ASSESSMENT OF THE EXISTING PREFABRICATED

RC RESIDENTIAL BUILDINGS IN BULGARIA AND

RECOMMENDATIONS FOR THEIR REHABILITATION

AND STRENGTHENING

Rangelova F.

Traykova M.

I.Characteristics of the

different categories of

prefabricated building systems

in Bulgaria

Abstract—In this paper the differentcategories of prefabricated buildingsystems for residential buildings inBulgaria are first discussed. Themost important problems of the“large-panel system” for residentialbuildings are commented.Contemporary techniques forrehabilitation and strengthening arepresented. Finally, a few generalconclusions and recommendationsare given.

Keywords—rehabilitation andstrengthening of existing buildings,FRP techniques

The concept of prefabricatedconstruction includes those buildingswhere the majority of structuralcomponents are standardized andproduced in plants in a location awayfrom the building, and thentransported to the site for assembly.These components are manufacturedby industrial methods based on massproduction in order to build a largenumber of buildings in a short time atlow cost.

Rangelova F.

Traykova M.

Figure 1.One story precast industrial building:

1 – column

2 - crane beam (beam that carries rail crane)

3 – RC trusses

4 - roof panels

5 - crane

The assembly elements areinterconnected by:

• Hardening accelerated concrete inthe joints between the elements. Abetter connection is obtained ifthere reinforcement in thehardening accelerated concrete,that sticks out from theneighboring elements;

• By welding in advance concretedin the elements steel units, calledembedded parts.

During the construction ofprefabricated industrial buildings areused different types of elements(Fig.1, Fig.2)

Rangelova F.

Traykova M.

Figure 2.Multistory concrete prefabricated industrial building:

1- foundation

2 – column

3- beam

4 - strengthen ties

5 – beam

6 - roof panel

Rangelova F.

Traykova M.

Many countries used variousprefabricated building systems duringthe second half of the 20th century toprovide houses for the growing urbanpopulation. They were very popularafter the Second World War andespecially in Eastern Europeancountries.

The most applied systems forprefabricated buildings in Bulgariaare: ―large-panel systems‖, ―framesystems‖ and ―lift-slab system withwalls‖.

Advantages of prefabricatedstructures [8]:

•The construction time is shortened,since structural work on the site isconfined to constructing thefoundations and erecting theprefabricated components. Because ofthe low moisture content the buildingdries out more rapidly than a buildingof conventional construction and issooner ready for service.

•The quantities of materials requiredare reduced, as formwork andscaffolding are largely eliminated.

Rangelova F.

Traykova M.

Favorable weight - saving

structural sections can be used, sothat less concrete and steel isneeded and the weight of thebuilding as a whole is reduced.

• Production of precast units inlarge series makes it practicable touse machines whereby therequired amount of manual laboris substantially reduced. Besidesthe units can be manufactured inthe most convenient position onthe casting bed.

• Less man power is needed, sincethe precast units are manufacturedin a factory or, at any rate, underfactory conditions on the buildingsite. Instead of skilled labor,unskilled workmen can be used,who do not have to travel aroundfrom site to site.

• Better quality of the products isobtained as a result of manufactureunder factory conditions withconstant quality control, the use ofmachines, and the better workingenvironment provided by thefactory.

Rangelova F.

Traykova M.

• Construction can proceed almostindependently of weatherconditions, since the units can bemanufactured in coveredbuildings which can be heated anderection of the units can alsocarried out in winter.

A. Large - panel systems

The designation ―large-panelsystem‖ refers to multistorystructures composed of large wall andfloor concrete panels connected in thevertical and horizontal directions sothat the wall panels encloseappropriate spaces for the roomswithin a building. These panels forma box-like structure. Both vertical andhorizontal panels resist gravity load.Wall panels are usually one storyhigh. Horizontal floor and roof panelsspan either as one-way or two-wayslabs. When properly joined together,these horizontal elements act asdiaphragms that transfer the lateralloads to the walls.

Rangelova F.

Traykova M.

Figure 3.

Depending on the wall layout (Fig.3),there are three basic configurations oflarge-panel buildings [1]: 1. Cross-wall system - the main walls thatresist gravity and lateral loads areplaced in the short direction of thebuilding; 2. Longitudinal-wall system- the walls resisting gravity and lateralloads are placed in the longitudinaldirection; 3. Two-way system- thewalls are placed in both directions.

