Prefabrication in construction

Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting them to the construction site where the structure is to be located.

1. DEEKSHA MITTAL PRATIKSHA AGGARWAL SHAILJA KUMARI VIBHA KACHROO PREFABRICATION OF CONCERETE

2. What is Prefabrication? Prefabrication  is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting  them to the construction site where the structure is to be located. The method controls construction costs by economizing on time, wages, and materials. Prefabricated units may include doors, stairs, window,  wall panels, floor panels, roof  trusses and even room-sized components .

3. Theory The theory behind the method is that time and cost is saved if similar construction tasks can be grouped, and assembly line techniques can be employed in prefabrication at a location where skilled labour is available, while congestion at the assembly site, which wastes time, can be reduced. The method finds application particularly where the structure is composed of repeating units or forms, or where multiple copies of the same basic structure are being constructed. The restricting conditions such as a lack of power, lack of water, exposure to harsh weather or a hazardous environment are avoided. Against these advantages must be weighed the cost of transporting prefabricated sections and lifting them into position.

4. What Material to Use? Materials that allow a large span construction shall be used. Actual manufactured components vary slightly when made from the specified dimensions , thus a material shall be used which gives minimum variation. Economic and cost– effective materials. Aesthetically sound material shall be used to improve the appearance of building. Material must be able to take the structural loads and stresses. A material that can be molded to variety of shapes shall be used. Ease of maintenance.

5. What is the Aim of Prefabrication? to speed up the construction time. to lower the labour cost. to allow the year round construction. to ensure precise conformity to standards. to ensure greater quality. to allow less wastage of materials. to ensure higher worker safety and comfort level than in site- built construction.

6. Prefabricated Building Material D oor frames W indow frames R oof slabs Lintels Ventilators W all partition panels C ompound wall panels C ement concrete S mall pillars for compound wall Sills T russes etc.

7. Prefabricated Building Material

8. Flow of Materials, Components & Prefabricated Elements

9. Components of Prefabrication The preferred dimensions of precast elements according to NBC shall be as follows: Flooring and Roofing Scheme - Precast slabs or other precast structural flooring units: Length - Nominal length shall be in multiples of 1 M; Width - Nominal width shall be in multiples of 0.5 M; and Overall Thickness - Overall thickness shall be in multiples of 0.1M. Beams Length - Nominal length shall be in multiples of 1 M; Width - Nominal width shall be in multiples of 0.1 M; and Overall Depth - Overall depth of the floor zone shall be in multiples of 0.1M.

10. Columns Height - Height of columns for industrial and other building 1 M; & Lateral Dimensions - Overall lateral dimension or diameter of columns shall be in multiples of 0.1 M. Walls Thickness - The nominal thickness of walls shall be in multiples of 0.1 M. Staircase Width - Nominal width shall be in multiples of 1M. Lintels Length - Nominal length shall be in multiples of 1 M; Width - Nominal width shall be in multiples of 0.1 M; and Depth - Nominal depth shall be in multiples of 0.1 M. Sunshades/ Chhajja Projections Length - Nominal length shall be in multiples of 1 M .

11. Types of Prefabrication Components The prefabricated concrete components such as those given below may be used which shall be in accordance with the accepted standards, where available: Reinforced/ Prestressed concrete channel unit. Reinforced/ Prestressed concrete slab unit. Reinforced/ Prestressed concrete beams. Reinforced/ Prestressed concrete columns. Reinforced/ Prestressed concrete hollow core slab, Reinforced concrete waffle slab/shells. Reinforced/ Prestressed concrete wall elements, Hollow/Solid blocks and battens, Precast planks and joists for flooring and roofing, Precast joists and trussed girders, Light weight/cellular concrete slabs, Precast lintel and chhajjas , Large panel prefabricates, Reinforced/ Prestressed concrete trusses, Reinforced/ Prestressed roof purlins , Precast concrete L-panel unit, Prefabricated brick panel unit, Prefabricated sandwich concrete panel, and Precast foundation.

12. On Site & Off S ite Prefabrication Manufacture : The manufacture of the components can be done in a factory for the commercial production established at the focal point based on the market potential or in a site- precasting yard set up at or near the site of work.   Factory prefabrication Factory prefabrication is resorted to in a factory for the commercial production for the 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.

