Prefabrication Unit i Introduction

8/9/2019 Prefabrication Unit i Introduction

1/30

Chapter 1INTRODUCTION

Definition:

Prefabrication is the practice of assembling components of a structure in

a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies 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 whennormal construction materials are not available.

Structures which are used repeatedly can be standardized; such as

mass housing, storage sheds, godowns, shelters, bus stand, securitycabins, 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 conventionalstructures.

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 andwith tight production control. This produces units with high quality performance andappearance. The designer can select from a range of nishes and be able to inspectand accept the units before they are xed in place. Factory production controlensures that the reinforcement is located accurately and that the units are made totight dimensional tolerances. The structural connections are designed to enableadjustments to be made on site, so the frame can be erected to very precisedimensions. This greatly assists the subsequent installation of cladding, windowsand other elements.

Speed of constructionpeed 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 thelayout and details are not too complex. "esigning for maximum repetition will ma#emanufacture of the precast units easier and construction faster, but precastconcrete can be used in complex and irregular structures, although it may not thenprovide the same e$ciency of construction as a rationali!ed design.

%rame cost

% & P a g e


2/30

'n addition to the economic advantages of faster construction, the capitalcost of a precast concrete frame can be less than that for alternative farmingmethods designed to give an equivalent performance. (lternative frames for a ve)storey commercial building have been compared.*%* These showed that the cost of aprecast concrete frame was +% less than a steel frame with steel dec# composite-oors, and % less than a more traditional steel frame with precast concrete

-oors. (nother comparison on a higher, seven)storey structure, suggested a similarsaving.*+* these savings may not be reali!ed on all projects, because the di/erencewill be in-uenced by the particular design parameters, but they demonstrate apotential cost saving which is worth considering.

&verall costThe 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 thebasic cost of the frame is important, speed of construction is often the dominantconsideration, particularly in times of high interest rates and land values. 0here thebuilding is to be let, extra rental can be obtained from earlier completion.

'hermal capacityThe high thermal capacity of a concrete structure can help to control

temperature -uctuations. This can reduce the ris# of condensation. Pea# demandson the heating, ventilating and air conditioning equipment may also be reduced andlead to cheaper service installations and lower running costs.

(uildabilityPrecast concrete frames can greatly improve buildability. 1ompared with

many other methods of construction, precasting removes many of the sensitive siteoperations to the more stable environment of the factory. 2ad weather has littlee/ect on the rate of frame construction and little protection is needed on site.

Precast concrete frames are precisely manufactured to improve speed oferection. The care ta#en in design and detailing of the connections ensures thaterection 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, is usuallyresponsible for the design, production and erection of the frame. 3inimi!ing thenumber of subcontractors simplies contract programming and can reducepressures on the management team. 1ontract periods may also be forecast morecondently because, with fewer operations, there is less to go wrong.

Structural eciencyPrecast concrete o/ers considerable scope for improving structural e$ciency.

4onger spans and shallower construction depths can be obtained by usingprestressed concrete for beams and -oors. The examples of buildings given later inthis ection and elsewhere in this publication, demonstrate the considerable-exibility that is possible in design. 3any of the available precast concrete unitshave been tested, both in the laboratory and in service, to obtain maximumstructural e$ciency.%ire resistance

+ & P a g e


3/30

r

e

p

a

r

a

t i

o

no

f

3

a

t

e

r i

a l

s

3

o

d

u

l

a

r

1

o

or

d

i

n

a

t i

o

n

t

a

n

d

a

r

d

i

!a

t

i

o

n

P

r

o

d

u

c

t

i

o

ns

y

s

t

e

m

T

r

a

n

s

p

o

r

t

at

i

o

n

5

r

e

c

t

i

o

n

1oncrete has its own built)in re resistance. This is present during allconstruction phases and does not depend on additional board or sprayed protection.6btaining a two)hour re resistance presents little problem, and four hours may alsobe achieved.

