Prefabricated (Precast) Construction

Prefabricated (Precast) Construction

Prefabricated building is the completely assembled and erected building, of which is the structural parts consist of prefabricated individual units or assemblies using ordinary or controlled materials.

Prefabricated construction is a new technique and is desirable for large scale housing programmes, yet this has to take a firm hold in the country. This type of construction serves the followings aims:

(i)      Prefabrication is used to effect economy in cost.

(ii)    This result in improvement in quality because components can be manufactured under controlled conditions.

(iii)   The speed of construction is increased since no curing period is necessary.

(iv)  Prefabrication helps in the use of locally available materials with required characteristics like light-weight; easy workability, thermal insulation, non-combustibility, etc. effect economy and improve quality.

MATERIALS OF CONSTRUCTION

                While choosing the materials for prefabrication, the following special characteristics are to be considered:

(a)   Easy availability;

(b)   Light weight for easy handling and transport, and to economise on sections and sizes of foundations  ;

(c)    Thermal insulation property ;

(d)   Easy workability ;

(e)    Durability in all weather conditions ;

(f)     Non-combustibility ;

(g)   Economy in cost, and

(h)   Sound insulation.

The material used on prefabricated components can b various and the modern trend is to use concrete, steel, treated wood, aluminum, cellular concrete, light weight concrete elements, ceramic products, etc. However, this chapter pertains to prefabricated concrete elements.

MODULAR CO-ORDINATION, ARCHITECTURAL TREATMENT AND FINISHES

Modular Co-ordination. The basic module is to be adopted. Basic module is the fundamental module used in the modular co-ordination, the size of which is selected for the general applications to buildings and its components. The value of the basic module chosen is 100 mm for maximum flexibility and convenience. The symbol used for basic module is M. After adopting this, further work is a necessary to outline suitable range of multi module with greater increments, often referred to as preferred increments. A set of rules as detailed below are adequate for meeting the requirements of conventional and prefabricated construction. These rules relate to the following basic elements:

(a)    The planning grid in both directions of the horizontal plan shall be:

(1)    3 M for residential and institutional buildings ;

(2)    For industrial buildings;

15 M for spans up to 12 m,

30 M for spans between 12 m and 18 m and 60 M for spans over 18 m.

                The centre lines of load bearing walls shall coincide with the grid lines;

(b)   In case of external walls, the grid lines shall coincide with centre line of the wall or a line on the wall 5 cm from the internal face;

(c)    The planning module in the vertical direction shall be 1 M  up to and including a height of 2.8 M ; above the height of 2.8 m it shall be 2 M;

(d)   Preferred increments for soil heights, doors, windows etc. shall be 1 M ; and

(e)   In the case of internal columns, the grid lines coincide with the centre line of the columns. In case of external columns and columns near the lift stair and stair walls, the grid lines shall coincide with centre lines of the column in the topmost storey or a line in the column 5 cm from the internal face of the column in the topmost storey.

Architectural treatment and finishes

                While deciding the type of architectural treatment and finishes for prefabricated buildings, the following the points should be kept in view;

(a)    Suitability for mass production techniques ;

(b)   Recognition of the constraints imposed by the level of workmanship available;

(c)    Possibility of using different types of finishes;

(d)   The use of finishes and architectural treatment for the creation of a particular architectural character in the individual buildings and in groups of buildings by the use of colour, texture, projections and recesses on surfaces , etc;

(e)   The incorporation of structural elements like joists, columns, beams, etc; as architectural features and the treatment of these for better overall performance and appearance;

(f)     Simultaneous design of structural sub-system and finishes;

(g)    Satisfactory finishes of surfaces; and

(h)   The use of light weight materials to effect economy in the structural system.

Some of the normally acceptable methods of finishes are:

(a)    Moulded concrete surface to design,

(b)   Laid-on finishes tiles fixed during casting,

(c)    Finishes obtained by washing, tooling,  grinding, grooving of hardened concrete,

(d)   Exposed aggregates in-situ, and

(e)   Finishes added in-situ.

COMPONENTS OF PREFAB CONSTRUCTION

The preferred dimensions of precast elements are as follows:

(a)   Flooring and Roofing Scheme

Precast slabs or other precast structural flooring units:

(1)    Length          Nominal length shall be in multiples of 3 M;

(2)    Width            Nominal width shall be in multiples of 1 M;

and

(3)    Overall Thickness. Overall thickness (that is, the thickness of structural flooring units plus in-situ concrete decking) shall be in multiples of M/4.

(b)   Beams

(1)    Length         Nominal length shall be in multiples of 3 M;

(2)    Width           Nominal width shall be in multiples of M/4;

and

(3)    Overall depth. Overall depth of the floor zone (that is, from the soffit of the beam to the top of in-situ decking) shall be in multiples of M/4.

