Concrete Chronicles

Concrete is one of the most commonly used building material in the construction industry today, although it is a relatively new material. Concrete's make-up is a mixture of several ingredients - cement, sand (fine aggregate), coarse aggregate and water - in varying ratios depending on the desired aim: strength or economy.

Cement acts as the bonding agent. In fact, hardened concrete is formed because of a chemical reaction between water and cement in a concrete mixture. A wet construction site will thus be advantageous because then the setting concrete need not be covered with a wet cloth to preserve moisture.

Here are some illustrations on how concrete is used in floor construction:

Ground floor construction.

Upper floor construction.

Concrete can be moulded into practically any form, as it mimics the shape of its formwork; it can even be pumped and laid in various different, difficult positions.

Advantages:

• Durable
• Fire resistant
• High compressive strength
• Low maintenance
• Not affected by corrosion
• Not susceptible to insect attacks
• Easy to handle when batching and mixing
• Reasonable price (economical)
• Widely available 

Disadvantages:

• Low tensile strength, cracks easily
• Fresh concrete shrinks upon drying (water evaporates), while hardened concrete expands upon contact with moisture
• Concrete expands and contracts with changes in temperature
• Concrete will disintegrate under alkali and sulphate attacks
• Lack of ductility - unsuitable for earthquake-resistant designs

The quality of concrete depends on:

• The quality of its ingredients
• The batching and mixing ratios
• The grades of aggregate used
• The amount of water used
• The quality of the compaction process
• The curing process

There are several types of concrete available for use. Some of them are:

• In-situ Concrete

The concrete is mixed and tested on site. This is how it s made:

• Pre-cast Concrete

The concrete is readily mixed, and pre-cast panels are made at a factory off-site then shipped to the construction site. This has the advantage of quickening the construction process (as we do not need to wait for the concrete to set and cure) and also reducing the cost of formwork.

• Pre-tensioned Concrete

Sometimes we require the concrete structures to withstand more force than what normal concrete structures can afford. In that case, we can pre-stress or pre-tension the concrete to increase its resistance to stress.  Illustrated below is the method to create pre-tensioned concrete:

Step 1: Stretch the steel pre-stressing strands tightly across the casting bed. 

Step 2: Cast the concrete around the strands and left to cure, during which the concrete will bond to the strands.

Step 3: After the concrete has adequately cured, cut the strands. The concrete will go into both compression and tension simultaneously. This will increase its strength.