B. Frame systemsPanel connections represent the keystructural components in thesesystems. Based on their locationwithin a building, these connectionscan be classified into vertical andhorizontal joints. Vertical jointsconnect the vertical faces of adjacentwall panels and primarily resistvertical seismic shear forces.Horizontal joints connect thehorizontal faces of the adjacent walland floor panels and resist bothgravity and seismic loads.

Rangelova F.

Traykova M.

The ―precast frame system‖ [1] canbe constructed using either linearelements or spatial beam-columnsubassemblages. Precast beam-column subassemblages have theadvantage that the connecting facesbetween the subassemblages can beplaced away from the critical frameregions; however, linear elements aregenerally preferred because of thedifficulties associated with forming,handling, and erecting spatialelements.

Figure 4.Number of residential buildings according to the material of

the

In the connections, the steel bars(dowels) that project from the edgesof the slabs are welded to the dowelsof the adjacent components andtransverse reinforcement bars areinstalled in place. The connections arethen filled with concrete that ispoured at the site. Most buildings ofthis type have some kind of lateralload-resisting elements, mainlyconsisting of cast-in-place or precastshear walls, etc.

Rangelova F.

Traykova M.

Figure 5. Residential buildings, according to statistic information-

until 2011 [4]

The housing in Bulgaria is relatively―young‖. About 30% of the residentialbuildings are aged below 30 years,about 50% are below 40 years andonly 4% are built before 1919.

The total amount of residentialbuildings in Bulgaria until 2011 is2060745 [5] (Fig.4, Fig.5). Theresidential buildings constructed as―large-panel system‖ are 21641. Morethan 20% of the apartments are inthis type of residential buildings.

Rangelova F.

Traykova M.

Figure 6. Sofia, Bulgaria – Lulindistrict

In the cities - regional centers arelocated 82.3% of the total fund ofpanel buildings. The largest sharebelongs to Sofia (Fig.6) (28.5% -200579 residential), followed byPlovdiv (8.2% - 57468 residential),Varna (8.1% - 56821 residential),Burgas (4.9% - 34714 residential) andRousse (4.2% - 29378 residential). Inthese five cities are concentrated53.8% (378 960 residential) of allprefabricated housing (707 441residential).

Rangelova F.

Traykova M.

In the first years of prefabricatedconstruction as a material for thesurrounding wall structures scoria-concrete is used. Later, single- layerpanels were manufactured by othermaterials. Three-layer front panels"sandwich" (concrete-light concrete-concrete) were applied also. Usuallythe thickness is respectively 50 mm -100 mm - 50 mm for load-bearingwall panels and 50mm- 80mm- 90mm for the blind facade bearing wallpanels.

Rangelova F.

Traykova M.

The ―large- panel system‖ is verypopular in the Bulgarian constructionpractice in the recent past. The firstbuildings constructed according thissystem are dated from 1958. They are4-storey. Since 1962 the 8-storeyvariant of this type of buildings isrealized. After 1972 the ―large-panelsystem‖ is one of the most popularsystems for residential buildings.

Figure 7. Berlin, Germany –Hellersdorf (Kaulsdorf-Nord)

Back in the 90's Senate Departmentfor Urban Development and theEnvironment - Berlin startedUmbauprogramm - a programme fortransformation of cities, whichincludes specific renovation ofprefabricated panel housing(Plattenbau). Experience is gained inthe center of Berlin on sub regions, anorganization that provides consultancyservices to German and foreigncompanies in the constructionindustry. In the largest socialistneighborhoods in the German capitalMarzahn and Hellersdorf (total 230000 people) 165 thousand apartmentsare completely renovated, 64thousand. - partially, and 41 thousand– expect remediation (Fig.7).

Rangelova F.

Traykova M.

One of the first countries, which setsout a strategy for the reconstruction ofold residential complexes is Germany.

Examples of “large - panel system” residential buildings in Bulgarian cities

Rangelova F.

Traykova M.

Rangelova F.

Traykova M.

Rangelova F.

Traykova M.

Rangelova F.

Traykova M.

II. Assessment of the actual

status and some problems of

the “large - panel system”

for prefabricated RC

residential buildings in

Bulgaria

Balkans is one of the most activeseismic unit in Europe. On thecurrent territory of Bulgaria, for aperiod of 536 year until now, ninelarge earthquakes are registered. Thebuilding structures in the country arealso subjected to the effects of theearthquakes occurring in the territoryof neighboring countries.

