TD6/3: CONCRETE

CONCRETE

Plain concrete, commonly known as concrete, is an intimate mixture of binding material, fine aggregate, coarse aggregate, and water.

This can be easily molded to the desired shape and size before it loses plasticity and hardens. Plain concrete is strong in compression but very weak in tension.

PLAIN CONCRETE

Major ingredients of concrete are:

1. Binding material (like cement, lime, polymer)
2. Fine aggregate (sand)
3. Coarse aggregates (crushed stone, jelly)
4. Water

Functions of Various Ingredients

Cement is the binding material. After the addition of water, it hydrates and binds aggregates and the surrounding surfaces like stone and bricks. Generally richer mix (with more cement) gives more strength.

Setting time starts after 30 minutes and ends after 6 hours. Hence concrete should be laid in its mould before 30 minutes of mixing of water and should not be subjected to any external forces till final setting takes place.

Coarse aggregate consists of crushed stones. It should be well graded and the stones should be of igneous origin.

They give mass to the concrete and prevent shrinkage of cement.

Fine aggregate consists of river sand. It prevents shrinkage of cement.

When surrounded by cement it gains mobility enters the voids in coarse aggregates and binding of ingredients takes place. It adds density to concrete since it fills the voids. Denser the concrete higher is its strength.

Water used for making concrete should be clean. It activates the hydration of cement and forms plastic mass. As it sets completely concrete becomes hard mass. Water gives workability to concrete which means water makes it possible to mix the concrete with ease and place it in final position. More the water better is the workability. However excess water reduces the strength of concrete.

PREPARING AND PLACING OF CONCRETE

The following steps are involved in the concreting:

1. Batching
2. Mixing
3. Transporting and placing and
4. Compacting

1. Batching: The measurement of materials for making concrete is known as batching. The following two methods of batching is practiced:
   • Volume batching
   • Weight batching.
   • Volume Batching: In this method cement, sand and concrete are batched by volume. A gauge box is made with wooden plates, its volume being equal to that of one bag of

• Weight Batching: This is the recommended method of batching. A weighing platform is used in the field to pick up correct proportion of sand and coarse aggregates. Large weigh batching plants have automatic weighing

2. Mixing: To produce uniform and good concrete, it is necessary to mix cement, sand and coarse aggregate, first in dry condition and then in wet condition after adding

The following methods are practiced:

• Hand Mixing
• Machine Mixing.

• Hand Mixing: Required amount of coarse aggregate for a batch is weighed and is spread on

an impervious platform. Then the sand required for the batch is spread over coarse aggregate. They are mixed in dry condition by overturning the mix with shovels. Then the cement required for the batch is spread over the dry mix and mixed by shovels. After uniform texture is observed water is added gradually and mixing is continued. Full amount of water is added and mixing is completed when uniform colour and consistency is observed. The process of mixing is completed in 6–8 minutes of adding water. This method of mixing is not very good but for small works it is commonly adopted.

• Machine Mixing: In large and important works machine mixing is preferred

1. Transporting and Placing of Concrete. After mixing concrete should be transported to the final position. In small works it is transported in iron pans from hand to hand of a set of workers. Wheelbarrow and hand carts also may be employed.

In large scale concreting chutes and belt conveyors or pipes with pumps are employed. In transporting, care should be taken to see that segregation of aggregate from matrix of cement do not take place.

Concrete is placed on form works. The form works should be cleaned and properly oiled. If concrete is to be placed for foundation, the soil bed should be compacted well and is made free from loose soil.

Concrete should be dropped on its final position as closely as possible. If it is dropped from a height, the coarse aggregates fall early and then mortar matrix. This segregation results into weaker concrete.

2. Compaction of Concrete: In the process of placing concrete, air is entrapped. The entrapped air reduces the strength of concrete up to 30%. Hence it is necessary to remove this entrapped air. This is achieved by compacting the concrete after placing it in its final position. Compaction can be carried out either by hand or with the help of

CURING OF CONCRETE

Curing may be defined as the process of maintaining satisfactory moisture and temperature conditions for freshly placed concrete for some specified time for proper hardening of concrete.

If curing is not done properly;

• the strength of concrete
• Cracks develop due
• The durability of concrete structure

The following curing methods are employed:

• Spraying of water
• Covering the surface with wet gunny bags, straw
• Ponding
• Steam curing and
• Application of curing

PROPERTIES OF CONCRETE

Concrete has completely different properties when it is the plastic stage and when hardened. Concrete in the plastic stage is also known as green concrete.

The properties of green concrete include:

1. Workability
2. Segregation
3. Bleeding
4. Harshness

The properties of hardened concrete are:

1. Strength
2. Resistance to wear
3. Dimensional changes
4. Durability
5. Impermeability