Precast Concrete system
What is precast concrete .
Precast concrete is a form of concrete that is prepared, cast and cured off-site, usually in a controlled factory environment, using reusable molds. Precast concrete elements can be joined to other elements to form a complete structure. It is typically used for structural components such as; wall panels, beams, columns, floors, staircases, pipes, tunnels, and so on.
Structural steel frames can provide an alternative for pre-fabricated structural components, but precast concrete can be more economical and sometimes more practical. Many buildings now include a mixture of both construction techniques, sometimes incorporating structural steel work, in-site concrete and precast concrete elements.
where we can use precast concrete
Precast concrete are usually used for structure elements such as :-
- Beams, walls, columns , floor slab and facade cladding for building .
- Precast pre-stressed structure system .
- For infrastructure work such as a bridge span.
- For any product with unusual forms such as boats , sculptures and so on .
Advantages of precast system
- Can cast concrete at any time because concrete casting in controlled climate location .
- The quality of precast concrete components can be very high .
- Can product a large number of structure elements in a short time .
- Reduced cost by reduced people in site and reduced rent or purchasing of materials such form work system and curing materials .
- Thermal efficient and low maintenance .
Disadvantages of precast system
- Since each piece is made separately, the structural frame or system is not monolithic or continuous like regular concrete construction. The joints between pieces create structural discontinuity. The forces of the building will pass through these joints, so they have to be designed to transfer these forces safely and properly. Note that precast concrete can be used for non-structural members too.
- Each precast component is usually large and heavy. This means that cranes are required to lift them in position; these cranes are required to operate over the entire building volume. Since there will only be a few cranes at site, the time taken by the cranes to pick up a piece and shift it to its final position becomes critical in determining the building schedule.
- As the building is made of discrete components, the joints between adjacent members have to be sealed with special sealants to make them waterproof
Precast process
General
The production of precast concrete elements takes place under controlled conditions in enclosed factories. This means that tolerances can be accurately controlled, waste can be minimized, and that a denser, stronger and better quality concrete can be produced.
MOLDS
- Molds for precast elements shall be prepared on fixed / rotating pallets .
- All forms/molds shall be perfectly cleaned .
- Opening should be in correct size and position .
- Line and level shall be as per design .
- Correct angle for batter .
- For every concrete pouring , integrity of mold /form work will be carefully examined . after cleaning the molds properly to remove the loose particles , suitable mold release oil will be applied to prevent adhesion of concrete .
Reinforcement.
- Reinforcement will be procured from approved suppliers. Test certificate for the procured lot will be provide for record and verification .
- The reinforcement will be clean and free from rust , dust , grease ,etc.
- The reinforcement will be cut and cold bent by experience worker and well be placed with a proper cover as per design .
- Reinforcement cage is placed in the mold support on plastic or concrete spacers to give the required concrete cover .
- Four point lifted anchors with the required number and capacity as given in design as well as the fixing accessories required for fixing the elements together with the steel inserts , if required will be fixed with the mold .
Pre-stressing.
- PC strand will procured from approved supplier . Test certificate for the procured lot will be provide for record and verification .
- The strand will be placed as per design .
- Pre-stressing will be carried out with calibrated equipment as per the design requirements.
Concrete work.
- Approved concrete mix as per design will be used .
- All required concrete test such as slump , concrete temperature , compressive strength to be check as per design .
- For pre-stressed elements strands will be cut only after the concrete on the last cast beam has attained the required strength as per design .
- Concrete will be poured in the moulds , spread , leveled using proper hand tools the compacting is done by vibration frame on to which the mould table is rolled and then fixed , the vibrate frame can both vibrate with a mortarboard and consolidated using vibrators .
- Concrete surface finishing as per design requirements.
Curing.
- Precast concrete can cured by many methods such as water , chemical curing compound or by curing chamber .
Stripping , loading, transportation , and stacking.
- Once concrete cubes pass transportation will be done on a flat bed trailers with proper support .
- Elements will be stacked on leveled surface with proper support at bottom to avoid the damages .
- Crane will be used for loading and off-load of the precast units .
Installation.
- Pre-casting can be carried out at a casting yard, in or near the site, or in a factory. A key aspect of determining whether to use site or factory pre-casting is transport costs. Factory work offers superior quality for obvious reasons, so if there is a factory close to the site, it makes sense to use it.
If a pre-casting yard is to be created, space must be laid out for the following activities:
- Storing the raw materials, such as cement, aggregate, sand, admixtures, water, reinforcement bars,
- and steel or plywood sheets for form-work.
- A form-work making and maintenance yard.
- A concrete mixing plant.
- A steel reinforcement yard to make rebar cages to be placed inside the concrete.
- A casting area.
- A curing area.
- A stacking area for finished components.
For infrastructure projects, a casting yard is created on a piece of open land in the city. It is important that this is located near a major highway, as the precast elements can be very large or heavy, and cannot be taken through narrow roads.
We will learn more about precast concrete installation methods through the following articles titled method statement .
In this article, we reviewed a type of concrete, which is precast concrete, and the way to implement it.
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