Selection of the proper quantity of cement, coarse aggregate, sand and water to obtain the desired quality is known as proportioning of concrete. Concrete is formed by successive filling of voids in aggregate by sand, the voids in sand by cement and voids in cement by water and undergoing a chemical reaction.
The concrete formed by proper proportioning of ingredients should satisfy the following properties:
- The fresh concrete should have adequate workability for uniform placement.
- The hardened concrete after setting should have the desired strength and durability.
- The concrete should be cheap considering the materials and labour.
There are two approaches in proportioning concrete. In the first method no preliminary tests are conducted. But based on experience, arbitrary ratios such as 1:2:4; 1:1½:3; 1:1:2, etc., are used. This method of proportioning by adopting an arbitrary ratio is called the mix method, and the concrete formed by this method is called ordinary concrete. This type of concrete is used for ordinary or common works such as columns and members subjected to medium loads, all general building RCC works, mass concrete work in culverts, retaining walls, compound walls, and ordinary machine bases. Ordinary concrete can also be used for long span arches with a mix of 1:1:2 and for heavy stressed members with a mix of 1:2:2.
In the second method, preliminary tests are conducted, the mix being designed by any one of the mix design methods to get the desired strength and durability. The concrete formed by this method is called controlled concrete. This type of concrete is used for all plain and reinforced concrete structures. The concrete mixes for controlled concrete are designed to have an average strength in preliminary strength test as shown in Table 8.11.
Table 8.11 Strength of concrete in preliminary strength test
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