Last edited 21 Sep 2021

Precast concrete


[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:

[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:

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:

[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:

[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:

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:

[edit] Related articles on Designing Buildings Wiki


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