one of the spillways it was tested to trowel the surface of the sprayed concrete on half thespillway. The reason for stopping the trowelling was a suspicion that it might reduce the bonding tothe underlying concrete. In the investigation [1] it could be established that the trowelling did notreduce the bonding strength.

4.1.2 Status today

Spillway A was subject for examination. After two years damages occurred when parts of thesprayed concrete came loose and were washed away. The damages were concentrated to partsaccording to figure 2. Previous damages where repaired with a polymer/silica-composite. Newlyarisen damages have not been repaired yet and therefore it was possible to establish the fact that thesprayed concrete there was thin (approx. 15mm).

SprayedConcrete

OldConcrete

OldRepairs

New damages

Test points

1.

2.

Figure 2. Damages on sprayed concrete repair in spillway A, Nämforsen.


98

As can be seen the damages are concentrated to the lower part of the spillway and near the jointbetween two sections. In figure 2 it can be seen where testing of bonding strength and measure ofsprayed concrete thickness were performed in 1990. The results were 1.3-1.4 MPa and the thicknesswas 26mm in point 1 and 15mm in point 2. Damages appeared and were repaired in point 2.

When knocking the surface with a hammer no areas with signs of debonding could be found. Noteven near areas where sprayed concrete is missing. No cracks were visible.

4.2 Hölleforsen

4.2.1 Background & repair

The building of the power station of Hölleforsen was completed in 1952. In 1990 it was decided thatthe spillway C was to be repaired. The repairs were performed during the winter with a heated tentover the spillway. There are some uncertainties about preparation and curing and what compositionthe sprayed concrete have. It is in any case clear that the concrete is reinforced with steel fibres (typeDramix 30/0.5) and most probably sprayed with the dry-mix method. The spraying sequence startedat the cage and from the right to the left in a section of approximately 3m down the spillway (seefigure 3). There seem to be joints in a vertical direction as well.

123456

789

101112

131415

161718

Figure 3. Assumed order of execution for sprayed concrete in spillway C, Hölleforsen.

4.2.2 Status today

In the upper parts of the spillway there is a serious damage of the sprayed concrete which has comeloose and the underlying reinforcement is uncovered (see figure 4.). But when knocking with ahammer no areas with debonding could be localised, only in close connection to the damaged area.The uncovered reinforcement is corroded but only slightly, and the steel fibres in the sprayedconcrete is corroded in the surface but unaffected inside the concrete. A test of carbonation depthgives a value less then 1mm. Cracks are visible in some of the joints and in a few places depositsfrom leaking water can be found.


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Figure 4. Damages on sprayed concrete repair in spillway C, Hölleforsen.

The reason for occurred damages can be many, but an important factor should be the construction ofjoints between the different sections (see figure 5.)

Direction of waterflow

Old concrete

Sprayed concrete

2

Figure 5. Detail of a joint between sections of sprayed concrete in spillway C, Hölleforsen.

As can be seen in figure 5 the section (2) applied below the previous upper one (1) is very thin at theupper end. This part is most likely to come loose and be washed away and water will then continueto erode the joint between the two sections and cause further damage. This is probably the mainreason to why damages have occurred. Another reason can be poor bonding between the oldconcrete and the sprayed concrete and/or between sprayed concrete layers from the beginning. Asmentioned in the beginning this object is not documented and there are uncertainties about thepreparatory work.


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4.3 Stadsforsen

4.3.1 Background & repair

In 1939 and 1956 the building of the power station of Stadsforsen was completed and in August1989 a repair of spillways was performed. The old damaged concrete was removed withhydrodemolition. Before spraying it was prescribed to moisten the concrete to a state described as“damp but slightly absorbing“. The sprayed concrete was applied with the dry-mix method and thereinforcement was both tied mesh and steel fibres (Dramix 30/0.6). This is possible since thethickness of the sprayed concrete approximately 60mm. Prescribed strength was 40 MPa (cube).The order of spraying was from above by the cage and downwards the spillway, there were nohorizontal joints. The different sections where divided with bulkheads and the first section sprayedwas in the middle of the spillway. After spraying the surface was steel trowelled and in the bottomparts also vacuum treated. Curing was done by watering the surface for 5 days after spraying.

4.3.2 Status today

For the time of writing the paper no visit has been made to the dam but by personal conference withpeople on site it can be established the fact that no visual damages has arisen. An inspection on siteis carried out in the end of April.

5. DISCUSSION

In this investigation three quite similar sprayed concrete repairs in spillways have been studied. Inspite of the similarity two of the objects show damages and one does not. Why is that?

When trying to explain the difference in durability one must take into consideration the time of theyear for construction. The two damaged structures are performed during wintertime with heatedtents over the spillways. This will of course make it more difficult to produce a good repair withsprayed concrete.

In both Nämforsen and Hölleforsen damages have arisen in connection to the joints betweendifferent sections of the sprayed concrete. In Nämforsen it seems like the designer/contractor at leasthave taken into consideration the risk for damages to occur in the joints. When spraying thespillways bulkheads were used to divide the sections when spraying, and this gives a possibility tokeep the same thickness of each layer of sprayed concrete. No overlap on previously sprayedsections that will give a risk for damage. Since the dimensions of the spillways in Nämforsen aresmaller than in Hölleforsen it was also practicable to leave out joints in the horizontal directionwhich probably will reduce the damage risk. The joints should also be trowelled to reduce theunevenness of the surface of the sprayed concrete.

Where the new damages have appeared in Nämforsen the sprayed concrete is very thin and theprobable reason for damage is debonding. Debonding can occur if rebound is not removed whenspraying on an adjacent section. If a delamination should appear the reason for the sprayed concreteto come loose is most likely due to underpressure caused by running water on the spillway.


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The spillways in Stadsforsen are the only ones with a sprayed concrete surface being completelysteel trowelled which will make it fairly smooth with few discontinuities. This will probably reducethe risk for damages to take root in joints.

6. CONCLUSIONS

After removal of damaged concrete the surface should be watered to a state of "damp but slightlyabsorbing" and no free water should be present on the surface to be sprayed. This will reduce therisk for capillary action on the fresh sprayed concrete from the old concrete which can cause areduced strength due to lack of water for the hydration of cement.

The joints should be designed in a way making them easy to construct with as few discontinuities aspossible. Before spraying an adjacent section the joints should be blasted to increase the bondingbetween the different sections.

Rebound from spraying should be collected or blown away by an assistant to the nozzle man. If therebound is not removed it can reduce the bonding strength between the old concrete and the sprayedconcrete. The system with a “blow man“ assisting the nozzle man is common in e.g. USA

After spraying trowelling of the surface is preferred since this will make the surface smooth andreduce the possibilities for a damage to take root.

It is also preferable to avoid thin layers of sprayed concrete which otherwise can be damaged moreeasily than thicker layers. If a thin sprayed concrete layer debonds the action of underpressure fromrunning water will be enough to make the concrete come loose.

7. REFERENCES

[1] Nordström, Erik. ”Durability of sprayed concrete - a literature study”(in Swedish). Elforsk, report 95:11. 1995.

[2] Täljsten, Björn. ”Control of sprayed concrete repairs at the Nämforsen power plant” (inSwedish). Vattenfall Utveckling AB, document 7990002. 1990.

Documents

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