1.2 Principles 3ost projects benet from disciplined design. This is inherent in the design ofa precast concrete frame, because the designer see#s as much repetition aspossible so that the precaster can ta#e advantage of greater mould re)use andstandardi!ation of details to reduce the cost of manufacture

1.3 Materials

(ll materials shall conform to Part 7 82uilding 3aterials9.:se of materials forplain and reinforced concrete shall satisfy the requirements of ' 7;.

< & P a g e


4/30

0hile selecting the materials for prefabrication, the following characteristics shallbe considered=

%. 5asy availability>+. 4ight weight for easy handling and transport>. 5asy wor#ability>

7. "urability>;. ?on)combustibility>@. ound insulation>A. 5conomy>

"ementsThe most common cements used are ordinary and rapid)hardening Portland

Bincluding whiteC, sulphate)resisting and high alumina cement. :niversally thechemistries, performance, colours, etc., vary over quite a large range, but they all

have to comply with a tandard such as 2, demanding minimum requirements.

)ggregateThese fall into two main types, each with several sub)groups=

?atural (ggregates=Flint,Dolcanic Bgranites, basalts, feldspars, etc.Candstone4imestone Bsedimentary, oolitic, etc.C3arble BcalciteC2arytes?atural sands Bsiliceous mainly, river, dune, wadi, marineC

PerliteDermiculite

ynthetic (ggregates=intered pulverised fuel ash5xpanded shale5xpanded slate5xpanded clayFoamed slag1rushed bric#s1alcined -intEeconstituted concrete

The selection of aggregates for any particular precast concrete operation is afunction of many factors of which economic availability and performancerequirements are probably the following reactions are most important.

Alkali-silica reaction

& P a g e


5/30

(ggregates containing silica minerals are susceptible to attac# by al#alis

B?a+6 and +6C from the cement or other sources. (l#ali)silica reaction causescrac#ing and reduces the strength of concrete.

5/ective means of reducing the ris# of al#ali aggregate reaction include=%. 1ontrol on the amount of cement used in the concrete mix>+. :se of a low al#ali cement>

andAlkali-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 tobe used, specialist attention should be ta#en.

Chlorides in concreteEeinforcing 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 +.


6/30

mould ensuring !ero segregation honey) combing and minimal vibration. :singmaterials that have passed strict quality control procedures, rapid hardeningcement is mixed with excellent quality aggregates of #nown source and purity, oftenin computer controlled batching and mixing plant> to produce concrete of peciedwor#ability and strength. 5ven the introduction of small quantities ofuncontaminated recycled concrete, usually from the factory*s own waste production,

super plastici!ers and po!!olana materials Bsuch as pulveri!ed fuel ashC has notreduced 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 bereinforced, although this amount 4s rarely used in favor of higher concretestrengths.

High tensioned hot rolled ribbed bar is used I7 of cases, even in shear lin#swhere mild steel bars to be used. The small cost di/erence compared to theadditional trength, i.e. ;G Ds +7G?JmmK, and the need lot consistency of habitwhen assembling cages, it ma#e more economical Tying wire is more secure aroundribbed bar ma#ing the cage more robust. 3ild steel is often used lot projectingloops, etc., because it is easier to hand ) bend tin site. bar diameters commonlyused are A and %G mm for common tirrups, %G and %+mm for beam stirrups andother distribution 6r and crac# bars, and %;, +G, +7,


7/30

to give an adequate wor#ability, and compaction by vibration needs to be moreenergetic than for a conventional mix.

Polypropylene and the other plastics bres being developed have the mostpromise, and give good impact resistance to products such as pipes, pontoons, etc.,and stabilised high air content systems in architectural products such as the

thixotropic Faircrete.

Llass bres have had a chequered history since their use was ta#en up in thelate sixties. From the thermodynamic point of view, the di/erent heats of formationof calcium and sodium silicate indicate that the lifetime in a weathering situationwill be limited. (n optimistic picture can only be painted for !irconium glass bre incement matrices.

