PREFABRICATION



PREFABRICATION AND SYSTEMS BUILDING

22·0 Foreword

22.0.1 Prefabrication, though desirable in terms of large scale housing, has yet to take a firm hold in the country. Two aspects of prefabrication specifically to be borne in mind are the system to be adopted for the different categories of buildings and the sizes of their components. Here the principle of modular co-ordination is of value and its use is recommended.

22.0.2 Advantages of recent trends in prefabrication have been taken note of and also the hazards attended to such construction. A few recommendations on the need to avoid 'progressive collapse’ of the structure have been included This has become necessary in view of such collapses in the past. A specific point to be borne in mind, therefore, is the need to make the structure reasonably safe against such a collapse. .

22.0.3 Prefabricated construction, being a new technique some of the essential requirements for the manufacture of the prefabricated components and elements are also included in this section.

22.0.4 Since the aim of prefabrication is to effect economy, improvement in quality and speed construction, the selection of proper materials for prefabrication is also an important factor in the popularisation of this new technique. The use of locally available materials with required characteristics and those material which, due to their innate characteristics like light/weight easy workability, thermal insulation, non-combustibility, etc, effect economy and improved quality may be tried. However, this section pertains to prefab elements with cementatious materials.

22.0.5 The design of prefabricated buildings shall include provision for all installations of piping, wiring and accessories for service equipment to be installed in the building.

22.0.6 This section was first published in 1970. In this revision the following main changes have been made:

a) A brief provision regarding importance of architectural treatment and finishes as applicable to prefabricated buildings is included;

b) A brief clause is added on the requirements of materials for use in prefabrication:

c) The clause on prefabrication systems and structural elements is elaborated;

d) The clause on testing of components is now revised to include testing of structure or part of structure,

e) A brief clause on the manufacture of cellular concrete is added.

22.1 Scope

22.1.1 This section gives recommendations regarding modular planning, component sizes, joints, manufacture, storage, transport and erection of prefabricated elements for use in buildings.

22.2 Materials, plans and specifications

22.2.1 MaterialsAll materials shall conform to prescribed specifications.

22.2.1.1 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;

f) Non-combustibility;

g) Economy in cost, and

h) Sound insulation.

22.2.1.2 The materials used in prefab components can be various and the modern trend is to use concrete, steel, treated wood, aluminium, cellular concrete, light weight concrete, ceramic products, etc.  However, this section pertains to prefab concrete elements.

22.2.2 Plans and specifications – Complete set of drawings as specified in byelaw shall be submitted to the Authority for approval.

22.2.2.1. Such drawings shall describe the elements of the structure or assembly including all required  data  of  physical properties of component materials.

22.2.2.2 Details of connecting joints shall be given to an enlarged scale.

22.2.2.3 Site or shop location of services, such as installation of piping, wiring or other accessories shall be shown separately.

22.2.2.4 The drawings shall also clearly indicate location of handling arrangements for lifting and handling the prefabricated elements.

22.3 Modular co-ordination, architectural treatment and finishes

 22.3.1 Modular co-ordination - The basic module is to be adopted. After adopting this, further work is necessary to outline suitable range of multimodules with greater increments, often referred to as preferred increments. A set of rules as detailed below would be 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 institutions buildings:

2) for industrial buildings:

15 M for spans up to 12m,

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

60 M for spans over 18 m.

The centre lines of load bearing-wall shall coincide with the grid lines:

b) In case of external walls, the grid line shall coincide with the 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 he I M upto and including a height of 2.8 M: above the height of 2.8.m. it shall be 2 M:

d) Preferred increments for sill height; doors, windows and other fenestration shall be 1 M: and

e) In the case of internal columns, the grid lines shall coincide with the centre line of columns. In case of external columns and columns near the lift and stairwells, 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.

