Precast concrete

1 Introduction
2 Applications
3 Advantages of precast construction
4 Disadvantages of precast construction
5 Manufacturing process
6 Installation
7 Site vs factory precasting
8 Connecting precast concrete elements
9 Related articles on Designing Buildings Wiki

[edit] Introduction

Precast concrete is a form of concrete that is prepared, cast and cured off-site, usually in a controlled factory environment, using reusable moulds. 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 steelwork, in-situ concrete and precast concrete elements.

[edit] Applications

Reinforced concrete is usually used for structural systems due to its strength, durability, and affordability. Precast concrete is used in the following ways:

To make beams, columns, floor slabs, foundations, and other structural members for buildings.
To make wall or cladding panels for buildings.
To make precast pre-stressed elements for buildings.
To make components for infrastructure projects: elements such as bridge spans, or metro line viaducts are often precast in a casting yard.
To make products such as precast water tanks, septic tanks, drainage chambers, railway sleepers, floor beams, boundary walls and water pipes.
As it can be moulded into any shape, it can also be used to create one-off unusual forms such as boats, sculptures and so on.

[edit] Advantages of precast construction

Precasting is good at producing large numbers of identical components. For example, building an affordable housing project with identical apartments could use precasting to produce wall slabs and floor slabs for all the apartments, and then lift them into place and connect them.

As it is done in a purpose-built precasting yard or factory, it makes construction easier for the following reasons:

The construction is done on the ground rather than at height.
It can be done inside a climate-controlled structure, eliminating problems of rain, dust, cold, or heat.
Specialised formwork (moulds) can be built for doing many repetitions of the same component.
Specialised equipment can be used to make, move, and pour the liquid concrete.
Curing takes place in a controlled environment.

This means that the quality of precast components can be very high.

Since the components can be made beforehand, construction can be very quick. In cast-in-situ construction, engineers have to build each set of components after the previous set has finished, which does take time, as concrete generally takes 28 days to reach its full strength.

[edit] Disadvantages of precast construction

There are a few main disadvantages of precast concrete construction:

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.
As the building is made of discrete components, the joints between adjacent members have to be sealed with special sealants to make them waterproof
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.

[edit] Manufacturing process

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 minimised, and that a denser, stronger and better quality concrete can be produced.

Concrete is cast into forms and left to cure. Precast forms are normally made of steel or plywood. Whereas plywood forms are usually limited to about 20-50 castings depending upon the complexity of the form, a virtually unlimited number of castings can be made by precasting using steel forms.

Precast elements generally incorporate steel reinforcement to resist loading stresses. A common cause of the deterioration of concrete structures is the corrosion of this reinforcement. It is important therefore, that they are properly designed and embedded in the concrete.

During the manufacturing process, admixtures can be included in the concrete. These can be water-reducing, air-entraining, retarders and accelerators (for faster curing time). The purpose of admixtures is to improve concrete quality in both its fresh and hardened state. Colour pigments can also be added, such as iron oxides (red and brown), chrome oxides (green) or cobalt oxides (blue).

An alternative form of precasting is prestressed concrete, where stresses are introduced into the structural member during fabrication as a way of improving both its strength and performance. For more information, see Prestressed concrete.

[edit] Installation

The on-site installation of precast components can be a high-risk activity involving the use of heavy plant, cranes and personnel working at height. Consideration should be given therefore to safeguarding against risks when receiving delivery, moving, and placing units.

Consideration should be given to:

The method and sequence of assembly and erection.
The method of providing temporary supports.
Structural connections and joint details.
Tollerances.
Handling and rigging requirements.
Site accessibility for delivery and storage.
Crane capacity and working clearance for hoisting.
Sample measurement to confirm the accuracy of critical dimensions.
Visual inspection of concrete finishes for defects.
Locations and conditions of lifting inserts for hoisting.

[edit] Site vs factory precasting

Precasting 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 precasting 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 precasting 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 formwork.
A formwork 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.

[edit] Connecting precast concrete elements

Precast concrete components can be connected in a number of ways:

They can be bolted together. In order to do this, steel connectors are embedded in the concrete at the time of casting. This must be done with great precision.
They can be grouted or concreted together. In this method, loops of steel reinforcement are left protruding out of the precast concrete members. Two members are placed in position, and reinforcement is threaded between the loops. Fresh concrete is then poured around this reinforcement, in a space left for this purpose.