Concrete work is carried out for monolithic structures or reinforced concrete structures and installations made of cement concrete. Concrete work includes the following activities: preparation of the concrete mix; transportation of the mix to the construction site; compaction of the mix in the formwork, curing of the concrete, etc. In this lecture, we will be discussing sub-activities involved in the concrete work and the materials used in each item.
1. Laying (PCC)
Plain cement concrete of 40 mm size graded aggregate with cement and sand in fixed proportion is to be provided. The surface on which the concrete is to be laid should be clean. A mix of 1:4:8 is used and it is mixed thoroughly in the concrete mixer to have a plastic mix of uniform color. PCC is laid in the layers and it is vibrated by the means of mechanical vibrators till dense concrete is obtained. Compaction should be done and it should be completed before the initial setting starts i.e 30 minutes of addition of water. After 24 hours of laying of concrete, the surface should be cured and curing shall be done for at least 14 days.
2. Laying (RCC)
A mixture of 20 mm size graded aggregate with cement and sand in fixed proportion is to be provided. The base on which concrete is to be laid should be clean and loose materials from the sides of the pit should be removed. M25 Grade of cement concrete is to be used and it is mixed thoroughly in the concrete mixer. Concrete is to be laid using a concrete pump and it is vibrated by the means of mechanical vibrators to ensure an even surface. Proper compaction should be done with the help of tamping rods into the corners of the formwork. After the concrete has begun to harden, curing should be done. It should be protected with moist gunny bags or any other materials. Curing should be done for a minimum period of 14 days.
3. Laying RCC (using Ready Mix Concrete)
M25 Grade of concrete is used for ready mix concrete by mixing cement, water and aggregates: sand, gravel, or crushed stone. Ready-mix concrete is transported and placed at the site. The concrete is to be poured to the surface and it is placed with the help of a concrete pump. Concrete should be placed from the corners of the formwork and compaction should be done immediately after it has been placed. Consolidation is to be done with hand tampers or iron rods and a vibrator are to be immersed into the concrete at regular intervals. After the concrete has been properly compacted, the top of the concrete is to be leveled for a smooth surface. Curing should be done continuously for 14 days for gaining strength.
4. Placing of Reinforcement
The reinforcement bars should be placed in position as directed by the Engineer. The bars should be tied together by crossing with each other and binding with annealed steel wire. The reinforcement bars in the concrete should be placed within tolerance limits. The bars are bent at the construction joints and afterward bent back into the original position. Authorized laps and chairs should only be measured and paid. Payment will be made on a length basis and converted into weight by using the standard IS coefficient.
5. Erection of Formwork
Formwork should be erected in the correct position when the concrete has gained sufficient strength to cast concrete structural members. The removal time of formwork is different for a different types of formwork. The structural members should be arranged in such a way that erection becomes simple and easy, without any damage. Before concreting, shuttering oil is to be applied to the inner surface of the formwork. The sequence and pattern of formwork removal should be approved by the Engineer.
6. Adding admixtures
Admixtures/Plasticizer is added to the concrete during the mixing process in addition to cement, sand, and aggregate. Liquid waterproofing admixture is provided as per the specifications of the Engineer. By adding admixtures, workability and concrete strength are increased.
Necessary Holes and openings for rainwater pipes & other pipes should be kept or the pipes should be provided at the time of concreting. No extra measurements should be paid for the same.
Necessary testing of design mix or ready mix is required in the approved laboratory in the scope of the contractor at his own cost. The materials for the design mix should be provided by the contractor at his own cost.
Concrete of grade M-15 & above should be either from the site installed weigh batching plant or from RMC only.
Provision for a mini batching plant or equivalent electronically operating mixer machine having a capacity of a minimum of 22 Cu.mt per hour is mandatory to be provided. It should also include the availability of operating manpower and accessories for the transportation of concrete to the workplace. No additional payments shall be allowed for the same