22.3.2 Architectural treatment and finishes - The process of architectural design requires the designer to relate the resources of knowledge of building technology to the human, social and cultural conditions at a particular stage of economic growth. In doing this, the designer seeks to make a comprehensible statement out of tangible matter and intangible ideas (bricks, timber, steel and cement plus the social order and cultural disciplines) and expresses this unity in plastic form. Adoption of industrialized approach in the execution of building programmes on mass scale becomes necessary if the objective is to achieve economy in cost, efficiency of design, reduction in time for construction, etc. This in turn requires development of prefabrication techniques which does necessarily mean that it is    not   possible   to   achieve   or   evolve aesthetically satisfying designs.  In fact, a careful and judicious handling of materials and use of finishes on a prefabricated building can help the designer a great deal in ensuring that the appearance of the building is not monotonous and unappealing. The purpose of finishes and architectural treatment is not only to give prefabricated buildings an individual character but also to effect better performance, and greater user satisfaction. Treatment and finishes have to be specified keeping in view the requirements of protection, function and aesthetics of internal and external spaces and surfaces. Thinking on these aspects must be incorporated right at the design inception stage so that the entire building or complex of buildings can be conceived of in totality - in terms of architectural expression, structural design, function, etc.

While deciding the type of architectural treatment and finishes for prefabricated buildings, the following 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 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 subsystem and Finishes;

g) Satisfactory finishing 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 finishing 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.

22.4. Components

22.4.1 The preferred dimensions of precast elements shall be as follows:

a) Flooring and roofing scheme - Precast slabs or other precast structure flooring units.:

1) Length - Nominal length shall be in rnultiples 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 1be 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 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 2.8m; and for heights above 2.8 m, it shall be in multiples of 2 M; and

2) Lateral dimensions - Overall lateral dimension or diameter of column shall be in multiples of M/4.

d) Walls

Thickness - The nominal thickness of walls shall be in 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 IM;

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

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

g) Sunshades, chajja projections

1) Length - Nominal length shall be in multiples of 1 M.

2) Projection—Nominal length shall be in multiples of 1 M.

22.4.2 Tolerances on the dimensions of components shall be as follows:

a) Length - ± 0.1  percent subject  to a minimum of ±5 mm and a maximum of ±10 mm.

b) Cross-sectional dimensions - ±3 m  or ± 0.1 percent, whichever is greater.

c) Straightness of bow - 1/750 of the length subject to a minimum of ± 5 mm and a maximum of ± 20 mm.

d) Squareness - When considering the squareness of the corner, the longer of the two adjacent sides being checked shall be taken as the base line. The shorter side shall not vary in length from the perpendicular by more than 5mm.

For the purpose of this requirement, any error due to lack of straightness shall be ignored; squareness shall be measured with respect to the straight lines which are nearly parallel with the features being checked when 'nominal angle is other than 90°; the included angle between the check lines should be varied accordingly.

e) Twist - Any corner shall not be more than the tolerance given below from the plane containing the other three corners:

Upto 60 cm in width And upto 6 m in length

5 mm

Over 60 cm in width and for any length

10 mm

f)  Flatness - The maximum deviation from a 1.5m straight edge placed in any position on a nominal plane surface shall not exceed 5 mm.

22.5 Prefabrication systems and structural schemes

22.5.1 The word 'system' is referred to a particular  method  of  construction  of buildings using the 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 systems there are a certain order and discipline.

22.5.2 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 numbers of components;

d) Limited opening 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;

j) Suitability for adoption in low rise and high rise blocks;

k) Ease of manufacturing, storing and transporting;

m) Speed and ease of erection; and

n) Simple joining system.

22.5.3 Prefabrication systems -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.

22.5.3.1 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,

j) Reinforced/prestressed concrete columns,

k) Precast lintels and chajjas,

m) Reinforced concrete waffle slabs/shells,

n) Room size reinforced/prestressed concrete panels.

p) Reinforced/prestressed concrete walling elements, and

q) Reinforced/prestressed concrete trusses.

Note -The elements may be 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 loads; and the columns may have  hinged  or  fixed  base connections depending upon the type of components used and the method of design adopted.

22.5.3.1.1 There are two categories of open prefab system depending on the extent of prefabrication used in the construction as given in 22.5.3.1.2 and 22.5.3.1.3.

22.5. 3.1.2 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.

22.5.3.1.3 Full prefab open system - In this system almost all the structural components are prefabricated. The filler, walls may be of bricks or of any other local material.

