Chapter 1 INTRODUCTION
Definition:
“Prefabri “Prefabricatio cation n is the practice practice of assembli assembling ng component components s of a struc structure ture in a factory or other manufacturing site, and transporting complete assemblies or subassemblies to the construction site where the structure is to be located”.
1.1 Need for prefabrication Prefabricated structures are used for sites, which are not suitable for normal construction methods; such as hilly region, and also when normal construction materials are not available. Structures which are used repeatedly can be standardized; such as mass housing, storage sheds, godowns, shelters, bus stand, security cabins, site oces, foot over bridges, tubular structures, concrete building blocks etc., can be made of prefabricated structures. Prefabricated structure facilities can also be created at near a site as is done to make concrete blocks used in plan of conventional structures. Speed in construction Less Lack of space for proper utilization of space! "ontrol over material. #ass production Production even dicult weather conditions
1.1.1 An Overview: Advantages of Prefabrication $uality and accuracy Precast concrete units are made in a factory in a favorable environment and with tight production control. This produces units with high quality performance performance and appearance. The designer can select from a range of nishes and be able to inspect and accept accept the units before before they are xed in place. place. Factory actory product production ion control control ensures that the reinforcement reinforcement is located accurately and that the units are made to tight tight dimensio dimensional nal tolerance tolerances. s. The structur structural al connectio connections ns are designed designed to enable enable adjustments to be made on site, so the frame can be erected to very precise dimensions. This greatly assists the subsequent installation of cladding, windows and other elements. Speed of construction peed of construction is a major consideration in most building projects and it is here that precast concrete frames excel. This advantage is maximi!ed if the layout and details are not too complex. "esigning for maximum repetition will ma#e manuf manufact actur ure e of the the preca precast st units units easier easier and and const construc ructio tion n faster faster,, but but preca precast st concrete can be used in complex and irregular structures, although it may not then provide the same e$ciency of construction as a rationali!ed design. %rame cost
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'n addition to the economic advantages of faster construction, the capital cost cost of a preca precast st concr concrete ete frame frame can be less less than than that that for alternat alternativ ive e farmin farming g methods designed to give an equivalent performance. (lternative frames for a ve) storey commercial building have been compared.*%* These showed that the cost of a precast concrete frame was +% less than a steel frame with steel dec# composite -oors -oors,, and % less less than than a more more tradit tradition ional al steel steel frame frame with with preca precast st concr concrete ete -oors. (nother comparison on a higher, seven)storey seven)storey structure, suggested a similar saving.*+* these savings may not be reali!ed on all projects, because the di/erence will be in-uenced by the particular design parameters, but they demonstrate a potential cost saving which is worth considering. &verall cost The total cost of a building is related to the speed of construction, the cost of the frame, other construction costs, land costs and interest rates. (lthough the basic cost of the frame is important, speed of construction is often the dominant consideration, particularly in times of high interest rates and land values. 0here the building is to be let, extra rental can be obtained from earlier completion. 'hermal capacity The high thermal capacity of a concrete concrete structure structure can help to control temperature -uctuations. This can reduce the ris# of condensation. Pea# demands on the heating, ventilating and air conditioning equipment may also be reduced and lead to cheaper service installations and lower running costs. (uildability Precas Precastt concret concrete e frames frames can greatly greatly improve improve buildabi buildability lity. 1ompar 1ompared ed with many other methods of construction, construction, precasting removes many of the sensitive site operations to the more stable environment of the factory. 2ad weather has little e/ect on the rate of frame construction and little protection protection is needed on site. Precas Precastt concret concrete e frames frames are precisely precisely manufact manufactured ured to improve improve speed speed of erection. The care ta#en in design and detailing of the connections ensures that erection is simple and rapid, and structural integrity is achieved during erection. 1rane hoo# time is #ept to a minimum. The precast concrete supplier, as a single subcontractor, subcontractor, is usually responsible for the design, production and erection of the frame. 3inimi!ing the numb number er of subc subcon ontr trac acto tors rs simp simpli lie es s cont contra ract ct prog progra ramm mmin ing g and and can can reduc educe e pressures on the management team. 1ontract periods may also be forecast more condently because, with fewer operations, there is less to go wrong. Structural eciency Precast concrete o/ers considerable scope for improving structural e$ciency. 4onge ongerr span spans s and and shal shallo lowe werr cons constr truc ucti tion on dept depths hs can can be obta obtain ined ed by usin using g prestressed concrete for beams and -oors. The examples of buildings given later in this this ectio ection n and elsewh elsewher ere e in this this public publicati ation, on, demon demonstr strate ate the consi consider derabl able e -exibility that is possible in design. 3any of the available precast concrete units have have been been tested tested,, both both in the labora laborator tory y and and in servic service, e, to obtain obtain maxim maximum um structural e$ciency. e$ciency. %ire resistance
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1oncr 1oncrete ete has its own built built)in )in re re resis resistan tance. ce. This This is prese present nt durin during g all all construction phases and does not depend on additional board or sprayed protection. 6btaining a two)hour re resistance presents little problem, and four hours may also be achieved.
1.2 Principles 3ost projects benet from disciplined design. This is inherent in the design of a preca precast st concr concrete ete frame, frame, becaus because e the the design designer er see#s see#s as much much repet repetiti ition on as possible so that the precaster can ta#e advantage of greater mould re)use and standardi!ation of details to reduce the cost of manufacture
3 P T 5 r rotr e d a a eo u p n d cn a d tlsu p iacra rotra r1td i n o toio o n a!n a tro s f d ity io3 is a n nto atn e e tm or na s
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1.3 Materials (ll materials shall conform to Part 7 82uilding 3aterials9. :se of materials for plain and reinforced concrete shall satisfy the requirements of ' 7;.
