Sara Ali, Architect and Project Asstt., CSIR-Central Building Research Institute, Roorkee.
Utkarsh Singh, Manipal University of Jaipur (MUJ), Jaipur.
Prefabrication has brought a substantial change in the development of construction industry worldwide over the last few decades. It ensures the strength, economy and environmental performance of the structures and hence is preferred over the onsite construction. Pre-assembly, prefabrication, modularisation, system buildings and industrialised buildings are the various terms used to describe the processes of manufacturing of modular units on-site or off-site. There are various types of modular precast building construction techniques prevalent worldwide. This article discusses the various types of prefabrication technologies along with available standards & codal provisions, its advantages, disadvantages.
Prefabrication usage large panel technology was initially developed in the mid 1960s. This is quick construction of huge numbers of building units at a minimal cost. It is referred as off-site construction and fabricating of some or all elements of structure in industrial units, and transporting and assembling them to the construction site where the building is to be made. It is learnt that this construction technique exists from prehistoric times and has not been evolved recently. “The Stonehenge” is a prominent example as it was also completed in different stages as shown in Fig. 1. Concentric rings are made by the arrangement of columns standing in height. To ensure the stability of structure Tenon and Mortise joints were used .
Presently, construction materials and techniques have a severe competition: concrete against steel, precast concrete against in situ concrete or steel . It can be produced with excellent quality construction as manufacturing is done under controlled conditions. Thus it’s a large potential of the future in the construction site . Precast concrete buildings systems are attracting people for wide applicability as they are accessible in numerous shape, sizes, as well as structural elements and unreinforced pieces. In construction industry of every country it is the backbone for the expansion of fresh ideas .Designing with prefab mechanism is not a barrier to creativity; on the contrary these are standardized components and provide group customization at lower costs through economies of high volume work .
The prefabricated construction system has been identified as one of the replacement to altering the speed of conventional construction methods at a rapid rate. These modules are then transported and installed on-site as load-bearing structural blocks of the structure. This type of prefabricated structure also provides environmental benefits, such as the decline of construction waste and CO2 emissions, and less interruption to the building site’s neighbours by minimizing on-site noise and dust. These advantages are the driving force within the European building industry for the expansion of prefabricated building systems. Moreover, due to increase in population, other countries (i.e. US, Canada, Japan, etc.) also use modular construction technology to construct houses, apartments, offices, etc . Sustainable growth has become globally essential as human race moves through the 21st century. The mainstream construction “work” is, consequently, transferred from the site to the manufacturing units. Assembly lines will carry out most of the work, which means numerous parallel activities can be carried out with no stoppage. The procedure is highly planned, which requires a smaller labour force at higher productivity. Prefabricated construction improves quality, safety, productivity, labour efficiency, construction timeframe, construction waste, noise, dust, and energy use. These advantages can improve the entire industry and benefit all stakeholders in the industry chain, making prefabricated construction more green, environmentally-friendly, and sustainable .
Prefabrication Adoption in India
The prefab production was pioneered by Hindustan Housing Factory. The corporation transformed its name due to the diversity of its operations and is now recognized as Hindustan Prefab Limited or HPL. The government-run corporation prefabricates mainly precast concrete for architectural and civil projects and is located in Delhi. When components are manufactured in a stable environment, quality of construction increases. Materials are used more resourcefully, are safer from climatic damage, and can be reused . In the last few decades, Indian Infrastructure and Construction sector has grown exponentially. It is main driving factor for the economic growth. The manufacturing is focusing to take up more versatile and technically intense projects. Consequently, the focal point has moved from cost efficiency to time and capability . On 25th June 2015 The Pradhan Mantri Awas Yojna (PMAY) was launched which involve manufacture of about 20 million houses by 2022 for urban poor while rural housing which envisages 10 million houses in next three years was launched on 1st April, 2016. In order to attain this enormous task, Ministry of Housing and Urban Development, GoI has adopted numerous innovative pre-fab construction systems, developed within the country. So, there is a need to change our gears in the direction of the prefabrication and pre-cast techniques which stress upon the reduced time and the enlarged productivity. It will not end-the product but would only have an effect on the procedure of construction. As it will offer opportunity for dealing with the lack of skilful labour and the deteriorating workmanship standards. The quality of construction is much superior when components are manufactured in a steady environment .