22.5.3.2 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, staircases, etc. The casting of the components could be at the site or off the site. Depending upon the extent of prefabrication, this system can also lend itself to partial prefab system and full prefab system.

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

a) Cross wall system - In this scheme, the cross walls 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 case, cross walls are non-load bearing whereas longitudinal walls are load-bearing walls. This system is suitable for low rise buildings.

Note - A combination of the above systems with all load bearing walls can also be adopted.

22.5.3.2.2 Precast concrete 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.

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

a) load bearing walls,

b) non-load bearing walls, and

c) shear walls

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

a) external  walls,  which  can  be  load bearing or non-load bearing depending upon the lay-out, are usually .non-homogeneous walls of sandwiched type to import better thermal comforts; and

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

22.5.3.2.5 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 lightweight 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 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 transfer loads in both the directions imparting loads on the four edges. The precast elements under this category are room sized panels, two wav ribbed or waffle slab systems, etc

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

22.5.3.3 Box type construction - In this system, room size units are prefabricated and erected at site. Toilet and kitchen blocks could also be similarly prefabricated and erected at site.

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

22.5.4 Design considerations -The precast structure should be analyzed as a monolithic one and the joints in them designed to take the forces of an equivalent discrete system. Resistance to horizontal loading shall be provided by placing shear walls (in diaphragm braced frame type of construction) in two directions at right angles or otherwise. No account is to be taken of rotational stiffness, if any the floor-wall joint in case of precast bearing wall buildings. The individual components shall be designed, taking into consideration the appropriate end conditions and loads at various stages of construction. The components of the structure shall be designed for loads in accordance with requirements. In addition members shall be designed for handling, erection and impact loads that might be expected during handling and erection.

22.5.4.1 In some conventional forms of construction, experience has shown that the structures are capable of safely sustaining abnormal conditions of loading and remaining stable after the removal of primary structural members. It has been shown that some forms of building structure and particularly some industrialized large panel systems have little reserve strength to resist forces not specifically catered for in the design.  In the light of this, therefore recommendations made in 6.4.2 to 6.4.9 should be kept in mind for ensuring stability in the structure.

22.5.4.2 Adequate buttressing of external wall panels is important since these elements are not fully restrained on both sides by floor panels. Adequate design precautions may be taken by the designer. Experience shows that the external, wall panel connections are the weakest points of a precast panel building.

22.5.4.3 It is equally important to provide restraint to all load-bearing elements at the corners of the building. These elements and the external ends of cross-wall units should be stiffened; either by introducing columns as connecting units or by jointing them to non-structural wall units, which in emergency may support the load. Jointing of these units should be done bearing in mind the need for support in an emergency.

22.5.4.4 In prefabricated construction, the possibility of gas or other explosions, which can remove primary structural elements leading to progressive collapse of the structure, shall be taken into account. It is, therefore necessary to consider the possibility of progressive collapse in which the failure or displacement of one element of a structure causes the failure or displacement of another element and results in the partial or total collapse of the building.

22.5.4.5 Provision in the design to reduce the probability of progressive collapse is essential in buildings of over six storeys and is of relatively higher priority than for buildings of lower height.

22.5.4.6 It is necessary to ensure that any local damage to a structure does not spread to other parts of the structure remote from the point of mishap and that the overall stability is not impaired, but it may not be necessary to stiffen all parts of the structure against local damage or collapse in the immediate vicinity of a mishap, unless the design briefs specifically requires this to be done.

22.5.4.7 Additional protection maybe required in respect of damage from vehicles; further, it is necessary to consider the effect of damage to or displacement of a load-bearing member by an uncontrolled vehicle.  It is strongly recommended that important structural members are adequately protected by concrete kerbs or similar method.

22.5.4.8 In all aspects of erection that affect structural design, it is essential that the designer should maintain a close liaison with the builder/contractor regarding the erection procedures to be followed.

22.5.4.9 Failures that have occurred during construction appear to be of two types. The first of these is the pack-of-cards type of collapse in which the absence of restraining elements, such as partitions, cladding or shear walls, means that the structure is not stable during the construction/period. The second is the situation in which one element falls during erection and lands on an element below. The connections of the lower element then give way under the loading, both static and dynamic, and a chain reaction of further collapse is set up.