Development of a legal basis forearthquake resistance of buildingsand facilities should accumulate

Rangelova F.

Traykova M.

knowledge of seismic hazards andmethods for reliable estimation of therisk, and the accumulation of new dataon the seismic regime in Bulgaria andneighboring countries.

Since the beginning of 2013 in parallelwith the national standards, and theEuropean codes (Eurocodes) forstructural design is required to beapplied in Bulgaria. By the end of 2014,they must completely replace nationalstandards in order to equalize theregulations in the European Union.Building structures are provided with acertain reliability impact as the seismiczoning map.

Until 1977 the seismic zoning mapwas developed based on theearthquakes occurred. After theVrancha earthquake (Romania) in1977 following the new changes,additions and instructions andterritories to which the seismic designof structures required, already occupy39% of the total area of the country.Attached map to the standards for thedesign of buildings and structures inseismic areas in 1987 for the first timehave predictive nature. It is designedfor maximum values of the expectedintensity scale MSK-64 with arecurrence period of 1000 years.

Rangelova F.

Traykova M.

Same map, according to which thestrict rules of mandatory earthquakeinsurance now apply to 98% of theBulgarian territory is used inRegulation 2 of the 2007. Nationalannex to Eurocode 8 includes threemaps of seismic zoning of theterritory of the country correspondingto the return period 90, 475 years.

Based on experience in pastearthquakes in Bulgarian, EasternEuropean and in Central Asiancountries [1] where ―large-panelsystem‖ have been widely used, it canbe concluded that the seismicperformance of the prefabricated RCbuildings of this system has beenfairly satisfactory.

The main problems of the existingprefabricated RC residential buildings(―large - panel system‖) in Bulgariacan be resumed in the following: 1.Reconstructions of the ground floorfor shops and offices- manyadditional openings in the wall panelsin the ground floor and removing ofpanels; 2. Bad maintenance of thejoints between the panels; 3.Incorporation of the balconies to therooms – additional load from glazingand brick masonry; 4. Badmaintenance of the water and thethermal insulation; 5. Not effectivethermal insulation.

Rangelova F.

Traykova M.

In precast concrete structures theconnections between the elements areof prime significance. There are twotypes of connections, one ―Wet‖connection (with mortar or in-siteconcrete), second is ―Dry‖ connection(with welding on bolting). Whilechoosing a connection, so manyfactors we have to consider. Theconnection must comply with allrequirements regarding thetransmission of forces and moments,and permissible deformation orrotation. The connection should alsosatisfy the technical, economical, andif required - aesthetical aspects.

The future programme for theexisting prefabricated RC residentialbuildings (―large-panel system‖) isnecessary to be developed on thebasis of the following measures: 1.Extending the remaining lifetime ofthe buildings as much as possible; 2.Upgrading the performance to suitmodern standards; 3. Improving ofthe seismic behavior and the globalstructural safety; 4. Improving theenergy efficiency; 5. Creating asustainable environment.

Rangelova F.

Traykova M.

Fiber-reinforced polymer (FRP) [7]systems have been used for more than20 years and are becoming a widelyaccepted method of strengtheningconcrete structures. The use of FRPcomposites in rehabilitating structureshas grown in popularity due to itsadvantages over conventionalmaterials and wide range of structuralapplications.

III. Principal structural and

contemporary technological

solutions for rehabilitation

and strengthening of

prefabricated RC residential

buildings

FRP systems for strengtheningreinforced or pre-stressed concretegirders consist of externally bondedlaminates or near-surface mountedbars. These systems may containeither carbon or glass fibers. Becauseof their light-weight and exceptionalformability, FRP reinforcements canbe quickly and easily bonded to eventhe most curved and irregularsurfaces. The high strength-to-weightratio of FRP composites makes themmore structurally efficient thantraditional strengthening materials.In addition, FRP composites arenoncorrosive,nonmagnetic,nonconductive, and generallyresistant to chemical attacks.

Rangelova F.

Traykova M.

The application of advancedpolymer composites [5] for thebuilding and civil engineeringindustry, can be convenientlydivided into some specific areas,as:

1) Building industry:

infill panels and new buildingstructures;

2) Civil engineering industry:

- civil engineering structures,fabricated entirely from advancedpolymer composite material,known as all-polymer/fibercomposite structures;

All these, other than the first,involve a combination of advancedpolymer composites and conventionalconstruction materials and aretherefore often termed compositeconstruction.