They also have the potential to deal with the problem of mass housing crisis in India that we face these days. There is a huge housing crisis in cities which has rendered millions to live in dilapidated conditions. The simple way to clear this build-up is mass housing. The idea of mass housing with a superior density and floor area ratio seems to resolve the setback considerably. This reduces cost on individual owners. Mass housing further economises by standardising resources and structural components, thus ensuing in well-organized management of materials and resources. Even though precast technology is extensively used all over the country but the utilization of this technology is restricted in some parts. At present it is used in business-related towers and a small number of government housing projects. Following are the reasons why precast construction is unpopular in India:
- Contractors’ prefer for employing low cost labour as against high capital investment
- Lack of appropriate transportation systems is main obstacle for precast technology as huge precast elements are transported from factory to construction site for erection.
- Less level of standardization of technology [13-14].
There is a recent advancement in design and technology along with increasing importance in the construction area to address the technical, social, and economic and sustainability issues of prefabricated construction techniques. This is a feasible solution compared to the existing conventional techniques. Majority of the advanced nations have by now accepted prefabricated construction technique and is gaining its advantages. Few techniques adopted by India are given in Table 1 .
Table 1: Prefabrication Technology
|Precast Sandwich Panel Systems||
|Light Gauge Steel Structural Systems||
|Precast Concrete Construction Systems||
|Steel Structural Systems||
Monolithic concrete construction system: In this system, using suitable grade of concrete RCC framed construction of columns and beams; all walls, floors, slabs, columns, beams, stairs, together with door and window openings are cast-in-place monolithically in one operation. This is made up of Aluminium/Plastic/Aluminium-Plastic Composite especially custom designed modular formwork is simple to handle with minimum labour & without use of any apparatus as shown in Fig. 2. Being modular formwork system, it facilitates in speedy construction of multiple/mass unit scale. Lightweight Aluminium formwork systems are used. In the concrete form a soft alloy weld wire is utilized in the weld process. Fixing of the formwork is finished using tie, pin & wedges system. Skilled labour is not required to do the work .
Modular tunnel form: Tunnel formwork is used to reduce cycle time and also the slab & the wall are cast monolithically, this system is one type of construction techniques used for multi storied building construction as shown in Fig. 3. Steel components are used. Its effectiveness also stems from the fact that no starter concrete is necessary for walls; it allows easy arrangement and de-shuttering, hot air curing to allow early stripping. It also favours a consistent working sequence to improve labour efficiency. The major element of the system is the half tunnel, it provides the firmness and smooth face needed to produce a consistently high quality finish to the concrete and Manufacturing is entirely done from steel. When two half tunnels are placed together this creates a tunnel. These tunnel sections are in two lengths, 1.25 and 2.5m and are set together to construct a tunnel length that suits the building dimensions. The tunnel is customized to the room width and height by the inclusion of infill sections which are sacrificed at the end of the job. These are not loose fittings but are an integral part of the tunnel .
- This utilizes a large steel formwork system.
- These forms are simple to install, durable, more accurate and produce higher quality structures. It gives the opportunity to repeat the entire construction phase within a period of only 48 hours.
- It uses a formwork system that allows the builder to cast foundations, walls, and ceilings in accordance to a pre-defined cycle. It is a combination of speed; quality and precision of factory/off site production with the flexibility and economy of in-situ construction.
- Other formwork systems are heavier than these forms.
- The wall forms can be removed within just five to eight hours .
Precast Sandwich Panel Systems
Advanced building system: These are industry made panels, consisting of self extinguishing expanded polystyrene sheet (generally corrugated) with minimum thickness not less than 60 mm and density of 15 kg/m3, sandwiched between two engineered sheet of welded wire fabric mesh. High strength galvanized wire of 2.5 mm to 3 mm dia is used in it. It is pierced entirely through the polystyrene .30 mm thick shotcrete of cement & coarse sand in the ratio of 1:4 applied to make these panels with minimum under pressure. Successfully used in many countries with involvement of different agencies and brand names Morocco, Algeria, South Africa, Kenya, Austria, Malaysia, Ireland, Romania & Australia . The system is shown in Fig. 6.