13.   Site prefabrication Prefabricated components produced at site or near the site of work as possible. This system is normally adopted for a specific job order for a limited period.   Semi-mechanized The work is normally carried out in open space with locally available labor force. The equipment machinery used may be minor in nature and moulds are mobile or stationary in nature.   Fully-mechanized The work will be carried out under shed with skilled labor. The equipments used will be similar to one of factory production. This type of precast yards will be set up for the production of precast components of high quality, high rate of production. Though there is 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.

14. The transportation of precast facades . The temporary supporting props to hold the precast facade in position. This prevents any movement of the panel and allows final adjustments to be made.

15. The worker is laying the services pipes and service conduits Lifting Precast Slab into Position

16. Advantages of Using Prefabricated Structures Factory made products. Independent of adverse weather conditions during construction. Reduced energy consumption. Environmentally friendly way of building with optimum use of materials, recycling of waste products, less noise and dust etc. Cost effective solutions. Reduction in construction time. Safety in construction. Increase in the quality of construction. Reduction of construction waste. Demountable structures. Accelerated curing techniques.

17. Disadvantages of Using Prefabricated Structures A small number of units required may prove to be uneconomical. Special connections, such as special bearings to transmit the vertical and horizontal loads, can add cost to the system. Waterproofing at joints. Transportation difficulties. Large prefabricated sections require heavy-duty cranes and precision measurement and handling to place in position.

18. Prefabrication Systems Open Prefabrication System Large Panel Prefabrication system

19. Open Prefabrication System I t allow the pre-casted to produce a limited number of elements with a predetermined range of product and at the same time maintaining architectural aesthetic value. Inspite of many advantages of open system, its adoption experiences one major setback. For example, joint and connection problem occur when two elements from different system are fixed together. This is why similar connection technology must be observed in order to achieve greater structural performance. There are two categories of open prefab system depending on the extent of prefabrication used in the construction as given in Partial and Full prefabrication systems: Partial prefabrication system This system basically uses precast roofing and flooring components and other minor elements like lintels, chhajjas , and kitchen sills in conventional building construction. The structural system could be in the form of in-situ framework or load bearing walls.   Full prefabrication system In this system almost all the structural components are prefabricated. The filler walls may be of brick/block masonry or of any other locally available material.

20. Large Panel Prefabrication 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 . M ultistory structures are composed of large wall and floor concrete panels , connected in the vertical and horizontal directions. Panels form a box-like structure. Both vertical and horizontal panels are load bearing . Wall panels are usually one story high. Horizontal floor and roof panels span either as one-way or two-way slabs. When properly joined together, these horizontal elements act as diaphragms that transfer the lateral loads to the walls.

21. Contains insulation, wiring, or pre-cut opening for windows and doors Construction of panelized homes includes whole walls with windows , doors, wires or outside siding prepared in factory and erected on site. Depending on the wall layout, there are three basic configurations of large-panel buildings: Cross wall system Longitudinal wall system Two way system Thickness of wall panels :120 mm ( interior walls) 300 mm ( exterior walls) Floor panel thickness : 60 mm. Wall panel length : room length (2.7m to 3.6m) Precast vertical boundary wall panel Precast wall panel Precast wall panel

22. Process : Precast concrete panels are cast face-downwards in purpose-built moulds, usually made of either timber or fibre glass. A steel reinforcement cage is placed in the mould prior to the concrete being poured. By inserting mats or rods in the base of the mould different designs and effects can be created - from false joints to intricate leaf patterns. Increasingly , maximum use of off-site fabrication is being made by fixing other elements, from insulation to entire windows, during manufacture of precast panels . Once de- moulded , the exposed surface of the unit can be treated in a variety of ways - acid-etched, smooth or coarse ground, grit or sand-blasted, rubbed or polished. Moulds Different designs and effects can be created