22.5.4.9.1 A precaution against the first form of failure is that the overall stability of a building shall be considered in all its erection stages as well as in its completed state. All joints that may be required to resist moments and shears during the erection- stage only shall be designed with these in mind. Temporary works required providing stability during construction shall be designed carefully.

22.5.4.9.2 To guard against the second form of failure, that is, the dropping of a unit during erection, particular attention shall be given to the details of all pre-formed units and their seating to ensure that they are sufficiently robust to withstand the maximum stresses that can arise from site conditions. Precast concrete  construction  generally shall  be capable of withstanding the impact forces that can rise from bad workmanship on site.

22.5.5 Bearing for precast units - Precast units shall have a bearing at least of 100 mm on

masonry supports and of 75 mm at least on steel or concrete. Steel angle shelf bearings shall have a 100-mm horizontal leg to allow for a 50mm bearing exclusive of fixing clearance. When deciding to what extent, if any, the bearing width may be reduced in special circumstances, factors, such as loading, span, height of wall and provision of continuity, shall be taken into consideration.

22.6. Joints

22.6.1 The design of joints shall be made in the light of their assessment with respect to the

following considerations:

a) Feasibility - The feasibility of a joint shall be determined by its load carrying capacity in the particular situation in which the joint is to function.

b) Practicability - Practicability of joint shall be determined by the amount and type of material required in construction; cost of material, fabrication and erection and the time for

fabrication and erection.

c) Serviceability - Serviceability shall be determined by the joints/ expected behaviour to repeat or possible over-loading and exposure to climatic or chemical conditions.

d) Fire - proofing

e) Appearance

22.6.2 The following are the requirements of an ideal structural Joint:

a) It shall be capable of beings 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 be 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 building; and

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

22.6.2.1 Precast structures may have continuous or hinged connections subject to providing sufficient rigidity to withstand horizontal loading. When only compressive forces are to be taken, hinged joints may be adopted. In case of prefabricated concrete elements, load is transmitted via the concrete.  When both compressive force and bending moment 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 friction resistance of the joint. Here load transmission is   accomplished   by   steel   inserted   parts together with concrete.

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

22.6.3 Joining techniques/materials normally employed are:

a) Welding of 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.

22.7 Tests for components/structures

22.7.1 Testing on individual components - The component should be loaded for one hour at its full span with a total load (including its own self weight) of 1.25 times the sum of the dead and imposed loads used in design. At the end of this time it should not show any sign of weakness, faulty construction or excessive deflection.  Its recovery one-hour after the removal of the test load should not be less than 75 percent of the maximum deflection recorded during the test. If prestressed, it should not show any visible cracks up to working load and should have a recovery of not less than 85 percent in one hour.                  ,

22.7.2 Load testing of structure or part of structure - Loading test on a completed structure should be made if required by the specification or if there is a reasonable doubt as to the adequacy of the strength of the structure.

22.7.2.1  In such tests the structure should be subjected to an imposed load equal to 1.25 times the specified imposed load used in design, and this load should be maintained for a period of 24 hours before removal. During the tests, struts equal in strength to take the whole load should be placed in position leaving a gap under the member.

22.7.2.1.1 If within 24 hours of the removal of the load, a reinforced concrete structure does not show a recovery of at least 75 percent of the maximum deflection shown during the 24 hours under load, test loading should be repeated after a lapse of 72 hours. If the recovery is less than 80 percent, the structure shall be deemed to be unacceptable.

22.7.2.1.2 If within 24 hours of the removal of the load, prestressed concrete structure does not show a recovery of at least 85 percent of the maximum deflection show7 during the 24 hours under load, the test loading should be repeated. The structure should be considered to have failed, if the recovery after the second test is not at least 85 percent of the maximum deflection shown during the second test.

22.8. Manufacture, storage, transport and erection of precast elements

22.8.1 Manufacture of   precast   concrete elements - A judicious location of precasting yard with storage facilities, suitable transporting and erection equipment and availability of raw materials are the crucial factors which should be carefully planned and provided for effective and economic use of precast concrete components in constructions.