Structural steel and boltstructural .steelwor# .sections are used in many type of precast elements,

especially at the connections. These include rolled rectangular and square hollowsections BEH, HC, solid billets, channels and angles, plates and welded)tees, etc."etails of how these ate used in practice are given in ections A. and I.+.tructural sections such as :niversal beams and columns B:2, :M may be cast intoprecast elements to enhance strength where the reinforced concrete capacity isexhausted. However, this may have severe cost implications that must be carefullyexamined.

Eolled steel sections and bent or -at steel plates ate welded to form steelconnectors in many highly stressed support situations where direct contact betweenconcrete surfaces 4s to be avoided. The steel used is grade < BmostlyC or grade 7G.0elded electrodes ate mostly grade 5 and

@ & P a g e


8/30

B+C hardeners. They are occasionally used as pressure= injections for crac# lling orto 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 Precast 1ladding+ gives. 5xtensive guidance on theuse 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#oulds3oulds are basically means by which=

%. 1oncrete is #ept to a required shape until it is strong enough to bedemoulded, or

+. 1oncrete is moulded on a machine and retains that shape on virtually instantdemoulding, or


9/30

0here, ( is the target dimension often called the wor# si!e.

Two important points need to be borne in mind=%. Tolerance is an easy thing to nd during construction but is a very di$cult

thing to lose. 2y this is meant that a product that is too large will generallycause more problems than a product that is too small, i.e. a joint can be lled

with mortar, sealant, etc., when the product is nearer ()y but needs cuttingbac# when there is too much (x.

+. 3oulds tend to grow in si!e with continuous usage.

Figure shows how a joint can be designed to cater for resistance to arries damageand 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 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. The symbol for the basic module is !.R

!odular Co-ordination"imensional co)ordination employing the basic module or a multi)module.

The purposes of modular co)ordination are=aC To reduce the variety of component si!es produced, andbC To allow the building designer greater -exibility in the arrangement of

components.

!odular +rid( rectangular coordinate reference system in which the distance betweenconsecutive lines is the basic module or a multi)module. This multi)modulemay di/er for each of the three orthogonal dimensions of the grid, two in planand one in vertical direction.

!ulti-module( module whose si!e is a selected multiple of the basic module.

I & P a g e


10/30

1.4.1 Modular Co-Ordination, Architectural Treatent and!inishes

#odular "o/ordinationThe basic module shall be adopted. (fter adopting this, further wor# isnecessary to outline suitable range of multi modules with greater increments,often referred to as preferred increments. ( set of rules as detailed belowwould be adequate for meeting the requirements of conventional andprefabricated construction.

These rules relate to the following basic elements=a. The planning grid in both directions of the hori!ontal plan shall be=

%C For industrial buildings%73 for span up to %+m

f. atisfactory nishing of surfaces> and

%G & P a g e


11/30

g. :se of light weight materials to e/ect economy in the structuralsystem.

ome of the acceptable methods of nishes integral with the pre casting are,a. 1oncrete surface moulded to designJshape>b. 4aid)on nishing tiles xed during casting>

c. Finishes obtained by washing, tooling>d. Lrinding, grooving of hardened concrete>e. 5xposed aggregates> andf. 6ther integral nishes.

Concept Of Dimensional Co ordination

The whole concept of dimensional coordination is represented in thefollowing chart which indicates the application of system and nal result

%% & P a g e


12/30

1.$ %tandari&ation '

.. tandardisationPrecast concrete construction should be planned wherever possible to utili!estandardi!ed precast concrete elements.

3ost prefabricated buildings will be unique and site specic. (t thetheoretical design stage, a basic layout plan should be developed which

%+ & P a g e


13/30

achieves a balance between architecturalJaesthetic requirements and a highdegree of standardi!ation. Therefore, close collaboration amongst di/erentdesign parties is essential during conceptual design to achieve the optimumstandardi!ation

)dvantages of 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 fromthe 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 the extent possible the largest si!e to be used which results in less

number of joints The si!e and the number of the prefabricates is limited by the weight in

overall dimension that can be handled by the transportation.