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0hile selecting the materials for prefabrication, the following characteristics shall be considered= %. 5asy 5asy avai availa labi bili lity ty>> +. 4ight 4ight weight weight for easy easy handlin handling g and transpo transport> rt> <. Thermal Thermal insulati insulation on proper property> ty> . 5asy 5asy wor# wor#ab abil ilit ity> y> 7. "ura "urabi bili lity ty>> ;. ?on)co ?on)combu mbusti stibi bilit lity> y> @. oun ound d insu insula lati tion on>> A. 5co 5conom nomy>
"ements The most common cements used are ordinary ordinary and rapid)hardening rapid)hardening Portland Portland Bincludi Bincluding ng whiteC, whiteC, sulphate) sulphate)res resisti isting ng and high alumina alumina cement. cement. :niversa :niversally lly the chemistries, performance, colours, etc., vary over quite a large range, but they all have to comply with a tandard such as 2, demanding minimum requirements.
)ggregate These fall into into two main types, types, each with with several sub)gr sub)groups= oups= ?atural (ggregates= Flint, Dolcanic Bgranites, basalts, feldspars, etc.C andstone 4imestone Bsedimentary, Bsedimentary, oolitic, etc.C 3arble BcalciteC 2arytes ?atural sands Bsiliceous mainly, river, dune, wadi, marineC Perlite Dermiculite ynthetic (ggregates= intered pulverised fuel ash 5xpanded shale 5xpanded slate 5xpanded clay Foamed slag 1rushed bric#s 1alcined -int Eeconstituted concrete The selection of aggregates aggregates for any particular precast precast concrete concrete operation is a func functi tion on of many many fact factor ors s of whic which h econ econom omic ic avai availa labi bili lity ty and and perf perfor orma manc nce e requirements are probably the following reactions are most important.
Alkali-silica reaction
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(ggregate (ggregates s containi containing ng silica silica minerals minerals are suscepti susceptible ble to attac# attac# by al#alis al#alis B?a+6 and +6C from from the cement cement or other sources. sources. (l#ali) (l#ali)sili silica ca reaction reaction causes crac#ing and reduces the strength of concrete. 5/ective means of reducing the ris# of al#ali aggregate reaction include= %. 1ontrol 1ontrol on the amount amount of cement cement used used in the concre concrete te mix> +. :se of a low al#ali al#ali cement cement>> <. :se of an appropriat appropriate e cement replacem replacement ent such as pulveri!e pulveri!ed d fuel ash BpfaC> and Alkali-carbonate Alkali-carbonate reaction ome carbonate aggregates may be susceptible to al#ali)carbonate reaction, which is similar to al#ali)silica reaction in its e/ects. 'f carbonate aggregates are to be used, specialist attention should be ta#en. Chlorides in concrete Eeinforcing steel is susceptible to corrosion with the presence of chloride in concrete. The total chloride content of the concrete mix arising from the aggregate, admixtures and any other source should not exceed the limits given in Table +.<. The total chloride content should be calculated from the mix proportions proportions and the measured chloride contents of each mix constituent. Table . Chlorides in concrete
*ater 't has often been stated that water which is t enough for drin#ing can be used for concrete but this is not always the case. everal tandards exist for water for concrete and the best answer to the question of the suitability of a source of wate waterr is to ma# ma#e the the prod produc uctt with with that that wate waterr and and see see if it has has the the requi equirred prope properti rties es 'n most most countr countries ies water water is the the cheap cheapest est ingre ingredie dient nt of the mix and production bonuses paid on quantity rather than quality encourage the use of too much water. 3achine)intensive processes generally will only accept a water content consi consiste stent nt with with good good mix mix design design practi practice. ce. The The labou labourr)inten )intensiv sive e proc process esses es are are di/erent and in these one should only have enough water to achieve the minimum wor#ability requirements.
"oncrete+ Preca Precast st concr concrete ete should should have have highes highestt possib possible le qualit quality> y> both both in terms terms of strength and durability. The concrete is accurately delivered to every part of the 7&Page
mould mould ensuring ensuring !ero !ero segregat segregation ion honey) honey) combing combing and minimal minimal vibratio vibration. n. :sing :sing materi materials als that that have have passed passed strict strict quali quality ty contr control ol proce procedur dures, es, rapid rapid harden hardening ing cement is mixed with excellent quality aggregates of #nown source and purity, often in computer controlled batching and mixing plant> to produce concrete of pecied wor#ab r#abil iliity and stren trengt gth. h. 5ven the the intr ntroduc oducttion ion of small quan uantiti tities es of uncontaminated recycled concrete, usually from the factory*s own waste production, super plastici!ers and po!!olana materials Bsuch as pulveri!ed fuel ashC has not reduced this standard.
Steel reinforcement as prestressing!+ Precast concrete elements can, if necessary, be heavily reinforced because they are cast hori!ontally. 2' permits up to %G per cent of the cross)section to be reinf einfor orce ced, d, alth althou ough gh this this amou amount nt 4s rar rarely ely used used in favo favorr of high higher er conc concrrete ete strengths. High tensioned hot rolled ribbed bar is used I7 of cases, even in shear lin#s where where mild mild steel steel bars to be used. used. The small small cost di/ere di/erence nce compar compared ed to the addition additional al trength trength,, i.e. ;G Ds +7G?Jmm +7G?JmmK, K, and the need lot consisten consistency cy of habit habit when assembling cages, it ma#e more economical Tying wire is more secure around ribbed ribbed bar ma#in ma#ing g the cage cage more more robus robust. t. 3ild 3ild steel steel is often often used lot proj project ecting ing loops, loops, etc., because because it is easier easier to hand ) bend tin site. bar diameters diameters commonly used are A and %G mm for common tirrups, %G and %+mm for beam stirrups and other distribution 6r and crac# bars, and %;, +G, +7, <+ and Gmm for main -exural bars. Eeinforcement is used for one or more of the following reasons= BaC tructural Bloading, re, earthqua#e, etc.C BbC Handling Bto withstand stac#ing, transport and erection stressesC BcC hrin#age Bto withstand di/erential stressesC
einforcement Spacers pacers are designed to maintain the specied minimum cover between the steel reinforcement and the mould and may be made of a variety of materials, vi!.= BaC 1oncrete or mortar bloc#s with or without tie wires. BbC (sbestos cement. BcC Plastics, usually lled polyethylene polyethylene or polyamide. BdC teel with end)leg protection.