(c)    Columns

(1)    Height.        Overall height (that is, floor to floor or the clear height) shall be in multiples of 1 M for heights up to 2.8 M; and

(2)    Lateral dimensions.                  Overall lateral dimensions or diameter of column shall be in multiple of M/4.

(d)   Walls

Thickness.          The nominal thickness of the walls shall be multiples of M/4.

(e)    Staircase

Width.                 Nominal width shall be in multiples of 1 M.

(f)     Lintels

(1)    Length.        Nominal Length shall be in multiples of 1 M;

(2)    Width.         Nominal width shall be in multiples of M/4;

and

(3)    Depth.         Nominal depth shall be in multiples of M/4;

(g)   Sunshade/Chajja Projections

(1)    Length.        Nominal depth shall be in multiples of 1 M.

(2)    Projection.                Nominal length shall be multiples of 1 m.

PREFABRICATION SYSTEMS

                The word ‘System’ is referred to a particular method of construction of buildings’ using prefabricated components which are inter-related in functions and are produced to a set of instructions. With certain constraints, several plans are possible, using the same set of components. The degree of flexibility varies from system to system. However, in all the system there is a certain order and discipline.

                The following characteristics, among others, are to be considered in devising a system:

(a)   Intensified usage of spaces;

(b)   Straight and simple walling scheme;

(c)    Limited sizes and number of components;

(d)   Limited openings in bearing walls;

(e)    Regulated locations of partitions;

(f)     Standardized service and stair units;

(g)   Limited sizes of doors and windows with regulated positions;

(h)   Structural clarity and efficiency;

(i)      Suitability for adoption in low rise and high rise blocks.

(j)     Ease of manufacturing, storing and transporting;

(k)    Speed and ease of erection; and

(l)      Simple jointing system.

Prefabrication System

                The system of prefabricated construction depends on the extent of the use of prefab components, their materials sizes and the technique adopted for their manufacture and use in building. The various prefabrication systems are outlined below:

(1)    Open prefab System

This system is based on the use of the basic structural elements to form whole or part of a

building. The standard prefab concrete components which can be used are:

(a)   Reinforced concrete channel units,

(b)   Hollow core slabs,

(c)    Hollow blocks and battens,

(d)   Precast planks and battens,

(e)    Precast joists and tiles,

(f)     Cellular concrete slabs,

(g)   Prestressed/reinforced concrete slabs,

(h)   Reinforced/prestressed concrete beams,

(i)       Reinforced/prestressed concrete columns,

(j)     Precast lintels and chajjas,

(k)    Reinforced concrete waffle slabs/shells,

(l)      Room size reinforced/prestressed concrete panels,

(m) Reinforced/prestressed concrete walling elements, and

(n)   Reinforced/prestressed concrete trusses

The elements may b cast at the site or off the site.

Foundation for the columns could be of prefabricated type or of the conventional cast in-situ type depending upon the soil conditions and load; and the columns may have hinged or fixed base connections depending upon the type of components used and the method of the design adopted.

There are two categories of open prefab systems depending on the extent of prefabrication used in the construction as given below:

(a)   Partial prefab open system.                This system basically emphasizes the use of precast roofing and

flooring components and other minor elements like lintels, chajjas, kitchen sills in conventional building construction. The structural system could be in the form of in-situ framework or load bearing walls.

(b)   Full prefab open system.  In this system almost all the structural components are

Prefabricated. The filler walls may be of bricks or any other local material.

(2)    Large Panel Prefab System

This system is based on the use of large prefab components. The components used are

Precast concrete large panels for walls, floors, roofs, balconies, staircase, etc. The casting of the components could be at the site or off the site.

                Depending upon the extent of fabrication, this system can also lend itself to partial prefab system and full prefab system.

Wall Systems

                Structural scheme with precast large panel walls can be classified as:

(a)   Cross Wall System.  In this scheme, the cross wall are load bearing walls whereas the facade

Walls are non-load bearing this system is suitable for high rise buildings.

(b)   Longitudinal Wall System.    In this scheme the cross walls are non-load bearing whereas

Longitudinal walls are load bearing walls. This system is suitable for low rise buildings.

                A combination of the above system with all load bearing walls can also be adopted.

                Precast walls could be:

(a)   Homogeneous Walls                      which could be solid hollow or ribbed ; and

(b)   Non-Homogeneous Walls            these could be composite or sandwich panels.