Hence it is preferable to have all the prefabricates approximately of sameweight very near to the lifting capacity of the equipment

1.( %)ste"s

Prefabrication Systems )nd Structural SchemesThe word 8system9 is referred to a particular method of construction ofbuildings using the prefabricated components which are inter)related infunctions and are produced to a set of instructions. 0ith certain constraints,several plans are possible, usingthe same set of components. The degree of -exibility varies from system tosystem. However, in all the systems there is a certain order and discipline.

The following aspects, among others, are to be considered in devising a 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


14/30

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-nitionThe system of prefabricated construction depends on the extent of the use ofprefabricated components, their materials, si!es and the technique adoptedfor theirmanufacture and use in building.

0.4.2 'ypes of Prefabrication "omponentsThe prefabricated concrete components such as those given below may beused which shall be in accordance with Part 7 82uilding 3aterials9 and theaccepted 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 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 theapproval 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


15/30

There are two categories of open prefab system depending on the extent ofprefabrication used in the construction as given as follows

0. Partial prefabrication systemThis system basically uses precast rooting and -ooring components and otherminor elements li#e lintels, 1H((, #itchen sills in conventional buildingconstruction. The structural system could be in the form of in)situ framewor#

or load bearing walls.2. %ull prefabrication system

'n this system almost all the structural components are prefabricated. Theller walls may be of bri c#Jbloc# masonry or of any other locally availablematerial.

6. Large Panel Prefabrication SystemThis system is based on the use of large prefab components. The componentsused are precast concrete large panels for walls, -oors, roofs, balconies,staircases, etc. The casting of the components could be at the site or o/ thesite. "epending upon the extent of prefabrication, this system can also lenditself to partial prefab system and

full prefab system.

0.4.6.0 Precast *allstructural 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 beclassied as=

aC 4oad bearing walls,bC ?on)load bearing walls, andcC hear walls.

2ased on construction, the precast walls may be classied as=aC Homogeneous walls N which could be solid, hollow or ribbed> andbC ?on)homogeneous walls N these could be composite or sandwich

panels.

2ased on their locations and functional requirements the precast walls mayalso classied as=

aC 5xternal walls, which may be load bearing orbC ?on)load bearing depending upon the lay)out>

These are usually non)homogeneous walls of sandwiched type to impartbetter thermal1omforts> and internal walls providing resistance against vertical loads,hori!ontal loads, re, etc> these are normally homogeneous walls.

0.4.6.2 Precast 3oors

%7 & P a g e


16/30

"epending upon the composition of units, precast -ooring units may behomogeneous or non homogeneous.

aC Homogeneous -oors may be solid slabs, cored slabs, ribbed orwaSe slabs.bC ?on)homogeneous -oors may be multilayered ones withcombinations of light

weight concrete or reinforcedJprestressed concrete, with ller bloc#s."epending upon the way the loads are transferred, the precast -oors may beclassied as one way or two way systems=aC 6ne way system transfers loads to supporting members in one direction

only. The precast elements which come under this category are channelslabs, hollow core slabs, channels and ties system, light weight cellularconcrete slabs, etc.

bC Two way systems transfer loads in both the directions imparting loads onthe four edges. The precast elements under this category are room si!edpanels, two way ribbed or waSe slab systems, etc.

0.4.6.6 Staircase systemstaircase system may consist of single -ights with inbuilt risers and treads inthe element. The -ights are normally unidirectional transferring the loads tosupporting 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. Toiletand #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 areformed by four ad/acent walls. 0alls are /ointed to make rigid connections amongthemsel'es. The bo) unit rests on foundation which may be of con'entional

type or precast type.

1.* Prod!ctionTransportation and Erection

%; & P a g e


17/30

0.9.0 #anufacture of Precast "oncrete :lements

%@ & P a g e


18/30

( judicious location of pre casting yard with concreting, initial curing Brequiredfor demouldingC, storage facilities, suitable transporting and erectionequipments and availability of raw materials are the crucial factors whichshould be carefully planned and provided for e/ective and economic use ofprecast concrete components in constructions.

