Strength of Concrete Strength of concrete is commonly considered its most valuable property, although in many practical cases, other characteristics, such as durability and permeability may in fact be more important. Nevertheless strength usually gives an overall picture of the quality of concrete because strength is directly related to the structure of the hydrated cement paste. Moreover, the strength of concrete is considered in structural design and is specified for this purpose. Strength of concrete could be defined as the ultimate load that causes failure (or is its resistance to rupture) and its units are force units divided by area (N/mm2, lb/in2).

Nature of strength of concrete The influence of voids in concrete on its strength has been repeatedly mentioned, and it should be possible to relate this factor to the actual mechanism of failure. For this purpose, concrete is considered to be a brittle material, even though it exhibits a small amount of plastic action, as fracture under static loading takes place at a moderately low total strain; a strain of 0.001 to 0.005 at failure has been suggested as the limit of brittle behavior. High strength concrete is more brittle than normal strength concrete but there is no quantitative method of expressing the brittleness of concrete, whose behavior in practice located between the brittle and the ductile types.

Fracture and failure of concrete Concrete specimens subjected to any state of stress can support loads of up to 40– 60 per cent of ultimate without any apparent signs of distress. Below this level, any sustained load results in creep strain, which is proportional to the applied stress and can be defined in terms of specific creep (i.e. creep strain per unit stress). Also the concrete is below the fatigue limit. As the load is increased above this level, soft but distinct noises of internal disruption can be heard until, at about 70–90 per cent of ultimate, small cracks appear on the surface (signs of distress).

At this stage sustained loads result in eventual failure At this stage sustained loads result in eventual failure. Towards ultimate, cracks spread and interconnect until, at ultimate load and beyond; the specimens are increasingly disrupted and eventually fractured into a large number of separate pieces. The formation and propagation of small microscopic 2-5 μm cracks long (microcracks) have been recognized as the causes of fracture and failure of concrete and the marked non-linearity of the stress–strain curve near and beyond ultimate.