Concrete Chronicles 2

The characteristics of set concrete and its further maintenance depend on:

• Its strength.

How can you know its strength? Simple! Just use this formula to calculate the maximum load resistance for one square unit of your concrete:

STRENGTH =   Pmax  N/mm2

                             A

Concrete is said to have achieved maximum strength upon 28 days of setting it.

• The compaction process.

Compaction is the process of removing entrapped air to form a homogenous dense mass. The compaction of concrete can be achieved in four ways:

Hand rodding

This is the crudest method to compact concrete. A steel rod is used to    compact the concrete to release trapped air at equal intervals. A predetermined number of strokes is used every time.

Mechanical vibration

Mechanical vibration compacts concrete equally and steadily, unlike hand rodding which is done manually. The picture on the left shows a mechanical vibrator.

Centrifugation (machine)

Centrifugation is usually used to produce concrete forms that are circular in cross-section, such as lamp posts.

Steps:

1) Concrete is fed into horizontal mould, which is spinning slowly.

2) Once adequate concrete has been fed, the speed is upped.

3) Water is forced out of the mix and flows out of the mould.

4) A small amount of dry cement is sprinkled to prevent upsetting the water:cement ratio.

5) Two round rods are held against each end-ring to finish the surface.

  - High pressure and shock

• The curing process.

The process of creating an environment favourable for the setting and hardening of concrete in a relatively short period of time is termed 'curing'. This takes place directly after the concrete has been placed and compacted.

The curing process controls the loss of water on the surface of the concrete.

• Its durability.

Its durability refers to the span of time concrete can withstand the ravages of time with minimum maintenance.