Last edited 30 Sep 2016

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.

Advantages of using precast concrete include:

  • It is manufactured in a controlled environment where greater accuracy and better quality control are possible.
  • Reusable factory moulds mean it is economical to produce repetitive units.
  • There is generally less waste.
  • On-site assembly can be very fast and is generally weather independent, reducing site delays and ensuring the construction programme can be maintained.
  • Precast concrete is very durable.
  • There are fewer local environmental impacts such as dust and noise pollution.

[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.

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