#anufactureThe manufacture of the components can be done in a factory for the commercialproduction established at the focal point based on the mar#et potential or in a sitepre casting yard set up at or near the site of wor#.

0.9.0.0%actory prefabricationFactory prefabrication is resorted to in a factory for the commercialproduction for the manufacture of standardi!ed components on a long)termbasis. 't is a capital intensive production where wor# is done throughout theyear preferably under a closed shed to

avoid e/ects of seasonal variations. High level of mechani!ation can alwaysbe 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 prefabricationPrefabricated components produced at site or near the site of wor# aspossible.This system is 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

The wor# is normally carried out in open space with locally available labourforce. The equipment machinery used may be minor in nature and mouldsare of mobile or stationary in nature.

0.9.0.7 %ully/mechanizedThe wor# will be carried out under shed with s#illed labor. The equipmentsused will be similar to one of factory production. This type of precast yardswill be set up for the production of precast components of high quality, highrate of production. Though there is denite economy with respect to cost oftransportation, this system su/ers from basic drawbac# of its non)suitabilityto any high degree of mechani!ation and no elaborate arrangements forquality 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 elementsThe various processes involved in the manufacture of precast elements maybe classied as follows=

0.9.2.0 #ain process

%A & P a g e


19/30

Providing and assembling the moulds, placing reinforcement cage inposition for reinforced concrete wor#, and stressing the wires in thecase 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 processProcess necessary for the successful completion of the processes covered bythe main process=

3ixing and manufacture of fresh concrete Bdone in a mixing station orby a batching plantC>

Prefabrication of reinforcement cage Bdone in a steel yard orwor#shopC>

3anufacture of inserts and other nishing items to be incorporated in

the main precast products> Finishing the precast products> and Testing of products.

0.9.2.6 Subsidiary process(ll other wor# involved in #eeping the main production wor# to a cyclicwor#ing=

torage of materials> Transport of cement and aggregates> Transport of green concrete and reinforcement cages> Transport and stac#ing the precast 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 ina systematic way to achieve the following=

aC ( cyclic technological method of wor#ing to bring in speed and economy inmanufacture>

bC 3echani!ation of the process to increase productivity and to improve quality>cC The optimum production satisfying the quality control requirements and to

#eep up the expected speed of construction aimed>dC 2etter wor#ing conditions for the people on the job> andeC To minimi!e the e/ect of weather on the manufacturing schedule.

The various stages of precasting can be classied as in Table %.< on the basis of theequipments required for the various stages.

This permits mechani!ation and rationali!ation of wor# in the various stages. 'n theprecasting, stages ; and @ given in Table %.< form the main process in themanufacture of precast concrete elements. For these precasting stages there aremany Technological processes to suit the concrete product under considerationwhich have been proved rational, economical and time saving.

%I & P a g e


20/30

The technological line or process is the theoretical solution for the method ofplanning the wor# involved by using machine complexes. Figure 7 illustratesdiagrammatically the various stages involved in a plant process.

+G & P a g e


21/30

+% & P a g e


22/30

#ethods of precastingThe various accepted methods of manufacture of precast units can be

broadly classied into two methods= 'he Stand #ethod


23/30

From experience in construction, it is clear that there will be very high percentagesof 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 onquality, proper storage and preservation of building materials, especially cement,coarse and ne aggregates, is of prime importance#oulds3oulds for the manufacture of precast elements may be of steel, timber, concreteand plastic or a combination thereof. For the design of moulds for the variouselements, special importance should be given to easy demoulding and assembly ofthe various parts. (t the same time rigidity, strength and water tightness of themould, ta#ing into consideration forces due to pouring of green concrete andvibrating, are also important.'olerancesThe moulds have to be designed in such a way to ta#e into consideration thetolerances 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 apossibility of the elements getting stuc# up with the mould at the timeof demoulding.I. (ccelerated Hardening'n most of the precasting factories, it is economical to use faster curing methods orarticial curing methods, which in turn will allow the elements to be demouldedmuch earlier permitting early re)use of the forms. (ny of the following methods maybe adopted=

aC 2y Heating the (ggregates and 0ater 2efore 3ixing the 1oncrete N 2y heatingof theaggregates as well as water to about @GU1 to AGU1 before ma#ing the concrete mix

andplacing the same in the moulds, su$ciently high earlier strengths are developed toallowthe elements to be stripped and transported.