einforcing -bers and meshes Fibres and meshes made from steel, polypropylene, glass and carbon are in curr current ent use in many many types types of preca precast st produ product. ct. Their Their main main applic applicati ations ons are are for concretes where thin sections, impact resistance or special thixotropic properties are required. 6nly carbon and steel bres and meshes are true reinforcing materials as they have strengths and moduli in excess of concrete, whereas polypropylene and glass have moduli either lower or of the same order as concrete. (part from steel meshes, steel bres have an application in specic products where where a combin combinati ation on of high high streng strength th and impac impactt resis resistan tance ce is requi requirred, vi!. vi!. explosion)resistant units. However, the bres require careful handling using steel) faced gloves, and slow addition to a wor#ing pan mixer otherwise the bres tend to form balls or clots. 5xtra water andJor the use of wor#ability admixtures is required ;&Page
to give an adequate wor#ability, and compaction by vibration needs to be more energetic than for a conventional conventional mix. Polypropylene and the other plastics bres being developed have the most promise, promise, and give good impact resistance to products such as pipes, pontoons, etc., and stabi stabilis lised ed high high air conte content nt syste systems ms in archi architec tectur tural al produ products cts such such as the the thixotropic Faircrete. Llass bres have had a chequered history since their use was ta#en up in the late sixties. From the thermodynamic point of view, the di/erent heats of formation of calcium and sodium silicate indicate that the lifetime in a weathering situation will be limited. (n optimistic picture can only be painted for !irconium glass bre in cement matrices.
Structural steel and bolts tructural .steelwor# .sections are used in many type of precast elements, especially at the connections. These include rolled rectangular and square hollow sections BEH, HC, solid billets, channels and angles, plates and welded)tees, etc. "eta "etail ils s of how how thes these e ate ate used used in prac practi tice ce are are give given n in ect ectio ions ns A. A. and and I.+. I.+. tructural sections such as :niversal beams and columns B:2, :M may be cast into precast elements to enhance strength where the reinforced concrete capacity is exhausted. However, this may have severe cost implications that must be carefully examined. Eolled steel sections and bent or -at steel plates ate welded to form steel connectors in many highly stressed support situations where direct contact between concrete surfaces surfaces 4s to be avoided. The steel used is grade < BmostlyC or grade 7G. 0elded electrodes ate mostly grade 5<. 0hen used to join grade < steel, the yield strength of the weld, Bpy N +%7 ?Jmm +. 0hen used in combination with grade 7G, steel steel grade grade 57l electr electrode odes s gives gives py N +77?Jm +77?Jmm m+ . The usual rules for lap lengths BtwC, throat thic#ness BG.@twC and returns and run)outs B+twC apply, where tw is the leg length. 'ntermittent llet welds are rarely used as weld lengths tend to be short.& Hot dipped galvani!ed steel is used for exposed connections, usually of secondary structural structural signicance, such as dovetail channels for bric# ties. The basic plate is grade < steel, and grade 7G 4s used in the more highly stressed plates. 2lac# bolts grade .; Bpy N %I7?Jnun in tension and %;G?Jmm+ in shearC and A=ABpy= 7G=<@7C are used in many connections. High strength friction grip bolts are used in special circumstances where the integrity and safety Bboth temporary and permanentC of oonncttions made with ordinary bolts in clearance holes cannot be guaranteed
on/cementitious on/cementitious materials 5poxy 5poxy)ba )based sed morta mortars rs are are used used to ma#e, ma#e, either either partia partially lly 6r compl complete etely ly,, + connections where where a rapid gain in strength is required, e.g. up to get G?Jmm in +) and
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B+C hardeners. They are occasionally used as pressure= injections for crac# lling or to restore tensile strength. 3anufacturer9s procedures should be strictly adhered to. ?eoprene, rubberO and mastics art= used for soft bearing, bac#ing strips, etc. The P1' 3anual on (rchitectural (rchitectural Precast Precast 1ladding+ gives. gives. 5xtensive guidance guidance on the use of these materials. (lthough they are not used extensively in precast structure, a typical range of applications is given in Table
1.3.1 E!ip"ents #oulds 3oulds are basically means by which= %. 1onc 1oncrrete ete is #ept ept to a requi equirred shape ape until it is strong enoug nough h to be demoulded, or +. 1oncrete 1oncrete is moulded moulded on a machine machine and retains retains that that shape on virtually virtually instant instant demoulding, or <. 1oncrete 1oncrete is shaped immediat immediately ely after casting casting using using additional additional or secondary secondary mould acting on previously un)moulded surfaces. The one thing that all moulding techniques and moulds have in common is dime dimens nsio ions ns.. 0het 0hethe herr thes these e are are crit critic ical al for for stru struct ctur ural al,, arch archit itec ectu tura rall andJ andJor or cont contra ract ctua uall reaso easons ns is a matt matter er that that caus causes es quit quite e a lot lot of argu argum ment. ent. The The specication for the product should state strictly what is required, bearing in mind what is practical and how the product is to t into the main construction. (ll too often precast products such as cladding are specied on a dimension such as=
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0here, ( is the target dimension often called the wor# si!e. Two Two important important points need to be borne borne in mind= %. Tolerance olerance is an easy thing to nd during during constructio construction n but is a very di$cult di$cult thing to lose. 2y this is meant that a product that is too large will generally cause more problems problems than a product product that is too small, i.e. a joint can be lled with mortar, sealant, etc., when the product is nearer ()y but needs cutting bac# when there is too much (x. +. 3oulds 3oulds tend tend to grow grow in si!e si!e with contin continuous uous usag usage. e. Figure shows how a joint can be designed to cater for resistance to arries damage and give apparent uniform joint thic#ness.