22.8.1.2 Manufacture - 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.

22.8.1.2.1 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 labor of permanent nature with regular benefits lead to huge establishment cost which adds to ultimate cost of production.

22.8.1.2.2 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 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 elaborate arrangements for quality control. Normal benefits of continuity of work are not available in this system of construction.

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

22.8.1.3.1 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: and

f) Curing (steam curing, if necessary).

22.8.1.3.2 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 inserts and other finishing items to be incorporated in the main precast products,

d) Finishing the precast products; and

e) Testing of products.

22.8.1.3.3 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 aggregates;

c) Transport of 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.

22.8.1.4 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.

22.8.1.5 The various stages of precasting can be classified as in Table 1 on the basis of the machine complexes required for the various stages. This permits mechanization and rationalization of work in the various stages. In the precasting. Stages 6 and 7 given in Table 1 form the main process in the manufacture of precast concrete elements. For these precasting stages there are many technological processes to suit the concrete product 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 1 illustrates diagrammatically the various stages involved in a plant process.

22.8.1.6 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

Figure 1 Plant process

Table  1  Stages of Precasting concrete products

[Clauses 22.8.1.5 and 22.8.11(8)]

Sl. No.

Precasting stage No.

Name of process

Operations involved

(1)

(2)

(3)

(4)

i

1

Procurement and storage of construction materials

Unloading and transport of cement, coarse and fine aggregates, and steel and storing them in bins, silos or storage sheds

ii

2

Testing of raw materials

Testing of all materials including steel

iii

3

Design of concrete mix

Testing of raw materials, plotting of grading curves and trial of mixes in laboratory

iv

4

Making of reinforcement cages

Unloading of reinforcement bars from wagons or lorries and stacking them in the steel yard cutting, bending, tying or welding the reinforcements and making in the form of a cage, which can be directly introduced into the mould.

v

5

Oiling and laying of moulds in position

Moulds are cleaned, oiled and assembled and placed at the right place

vi

6

Placing of reinforcement cages,  inserts and fixtures

The reinforcement cages are placed in the moulds with spacers, etc.

vii

7

Preparation of green concrete

Taking out aggregates and cement from bins, silos, etc, batching and mixing

viii

8

Transport of green concrete

Transport of green concrete from the mixer to the moulds.  In the case of precast method involving direct transfer of concrete from mixer to the mould or a concrete hopper attached to the mould this prefabrication stage is not necessary.

ix

9

Pouring and consolidation of concrete

Concrete is poured and vibrated to a good finish

x

10

Curing of concrete and demoulding

Either a natural curing with water or an accelerated curing using steam curing and other techniques.  In the case of steam curing using trenches or autoclaves, this stage involves transport of moulds with the green concrete into the trench or autoclave and taking them out after the curing and demoulding elements from the mould.  In the case of pre-tensioned elements cutting of protruding wires also falls in this stage.  In certain cases the moulds have to be partly 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 elements

Lifting of precast elements from the mould and transporting to the stacking yard for further transport by trailer or rails is part of this stage.

xii

12

Testing of finished components

Tests are carried out on the components 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, stages 1, 2, 3 and 7 are not applicable.

Table 2 Precasting methods

(Clauses 22.8.1.6 and 22.8.9.1)

Sl. No. Precasting method Where used Dimensions and weights Advantages and remarks
1 2 3 4 5
1 Individual mould method (precasting method using moulds which may be easily assembled out of bottom and sides, transportable, if necessary.  This may be either in timber or in steel using needle or mould vibrators and capable of taking prestressing forces) a) Rib slabs, beams, girders, window panels, box type units and special elements Any desired dimensions and weight up to 20 tonnes, except for prestressed elements, as below: a) Strengthening of the cross section possible
b) Prestressed railway sleepers, parts of prestressed girders, etc Length:
Less than 7200 mm
b) Openings are possible in two planes
Breadth :
Less than 1800 mm
 