Based on the structural functions of the walls, the walls could be classified as:

(a)   Load bearing walls,

(b)   Non-load bearing walls,

(c)    Shear walls,

Based on their locations and functional requirements the walls are also classified as:

(a)   External walls,           which can b load bearing or non-load bearing depending upon the lay-out and are usually non-homogeneous walls of sandwich type to impart better thermal comforts; and

(b)   Internal walls providing resistance against vertical loads, horizontal loads, fire, etc. and are normally homogeneous walls.

Types of Precast Floors

                Depending upon the composition of units, precast flooring units could be homogeneous or non homogeneous.

(a)   Homogeneous floors could be solid slabs, cored slabs, ribbed or waffle slabs.

(b)   Non-homogeneous floors could be multi-layered ones with combinations of light weight    

Concrete or reinforced/prestressed concrete, with filler blocks.

                Depending upon the way the loads are transferred, the precast floors could be classified as one way or two way systems.

                One way system transfers loads to the supporting members in one direction only. The precast elements which come under this category are: channel slabs, hollow core slabs, hollow blocks and battens, battens plank system, channels and tiles system, light weight cellular concrete slabs, etc.

                Two way systems transfers load in both directions imparting loads on the four edges. The precast elements under this category are room sized panels, two way ribbed or waffle slab systems, etc.

Staircase Systems

                Staircase system could consist of single flights with in-built risers and treads in the element only. The flights are normally unidirectional transferring the loads to supporting landing slabs or load bearing walls.

Box Type Construction

                In this system, room size units are prefabricated and erected at site. Toilets and kitchen blocks also be similarly prefabricated and erected at site.

                This system derives its stability and stiffness from the box unit which are formed by the four adjacent walls. Walls are jointed to make rigid connections among themselves. The box unit rests on the plinth foundation which may be of conventional type or precast type.

JOINTS

                The joints should be provided in the light of their assessment with respect to the following considerations:

(a)   Feasibility.   The feasibility of joint shall be determined by its loads-carrying capacity  in the

particular situation in which the joints is to function.

(b)   Practicability.             Practicability of joint shall be determined by the amount and type of

Material, fabrication and erection and the time for fabrication and erection.

(c)    Serviceability.            Serviceability shall be determined by the joints/expected behavior to

Repeated or possible overloading and exposure to climatic or chemical conditions.

(d)   Fire-Proofing

(e)    Appearance

The following are the requirements of an ideal structural joint.

(a)   It shall be capable of bearing designed to transfer the imposed load and moments with a known margin of safety;

(b)   It shall occur at logical locations in the structure and at points which may b most readily analysed and easily reinforced;

(c)    It shall accept the loads without marked displacement or rotation and avoid high local stresses;

(d)   It shall accommodate tolerances in elements;

(e)    It shall require little temporary support, permit adjustment and demand only a few distinct operation to make;

(f)     It shall permit effective inspection and rectification;

(g)   It shall be reliable in service with other parts of the buildings ; and

(h)   It shall enable the structure to absorb sufficient energy during earthquake so as to avoid sudden failure of the structure.

Precast structure may have continuous or hinged connections subject to providing sufficient rigidity to withstand horizontal joints may be adopted. In case of prefabricated concrete elements, load it transmitted via the concrete. When both compressive force and bending movements are to be taken, rigid or welded joints may be adopted; the shearing force is usually small in the column and can be taken up by the frictional resistance of the joint. Here load transmission is accomplished by steal inserted parts together with concrete.

When considering thermal shrinkage and heat effects, provision of freedom of movement or introduction of restraint may be considered.

Joint techniques/materials normally employed are:

(a)   Welding if cleats or projecting steel.

(b)   Overlapping reinforcement, loops and linking steel grouted by concrete.

(c)    Reinforced concrete ties all round a slab.

(d)   Prestressing.

(e)    Epoxy grouting.

(f)     Bolts and nuts connection, and

(g)   A combination of the above.

MANUFACTURE OF PRECAST CONCRETE ELEMENTS

                A judicious location of precasting yard with storage facilities, suitable transporting and erection equipments and availability of raw materials are to crucial factors which should be carefully planned and provided for effective and economic use of precast concrete components in construction.

                The manufacture of the components can be done in a centrally located factory or in a site precasting yard set up at or near the site of work.

Factory Prefabrication

                Factory prefabrication is resorted to in a centrally located plant for manufacture of standardized components on a long term basis. It is a capital intensive production where work is done throughout the year preferably under a closed shed to avoid effects of seasonal variations. High level of mechanization can always be introduced in this system where the work can be organized in a factory-like manner with the help of a constant team of workmen.

                The basic disadvantage in factory prefabrication is the extra cost incidence of transportation of elements from plant to site of work where sometimes even the shape and size of prefabricates get limited due to lack of suitable transportation equipment, road contours, etc. The organized labour of permanent nature with regular benefits leads to huge establishment cost which add to ultimate cost of production.