Fig %.% 1hamfered joint to cater for tolerances and arris damage.
1.# Mod!lar coordination !odule ( unit of si!e used in dimensional co)ordination. "asic !odule = The fundamental fundamental module used in modular co)ordination, the si!e of which is selected for general application to building and its components. Q#$T% & The 'alue of the basic module has been chosen as (( mm for the ma)imum *e)ibility and con'enience. con'enience. The symbol for the basic module is !. R
!odular Co-ordination "imensional "imensional co)ordinati co) ordination on employing the basic module or a multi)module.
The purposes purposes of modular modular co)ordination co)ordination are= aC To reduce the variety of component si!es produced, and bC To allow the building designer greater -exibility in the arrangement of components. !odular +rid ( rectangular coordinate reference system in which the distance between consecutive lines is the basic module or a multi)module. This multi)module may di/er for each of the three orthogonal dimensions of the grid, two in plan and one in vertical direction. !ulti-module ( module whose si!e is a selected multiple of the basic module.
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1.4.1 Modular Co-Ordination, Architectural Treatent and !inishes #odular "o/ordination The basic module shall be adopted. (fter (fter adopting this, further wor# is necessary to outline suitable range of multi modules with greater increments, increments, often referred to as preferred increments. ( set of rules as detailed below woul would d be adeq adequa uate te for for meet meetin ing g the the requi equirremen ements ts of conv conven enti tion onal al and and prefabricated construction.
These rules relate to the the following basic basic elements= elements= a. The planning planning grid grid in both directions directions of the hori!ontal hori!ontal plan plan shall shall be= %C For indust industria riall buildin buildings gs %73 for span up to %+m
)rchitectural 'reatment 'reatment and %inishes %inishes Treatment Treatment and nishes nishes have to be specied #eeping #eeping in view the requirements requirements of protection, function and aesthetics of internal and external spaces and surfaces. 0hile deciding the type of architectural treatment and nishes for prefabricated buildings, the following points should be #ept in view= a. uitabili uitability ty for mass mass produc production tion techni techniques ques>> b. Eecogn ecogniti ition on of the constra constraint ints s impos imposed ed by the level level of wor#m wor#mans anship hip available> c. Possibi Possibility lity of of using using di/erent di/erent types types of nish nishes es d. :se :se of nis nishe hes s and and arch archit itec ectu tura rall trea treatm tmen entt for for the the crea creati tion on of a particular architectural character in individual buildings and in groups of buildings by the use of colour, texture, projections and recesses on surfaces, etc> e. 'ncorporation of structural structural elements elements li#e li#e joists, joists, columns, columns, beams, etc, etc, as architec architectura turall features features and the treatme treatment nt of these for better better overall overall performance and appearance> f. atisf atisfact actory ory nis nishin hing g of surfac surfaces> es> and and %G & P a g e
g. :se :se of ligh lightt weig weight ht mate materi rial als s to e/ec e/ectt econ econom omy y in the the stru struct ctur ural al system. ome a. b. c. d. e. f.
of the acceptable methods of nishes integral with the pre casting are, 1oncrete 1oncrete surfac surface e moulded moulded to to designJ designJshap shape> e> 4aid)on 4aid)on nishi nishing ng tiles tiles xed xed during during casting casting>> Finishes inishes obtain obtained ed by washing, washing, tooling> tooling> Lrinding Lrinding,, groovin grooving g of hardene hardened d concret concrete> e> 5xpose 5xposed d aggr aggrega egates tes>> and and 6the 6therr int integ egra rall ni nish shes es..
Concept Of Dimensional Co ordination
The whole concept of dimensional dimensional coordination coordination is represented represented in the following chart which indicates the application of system and nal result
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1.$ %tandari&ation ' .. tandardisation tandardisation Precast concrete construction should be planned wherever possible to utili!e standardi!ed precast concrete elements. 3ost 3ost pref prefab abri rica cate ted d buil buildi ding ngs s will will be uniq unique ue and and site site spec speci ic. c. (t the the theor theoreti etical cal design design stage, stage, a basic basic layout layout plan plan should should be develo developed ped which which %+ & P a g e
achieves a balance between architecturalJaesthetic requirements and a high degree of standardi!ation. Therefore, close collaboration amongst di/erent design parties is essential during conceptual design to achieve the optimum standardi!ation
)dvantages )dvantages of standardization standardization
5asier design 5asier manufacture 5asier erection and completion
0.1.2 %actors in3uencing the standardization
the most rational type of member for each element is selected from the point of production from the assembly serviceability and economy The number of types of elements will be limited and they should be used in large quantities. To To the extent possible the largest largest si!e to be used which results in less number of joints The si!e and the number of of the prefabricates prefabricates is limited by the the weight in overall dimension that can be handled by the transportation.
Hence it is preferable to have all the prefabricates approximately of same weight very near to the lifting capacity of the equipment
1.( %)ste"s Prefabrication Systems )nd Structural Schemes The word 8system9 is referred referred to a particular method of construction construction of building buildings s using using the prefabr prefabricat icated ed componen components ts which which are inter inter)related )related in functions and are produced to a set of instructions. 0ith certain constraints, several plans are possible, using the same set of components. The degree of -exibility varies from system to system. However, in all the systems there is a certain order and discipline. The following following aspects, among among others, are are to be considered considered in devising devising a system= system= 5/ective utili!ation of spaces> traight and simple walling scheme> 4imited si!es and numbers of components> 4imited opening in bearing walls> Eegulated locations of partitions> tandardi!ed service and stair units> %< & P a g e
4imited si!es of doors and windows with regulated positions> tructural clarity and e$ciency> uitability for adoption in low rise and high rise building> 5ase of manufacturing, storing and transporting> peed and ease of erectiow and imple jointing system.