Thickness:
Less than 300 mm
 
Weight:
Up to 5 tonnes
 
2 Battery form method (the shuttering panels may be adjusted into the form of a battery at the required distances equal to the thickness of the concrete member) Interior wall panels, shell elements, reinforced concrete battens, rafters, purlins and, roof and floor slabs Length : 18 m Specially suitable for mass production of wall panels where shuttering cost is reduced to a large extent and autoclave or trench steam curing may be adopted by taking the steam pipes through the shuttering panels
Breadth: 3m
Weight: 5 tonnes
3 Stack method Floor and roof slab panels Length: Any desired length For casting identical reinforced or prestressed panels one over the other with separating media interposed in between
Breadth: 1 to 4 m
Weight: 5 tonnes
4 Tilting, mould method (this method is capable of being kipped vertically using hydraulic jacks) Exterior wall panels where special finishes are required in one face Length: 6 m Suitable for manufacturing the external wall panels
Breadth: 4 m
Weight: 5 tonnes
5 Long line prestressing bed method Double tees, rib slabs, purlins, piles and beams Length: Any desired  Ideally suited for pretensioned members
Breadth: 2 m
Height: 2 m
Weight: Up to 10 tonnes
6 Extrusion method (long concrete mould with constant cross section concreting and vibration will be done automatically just as in concrete roads) Roof slabs, foam concrete wall panels and beams Length: Any desired May be used with advantage in the case of un-reinforced blocks, foam concrete panels
Breadth: Less than 2 m
Height: Less than 3 m

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 2 with details regarding the elements that can be manufactured by these methods.

22.8.2 Preparation and storage of materials - Storage of materials is of considerable importance in the precasting industry, as a mistake in planning in 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.

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

22.8.2.2 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 metres in height, is considered to be the most convenient and economical. The dividing walls can be made up of precast R.C.C. retaining walls or steel or timber panels 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 mixing 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 the storage of coarse and fine aggregates, bins, silos, etc, shall have a minimum  storage  capacity  and  shall  be designed to suit the supply requirements of the factory. As far as batching silos are concerned, 2 to 4 hours storage capacity shall be provided.

22.8.3 Moulds

22.8.3.1 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 demoulding and assembly of the various parts. At the same time rigidity, strength and water tightness of the mould, taking into consideration forces due to pouring of green concrete and vibrating, are also important.

22.8.3.2 Tolerances -The moulds have to be designed in such a way to take into consideration the tolerances given in 5.

22.8.3.3 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.

22.8.4 Accelerated hardening - In most of the precasting 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 aggregates and water before mixing the concrete - By heating of the aggregates as well as water to about 70° C to 80° C before making the concrete mix and placing the same in the moulds, sufficiently 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 800C. This type of curing shall be done as specified in 22.8.5.2. For lightweight concrete products when steam cured under high pressure, the drying shrinkage is reduced considerably. Due to this reason, high-pressure steam curing in autoclave is specified for lightweight low densities ranging from 300 to 1000 kg/m3. For normal heavy concretes as well as light weight concretes of higher densities, low pressure steam curing may be desirable as it does not involve using high  pressures  and temperatures requiring high investment in an autoclave (see also 22.8.5.2).

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

d) Heated 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 especially useful for lightweight 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 accelerates curing. In this method, the concrete is heated up by an 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, effect 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 dense wear resistant concrete is obtained.

j) Vacuum treatment - This method removes the surplus air and water from the newly placed concrete as in slabs and similar elements. Suction up to about 70 percent of an atmosphere is applied for 20 to 30 minutes per centimetre thickness of the units.

k) 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 lift may be up to as much as half the depth of the mould

22.8.4.1 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. 

22.8.5 Curing

22.8.5.1 The curing of the prefabricated elements can be effected 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 specially made water tanks.

22.8.5.2 Steam curing

22.8.5.2.1 The steam curing of concrete products shall take place under tarpaulin in tents, under hoods, under chambers, in tunnels or in special autoclaves. The steam shall have a uniform quality throughout the length of the member. The precast elements shall be so stacked, with sufficient clearance between each other and the bounding enclosure, so as to allow proper circulation of steam.