Site Prefabrication

                In this scheme, the components are manufactured at site or as near the site of work as possible.

                This system is normally adopted for a specific job order for a short period. The work is normally carried out in open space with locally available labour force. The equipment machinery and moulds are of mobile nature.

                Though there is a definite economy with respect to cost of transportation, this system suffers from basic drawback of its non-suitability to any high degree of mechanization and no elaborates arrangements for quality control. Normal benefits of continuity of work is not available in this system of construction.

Processes of manufacture

                The various processes involved in the manufacture of precast elements may be classified as follows.

Main Process

(a)   Providing and assembling the moulds, placing reinforcement cage in position for reinforced concrete work, and stressing the wires in the case of prestressed elements;

(b)   Fixing of inserts and tubes, where necessary;

(c)    Pouring the concrete into the moulds;

(d)   Vibrating the concrete and finishing;

(e)    Demoulding the forms and stacking the precast products;

(f)     Curing (steam curing, if necessary).

Auxiliary process

                Process necessary for the successful completion of the processes covered by the main process:

(a)   Mixing and manufacture of fresh concrete (done in a mixing station or by a batching plant);

(b)   Prefabrication of reinforcement cage (done in a steel yard or workshop);

(c)    Manufacture of insets and other finishes items to be incorporated in the main precast products;

(d)   Finishes the precast products ; and

(e)    Testing of products.

Subsidiary process

                All other work involved in keeping the main production work to a cyclic working:

(a)   Storage of materials;

(b)   Transport of cement and aggregate;

(c)    Transport of concrete green concrete and reinforcement cages;

(d)   Transport and stacking the precast elements;

(e)    Repairs and maintenance of tools, tackles and machines; and

(f)     Generation of steam, etc.

For the manufacture of precast elements all the above processes shall be planned in a  

Systematic way to achieve the following:

(a)   A cyclic technological method of working to bring in speed and economy in manufacture.

(b)   Mechanization of the process to increase productivity and to improve quality.

(c)    The optimum production satisfying the quality control requirements and to keep up the expected speed of construction aimed.

(d)   Better working conditions for the people on the job ; and

(e)    To minimize the effect of weather on the manufacturing schedule.

The various stages of precasting can be classified as in Table 20.1 on the basic of the machine

Complexes required for the various states. This permits mechanization and rationalization of work in the various stages. In the precasting, stage 6 and 7 given in the elements. For these precasting stages there are many technological processes to suit the concrete products under consideration which have been proved rational, economical and time saving. The technological line or process is the theoretical solution for the method of planning the work involved by using machine complexes. Figure 20.1 illustrates diagrammatically the various stages involved in a plant process.

                The various accepted methods of manufacture of precast units can be broadly classified into two methods:

(a)   The ‘Stand Method’ where the moulds remain stationary at places, when the various processes involved are carried out in a cyclic order at the same place, and

(b)   The ‘Flow Method’ where the precast unit under consideration is in movement according to the various processes involved in the work which are carried out in an assembly-line method.

The various accepted precasting methods are listed in Table 20.2 with details regarding the

elements that can be manufactured by these methods.

TABLE 20.1          Stages of precasting concrete products

Sl.           Precasting                           Name of process                                              Operations Involved 

No.         Stage No.                           

(1)               (2)                                                 (3)                                                                              (4)

(i)                  1                                      Procurement and                            Unloading and transport of cement,

                                                                Storage of const-                             coarse and fine aggregates, and steel,

                                                                ruction materials                              and storing them in bins, solid or 

                                                                                                                                Storage shade.

(ii)                2                                      Testing of raw                                   Testing of all materials including steel

Materials

(iii)               3                                      Design of concrete                          Testing of raw materials, plotting of gra-

Mix                                                        ding curves and trial of mixes in laboratory

(iv)              4                                      Making of reinforcement             Unloading of reinforcement bars from

Cages                                                    wagons or Lorries and stacking them

                                                                                                                                In the steel yard, cutting, bending, tying

                                                                                                                                or welding the reinforcements and ma-

                                                                                                                                king in the form of a cage, which can be

                                                                                                                                directly introduced into the mould.

(v)                5                                      oiling and laying                                Moulds are cleaned, oiled and assem-

Of moulds in                                      bled and placed at the right place.

Position.

(vi)             6                                       Placing of reinforcement              The reinforcement cages are placed

Cages, inserts and                           in the moulds with spacers, etc. 

Fixtures

(vii)            7                                       Preparation of green                      Taking out aggregate and cement from

Concrete                                             bins, silos, etc. batching and mixing.