0.4.0 Prefabrication Systems+ 5e-nition The system of prefabricated prefabricated construction construction depends on the extent extent of the use of prefabricated components, their materials, si!es and the technique adopted for their manufacture and use in building.
0.4.2 'ypes of Prefabrication "omponents The prefabricated prefabricated concrete concrete components such as those given below may be used which shall be in accordance with Part 7 82uilding 3aterials9 and the accepted standards Q;)@(BlCR, where available= EeinforcedJPrestressed concrete channel unit, EeinforcedJPrestressed concrete slab unit, EeinforcedJPrestressed concrete beams, EeinforcedJPrestressed concrete columns, EeinforcedJPrestressed concrete hollow core slab, Eeinforced concrete waSe slabJshells, EeinforcedJPrestressed concrete wall elements, HollowJolid HollowJolid bloc#s and battens, Precast plan#s and joists for -ooring and roong, Precast joists and trussed girders, 4ight weight cellular concrete slabs, Precast lintel and chajjas, 4arge panel prefabricates, EeinforcedJPrestressed concrete trusses, EeinforcedJPrestressed roof purlins, Precast concrete 4)panel unit, Prefabricated bric# panel unit, Prefabricated sandwich concrete panel, and Precast foundation. There may be other types of components which may be used with the approval of the (uthority. Q#$T% & The elements may be cast at the site or o the site. R
0.4.6 "ategories of &pen Prefab System % & P a g e
There are two categories of open prefab system depending on the extent of prefabrication used in the construction as given as follows 0. Partial artial prefabri prefabricati cation on system system This system basically basically uses uses precast rooting rooting and -ooring -ooring components components and other minor minor elements elements li#e li#e lintels, lintels, 1H(( 1H((,, #itchen #itchen sills sills in conventio conventional nal building building construction. The structural system could be in the form of in)situ framewor# or load bearing walls. 2. %ull prefa prefabric brication ation system system 'n this system almost all the structural components are prefabricated. The ller ller walls walls may may be be of of bri bri c#Jblo c#Jbloc# c# masonr masonry y or of any other locally locally avail availabl able e material. 6. Large Large Panel Panel Prefab Prefabrica rication tion System System This system is based on the use of large large prefab prefab components. components. The components components used are precast precast concrete concrete large large panels panels for walls, walls, -oors, -oors, roofs, roofs, balconie balconies, s, staircases, etc. The casting of the components could be at the site or o/ the site. "epending upon the extent of prefabrication, this system can also lend itself to partial prefab system and full prefab system.
0.4.6.0 Precast *alls tructural scheme with precast large panel walls can be classied as given as follows 2ased on the structural functions of the walls, the precast walls may be classied as= aC 4oad bearing walls, bC ?on)load bearing walls, and cC hear walls. 2ased on construction, the precast walls may be classied as= aC Homogeneous walls N which could be solid, hollow or ribbed> and bC ?on)homogeneous walls N these could be composite or sandwich panels. 2ased on their locations and functional requirements the precast walls may also classied as= aC 5xterna 5xternall walls, walls, which which may may be load bearin bearing g or bC ?on)load ?on)load bearin bearing g depending depending upon upon the lay)out> lay)out> These are usually non)homogeneous non)homogeneous walls of sandwiched sandwiched type to impart better thermal 1omforts 1omforts>> and interna internall walls walls providi providing ng resista resistance nce against against vertical vertical loads, loads, hori!ontal loads, re, etc> these are normally homogeneous walls.
0.4.6.2 Precast 3oors
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"epend "epending ing upon upon the compos compositi ition on of units units,, preca precast st -oori -ooring ng units units may may be homogeneous or non homogeneous. aC Homo Homogen geneou eous s -oors -oors may may be solid solid slabs slabs,, cored cored slabs, slabs, ribbe ribbed d or waSe slabs. bC ?o ?on n)hom )homo ogeneo eneou us -oors ors may be multil ltila ayered ered ones nes with ith combinations of light weight concrete or reinforcedJprestressed concrete, with ller bloc#s. "epending upon the way the loads are transferred, transferred, the precast -oors may be classied as one way or two way systems= aC 6ne way system system transfer transfers s loads to supportin supporting g members members in one direction direction only. The precast elements which come under this category are channel slabs, hollow core slabs, channels and ties system, light weight cellular concrete slabs, etc. bC Two way systems transfer loads in both the directions imparting loads on the four edges. The precast elements under this category are room si!ed panels, two way ribbed or waSe slab systems, etc.
0.4.6.6 Staircase systems taircase system may consist of single -ights with inbuilt risers and treads in the element. The -ights are normally unidirectional transferring the loads to supporting landing slabs or load bearing walls.
0.4.6.7 (o8 'ype "onstruction 'n this system, room si!e units are prefabricated and erected at site. Toilet and #itchen bloc#s could also be similarly prefabricated and erected at site. #$T% & This system deri'es its stability and stiness from the bo) units which are formed by four ad/acent walls. 0alls are /ointed to make rigid connections among themsel'es. The bo) unit rests on foundation which may be of con'entional type or precast type .
Transportation ansportation and Erection 1.* Prod!ction Tr
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0.9.0 #anufacture of Precast "oncrete :lements
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( judicious location of pre casting yard with concreting, initial curing Brequired for demou demouldi ldingC ngC,, storag storage e facili facilitie ties, s, suitab suitable le transp transport orting ing and erect erection ion equipments and availability of raw materials are the crucial factors which should be carefully planned and provided for e/ective and economic use of precast concrete components in constructions.
