Preparation of cement concrete

PREPARATION OF CEMENT CONCRETE

1. MIXING CONCRETE

First, measure the ingredients, viz., Coarse and Fine Aggregates and Cement in proper proportions. Mixing of the ingredients with water should be done properly so that the mass becomes uniform and homogeneous. There are two methods of mixing.

1. Hand Mixing: For small works, mixing of concrete is done by hand. Mixing of concrete should not be done on ground. It should be done on a hard surface like a concrete floor.

2. Machine Mixing: For large scale work, Concrete Mixers, viz., Rotary Mixer or Tilting Mixer are used. The quality of machine-mixed concrete is better than that of hand mixed.

Transporting Concrete: After mixing, the concrete is transported to the place of work. place of mixing and the place of work should be quite nearer to each other.

2. PLACING AND COMPACTING CONCRETE

The concrete is placed and compacted before setting commences. Segregation should not take place while placing the concrete.

Compaction or Consolidation of Concrete: The object of Compaction of Concrete is to eliminate air bubbles and thus to obtain maximum density to concrete. The strength of concrete is very seriously affected by the degree of its Compaction.

Concrete shall be thoroughly compacted during placing and thoroughly worked around the reinforcement and into the corners of the Form Work. [Form Work: For all concrete constructions, some sort of temporary staging, either of timber or steel, is required so as to form mould or forms to hold the concrete till it sets. Such a temporary staging is known as Form Work or Centering.] Compaction is done either by Manual Method or by Mechanical Method.

3. CURING OF CONCRETE

Curing of Concrete: Curing of Concrete is the process of activating the hydration of concrete.

Purposes of Curing

(i) The process of hardening of concrete requires moisture. Water used for mixing may get evaporated. Surface cracks may occur. Strength of concrete comes down, if the concrete is allowed to dry. Hence, it is essential to maintain the newly placed concrete moist for about 2–3 weeks.

(ii) Curing reduces shrinkage.

(iii) It increases durability and impermeability of concrete.

(iv) It also increases resistance against abrasion.

Methods of Curing: During curing period, care should be taken to see that the concrete surface does not become dry. Curing may be done by sprinkling of water on the concrete surface. Vertical surfaces may be kept moist by wet gunny bags. Horizontal surfaces may be kept damp either by storing water on the surfaces or by placing wet gunny bags, etc., over the surfaces.