Concrete

Concrete is mixture of sand, crushed stone or lime and water. Sand forms the fine aggregate, crushed stone and gravel forms the coarse aggregate. The strength of the concrete depends up on the proportions of fine and coarse aggregates.

Types of concrete

>plain cement concrete

>Reinforced cement concrete

>Pre-stressed concrete.-

(a)Plain cement concrete

It is a mixture of cement, sand, pebbles (or) crushed rock and water. It has a high compressive strength. It is weal in tensile strength. It is free from corrosion. It is used for floorings, gravity dams, simple strip foundation etc. and other mass concreting works.

(b) Reinforced-cement concrete.

Cement concrete prepared with steel Reinforced is called reinforced cement concrete (R.C.C). it can be used for the structural members subjected to both compressive and tensile stresses. Steel is strong in tension and introduced in the places where tensile stresses are developed. It is used for roofs slabs, beams, columns, dams, pile foundation, railways etc.

(c) Pre-stressed concrete

In this type of concrete, high tensile steel wire are used as reinforced instead of mild steel bars. There are two types of pre-stressing namely pre-tensioning and post tensioning. In the pre-tensioning method wires are initially stressed and the concrete is built around the wires. The wires are released after the concrete attains is strengths. In post tensioning, the wires are placed inside the concrete and then stressed.

(d) Fibre Reinforced concrete

It consists of cement fibre, sand and water. Nylon, glass, Asbestos is the fibres used for reinforcement. It is more durable. Production rate is less and less maintenance cost.

Mix proportion of concrete and its uses

Cement: fine aggregates (FA): course aggregate (CA)

Aggregate is nothing but crushed stones.

Preparation of concrete

Mixing of concrete involves the following operations namely

(a) Batching

It is the collection of measuring of concrete ingredients, (cement/FA/CA/water/)

(b) Mixing

There are two types of mixing namely hand mixing and machine mixing. In manual mixing 10% of extra cement should be used.

(c) Transporting

Prepared concrete is conveyed to the work spot manually or by belt conveyors.

(d) Laying (placing)

The concrete should be placed in the form work always by vertical dropping to avoid segregation.

(e) Compaction

The strength of the concrete depends on the degree of compaction. Compaction is done manually (hand compaction) or by using vibrator.

Test on concrete

Fresh concrete: To check the workability of concrete and properties of segregation and beeding in concrete.

Hardened concrete: To check the compressive strength of concrete and its durability.

Slump test:

>Slump test is the most commonly used method of measuring consistency (workability) of concrete.

>Slump test is to test the water cement ratio.