#anufacture The manufacture manufacture of the components can be done in a factory for the commercial commercial production established at the focal point based on the mar#et potential or in a site pre casting yard set up at or near the site of wor#.
0.9.0.0%actory prefabrication Facto actory ry pref prefab abri rica cati tion on is resor esorte ted d to in a fact factor ory y for for the the comm commer erci cial al production for the manufacture of standardi!ed components on a long)term basis. 't is a capital intensive production where wor# is done throughout the year preferably under a closed shed to avoid e/ects of seasonal variations. High level of mechani!ation can always be introduced in this system where the wor# can be organi!ed in a factory) li#e manner with the help of a constant team of wor#men.
0.9.0.2 Site prefabrication prefabrication Prefa Prefabri bricat cated ed compo componen nents ts produ produced ced at site site or near near the site site of wor# wor# as possible. This system is normally normally adopted for a specic job order for a limited period. :nder this category there are two types that is semi)mechani!ed and fully) mechani!ed.
0.9.0.6 Semi/mechanized Semi/mechanized The wor# is normally normally carried out in open space with locally available labour force. The equipment machinery used may be minor in nature and moulds are of mobile or stationary in nature.
0.9.0.7 %ully/mechanized The wor# will be carried out under shed with s#illed labor. labor. The equipments used will be similar to one of factory production. This type of precast yards will be set up for the production of precast components of high quality, high rate of production. Though there is denite economy with respect to cost of transportation, this system su/ers from basic drawbac# of its non)suitability to any high high degre degree e of mecha mechani ni!at !ation ion and no elabor elaborate ate arran arrangem gement ents s for quality control. ?ormal benets of continuity of wor# are not available in this system of construction.
0.9.2 Processes involved in the manufacture of precast elements The various processes processes involved in the manufacture manufacture of precast elements may be classied as follows=
0.9.2.0 #ain process %A & P a g e
Providing and assembling the moulds, placing reinforcement cage in position for reinforced concrete wor#, and stressing the wires in the case of prestressed elements> Fixing of inserts and tubes, where necessary Bfor handlingC> Pouring the concrete into the moulds> Dibrating the concrete and nishing> 1uring Bsteam curing, if necessaryC> and "emoulding the forms and stac#ing the precast products.
0.9.2.2 )u8iliary process Process necessary for the successful completion of the processes covered by the main process= 3ixing and manufacture of fresh concrete Bdone in a mixing station or by a batching plantC> Pref Prefab abri rica cati tion on of reinf einfor orce ceme ment nt cage cage Bdon Bdone e in a stee steell yar yard or wor#shopC> 3anufacture of inserts and other nishing items to be incorporated in the main precast products> Finishing the precast products> and Testing Testing of products. products.
0.9.2.6 Subsidiary process (ll other other wor# wor# involv involved ed in #eeping eeping the main main produ producti ction on wor# wor# to a cyclic cyclic wor#ing= torage of materials> Transport Transport of cement cement and aggregates aggregates>> Transport Transport of green green concrete concrete and reinforc reinforcement ement cages> Transport Transport and stac#ing the the precast elements> elements> Eepairs and maintenance of tools, tac#les and machines> Eepairs and maintenance of moulds, and Leneration of steam, etc. For the manufacture of precast elements all the above processes shall be planned in a systematic way to achieve the following= aC ( cyclic technological method of wor#ing to bring in speed and economy in manufacture> bC 3echani!ation 3echani!ation of the process process to increase increase productivity productivity and and to improve improve quality> quality> cC The optimum optimum production production satisfying satisfying the quality quality control control requirement requirements s and to #eep up the expected speed of construction aimed> dC 2etter 2etter wor#ing wor#ing condition conditions s for the people people on the job> job> and eC To minimi!e minimi!e the e/ect e/ect of weather weather on the manufacturing manufacturing schedule. The various stages of precasting precasting can be classied as in Table Table %.< on the basis of the equipments required for the various stages. This permits mechani!ation mechani!ation and rationali!ation rationali!ation of wor# in the various stages. 'n the prec precas asti ting ng,, stag stages es ; and and @ give given n in Table able %.< %.< form form the the main main proc proces ess s in the the manufacture of precast concrete elements. For these precasting stages there are many Technologi echnological cal process processes es to suit the concre concrete te produc productt under under consider consideratio ation n which have been proved rational, economical and time saving. %I & P a g e
The technological technological line or process is the theoretical theoretical solution for the method of planni planning ng the wor# wor# involv involved ed by using using machi machine ne compl complex exes. es. Figure igure 7 illus illustra trates tes diagrammatically the various stages involved in a plant process.
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#ethods of precasting The various accepted methods of manufacture manufacture of precast units can be broadly classied into two methods= 'he Stand #ethod< where the moulds remain stationary at places, when the various processes involved are carried out in a cyclic order at the same place, and 'he %low %low #etho #ethod< d< wher where e the the prec precas astt unit unit unde underr cons consid ider erat atio ion n is in movement according according to the various processes involved involved in the wor# which are carried out in an assembly)line method. The various accepted accepted precasting precasting methods are are listed in Table Table < with details regarding regarding the elements that can be manufactured by these methods.
Preparation Preparation and Storage of #aterials torage of materials is of considerable importance in the precasting industry, as a mist mista# a#e e in plan planni ning ng in this this aspe aspect ct can can grea greatl tly y in-u in-uen ence ce,, the the econ econom omic ics s of production. ++ & P a g e
From experience in construction, it is clear that there will be very high percentages of loss of materials as well as poor quality due to improper storage and transport. o, in a precast factory where everything is produced with special emphasis on quality, proper storage and preservation of building materials, especially cement, coarse and ne aggregates, is of prime importance
#oulds 3oulds for the manufacture of precast elements may be of steel, timber, concrete and plastic or a combination thereof. For the design of moulds for the various elements, special importance should be given to easy demoulding and assembly of the various parts. (t the same time rigidity, strength and water tightness of the mould mould,, ta#ing ta#ing into into consid considera eratio tion n forces forces due due to pourin pouring g of green green concr concrete ete and vibrating, are also important.
