The failure of an over-reinforced concrete beam is due to Compression failure.

Reinforced concrete beams may be classified either as under-reinforced or over-reinforced.

The following beam conditions are identified by their ultimate strength actions:

(a) Over-reinforced beams:- These are heavily reinforced beams whose brittle failure is due to the crushing of the concrete while the steel reinforcement is stressed below its yield strength.

The concrete fails in compression before the steel reaches its yield point. Hence, this type of failure is termed compression failure. The failure occurs (often, explosively) without warning.

Over-reinforced beams give no warning of failure which is sudden and catastrophic. While over-reinforced beams are stronger, there are no advantages in designing over-reinforced beams because the Code discourages the design of over-reinforced beams by imposing certain design penalties to be considered later.

(b) Under-reinforced beams in which the area of tensile reinforcement and hence the steel ratio is such that all the tensile reinforcement will have yielded prior to crushing of the concrete.

The failure of an under-reinforced beam is termed tension failure so called because the primary cause of failure is the yielding in the tension of the steel.

Under-reinforced beams are said to behave in a ductile manner. It is possible to imagine that once the reinforcement has yielded, it continues to elongate rapidly. Since this occurs prior to the crushing of the concrete and hence beam failure, very pronounced beam deflection can be observed while the beam is still able to carry the applied loads. The exaggerated beam deflection gives ample warning of impending failure and the ductile beam behavior allows redistribution of moments in indeterminate structures. Similar “plastic” behavior is observed in steel structures.

(c) Balanced beams in which simultaneous yielding of the tensile reinforcement and crushing of the concrete will occur. Balanced conditions are idealized conditions.