bC team 1uring N team curing maybe done under high pressure and hightemperature inan autoclave. This technique is more suited to smaller elements. (lternatively, thiscouldbe done using low pressure steam having temperature around AGU1. This type ofcuringshall be done as specied in I.7.+. For light weight concrete products when steam

curedunder high pressure, the drying shrin#age is reduced considerably, "ue to thisreason, highpressure steam curing in autoclave is specied for light weight low densities rangingfrom


24/30

pressure saturated steam is injected into the mixer while the aggregates are beingmixed. This enables the heating up of concrete to approximately ;GU1. uch aconcrete after being placed in the moulds attains high early strength. Heated (ir3ethod N 'n this method, the concrete elements are #ept in contact with hot airwith a relative humidity not less than AG percent. Tlis method is specially useful forlight weight concrete products using porous coarse aggregates.

Hot 0ater 3ethod N 'n this method, the concrete elements are #ept in a bath of hotwater around 7GV1 to AGV1. The general principles of this type of curing are notmuchdi/erent from steam curing.5lectrical 3ethod N The passage of current through the concrete panels generatesheatthrough its electro)resistivity and accelerates curing. 'n this method, the concrete isheatedup by an alternating current ranging from 7G volts for a plastic concrete andgraduallyincreasing to +


25/30

I.7.% The curing of the prefabricated elements can be e/ected by the normalmethods of curing by sprin#ling water and #eeping the elements moist. This canalso be done in the case of smaller elements by immersing them in a speciallymade water tan#s.I.7.+ team 1uringI.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 steamshall have a uniform quality throughout the length of the member. The precastelements shall be so stac#ed, with su$cient clearance between each other and thebounding enclosure, so as to allow propercirculation of steam. 2efore the concrete products are subjected to any acceleratedmethod of curing, the cement to be used shall be tested in accordance withaccepted standardsBsee Part 7 2uilding 83aterialsC especially for soundness, setting time and suitabilityfor steam curing. 'n the case of elements manufactured by accelerated curingmethods, concrete admixtures to reduce the water content can be allowed to beused. The normal aeration agents used to increase the wor#ability of concreteshould 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 chamberor 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 adamp)proof layer to maintain the humidity of steam. 3oreover, proper slope shallbe given to the -oor and the roof to allow the condensed water to be easily drainedaway. (t rst, when steam is let into the curing chambers, the air inside shall beallowed to go out through openings provided in the hoods or side walls which shallbe 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 perforatedpipelines 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 setbefore allowing the concrete to come into contact with steam. The regular heatingup of fresh concrete product from about +GU1 to


26/30

The third stage of steam curing is to maintain the uniform temperature andpressure 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 ofhigh pressure steamcuring.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 tobring it at par with outside air. The maximum cooling rate, which is dependent onthe thic#ness of the member, should normally not exceed


27/30

location and orientation mar#s as and where necessary.The date of manufacture shall also be mar#ed on theunits.I.A.% The identication mar#ings on the drawingsshall be the same as that indicated in the manufacturer9sliterature and shall be shown in a table on the setting

schedule together with the length, type, si!e of the unitand the si!es and arrangement of all reinforcement.I.I TransportTransport of precast elements inside the factory and tothe site of erection is of considerable importance notonly from the point of view of economy but also fromthe point of view of design and e$cient management.Transport of precast elements must be carried out withextreme care to avoid any jer# and distress in elementsand handled as far as possible in the same orientationas it is to be placed in nal position.I.I.% Transport 'nside the Factory

Transport of precast elements moulded inside thefactory depends on the method of production, selectedfor the manufacture as given in Table