Panel prefab system: Precast construction system is in general a large panel system, modular system or a grouping of both. Precast large construction panel system consists of a range of precast elements such as walls, beams, slabs, columns, staircase, landing and a few personalized elements that are standardized and planned for stability, durability and structural integrity of the structure as shown in Fig. 7. Designing, strategic yard planning, lifting, handling and transportation of precast elements is involved in precast residential buildings. This technology is appropriate for construction of high-rise buildings resisting seismic and wind induced lateral loads along with gravity loads. Maximum number of repetitions of moulds is obtained in planning of building frame. These elements are cast in a factory which is developed at or near the site which provides a cost-effective solution in terms of storage space and transportation .
Ferrocement sandwich panel: In developed countries, ferrocement is a significant laminated unit of building constructions as shown in Fig. 8. It might be used as an independent part of structures like water tanks, walls, infill frames, chemises, silos and marine structures. Cement, sand, wire mesh and water are constituents of Ferrocement and have some attractive properties such as fire-resistance, antirust, seismic resistance, and rot or blow down in hurricanes. It is used in repairing of damaged buildings or retrofitting also. Ferrocement acquire large tensile strength and supreme cracking behaviour if compared to regular reinforced concrete .
Structural insulated panels (SIPs): SIP is a sandwich panel utilised as construction member such as wall, roof, and floor for concrete structures as shown in Fig. 9. It vary in altered thicknesses of two layers of rigid material as skin and a thicker layer as core. Based on its appliance, it can be made of a variety of materials. It is usually made of plastic foam such as Polyurethanes (PUR). PUR foam has superior performance against fire, flaming, and smoke rating. Injected PUR foam can be easily adhered to all SIP components such as skin material, top plates, and electrical boxes. Thus, it allows tough bond between mating surface and the foam .
Glass fibre reinforced gypsum (GFRG) panel system: Glass Fibre Reinforced Gypsum (GFRG) Panel also identified as Rapid wall is made-up of calcined gypsum plaster, reinforced with glass fibres as shown in Fig. 10. The panel was initially developed in 1990 in Australia for mass scale building construction. In recent times, these panels are being created in India and are being used. The thickness of panel is 124mm to a length of 12m and height of 3m, contains cavities that may be fully filled, partially filled or unfilled with reinforced concrete as per structural requirement. Filling these with plain reinforced concrete possesses substantial strength capable of resisting lateral loads due to earthquake and wind. GFRG panel can also be used favourably as in-fills (non-load bearing) in grouping with RCC framed columns and beams. GFRG Panel is manufactured in semi-automatic plant using water repellent emulsion and glass fibre rovings, cut, spread and imbedded uniformly into the slurry with the help of screen roller. Before shifting to storage area or the cutting table the panels are dried at a temperature of 275oC. These panels can be cut as per dimensions & requirements of the building planned. It is suitable for low rise to medium rise building..
Light Gauge Steel Structural Systems
Pods: Small rooms of light steel frame with all fittings and finishing. It is based on factory made galvanized light gauge steel components, designed as per code requirements as shown in Fig. 11. Cold forming method is used to produce the panels and assembled forming structural steel framework of a building of varying sizes of wall and floor. Special types of screws and bolts are used in joining. In residential floors, industrial buildings, commercial buildings, hotels Cold formed sections are broadly used. LGSF is gaining ground in India due to its flexibility, fast construction and durability after being used in North America, Australia and Japan. It is usually ideal for one to three storey high buildings (residential and commercial). Advisable maximum span for these buildings should be 7.5 m. These could be used for short-term or permanent structures such as schools and classroom, military and civil housing needs, post – disaster relief structures and industrial units .
