Monday, January 9, 2012

PRECASTING & PREFABRICATION

Prefabrication or Precasting is essentially the creation of units & elements (most often concrete elements) either in a factory or in a site yard instead of casting them in-situ at the required place. Once the elements are cast, they are cured properly and then lifted / placed into position during the time of construction. In building construction, the most common precast elements are wall panels, roof panels, beams, staircases, domes, retaining walls, even furnitures. Nowadays in the infrastructure segments almost all construction elements of flyovers, bridges etc are precast & then erected into position. Through precasting, we are able to avoid time consuming activities like shuttering, formworks, placing of concrete etc. leading to faster completion of projects . Also, it is possible to have a much better quality control & even reduce wastages thereby reducing the construction costs.
Precast Roofing elements in Bangalore International Airport

Precast Roofing elements in Bangalore International Airport

A Precast Staircase in AEI, Auroville
One of the basic principles of precasting is that the entire design has to be modular. By modular I mean that the different elements have to be standardised to uniform dimensions, specification etc. So that it will be easier to fabricate. It is also possible to prefabricate a few design elements alone in a building, but even then it has to be well thought out right from the design stage itself. Details like how to join the precast element with adjacent elements will have to be properly designed. During the construction phase proper planning has to be done to ensure smooth execution. So even things like planning the location of the precast yard plays an important role as that will decide the distance the precast element will have to be lifted & transported. Also, a workable design of the mould for precasting will have to be done as the mould is very important in how the final quality if the elements turn out.
ADVANTAGES OF PRECASTING –
  • Ensures better quality - It is possible to have a good quality control over the entire process especially since the process is repetitive. Thus, it is possible to have good control & supervision of the quality of moulds used, quality of materials used, proper placing of materials, proper curing after placing etc and ensure that the elements are of good quality.
  • Savings in time – Time consuming activities like shuttering, scaffolding, formwork, deshuttering etc can be totally eliminated. Also, since the process is repetitive, the cycle times generally tend to improve as productivity increases.
  • Independent of other activities – Once the precast yard is established, we can start manufacturing of precast elements even before the actual start of other building works. Thus, proper control of the construction activities is possible.
  • Savings in cost – if the design of the precast elements are done properly, a lot of optimisation can be done in materials, dimensions etc, which will help in reducing the cost. Savings are also made on cost of shuttering, scaffolding etc. The formworks used in precasting can be used for a much larger number of repetitions compared to cast in-situ work. Also, indirectly, faster completion of projects will ensure faster occupation of buildings and hence quicker return of capital invested.
  • Clean work fronts – usually since all the elements are precast, the in-situ work done at site is reduce leading to a much cleaner work area.

Precast Treads

Mould for precast Ferrocement channel
Precast septic tank in CSR, Auroville

ADDITIONAL THINGS TO BE CONSIDERED –
  • Lifting requirements – all precast elements will be lifted and then placed into position. For this, additional strengthening & lifting hooks will have to be designed & provided before casting itself.
  • Proper Planning – the entire process of precasting & also the subsequent erection will have to be carefully planned to avoid unnecessary delays & problems. Also, once the precast elements are ready for erection, their storage becomes crucial. Thus, the precast production & subsequent installation are to be planned very carefully to ensure a smooth construction process. This is very important as storage spaces in a construction site are always at a premium. So if quantity of precast elements manufactured are more and are not used immediately for construction, lot of storage space will be required which will lead to complications. Also, the lifting & erection process will also have to be planned carefully
  • Joints – Additional care will have to be taken to ensure that the joints of the precast elements are done properly and are waterproofed properly to avoid leakages.
Basic sequence of works in making a concrete precast element in a nutshell–
  • Establishing the precasting yard & preparation of the moulds. The formwork or moulds are to be made in the negative shape of the elements and are to be structurally sound to take up the load of the element.
  • Tying of the reinforcement as per the structural reinforcement details.
  • Placing of the reinforcement on the mould ensuring provision for suitable cover is to be supported in correct position to avoid displacement during placement of concrete. Along with this, lifting inserts should also be placed & fixed properly to the reinforcement.
  • Placing of concrete & suitable compaction using vibrators. Concrete must be placed in a uniform manner & properly spread over the area before commencing vibration.
  • Curing of the element on the mould till the initial setting is achieved for lifting the element from the mould.
  • Removing the element from the mould & carrying out the final curing. The strength, watertightness and durability of concrete depends on the concrete being adequately cured. Curing agents may be used for this purpose.
  • Proper storage of the precast elements in the yard. The storage area should be large enough for elements to be stored properly with adequate room for lifting equipments like cranes.

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