(viii)           8                                        Transport of green                          Transport of green concrete from the                

                                                Concrete.                                            Mixer to the moulds. In the case of pre-

                                                                                        Cast method involving directs transfer

                                                                                                                Of concrete hopper attached to the mo-

                                                                                                                Uld this prefabrication stage is not

                                                                                                                                Necessary.

(ix)            9                                        Pouring and consolidation            Concrete is poured and vibrated to a

Of concrete.                                       Good finish

(x)              10                                     Curing of concrete and                  Either a natural curing with water or an

Demoulding                                       accelerated curing using steam curing

                                                                                                                                And other techniques. In the case of st-

                                                                                                                                Eam curing using trenches or autoclaves

                                                                                                                                 this stage involves transport of moulds                

                                                                                                                                with the green concrete into the trench

                                                                                                                                or autoclave and taking them out from

                                                                                                                                the mould. In the case of pre-tensioned

                                                                                                                                element cutting of protruding wires also

                                                                                                                                falls in this stage. In certain cases the

                                                                                                                                moulds have to be partly removed and

                                                                                                                                inserts have to be removed after initial

                                                                                                                                set. The total demoulding is done after

                                                                                                                                a certain period and the components

                                                                                                                                are then allowed to be cured. All these

                                                                                                                                fall in this operation.

(xi)          11                                       Stacking of precast                          Lifting of precast elements from the

Elements                                             mould and transporting to the stacking

                                                                Yard for further transport by trailer or

                                                                Rail is part of the stage

(xii)         12                                       Testing of finished                           Tests are carried out on the compone-

Components.                                    nts individually and in combination to

                                                                Ensure the adequacy of their strength

(xiii)        13                                        Miscellaneous                                   (a)          Generation of steam involving

Storing of coal or oil necessary

For generation of steam and providing steam pipe connection up to the various technological lines

                                                                                                                                (b)          Repair of machines used in the

                                                                                                                                                Production

Note. For ready mixed concrete, stage 1, 2, 3 and 7 are not applicable.

Table 20.2. Precasting methods

Sl.           Precasting Method            Where used                     Dimensions and                                Advantages and

No.                                                                                                                weights                                  Remarks

(1)                          (2)                                   (3)                                                 (4)                                          (5)         

(i)            Individual mould              (a) Rib slabs, beams,       Any desire dimesnions                                 

                Method (precasting        girders window pan-      weight up to 20 tonnes,               

                Using mould which          els, box type units          except for prestressed      

                May be easily assemb-  and special elements.    Elements as below:             (a)      Strengthening

                led out of bottom and   (b) Prestress railway       Length:                                      Of the cross section

                Sides, transportable, if  sleepers, parts of pre-   Less than 7200 mm                possible.

                If necessary. This may    stressed girders, etc.      Breadth:                                    (b)    Openings are

                Be either in timber or                                                     Less than 1800 mm                   possible in two        

                in steel using needle or                                                                 Thickness:                                           panels.

                Mould vibrators and                                                       Less than 300 mm

                Capable of taking pres-                                                 Weight:

                tressing forces.                                                                 Up to 5 tonnes

(ii)           Battery form method       Interior wall                      Length:  18 m                           specially suitable for

                shuttering panels               (The panels, shell           Breadth:  3 m                           mass production of  

                May be adjusted in            elements reinforced   Weight:  5 tonne                    wall panels where

                The form of a battery         concrete battens,                                                              shuttering cost is re-  

                At the required dista-         rafters, purlins and                                                           duced to a large ext-

                Nces equal to the thi-         roof and floor slabs.                                                         ent and autoclave or

                Ckness of the concre-                                                                                                           trench steam curing

                Te members)                                                                                                                            may be adopted by

                                                                                                                                                                       Taking the steam pipes through the shuttering panels.

(iii)          Stack method                    Floor and roof                   Length: Any desire-        For casting identical       

                                                                Slab panels                         able length.                        Reinforced or pre-

                                                                                                                Breadth:  1 to 4 m            stressed panels one

                                                                                                                Weight:  5 tonnes            over the other with

                                                                                                                                                                Separating media inter-

                                                                                                                                                                Posed in between.

(iv)         Tilting,                                   mould Exterior  Wall       Length:    6 m                     suitable for manufactu-

                Method                                               (This panel method is     Breadth:  4 m                     ring the external wall

                                                                Capable special finishes                Weight:                    5 tonnes          panels.