'olerances The moulds have to be designed in such a way to ta#e into consideration consideration the tolerances given in 7.
Slopes of the #ould *alls For easy demoulding of the elements from the mould with xed sides, the required slopes have to be maintained. 6therwise there is a possibility possibility of the elements getting stuc# up with the mould at the time of demoulding. I. (ccelerated Hardening 'n most of the precasting factories, it is economical to use faster curing methods or articial curing methods, which in turn will allow the elements to be demoulded much earlier permitting early re)use of the forms. (ny of the following methods may be adopted= aC 2y Heating the (ggregates and 0ater 2efore 3ixing the 1oncrete N 2y heating of the aggregates as well as water to about @GU1 to AGU1 before ma#ing the concrete mix and placing the same in the moulds, su$ciently high earlier strengths are developed to allow the elements to be stripped and transported. bC team team 1uring 1uring N team team curin curing g mayb maybe e done done under under high high press pressur ure e and high temperature in an autoclave. This technique is more suited to smaller elements. (lternatively, this could be done using low pressure steam having temperature around AGU1. This type of curing shall be done as specied in I.7.+. For light weight concrete products when steam cured under under high pressure pressure,, the drying drying shrin#a shrin#age ge is reduced reduced considerab considerably, ly, "ue to this reason, high pressure steam curing in autoclave is specied for light weight low densities ranging from
pressure saturated steam is injected into the mixer while the aggregates are being mixed. This enables the heating up of concrete to approximately ;GU1. uch a concrete after being placed in the moulds attains high early strength. Heated (ir 3ethod N 'n this method, the concrete elements are #ept in contact with hot air with a relative humidity not less than AG percent. Tlis method is specially useful for light weight concrete products using porous coarse aggregates. Hot 0ater 3ethod N 'n this method, the concrete elements are #ept in a bath of hot water around 7GV1 to AGV1. The general principles of this type of curing are not much di/erent from steam curing. 5lectrical 3ethod N The passage of current through the concrete panels generates heat through its electro)resistivity electro)resistivity and accelerates curing. 'n this method, the concrete concrete is heated up by an alter alternat nating ing curre current nt rangin ranging g from from 7G volts volts for a plasti plastic c concr concrete ete and and gradually increasing to +
I.7.% I.7.% The The curin curing g of the prefa prefabri bricat cated ed eleme elements nts can be e/ecte e/ected d by the norma normall methods of curing by sprin#ling water and #eeping the elements moist. This can also be done in the case of smaller elements by immersing them in a specially made water tan#s. I.7.+ team 1uring I.7.+.% The steam curing of concrete products shall ta#e place under tarpaulin in tents, under hoods, under chambers, in tunnels or in special autoclaves. The steam shall have a uniform quality throughout the length of the member. The precast elements shall be so stac#ed, with su$cient clearance between each other and the bounding enclosure, so as to allow proper circulation of steam. 2efore the concrete products are subjected to any accelerated meth method od of curi curing ng,, the the ceme cement nt to be used used shal shalll be test tested ed in acco accord rdan ance ce with with accepted standards Bsee Part 7 2uilding 83aterialsC 83aterialsC especially for soundness, soundness, setting time and suitability for steam steam curin curing. g. 'n the case case of eleme elements nts manuf manufact actur ured ed by accele accelera rated ted curin curing g methods, concrete admixtures to reduce the water content can be allowed to be used. used. The normal aeration aeration agents agents used to increase increase the wor#abi wor#ability lity of concret concrete e should not be allowed to be used. :se of calcium chloride should be avoided for reinforced concrete elements. I.7.+.+ The surrounding walls, the top cover and the -oor of steam curing chamber or tunnel or hood shall be so designed as not to allow more than % #calJm+JhJ61. I.7.+.< The inside face of the steam curing chamber, tunnel or hood shall have a damp)proof layer to maintain the humidity of steam. 3oreover, proper slope shall be given to the -oor and the roof to allow the condensed water to be easily drained away. (t rst, when steam is let into the curing chambers, the air inside shall be allowed to go out through openings provided in the hoods or side walls which shall be closed soon after moist steam is seen jetting out. I.7.+. 't is preferable to let in steam at the top of the chamber through perforated pipelines to allow uniform entry of steam throughout the chamber. I.7.+.7 The fresh concrete in the moulds should be allowed to get the initial set before allowing the concrete to come into contact with steam. The regular heating up of fresh concrete product from about +GU1 to <7V1 should start only after a waiting period ranging from + to 7 h depending on the setting time of cement used. 't may be further noted that steam can be let in earlier than this waiting period provided provided the temperature of the concrete product does not rise beyond <7V1 within this waiting period. I.7.+. I.7.+.; ; The The second second stage stage in steam steam curin curing g proce process ss is to heat heat up the concrete concrete elements, moulds and the surroundings in the chamber= aC bC I.7.+.@ 'n the low pressure steam curing the airspace around the member is heated up to a temperature of @7V1 to AGU1 at a gradual rate, usually not faster than
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The third stage of steam curing is to maintain the uniform temperature temperature and pressure for a duration depending upon thic#ness of the section, This may vary from < h to 7 h in the case of low pressure steam curing and h to @ h in the case of high pressure steam curing. I.7.+.A The fourth stage of steam curing is the gradual cooling down of concrete products and surroundings in the chamber and normali!ation of the pressure to bring it at par with outside air. The maximum cooling rate, which is dependent on the thic#ness of the member, should normally not exceed
