Concrete Mix Ratio

Compressive strength is the most important property of hardened concrete. It can be measured easily on standard sized cube (15 cm) and is often taken as an index of the overall quality of concrete. The following things are co-related with compressive strength are as under.

Water Cement Ratio

Among the materials and mix variables, water-cement ratio is the most important parameter governing compressive strength. The compressive strength of concretes at a given age and under normal temperature depends primarily on the water-cement ratio; lower the water cement ratio, greater is the compressive strength and vice- versa. In other words with given materials and conditions of test, the ratio of the quantity of mixing water to the quantity of cement alone determines the strength of concrete so long as the mix is of a workable plasticity.

Aggregate Cement Ratio

As long as workability of the concrete is maintained at a satisfactory level, compressive strength of concrete increases with aggregate-cement ratio, the water-cement ratio being held constant. However, in high- strength concrete mixes of lower workability, or in such situations where due to increase in aggregate-cement ratio the workability is reduced to such an extent that concrete cannot be properly placed and thoroughly compacted, the above will not be true.

Cement Content and Characteristics

The water cement ratio and aggregate cement ratio together determine the cement content of the concrete mix. Generally, the cement content itself would not have a direct role on the strength of concrete; if increase in cement content is required to improve the workability of concrete mix for a given water-cement ratio, then compressive strength may increase with the richness of the mix. However, for particular water cement ratio there would always be optimum cement content resulting in 28-days compressive strength being the highest.

Size of Aggregates

Cement is the binding material in the concrete mix. A better coating of cement to the particles can provide more strength and this can be achieved when coarse aggregate is used.
When proper graded materials are used, lesser the voids in concrete and tightness of the mix provide more strength.

Effect of age of testing

Cement concrete starts gaining strength at the time of laying the mix and gradually increases with the time by continuing hydration, the later age strength of concrete would generally be higher than at 28 days; however the exact increase will depend upon the type of cement, mix composition and the extent of curing.

Effect of compaction

When the fresh concrete is compacted by vibration, particles are set in motion reducing inter particle friction so that concrete is compacted by vibration, the particles are set in motion reducing inter particle friction so that concrete is easily placed. Vibration eliminates most air pockets on the surf

ace of concrete.

Curing

Hydration of cement can take place only when the capillary pores remain saturated. If water is inadequate or it is not properly cured than it will not hydrate completely, which leads to inadequate strength. So cement concrete should be cure properly. Horizontal surface of cement concrete member such as raft, slab etc shall be cured by pounding the water where as vertical surface should be kept wet by wetting the gunny bags.

The following table shows compressive strength of some mix of concrete:

1	M-10	1:3:6	100 Kg. per cm square
2	M-15	1:2:4	150 Kg. per cm square
3	M-20	1:1.5:3	200 Kg. per cm square

Note:

Cement concrete cubes filled in for the above cement ratios should be tested after curing for 7 days and the strength should not be less than 70%.

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How to get more strength with same quantity of cement?

The same cement can provide better coating to particle and hence more strength when coarse aggregate is used.

What is cement concrete and which are various ratios according to the site requirement?

The cement concrete is a mixture of cement fine aggregate, coarse aggregate and water.

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Concrete is strong in compressive stress but is weak in tensile stress.

What quantity of water is required for cement concrete?

The water for nominal mix concrete shall generally be equal to 5% by weight of aggregate and 30% by weight of cement.

What precautions should be taken while mixing/laying cement concrete mix to the required site?

For general work the different materials required for cement concrete mix is measured loose through measuring box by volume for 1 cement bag at one time.

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