# Compressive strength

Compressive strength is the measure of the capacity of a material to withstand axially directed crushing forces. The forces may be caused either by 'live' or 'dead' loads. It is an indication of the maximum compressive stress that a material is capable of developing.

The compressive strength depends on the type of material in question. A brittle material fails in compression by fracturing. In such cases, the compressive strength has a definite value. However, in the case of materials which are ductile, malleable or semiviscous, the value denoting the compressive strength depends on the levels of distortion of the material.

## Use in civil engineering

The values calculated to represent compressive strength of materials are of particular importance in civil engineering. The construction process involves elaborate calculation of the compressive strength of the cement and reinforced concrete that is used in structures like buildings and bridges.

### Units of compressive strength

Compressive strength has units of stress, i.e. force per unit area and is usually expressed with negative values to indicate the compaction involved. The strength of concrete is in the order of several mega-pascal (MPa). 1 pascal is equal to 1 newton per meter square.

### In construction processes

The compressive strengths of concrete used in the period around the 1970s would usually be a few dozen MPa. With recent advancements in concrete technology, compressive strengths as high as 130 MPa have been achieved. The American Concrete Institute defines high-strength concrete as that which possesses compressive strength in excess of 41 MPa.

### Manufacture of high compressive strength concrete

High-quality portland cement is used along with optimized aggregates and suitable quantities of water, cement and admixtures such as silica fume and fly ash. The admixtures impart added strength to the concrete.

Another method of producing great compressive strength in concrete is by pouring cement and aggregates over stretched bars of steel, in a process called prestressing. The anchors to the steel bars are removed once the concrete has set so that the tendency of the bars to return to their original length causes the concrete to compress. This results in construction material that is both light in weight and high in compressive strength.

The most common application of high-strength concrete is in the building of skyscrapers. High-strength concrete, which also results in lowered costs, is used for the construction of bridges as well.