CONCRETE

Concrete structures have become very common in Civil engineering construction Concrete has established as a universal building material because of its high compressive strength, its adoptability to take any form and shape and resistant to fire and corrosion with negligible maintenance cost. Concrete is very strong in compression but very weak in tension. Its low tensile strength is compensated by introducing steel reinforcement in the tension zone. Thus, the concrete is strengthened by steel and the resultant composite mass is known as Reinforced Cement Concrete. Thus RCC is used extensively in construction of buildings, bridges, tanks, dams etc. The design of these modern reinforced concrete structures may appear to be complex.
Concrete is a composite maternal consisting of cement, aggregate and water in suitable proportions. The chemical interaction between cement and water binds the aggregates in to a solid mass. Fresh concrete will be plastic, so that it can be moulded to any desired shape in the moulds and compacted to form a dense mass Water has to be applied for few days over the concrete surface soon after its setting because the hydration reactions between cement and water continue for a longer period due to which hardening of concrete takes place. The process of solidification of concrete from plastic stage is called setting while gaining of strength after setting is called hardening Usually, setting completes within a maximum duration of 10 hours, while about 90% of hardening is completed by 28 days.

INGREDIENTS OF CONCRETE

• CEMENT
• AGGREGATE
• WATER
• ADMIXTURE
CEMENT
Cement is the binding material which is obtained by burning calcareous, siliceous and argillaceous materials together in definite proportions at high temperature and grinding the resultant clinker in to a fine powder. Various types of cements have been developed for the use in different types of structures under different situations. According to IS: 456-2000, the types of cements and their suitability for a specific situations are given in Table
AGGREGATES
Around 75% volume of concrete is occupied by the aggregates. The aggregates used for the concrete should be durable, strong, hard, chemically inert ar well graded.Aggregates whose particle size varies from 0.075 mm to 4.75 mm are called as fine aggregate Aggregates with particle sizes more than 4.75 mm are called as coarse aggregates. Usually sand is used as fine aggregate where as crushed rock and gravel is used as coarse aggregate.
The nominal maximum size of the coarse aggregate shall be as large as possible but it should be limited to th of the minimum thickness of the member.

WATER
Water plays an active role in the chemical process of hydration of cement and in curing of concrete. Hence, the water used for mixing and curing of concrete should be clean and free from injunous amount of oils, acids, alkalies, salts, organic matter etc The physical and chemical tests for water should be done as per 15: 3025