STRENGTHS OF HARDENED CONCRETE

STRENGTH OF CONCRETE

Is the property of hardened concrete to resist excessive strains due to induced stresses that could result to its failure, these stresses can be from internal or external factors.

Strengths of hardened concrete can be distinguished from one to another regarding to type of stress or load that a concrete is required to resist, as follows

1. Compressive strength

This is the ability of hardened concrete to resist compression stresses; the compression strength of concrete is the one which denotes grade of concrete, this is very important strength property of concrete than other strength properties because

a). Easy to measure

b). Other strength properties such as flexural shear and bond strengths they can be deduced from compressive strength through established correlations.

Compression strength of concrete is measured through standard uniaxial compression test, which can be performed on concrete cubes or cylinders.

On going compression strength testing of concrete cube.

Compression Failure mechanism

Normally concrete is composed of micro-cavities or pores inside concrete or micro-cracks around aggregate, failure occur due to extension or expansion of these cavities and cracks, greater compression load is required to cause expansion of these voids to extent that could lead to failure, this make to be good compression resisting material that is concrete is strong in compression.

Concrete stages until failure in compression

At first concrete has micro cracks around aggregates and in pores in seen (A) mostly these are due to hydration process and shrinkage, on loading theses cavities (cracks) tends to expand which weakens bond strength (B), further loading lead to cavities expansion extends to a point they connect one to another to form vertical cracks (C) which lead to its total failure.

2. Tensile strength

This is the one important property of concrete that should be considered during structural design especially of pre-stressed concrete structures, roads and runways, this describes ability of concrete to withstand axial tensile stresses (puling forces) applied on it.

Tensile strength of concrete is about 10 to 12 percent   of the compressive strength, in laboratory tensile strength of concrete can be determined through split cylinder test of concrete.

Tension Failure Mechanism

 

Failure stages of concrete in tension

Before uniaxial tensile loading concrete remains at its state (A), on application of uniaxial tensile loading there is formation and localization of some cracks as different point within concrete (B), further loading to its limit state localized cracks join one to the other forming major cracks which expand and separate the concrete (C).

3. Flexural strength

Flexural strength is also known as modulus of rupture, this measures ability of concrete flexural member to resist extreme fibre stress (stresses in tensile face) on bending.

Flexural loading system.

These flexural stresses in flexural members such as beam or slabs can be caused by internal factors such as shrinkage and creep and external loading.

The flexural strength of concrete ranges only about 10% to 20% of the compressive strength of concrete, this means concrete has weak flexural strength with strong compression strength.

The variation in flexural strength of one hardened concrete to the other, is dependent to its mix proportion and its coarse aggregates properties such as type, size and its contribution to concrete.   

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