                                                                Of being kipped are req-

                                                                uired in hydraulic jacks)

(v)          Long line pre-                    Double Tees, rib slabs    Length: Any desire          ideally suited for pre-

                Stressing bed                     purlins, piles and              Breadth: 2 m                      tensioned members

                Method.                              Beams                                  Height: 2 m

                                                                                                                Weight: up to 10 tonnes

(vi)         Extrusion method            Roof slabs, foam              Length:  Any desired      May be used with

                (Long concrete m-           concrete wall                     Breadth: Less than 2 m  advantage in the case

                ould with constant          panels and beam             Height :                Less than 3 m    of unreinforced blocks

                cross section and                                                                                                             foam concrete panels

                vibration will be

                done automatically

                just as in concrete

                roads)

Preparation and storage of materials                                           

Storage of materials is of considerable importance in the precasting industry, as a mistake in planning gin this aspect can greatly influence the economics of production. 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 bad storage and transport. So in a precast factory where everything is produced with special emphasis on quality, proper storage and preservation of building materials especially cement, coarse and fine aggregates, is of prime importance.

Storage of cement

Storage of cement can be effected either in especially erected storage sheds where cement can be stored in the form of bags or in silos where it is stored loose.

Storage of coarse and fine aggregates

The coarse and fine aggregates can be stacked either in open or in bunkers. In the case of open storage, the “Parallel-Boxes” method with dividing walls up to about 3 meters in height, is considered to be most convenient and economical. The dividing walls can be inserted between the columns. In Planning this method of storage, the following points shall be kept in mind:

(a)   The stored aggregate shall be protected from missing up with the local earth, clay or coal, and

(b)   The various bins or boxes shall be properly designated about the size and type of material to be stored. Mistakes occurring due to dumping of one class/size of aggregates in the wrong bin should be avoided.

Yet another method of open storage is by heaps under which a tunnel is provided with conveyor belt system to extract from the heap whatever material is required for preparation and mixing of concrete.

In planning storage of coarse and fine aggregates, bins silos, etc. shall have a minimum storage capacity and shall be designing silos are concerned, 2 to 4 hours storage capacity shall be provided.

Moulds

                Moulds 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 de-moulding and assembly of the various parts. At the same time rigidity, strength and watertightness of the mould, taking into consideration forces due to pouring of green concrete and vibrating, are also important.

                Slopes of the mould walls, for easy demoulding of the elements from the mould with fixed sides, the required slopes have to be maintained. Otherwise there is a possibility of the elements getting stuck up with the mould at the time of demoulding.

Curing

                Accelerated hardening

                In most of the pre casting factories, it is economical to use faster curing methods or artificial curing methods, which in turn will allow the elements to be demoulded much earlier permitting early re-use of the forms. Any of the following methods may be adopted:

                (a)          By heating the aggregate and water before mixing the concrete. By heating of the aggregate as well as water to about 70° C to 80° C before making the concrete mix and placing the same in the moulds, sufficiency high earlier strengths are developed to allow the elements to be stripped and transported.

                (b)          Steam curing.    Steam curing may be done under high pressure and high temperature in an autoclave. This technique is more suited to smaller elements. Alternatively this could be done using low pressure steam having temperature around 80° C. for light weight concrete products when steam cured under high pressure, the drying shrinking is reduced considerably. Due to this reason, high pressure steam curing in autoclave is specified for light weight low densities ranging from 300 to 1,000 kg/m². For normal heavy concrete as well as light weight concretes of higher densities, low pressure steam curing may be desirable as it does not involve using high pressure and temperature requiring high investment in an autoclave.

                (c)           Steam injection during mixing of concrete. In this method low pressure saturated steam is injected into the mixer while the aggregate are being mixed. This enables the heating up of concrete to approximately 60° C. Such as concrete after being placed in the moulds attains early strength.

                (d)          Heat air method.              In this method, the concrete elements are kept in contact with hot air with a relative humidity not less than 80 percent. This method is specially useful for light concrete products using porous coarse aggregates.

                (e)          Hot water method.         In this method the concrete elements are kept in a bath of hot water around 50° C to 80° C. The general principles of this type of curing are not much different from steam curing.

                (f)           Electrical method.            The passage of current through the concrete panels generates heat through its electro-resistivity and alternating current ranging from 50 volts for a plastic concrete and gradually increasing to 230 V for the set concrete. This method is normally used for massive concrete products.

                (g)          Consolidation by spinning. Such a method is generally used in the centrifugal moulding of pipes and such units. The spinning motion removes excess water, effects consolidation and permits earlier demoulding.

                (h)          Pressed concrete.           This method is suitable for fabrication of small or large products at high speed of production. A 100-200 tonnes press compresses the wet concrete in rigid moulds and expells water. Early handling and a dense wear resistant concrete is obtained.

                (i)            Vacuum treatment.        This method removes the surplus air and water from the newly places concrete as in slabs and similar elements. A suction up to about 70 percent of an atmosphere is applied for 20 to 30 minutes per centimeter thickness of the units.