Precast Concrete Construction Systems
Industrialized 3-S System using cellular light weight concrete slabs & precast columns: This technology is being used since 1972, and is based on industrial unit mass manufactured structural prefab components meeting the requirements of Indian Standards. The major precast elements are:
- RCC hollow columns with notches
- RCC solid beams (T/L/Square Shape)
- RCC precast slab
- AAC precast slab
- AAC precast block
Wafflecrete building system: It consists of huge, structural, ribbed panels of reinforced precast concrete, bolted together and the joints between the panels are caulked to form the walls, floor and pitched or flat roofs of buildings as shown in Fig. 13. The surface of each panel consists of 51 mm thick slab with overall panel thickness of 152 mm or 203 mm. In single storey buildings, floors are constructed using precast reinforced concrete floor panels supported on precast concrete grade beams on well- compacted earth is used in single storey buildings. While for buildings of more than one storey, the walls are supported on foundations designed as per the soil condition. Where there is a danger of water or wind erosion of the ground adjacent to the building a concrete apron is laid around the perimeter of building. Internal walls consist of either reinforced precast concrete ribbed panels, conventional masonry walls or concrete walls. Before the walls are lined services like water supply and electricity shall be normally accommodated in preformed slots in the ribs of panels. Trapping the moisture generated from the concrete curing time is reduced. The structure after construction can be shifted from one place to another as the structure is joined using bolt connections .
Pre-stressed precast system using hollow core slab, beams, columns etc: The current and future market demand for the building industry can be fulfilled by the precast pre stressed hollow core slab. A precast hollow core slab is a pre-stressed concrete member with constant voids which extend all through the length of the slab, provided to decrease the weight and, hence the cost and as a side advantage, to use for covered electrical or mechanical runs. Mainly used as floor or roof deck systems, hollow core slabs also have appliance as wall panels, spandrel, members and bridge deck units. Hollow core slab Span length reach up to (18m) with no columns or any supporting. This arrangement can be preferably used in residential, commercial, car parks or repetitive construction projects. Precast prestressed hollow core slabs provide maximum structural efficiency with the use of high strength concrete, yet at the same time requiring less material utilization . The system is shown in Fig. 14.
Steel Structural Systems
Speed floor system: These are suspended concrete flooring system using a roll formed steel joist as an essential part of the final concrete and steel composite floor. An integrated continuous one-way slab and a hybrid concrete/steel tee-beam in other direction as shown in Fig. 15. The joists of altered depths are manufactured from pre-galvanized high tensile steel in a one pass roll former, where it is roll formed, punched, pushed and slotted in a fully mechanized instrument. Depending on the span. The joist depth and the concrete thickness may vary, forced loads and other efficient considerations. It’s suitable for use in all types of construction.
Timber-concrete prefabricated compo- site wall system: Wood has been used as building material mainly attached with brickwork or stone in Europe. In order to develop the seismic resistance of masonry buildings wooden structural elements has been in a practice. It was composed basically of two parts, a slab of reinforced concrete (RC) with a thickness of 50 mm connected with particular connectors. A structure made of CGF panels for load-bearing walls and floors is a modular system in which the panels are prefabricated. The panels are then assembled providing insulation inside the frames and then are easily transported to the site as shown in Fig. 17. After having a foundation curb in the ground, the panels are hooked to it and to each other with nails and screws .
Sim[PLY] framing system: In these framing system plywood subcomponents, fastened together with tab and slot joints with hand-fastened steel cable ties are composed together to form structural members in the system as shown in Fig. 18. Due to its renewability, low-carbon footprint, ease-of-use, minimum weight, and affordability Wood is the perfect choice for this sustainable light-frame system. Furthermore, the CNC procedure allows for systems within a given house, including electrical wiring and plumbing. This promotes quick and exact assembly. Mainly are used to maximize material efficiency and lessen plywood waste. The prefabrication allows for disassembly of the house to be rapid and simple, taking only three days. Small, efficient, and mass-produced structures use this Sim [PLY] system .
Ultra-thin phase change material technology: To improve the thermal performance of building envelops and to achieve the goal of energy saving, thermal energy storage (TES) is one of the best ways to get better thermal performance. Ultra-thin phase change materials (PCMs) are a series of functional materials that give high-energy storage density in a thin temperature interval. PCM the appliance areas are mostly some cities of Europe .