22.8.5.2.2 The surrounding walls, the top cover and the floor of steam curing chamber or tunnel or hood shall be so designed as not to allow more than 1 kcal / m2 / h / 0C.

22.8.5.2.3 The inside face of the steam curing chamber, tunnel or hood shall have a damp proof layer to maintain the humidity of steam. Moreover, proper slope shall be given to the floor and the roof to allow the condensed water to be easily drained away. At first, when steam is let into the curing chambers, the air inside shall be allowed to go out through openings provided in the hoods or side walls which shall be closed soon after moist steam is seen jetting out.

22.8.5.2.4 It is preferable to let in steam at the top of the chamber through perforated pipelines to allow uniform entry of steam throughout the chamber.

22.8.5.2.5 The fresh concrete in the moulds should be allowed to get the initial set before allowing the concrete to come into contact with steam. The regular heating up of fresh concrete product from about 20° C to 35° C should start only after a waiting period ranging from 2 to 5 hours depending on the setting time of cement used. It may be further noted that steam can be let in earlier than this waiting period provided the temperature of the concrete product does not rise beyond 35° C within this waiting period.

22.8.5.2.6 The second stage in steam curing process is to heat up the concrete elements, moulds and the surroundings in the chamber:

a) In the low pressure steam curing the airspace around the member is heated up to a temperature of 75°C to 80°C at a gradual rate, usually not faster than 30 deg per hour.

This process takes around 1½ to 2½ hours depending upon outside temperature.

b) In the case of curing under high-pressure steam in autoclaves, the temperature and pressure are gradually built up for a period of about 4 hours.

22.8.5.2.7 The third stage of steam curing is to maintain the uniform temperature and pressure for duration depending upon thickness of the section. This may vary from 3 to 5½  hours in the case of low pressure steam curing and 4 to 7 hours in the case of high pressure steam curing.

22.8.5.2.8 The fourth stage of steam curing is the gradual cooling down of concrete products and surroundings in the chamber and normalization of the pressure to bring it at par with outside air. The maximum cooling rate, which is dependent on the thickness of the member, should normally not exceed 30 deg per hour.

22.8.5.9 Before the concrete products are subjected to any accelerated method of curing, the cement to be used shall be tested in accordance with accepted standards especially for soundness, setting time and suitability for steam curing.

22.8.5.3.1 In the case of elements manufactured by accelerated curing methods, concrete admixtures to reduce the water content can be allowed to be used. The normal acration agents used to increase the workability of concrete should not be allowed to be used. Use  of calcium chloride should be avoided for reinforced, concrete elements.

22.8.5.3.2 In all these cases, the difference between the temperature of the concrete product and the outside temperature should not be more than 600C for concretes up to M 30 and 75°C for concretes greater than M 45 In the case of light weight concrete, the difference in temperature should not be more than 600C for concretes less than M25. For concretes greater than M 50, the temperature differences can go up to 75°C.

22.8.6 Stacking during transport and storage  - Every precaution shall be taken against overstress or damage, by the provision of suitable packings at agreed points of inherent dangers of breakage and damage caused by supporting other than at two positions, and also by the careless placing of packings (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.

22.8.6.1 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 surfaces 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.

22.8.7 Handling arrangements

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

 

22.8.7.2 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 in concrete shall be permissible.

22.8.8 Identification and marking – All precast units shall bear an indelible identification, location and orientation marks as and where necessary. The date of manufacture shall also be marked on the units.

22.8.1 The identification markings on the drawings shall be the same as that indicated in the manufacturer's literature and shall be shown in a table on the setting schedule together with the length, type, size of the unit and  the sizes and arrangement of all reinforcement.

22.8·9 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 final position.

22.8.9.1 Transport inside the factory - Transport of precast elements moulded inside the factory depends on the method of production, selected for the manufacture as given in Table 2.

22.8.9.2 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  availability of suitable transport equipment, such as tractor-cum-trailers, to suit the load and dimensions of the member in addition to the load-carrying capacity of the bridges on the way.

22.8.9.2.1 While transporting elements in various systems, that is, wagons, trucks, bullock carts, care 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.