+@ & P a g e


28/30

Tying up of erection ropes connecting to theerection hoo#s>1leaning of the elements and the site oferection>1leaning of the steel inserts beforeincorporation in the joints, lifting up of the

elements, setting them down into the correctenvisaged position>(djustment to get the stipulated level, line andplumb>0elding of cleats>1hanging of the erection tac#les>Putting up and removing of the necessarysca/olding or supports>0elding of the inserts, laying of reinforcementsin joints and grouting the joints> andFinishing 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# invarious construction jobs using prefabricated concreteelements di/ers from place to place depending on thesite conditions, safety precautions in the wor# are ofutmost importance. Hence only those s#illed foremen,trained wor#ers and tters who have been properlyinstructed about the safety precautions to be ta#en shouldbe employed on the job. For additional information, seePart @ 81onstructional Practices and afety9.I.%G.+ Transport of people, wor#ers or visitors, byusing cranes and hoists should be strictly prohibitedon an erection site.I.%G.< 'n the case of tower rail mounted cranes runningon rails, the trac# shall not have a slope more thanG.+ percent in the longitudinal direction. 'n the transversedirection the rails shall lie in a hori!ontal plane.I.%G. The trac# of the crane should be daily chec#edto see that all sh plates and bolts connecting them tothe sleepers are in place and in good condition.I.%G.7 The operation of all equipment used forhandling and erection shall follow the operationsmanual provided by the manufacturer. (ll safetyprecautions shall be ta#en in the operations of handlingand erection.%G 5M:'P35?T%G.% LeneralThe equipment used in the precast concrete industryJ?(T'6?(4 2:'4"'?L 16"5 6F '?"'(construction may be classied into the followingcategories=aCbC

+A & P a g e


29/30

cCdCeCfCXhC

jC#C3achinery required for quarrying of coarseand ne aggregates>1onveying equipment, such as, belt conveyors,chain conveyors, screw conveyors, buc#etelevators, hoists, etc>1oncrete mixing machines>1oncrete vibrating machines>5rection equipment, such as, cranes, derric#s,hoists, chain pulley bloc#s, etc>Transport machinery, such as, tractor)cumtrailers,

dumpers, lorries, locomotives, motorboats and rarely even helicopters>0or#shop machinery for ma#ing and repairingsteel and timber moulds>2ar straightening, bending and weldingmachines to ma#e reinforcement cages>3inor tools and tac#les, such as, wheelbarrows, concrete buc#ets, etc> andteam generation plant for accelerated curing.'n addition to the above, pumps and soil compactingmachinery are required at the building site for theexecution of civil engineering projects involvingprefabricated components.5ach of the above groups may further be classiedinto various categories of machines and further tovarious other types depending on the source of powerand capacity.%G.+ 3echani!ation of the 1onstruction and5rection ProcessesThe various processes can be mechani!ed as in anyother industry for attaining the advantages of massproduction of identical elements which in turn willincrease productivity and reduce the cost of productionin the long run, at the same time guaranteeing qualityfor the end)product. 6n the basis of the degree ofmechani!ation used, the various precasting factoriescan be divided into three categories=aC 0ith simple mechani!ation,bC 0ith partial mechani!ation, andcC 0ith complex mechani!ation leading toautomation.%G.+.% 'n simple mechani!ation, simple mechanically

+I & P a g e


30/30

operated implements are used to reduce the manuallabour and increase the speed.%G.+.+ 'n partird mechani!ation, the manual wor# ismore or less eliminated in the part of a process. Forexample, the batching plant for mixing concrete, hoiststo 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 leadingto automation, a number of processes leading to theend)product are all mechani!ed to a large extentBwithout or with a little manual or human elementinvolvedC. This type of mechani!ation reduces manualwor# to the absolute minimum and guarantee the massproduction at a very fast rate and minimum cost.%G.+. The equipment shall conform to acceptedstandards as listed in Part @ 81onstructional Practicesand afety9.

Prefabrication Unit i Introduction

Documents

Transcript of Prefabrication Unit i Introduction