location and orientation mar#s as and where necessary. The date of manufactur manufacture e shall also be mar#ed mar#ed on the units. I.A.% The identication mar#ings on the drawings shall be the same as that indicated in the manufacturer9s literature literature and shall be shown in a table on the setting schedule together with the length, type, si!e of the unit and the si!es and arrangement of all reinforcement. I.I Transport Transport Transport of precast precast elements elements inside inside the factory and to the site of erection is of considerable importance not only from the point of view of economy but also from the point of view of design and e$cient management. Transport Transport of precast precast elements elements must must be carried out with extreme care to avoid any jer# and distress in elements and handled as far as possible in the same orientation as it is to be placed in nal position. I.I.% Transport 'nside the Factory Transport Transport of precast precast elements elements moulded moulded inside the the factory depends on the method of production, selected for the manufacture as given in Table <. I.I.+ Transportfrom tac#ing Ward 'nside the Factory to the ite of 5rection 5rection Transport Transport of precast precast concrete concrete elements elements from from the factory to the site of erection should be planned in such a way so as to be in conformity with the tra$c rules and regulations as stipulated by the (uthorities. The si!e of the elements is often restricted by the availability of suitable transport equipment, such as tractor)cumtrailers, to suit the load and dimensions of the member in addition to the opening dimensions under the bridge and load carrying capacity while transporting the elements over the bridge. I.I.+.% 0hile transporting elements in various systems, that is, wagons, truc#s, bulloc# carts, care should be ta#en to avoid excessive cantilever actions and desired supports are maintained. pecial care should be ta#en at location of sharp bends and on uneven or slushy roads to avoid undesirable stresses in elements. I.I.+.+ 2efore loading the elements in the transporting media, care should be ta#en to ensure that the base pac#ing for supporting the elements are located at specied positions only. ubsequent pac#ings must be #ept strictly one over the other. +G I.%G 5rection 'n the 8erection of precast elements9, all the following items of wor# are meant to be included= linging of the precast element> +@ & P a g e
Tying Tying up of of erection ropes ropes connecting connecting to the erection hoo#s> 1leaning of the elements and the site of erection> 1leaning of the steel inserts before incorporation in the joints, lifting up of the elements, setting them down into the correct envisaged position> (djustment (djustment to get the stipulated level, line and plumb> 0elding of cleats> 1hanging of the erection tac#les> Putting up and removing of the necessary sca/olding or supports> 0elding of the inserts, laying of reinforcements in joints and grouting the joints> and Finishing the joints to bring the whole wor# to a wor#manli#e nished product. I.%G.% 'n view of the fact that the erection wor# in various construction jobs using prefabricated concrete elements di/ers from place to place depending on the site conditions, safety precautions in the wor# are of utmost importance. Hence only those s#illed foremen, trained wor#ers and tters who have been properly instructed about the safety precautions to be ta#en should be employed on the job. For additional information, see Part @ 81onstructional Practices and afety9. I.%G.+ Transport of people, wor#ers or visitors, by using cranes and hoists should be strictly prohibited on an erection site. I.%G.< 'n the case of tower rail mounted cranes running on rails, the trac# shall not have a slope more than G.+ percent in the longitudinal direction. 'n the transverse direction the rails shall lie in a hori!ontal plane. I.%G. The trac# of the crane should be daily chec#ed to see that all sh plates and bolts connecting them to the sleepers are in place and in good condition. I.%G.7 The operation of all equipment used for handling and erection shall follow the operations manual provided by the manufacturer. (ll safety precautions shall be ta#en in the operations of handling and erection. %G 5M:'P35?T %G.% Leneral The equipment equipment used in the the precast concrete concrete industryJ industryJ ?(T'6?(4 2:'4"'?L 16"5 6F '?"'( construction may be classied into the following categories= aC bC +A & P a g e
cC dC eC fC X hC jC #C 3achinery required for quarrying of coarse and ne aggregates> 1onveying equipment, such as, belt conveyors, chain conveyors, screw conveyors, buc#et elevators, hoists, etc> 1oncrete mixing machines> 1oncrete vibrating machines> 5rection equipment, such as, cranes, derric#s, hoists, chain pulley bloc#s, etc> Transport Transport machinery, machinery, such such as, tractor)cumtrailers tractor)cumtrailers,, dumpers, lorries, locomotives, motor boats and rarely even helicopters> 0or#shop machinery for ma#ing and repairing steel and timber moulds> 2ar straightening, bending and welding machines to ma#e reinforcement cages> 3inor tools and tac#les, such as, wheel barrows, concrete buc#ets, etc> and team generation plant for accelerated curing. 'n addition to the above, pumps and soil compacting machinery are required at the building site for the execution of civil engineering projects involving prefabricated components. 5ach of the above groups may further be classied into various categories of machines and further to various other types depending on the source of power and capacity. %G.+ 3echani!ation of the 1onstruction and 5rection Processes The various various processes processes can be mechani!ed mechani!ed as in any other industry for attaining the advantages of mass production of identical elements which in turn will increase productivity and reduce the cost of production in the long run, at the same time guaranteeing quality for the end)product. 6n the basis of the degree of mechani!ation used, the various precasting factories can be divided into three categories= aC 0ith simple mechani!ation, bC 0ith partial mechani!ation, and cC 0ith complex mechani!ation leading to automation. %G.+.% 'n simple mechani!ation, simple mechanically +I & P a g e
operated implements are used to reduce the manual labour and increase the speed. %G.+.+ 'n partird mechani!ation, the manual wor# is more or less eliminated in the part of a process. For example, the batching plant for mixing concrete, hoists to lift materials to a great height and bagger and bulldo!er to do earthwor# come under this category. %G.+.< 'n the case of complex mechani!ation leading to automation, a number of processes leading to the end)product are all mechani!ed to a large extent Bwithout or with a little manual or human element involvedC. This type of mechani!ation reduces manual wor# to the absolute minimum and guarantee the mass production at a very fast rate and minimum cost. %G.+. The equipment shall conform to accepted standards as listed in Part @ 81onstructional Practices and afety9.
