BATCHING AND MIXING
Cement, aggregate, sand and water, and sometimes admixture are prepared in the correct proportions as given in the mix design. Mixing can take place at the concrete plant, in a truck-mixer or, on a small job, in a rotating hand-mixer.
Formwork forms the mould to shape the concrete and must be carefully constructed before placing. A special liquid or release agent must be applied before pouring the concrete, just like greasing a cake tin. Formwork is specially designed to achieve both the required finish and the strength to withstand very high concrete pressures.
Concrete must be poured into its final position as soon as possible after mixing the concrete will begin to set as soon as the cement comes into contact with water. It is placed in-situ by a pump, skip or directly from the truck-mixer.
As much entrapped air as possible must be removed from the placed concrete. Large air bubbles will reduce the strength and durability of the hardened concrete as well as resulting in a poor quality finish. This is done by vibrating the fresh concrete, usually by using an internal poker vibrator. External vibration is often used in the manufacture of precast units.
Covering the concrete surface as soon as placing and compaction have taken place is essential to prevent water escaping. This will allow the water to react fully with the cement, ensuring a hard-wearing surface and minimising cracking.
Methods of curing include covering with a plastic sheet, damp Hessian or damp sand, or spraying with an impermeable liquid. Formwork can also be used to cure the concrete surface.