23. PRECAST CONNECTIONS

24. Precast boundary walls are similar to precast wall panels but are typically of smaller sizes. PRECAST BOUNDARY WALLS quite ideal for pre casting as the smaller panels fewer problems with handling, transportation and installation on site. Joint details The considerations for proper joint details in the precast boundary panels are similar to those for precast wall panels. The design of the wall joints will include the following considerations: • water tightness • installation method • structural movement • type of wall finishes • panel sizes • weathering • tolerance

25. Finishes wide range of design flexibility and innovation . wide range of surface finishes on the precast boundary wall design. The most common techniques used are modeling techniques like sand blasting, acid washing, polishing and honing, hammering and chipping to create the required effects. These finishes can then be treated with appropriate protective coatings to prevent weathering and staining problems. A wide range of colours for precast boundary wall panel can be derived from aggregates, cement or pigments . Aggregate can provide colour to the final finishes. Cement with different colours can also give the desired colour for the boundary wall panel. Another form of colour finishes are colour and oxide pigments.

26. Installation of wall base Precast walls are placed and propped

27. Completed view of the precast boundary panel walls

28. The precast staircases proposed here are of standard sizes with tread sizes between 225mm to 250mm and risers between 150mm to 175mm. On the other hand, steel staircases can come in non-standard sizes to suit the architectural design. Precast/ steel staircases can be fabricated to a range of various forms and shapes. There are three basic staircases profiles: • Curved • Straight • Spiral profile In the case of the terrace and semi-detached houses, these staircases can be fabricated in three ways: • Type A for flight only • Type B flight and top landing • Type C flight and base landing The erection of the staircase can either be on the critical path or non-critical path of the construction sequence. In each case, prefabricated staircases will result in better quality, accuracy and productivity. The prefabricated staircases can be installed quickly and messy, cast-in-situ works can be eliminated. PREFABRICATED STAIRCASE

29. Precast concrete Stairs Uniquely shaped structural elements for a sports stadium

30. There are two main methods for fixing and installing the prefabricated staircases. The staircase can either be prefabricated together with the landing as a complete unit or it can be prefabricated separately and installed on site. The prefabricated staircase should be designed to ensure easily transportation and hoisting. However, in most cases, the size and weight are usually within the manageable capacity of the cranes. Precast Flight Cantilevered precast treads

31. Details to staircase to frame connections

32. PRECAST WINDOW ELEMENTS

33. Precast Flooring Before any system of precast concrete flooring can be considered in detail the following factors must be taken into account: Maximum span Nature of support Weight of units Thickness of units. Thermal insulation properties Sound insulation properties Fire resistance of units. Speed of construction. Amount of temporary support required.

34. With the development of precast concrete construction techniques, precast units e.g. floor slabs are successfully manufactured. Advantages: Construction is quick Sound proof Members are lightweight Does not require formwork No time wasted for curing of concrete Good thermal insulation properties Disadvantages: Requires uniform spacing of columns which can be difficult Great care has to be taken in construction in order for precast members to be able to resist handling stress May be wastages due due to breakages in transportation to site Careful supervision is required during manufacturing Formwork for manufacturing is usually costly – only economical when manufacturing in large scale.

35. Precast Slab SLAB TO SLAB PRECAST CONNECTION

36. WALLPANEL CONNECTED TO IN-SITU CONCRETE

37. CONNECTION OF WALL PANELS TO COLUMNS

38. Precast walls are propped before casting of joints – proper planning is required for perfect alignment The pour strip between 2 pieces of precast walls are cast Precast wall units PRECAST WALLS

39. Precast planks are installed in place Preparation of welded mesh and services for cast in-situ topping Concreting to topping

40. Precast Beams,Griders,Corbel & Nib Reinforced concrete nib Reinforced concrete corbel

41. Precast beam-column connection using cleat angle

42. BEAM TO COLUMN CONNECTION

43. SLAB TO BEAM CONNECTION

44. COLUMN TO COLUMN CONNECTION

45. WALL TO SLAB

46. Precast concrete structure consisting of solid wall panels and hollow core slabs.

47. FOUNDATIONS WITH BOLTED FLANGES FOUNDATIONS WITH PROTRUDING BARS FOUNDATIONS WITH BOLTED SOCKETS

48. Precast Column Precast Beam Precast Decking/Slabs Precast Corbel Assembly of Precast Members