Mix design of concrete is the process of selecting the required ingredients of concrete and finding their relative proportions with the aim of producing an economical concrete of certain strength and durability.
It has been discussed earlier that concrete is based on two phases, viz., aggregate phase and paste phase. Workability of the concrete depends on the lubricating effect of the paste phase. The strength of concrete is predominantly governed by the aggregate paste and the contribution by the paste phase is limited. But the permeability of concrete is based on the quality and continuity of the paste phase. Further, the paste phase fully contributes to the control of drying shrinkage of the concrete.
For a given set of materials, the four factors to be considered in the design of concrete mix are:
- Water–cement ratio or cement content
- Cement–aggregate ratio
- Gradation of aggregate
- Consistency
In general, all the four factors are inter-related and can not be dealt with individually to get the best concrete. However, two or three factors are fixed and the others are adjusted to get the required workability and economy.
The water–cement ratio represents the dilution of the paste, and cement concrete depends on the amount of paste. The gradation of the aggregate is done by adjusting the quantity of given fine and coarse aggregates. The required consistency or workability is obtained on the site of placement of concrete.
The effort in proportioning is to use a minimum quantity of paste which will lubricate the mixture while fresh, harden afterwards, will bind the aggregate particles together and fill the space between them. Excess cement should be avoided as it leads to greater cost, high shrinkage, high permeability, and more weathering. All these defects can be nullified by proper gradation.
There are over twelve mix designs of which two are explained, viz., the American Concrete Institute Method and the Bureau of Indian Standards Method.
Leave a Reply