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.
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.
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.
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.
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.
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
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|