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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BUBBLEDECK - INTERESTING CONCEPT FOR SLABS

BubbleDeck is a revolutionary method of virtually eliminating concrete from the middle of a floor slab not performing any structural function, thereby dramatically reducing structural dead weight. BubbleDeck is based on a new patented technique - the direct way of linking air and steel. Void formers in the middle of a flat slab eliminates 35% of a slabs self-weight removing constraints of high dead loads and short spans.

Incorporation of recycled plastic bubbles as void formers permits 50% longer spans between columns. Combination of this with a flat slab construction approach spanning in two directions – the slab is connected directly to insitu concrete columns without any beams - produces a wide range of cost and construction benefits.

ADVANTAGES -

v   Design Freedom – flexible layout easily adapts to irregular & curved plan layouts.

v   Reduced Dead Weight – 35% removed allowing smaller foundation sizes.

v   Longer spans between columns – up to 50% further than traditional structures.

v   Downstand Beams eliminated – quicker & cheaper erection of walls and services.

v   Load bearing walls eliminated – facilitating MMC with lightweight building envelopes.

v   Reduced concrete usage – 1 kg recycled plastic replaces 100 kg of concrete.

v   Environmentally Green and Sustainable – reduced energy & carbon emissions.

GREEN CREDENTIALS –

By virtually eliminating concrete in the middle of a slab BubbleDeck makes a significant contribution to reducing environmental impact. Guidance from the ODPM requires the direct environmental effects of buildings

to be considered, including usage of natural resources and emissions resulting from construction. Not only is concrete usage reduced by up to 50% within a buildings structure but knock-on benefits can be realized through reduced foundation sizes.

Every 5,000 m2 of BubbleDeck floor slab can save:-

v 1,000 m3 site concrete.

v 166 ready mix lorry trips.

v 1,798 Tonnes of foundation loads – or 19 less piles.

v 1,745 GJ energy used in concrete production & haulage.

278 Tonnes of CO2 – green house gases – emissions

Atleast that is what the theoretical part is. We now need to see how this concept works when implemented.

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FILLER SLABS

• In a simply supported RCC slab, the upper part of the slab is subjected to compressive forces while the lower portion is subjected to tensile forces. Concrete is very good in compression while it is weak in tension. Hence the steel reinforcement is placed in the tension zone. The concrete in this portion is only for holding together the steel reinforcement and has no structural purpose. This 'un-productive' concrete can be replaced by a 'Filler' material which will reduce the weight of the slab and also reduce the cost by reducing the amount of concrete used. Also, since the weight of the slab is thus reduced, lesser steel is also required for reinforcement, further reducing the cost.
• There is no reductoin in the structural strength of the slab at all and it will act as a normal load bearing slab.
• Common filler materials are light weight materials like Mangalore tiles, clay pans, bricks, waste bottles, coconut shells, thermocol etc.
•  Reduces the cost of the slab by around 25-30%
•  Creates interesting designs and patterns on the ceiling
• Helps in better thermal insulation of the slab due to the air pockets present.
•  Reduces the load on the foundation

A Typical Filler slab in Centre for Development Studies, Trivandrum

A filler slab roof using clay pans

Reinforcement details & clay pan positions

view showing reinforcemnt with roof tiles in between

Typical cross-section



View of finished ceiling






FERROCEMENT

Ferrocement is a form of reinforced concrete using closely spaced multiple layers of mesh and/or small dia rods completely infiltrated with mortar.

It is thinner and lighter than RCC structures and has greater tensile strength and more flexibility than RCC. /one can create a lot of interesting shapes with ferrocement. It is water tight and thus finds application in structures like water tanks, roofs etc.

It also uses lesser quantity of cement and steel and is hence cost effective compared to RCC.

Typical Cross section

Ferrocement Roofing using ferrocement channels

Roofing using ferrocement U sections

With Ferrocement, it is possible to make slabs of thicknesses as less as 20mm, which can be used in applications like door shutters, water tanks etc.

Ferrocement is especially useful for cost effective housing projects. The technology is easy to master and even ordinary masons can be trained to use this. There is a large scope for application in developing countries, where the resources are scarce.

APPLICATIONS -

v Doors and Windows- Frames and Shutters.

v Wall panels, Roofing Elements, Flooring channels, Precast Toilets

v For setting up Temporary shelters for workers toilets, Site offices, etc.

v Water tanks, Manhole covers, Stair Treads, cupboard shelves etc.

A ferrocement door shutter

Water tank

Ferrocement Toilet

PROCESS –

The first thing required is a base mould over which casting can be done. The negative mould (because it is in the opposite shape of the final structure), is often made of brick, sand and mortar. The mould is made first and is allowed to set

Mould for Ferrocement channel

A portable mould

Once the mould is ready, the basic reinforcement cage is made using 8mm rods and chicken mesh. 2-3 layers of chicken mesh are spread over the reinforcement and tied in place by binder wires. Once the basic reinforcement is ready, waste oil is applied on top of the mould. This is to facilitate easy debonding during the final removal of the finished structure.

A marginal layer of mortar is laid over the mould, over which the reinforcement layer is placed.

Finally, once the chicken mesh reinforcement is properly in place, mortar is then applied from the top and finished in the desired shape. The thickness of this mortar layer is around 10-15mm.

The finished structure is allowed to set completely and then removed from the mould. After this, regular curing is to be carried out using hessian bags and water. Normally, the structure is cured for around 28 days for complete structural integrity.