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.

Cemented Confusion

It is not unusual for those who are unfamiliar with the construction industry to confuse concrete with mortar and grout. Some might even mistakenly think that they are the same thing, albeit used for different purposes.

Well, that's wrong.

The difference lies in their ingredients. Let's see.

1. Concrete  is made up of:

       concrete                        water                        fine aggregate (sand)          coarse aggregate (gravel)


2. Mortar does not have coarse aggregates:

       concrete                        water                        fine aggregate (sand)          

3. Grout has neither fine nor coarse aggregates:

       concrete                        water 


So to sum it up...

	CEMENT	WATER	FINE AGGR.	COARSE AGGR.
CONCRETE	X	X	X	X
MORTAR	X	X	X
GROUT	X	X

Now you know that they are not the same!

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.