                (j)           Consolidation by shock.                This method is suitable for small concrete units dropped repeatedly from a height in strong moulds. The number of shocks required to remove excess water and air may vary from 6 to 20 and the height of the lift may be up to as much as half the depth of the mould.

                After the accelerated curing of the above products by any of the above accepted methods, the elements shall be cured further by normal curing methods, to attain full final strength.

                The curing of the prefabricated elements can be affected by the normal methods of curing by sprinkling water and keeping the elements moist. This can also be done in the case of smaller elements by immersing them in a specially made water tanks.

Stacking during transport and storage

                Every precaution shall be taken against overstress or damage, by the provision of suitable packings at agreed points of support. Particular attention is directed to the inherent dangers of breakage and damage caused by supporting other than at two positions, and also by careless placing of packing (for example, not vertically one above the other). Ribs, corners and intricate projections from solid section should be adequately protected. Packing pieces shall not discolour, disfigure or otherwise permanently cause mark on units or members. Stacking shall be arranged or the precast units should be protected, so as to prevent the accumulation of trapped water or rubbish, and if necessary to reduce the risk of efflorescence.

                The following points shall be kept in view during stacking:

(a)          Care should be taken to ensure that the flat elements are stacked with right side up. For identification, top surface should be clearly marked.

(b)          Stacking should be done on a hard and suitable ground to avoid any sinking of support when elements are stacked.

(c)           In case of horizontal stacking, packing materials must be at specified locations and must be exactly one over the other to avoid cantilever stress in panels.

(d)          Components should be packed in a uniform way to avoid any undue projection of elements in the stack which normally is a source of accident.

Handling arrangements

                Lifting and handling positions shall be clearly defined particularly where these sections are critical. Where necessary special facilities, such as bolt holes or projecting loops, shall be provided in the units and full instructions supplied for handling.

                For precast prestressed concrete members, the residual prestress at the age of particular operation of handling and erection shall be considered in conjunction with any stresses caused by the handling or erection of member. The compressive stress thus computed shall not exceed 50 percent of the cube strength of the concrete at the time of handling and erection. Tensile stresses up to a limit of 50 percent above those specified shall be permissible.

Transport

                Transport of precast elements 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 efficient management. Transport of precast elements must be carried out with extreme care to avoid any jerk and distress in elements and handled as far as possible in the same orientation as it is to be placed in the final position.

                Transport inside the factory.       Transport of precast elements moulded inside the factory depends on the method of production selected for the manufacture as given Table 20.2.

                Transport from stacking yard inside the factory to the site of erection.    Transport of precast concrete elements from the factory to the site of erection should be planned in such a way so as to be in conformity with the traffic rules and regulations as stipulated by the authorities. The size of the elements is often restricted by the trailers, to suit the load and dimensions of the member in addition to the load-carrying capacity of the bridges on the way.

                While transporting elements in various systems, that is, wagons, trucks, bullock carts, cares should be taken to avoid excessive cantilever actions and desired supports are maintained. Special care should be taken at location of sharp bends and on uneven or slushy roads to avoid undesirable stresses in elements.

                Before loading the elements in the transporting media, care should be taken to ensure the base packing for supporting the elements are located at specified positions only. Subsequent packings must be kept strictly one over the other.

Erection

                In the erection of precast elements; all the following items of work are meant to be included:

                (a)          Slinging of the precast elements;

                (b)          Tying up of erection ropes connecting to the erection hooks;

                (c)           Cleaning of the elements and site of erection;

(d)          Cleaning of the steel inserts before incorporation in the joints, lifting up of the elements, setting them down into the correct envisaged position;

(e)          Adjustment to get the stimulated level, line and plumb;

(f)           Welding of cleats,

(g)          Changing of the erection tackles;

(h)          Putting up and removing of the necessary scaffolding or support;

(i)            Welding of the inserts, laying of reinforcement in joints and grouting the joints; and

(j)           Finishing the joints to bring the whole work to a workman like finished product.

In view of the fact that erection work in various construction jobs using prefabricated concrete

elements differs from place to place depending on the site conditions, safety precautions in the work are of utmost importance. Hence only those skilled foremen, trained workers and fitters who have been properly instructed about the safety precautions to be taken should be employed on the job.

Autoclaved cellular concrete

The manufacture of the cellular concrete products differs from that of dense concrete in certain

Respects as given below:

                (a)          The manufacture of cellular concrete being a highly controlled process has to be done in

                                a factory;

(b)          The principal raw material are cement or lime and fine materials (silicious sand, fly ash, grannlated blast furnace slag);

(c)           The silicious material is ground finely in a ball-milland the alurry is prepared with predertmined quantity of cement or lime and water. Gas generating materials and harmless additives are also added in the required amount before the concrete is poured into the moulds;

(d)          The cellular concrete is cast in structural moulds and the various components are cut to the required size before it is autoclaved.