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

22.8.9.10 Erection - In the ‘erection of precast elements', all the following items of work are meant to be included:

a) Slinging of the precast element;

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

c) Cleaning of the element and the 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 stipulated level, line and plumb;

f) Welding of cleats;

g) Changing of the erection tackles;

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

j) Welding of the inserts, laying of reinforcements in joints and grouting the joints; and

k) Finishing the joints to bring the whole work to a workmanlike finished product.

22.8.10.1 In view of the fact that the 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.

22.8.l0.2 Transport of people, workers or visitors, by using cranes and hoists should be strictly prohibited on an erection site.

22.8.10.3 In the case of tower cranes running on rails, the track shall not have a slope more than 0.2 percent in the longitudinal direction. In the transverse direction the rails shall lie in a horizontal plane.

22.8.10.4 The track of the crane should be daily checked to see that all fishplates and bolts connecting them to the sleepers are in place and in good condition.

22.8.10.5 The operation of all equipment used for handling and erection shall follow the operations manual provided by the manufacturer. All safety precautions shall be taken in the operations of handling and erection.

22.8.11 Autoclaved cellular concrete – The manufacture of cellular concrete product: 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 materials are cement or lime and fine materials (silicious sand, fly ash, granulated blast furnace slag);

c) The silicious material is ground finely in ball-mill and the slurry is prepared with predetermined 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 standard moulds and the various components are cut to the required size before it is autoclaved;

e) Curing is done in autoclaves at high temperatures (180°C to 2000C) and at high-pressures (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 1.

22.9. Equipment

22.9.1 General - The equipment used in the precast concrete industry can be classified into the following categories:

a) Machinery required for 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;

j) Minor tools and tackles, such as wheel barrows, concrete buckets, etc; and

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

22.9.2 Mechanization of the construction and erection processes - The various processes can be mechanized as in any other industry for attaining the advantages of mass production 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.

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

22.9.2.2 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 materials to a great height and bagger and bulldozer to do earthwork come under this category.

22.9.2.3 In the 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 guarantee the mass production at a very fast rate and cheap price.

22.9.2.4. The   equipment   shall   conform   to accepted standards.

 22.10 Prefabricated structural units

22.10.1 For the design and construction of composite structures made up of prefabricated structural units and cast in-situ concrete, reference may be made to good practice.

22.10.2 For design and construction of precast the reinforced and prestressed concrete triangulated trusses reference may be made to good practice.

22.10.3 For brief design and construction of floors and roofs using precast doubly-curved shell units, waffle units, ribbed or cored units reference may be made to   good practice

22.10.4 For construction of floors and roofs with joists and filler blocks reference may be .made to good practice

22.10.5 For the requirements of autoclaved reinforced cellular concrete floor, roof and wall slabs reference may be made to accepted standards.

Annexure 22-A.1

LIST OF STANDARDS

1 18:3935-1966 Code  of  practice  for composite construction
2 IS: 3201-1965 Criteria for the design and construction of precast concrete trusses
3 15:6332-1971 Code  of  practice  for construction of floors and roofs using precast doubly-curved shell units
IS: 10297-1982 Code  of practice  for design and construction of floors an roofs using precast reinforced/prestressed concrete ribbed or cored units
IS: 10505-1983 Code  of practice  for construction of floors and roofs using precast reinforced concrete waffle units
4 IS: 6061(Part 1)-1971 Code of practice for construction of floor and roof with joists and hollow filler blocks : Part1 With hollow concrete filler blocks
IS: 6061(Part II) -1971 Code of practice for construction of floor and roof with joists and hollow Filler blocks : Part 1I With hollow clay filler blocks
5 13: 6073-1971 Specification  for  autoclaved reinforced cellular concrete floor and roof slabs
18: 6072-1971 Specification to autoclaved reinforced cellular concrete wall slabs
IS: 6441(Part Vl)-1973 Methods of test for autoclaved cellular concrete product; Part  VI  Strength,  deformation  and cracking of flexural members subject to bending - Short duration loading test
IS.6441(Part Vll)-1973 Methods of test for autoclaved cellular concrete product: Part  VII  Strength, deformation and cracking of flexural members subject to bending - Sustained loading test


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