Prefabrication is a promising strategy to realize lean construction. Among the diverse degrees of prefabrication, modular buildings make the most of the gain in time savings, because they are prefabricated to a better degree of finish. These are based on local building codes and standards, in the similar way as on-site built construction; thus, of equal quality to an on-site built construction. Although, maintenance during occupancy, deconstruction, and recycle or reuse of modular buildings as they are just not limited to design, manufacturing, and construction stages . Abundant projects incorporating prefab (on various levels) have already been completed successfully, and numerous more are planned. The potential for enlargement in the building economy; embracing higher output, total sustainability, improving place of work and workforce security, was hypothetical some ages ago, but is a practical recognition today and in future, through prefab. The effective factors are shown in Table 2.
|Table 2: Effective factors in increased used of prefabrication systems in future|
|Effective factor||Effect of using prefabrication|
|Program||The ability to progress work as a parallel operation in a factory and on a construction site|
|Factory tolerances and workmanship||They will show a higher quality and consistency to that achieved on site|
|Energy consumption||Reduction in energy consumption due to the automation|
|Labour markets||Access to cheaper labor markets according to factory base systems|
|Program certainty||Greater program certainty as a result of computerization|
|Safety||The factory environment can allow better safety than the construction site|
Issues in Prefab Construction
The joints to be provided linking the core structure and the components should be well-built enough to transmit all types of stresses. The strength and stoutness of the entire building depends totally on the strength of the joint. Therefore, it is necessary to have comprehensive studies on entire system rather than component based study. Requirement of skilled labor at site and shortage of onsite automation is one of the major issues in acceptance of prefabrication technologies in construction which requires accuracy and preciseness. Thus, skill development and native automation is mandatory for installation of the prefabricated systems. During erection or transportation of heavy machinery units are likely to get damaged thus the arrangement of the units has to be done precisely and this procedure becomes clumsy in a congested area. Labor maintenance is one more issue as skilled labor is required in the prefabricated construction as it is different from in-situ construction, which requires machine oriented skills both on-site and in the manufacturing process .
Advantages of Prefab Construction
Prefabrication Technology has various advantages like overhauling the view with energy efficiency, minimal wastage and inspection struggle, dependable construction, pace of work, security, sustainability and quality .
- The requirement for formwork, shuttering and scaffolding is significantly reduced as Self-supporting ready-made components are used.
- Construction time is reduced thus resulting in lower labour costs.
- Reduced amount of waste materials than in site built construction.
- Reduction in Construction time allowing an earlier return of the principal invested.
- Building ensures accurate conformity to building standards and superior quality assurance.
- High-energy efficiency along with quality control and factory sealing.
- Prefabrication site can be positioned where skilled labour is more readily accessible and the expenses of labour, power, materials, space and overheads are minimized.
- Prefabrication allows construction all over the year irrespective of the weather (related to excessive cold, heat, rain, snow, etc.).
- Construction material wastage is less.
- Independent of climatic condition.
- In off-site construction safety and comfort level of worker are higher .
It is known to have the subsequent limitations limited options in design, decreased resell value, high initial investment, non-suitability for foundation and transportation of precast units .There are more disadvantages too. They are as follows:-
- At joints in prefabricated components leakage occurs.
- Transportation costs may be high for huge prefabricated sections.
- Increased production volume is necessary to make sure affordability through prefabrication.
- Initial construction cost is higher.
- The initial design development is time consuming.
- Huge prefabricated sections need heavy-duty cranes and accuracy measurement from handling to place in site.
- Local jobs may be lost, as it requires skilled labour.
- Design and construction of modular buildings, require high levels of collaboration among project parties, especially architect, structural engineer and manufacturer.
- Due to its shorter economic life these buildings typically depreciate more quickly than traditional site-built housing .
This mainly focuses on the challenges faced by the construction sector at national, as well as in the international level and its adoption in construction sector. It also discusses the new prefab-technologies developed, along with benefits of this technique in construction industry. Therefore, the paper comes to a conclusion that prefabrication technology with huge advantages is an essential technological up gradation in construction sector to defeat the present challenges world-wide and it has the capacity to make a difference in the sector in terms of financial, social, ecological sustainability in India as well.
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