(e)          Curing is done in autoclave at high temperatures (180° C to 200° C) and at high pressure (7 to 15 kgf/cm²). The components are taken out after they are fully autoclaved.

(f)           Each slab is provided with tongue at one side and groove at the other or any other provision is made to transfer load from one unit to another; and

(g)          In view of the above there will be some changes in the stages of manufacture given in table 20.1

Equipment

General.              The equipment used in the precast concrete industry can be classified into the

Following categories:

                (a)          Machinery required for the quarrying of coarse and fine aggregates.

(b)          Conveying equipment, such as belt conveyors, chain conveyors, screw conveyors, bucket elevators, hoists, etc;

(c)           Concrete mixing machines.

(d)          Concrete vibrating machines.

(e)          Erection equipment, such as cranes, derricks, hoists, chain, pulling blocks, etc.

(f)           Transport machinery, such as tractor-cum-trailers, dumpers, lorries, locomotives, motor boats and rarely even helicopters.

(g)          Workshop machinery for making and repairing steel and timber moulds.

(h)          Bar straightening, bending and welding machines to make reinforcement cages.

(k)          Steam generation plant for accelerated curing.

In 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.

                Each of the above groups can further be classified into various categories of machines and further to various other types depending on the source of power and capacity.

Mechanization of the construction and erection process

                The various processes can be mechanized as in any other industry for attaining the advantage 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. On the basis of the degree of mechanization used, the various precasting factories can be divided into three categories:

                (a)          With simple mechanization,

                (b)          With partial mechanization, and

                (c)           With complex mechanization leading to automation.

                In simple mechanization, simple mechanically operated implements are used to reduce the manual labour and increase the speed.

                In partial mechanization, the manual work is more or less eliminated in the part of a process. For example, the batching plant for mixing concrete, hoists to lift material to a great height and bulldozer to do earthwork come under this category.

                In case of complex mechanization leading to automation, a number of processes leading to the end-product are all mechanized to a large extent (without or with a little manual or human element involved). This type of mechanization reduces manual work to the absolute minimum and guarantees the mass production at a very fast rate and cheap price.

Prefabricated structural unit

                Some of the structural members of a building which can be constructed with the help of prefabricated units are described below briefly.

                (1)          Walls and columns

                As described chapter 7, walls and columns made of hollow block masonry are advantageous as they are easy to construct, are cheaper and have a got a great thermal insulation effect. They considerably save mortar compared to brickwork since the numbers of joints are less. Internal plastering is reduced since a good finish can be obtained with one coat only; instead of two.

                These blocks are built in concrete, the forms of which can be reused a number of times. Special types of forms are used keeping in view the shape of the hollow space to be left in the blocks.

                (2)          Lintels

                Prefabricated RCC lintels can be used conveniently over window and door openings, thereby accelerating the speed of construction by eliminating curing period.

                (3)          Door and window frame

                Precast concrete door and window frames can also be built. Steel bars of about 4 mm to 4o mm in diameter run through them. Sitable hard wood blocks are used for the fixing hinges, etc. to them. Concrete is vibrated on a table vibrator and is then poured into the mould, thereby forming a durable mix.

                (4)          Roofing and flooring elements

                Prefabricated reinforced concrete battens and plain concrete tiles can b used for roofing and flooring for flat roofs, instead of wooden sections and brick tiles. For sloping roof, precast reinforced and prestressed concrete triangulated trusses can b used.

                Plain concrete or lightly reinforced concrete can be used in the form of precast shells for roofing. Plain concrete, doubly curved shells have been developed at C.B.T.I.; Rookee. While making them, a suitable frame is built over which hessian cloth is spread. This cloth has to be given a sort of sag and internal tension is controlled by the depth indicating frame. Over this, a thin frame of about 2.5 cm height is placed to retain the wet concrete and regulates its thickness. A piece of chicken mesh is next placed on the mould to act as reinforcement. Cement concrete is poured over the hessian and manually compacted. The frame is made to rest to permit the hessian to sag with the wet concrete. Moulds are set up for the edge beams and suitable steel rods are placed within these moulds. These beams are then cast. The beam sides are demoulded after an hour of casting. The unit is cured for about two days and is inverted for use. The final shape of the unit, as placed on small T-beams, is shown in Fig. 20.3. The top surface of the roof thus formed is wavy and as such the depressions are filled with concrete or earth so that level surface may b obtained. These units can be of 1m x 1m to about 3m x 3m in size.

                (5)          Stairs

                Treads of pre-cast plain concrete slabs and the risers of small, precast concrete blocks can be used for construction of stairs. The details are shown in fig. 14.14 in chapter 14.