The new techniques are and onecontemporary solution for upgradingthe performance of the ―large-panelsystem‖ buildings.

The purpose of [2], [3] is toexperimentally evaluate the seismicload bearing capacity of aprefabricated RC wall panel withlarge openings retrofitted postdamage using externally bondedcarbon fiber strips (CFRP) (Fig.8).

Rangelova F.

Traykova M.

- bridge enclosures and fairingsbridge decks;

- external reinforcementrehabilitation and retrofittingto RC structures (including FRPconfining of concrete columns);

- external reinforcementrehabilitation and retrofittingto steel structures;

- internal reinforcement toconcrete members;

- FRP/concrete duplex beamconstruction;

- polymer bridge bearings andvibration absorbers.

Figure 8. Experimental stand [3]

Rangelova F.

Traykova M.

Cut-out openings are often requiredto facilitate direct access from outsideor between adjacent apartments,predominantly at the ground floor,where both gravity and seismiccapacity demand is maximum. Cut-out openings performed in structuralwalls results in the modification ofthe internal force flow paths, loss ofload bearing capacity and reducedstructural safety.

Figure 9. The TRM strategy for the wall with cut-out [9]

reestablish about 90 % of the initialload bearing capacity of the elementand increase the drift ratio.

The proposed retrofitting strategy isviable alternative to classicprocedures.

The application of Textile ReinforcedMortar (TRM) was investigated in [9]as a rehabilitation manner, in order torestore the load bearing capacity of thespecimen first tested beforestrengthening (Fig.9). TRM systemcan be an effective solution forstrengthening elements. The punctualanchorage system used turned to beinefficient, allowing for system localdebondings. Very often a mortar withglass fiber grid for reinforcement isapplied.

Rangelova F.

Traykova M.

The presented experimental researchesshow that the retrofitting with carbonfiber strips externally bonded, in thecase of panels with openings, is able to

1) The existing prefabricated RCresidential buildings in Bulgariarepresent a considerable part ofthe total amount of residentialbuildings. The most popularsystem in the recent past is the―large-panel system‖.

2) The rehabilitation and thestrengthening of the residentialbuildings should aim to extendthe remaining lifetime andgenerally represent thesustainable solution forobtaining significant social effect.

Rangelova F.

Traykova M.

IV. Conclusions and

recommendations

3) The programme for rehabilitationand strengthening of the existingprefabricated RC buildings canserve as a basis of the nationalmeasures for renovation of theresidential buildings.

4) Using new techniques is the bestway for improvement theperformance including also theseismic performance of existingbuildings.

[1]. S. Brzev, T. Guevara-Perez , ―Precast Concrete Construction‖

[2]. I. Demeter, T. Nagy-Gyorgy, S. Valeriu, C. Daescu, D. Dan, ―Seismic retrofit of precastRC wall panels with cut-out openings using FRP composites‖, Department of CivilEngineering, Politehnica University of Timisoara, Romania, FRPRCS, 15 July 2009,Sydney, Australia

[3]. M. Fofiu, I.A.Bindean, E. Partene, V. Stoian, ―Retrofitting of Precast ReinforcedConcrete Wall Panel using Carbon Fiber Strips‖, Advances in Engineering Mechanicsand Materials

[4]. Z. Georgiev, E. Papin- Taneva , ―National building typology‖, 2012

[5]. Gangarao Hota and Ruifeng Liang, ―Advanced fiber reinforced polymer composites forSustainable civil infrastructures, Constructed Facilities Center‖, West VirginiaUniversity, Morgantown, WV 26505, USA

[6].National Statistic Institute of Bulgaria, Book 1 ―Residential buildings‖, 2012

[7]. N C H R P, ―Design of FRP Systems for Strengthening Concrete Girders in Shear‖, TRB,2011

[8]. M S Palanichamy, K L Muthuramu, G Jeyakumar, ―Prefabrication techniques forresidential building‖, 27th Conference on Our World in Concrete & Structures,Singapore, 2002.

[9]. D. Todut, V. Stoian, ―TRM strengthening of precast reinforced concrete wall panel withcut-out opening -experimental investigation‖, Advances in Engineering Mechanics andMaterials

Rangelova F.

Traykova M.References

Rangelova F.

Traykova M.