Question

What is truly elastic material? Is concrete truly elastic? If not, why? Describe the various stages of micro cracking when a concrete specimen is loaded to failure (please use sketches to explain the behavior and the micro cracking).

Answer 1

Elasticity is the property by vitue of which a material can regain its original shape and size once the applied load is removed. Those materials which express this property is termed as elastic material.

As concrete does not fully regain its original shape and size on unloading ,it is not truly elastic in nature . A truly elastic material shows a linear stress - strain curve whereas a non- linear stress - strain curve is exhibited by non- elastic material.

The value of Young's modulus and poissons ratio is different for different type of concrete.High strength concrete show low deflection as its modulus of elasticity is high.The elasticity of concrete depends on various factors:

a) elastic properties of aggregate used in concrete

b)age of concrete

c) mix proprtion

d) curing

e) type of concrete used.

Low intensity load for short duration exhibits a linear stress - strain curve of compression ,justifying the use of modulus of elasticity of concrete. But on sustained loading even at low stresses in non linear stress- strain curve is exhibited.The dynamic modulus of elasticity is obtain from initial tangent modulus.

Microcracking in concrete  refers to small fissures thst occur due to stress and strain concentration resulting due to incompatibility of elastic modulus of the aggregate and rest of concrete paste.

Three stages of microcracking

1) 1st stage occur due to shrinkage and thermal movement that result in a strain concentration at the aggregate - cement paste intereface. Once the stress and strain become stable the cracking stops and not propogate further.

2) 2nd stage occur when loading begins. The early formed cracks propogate further. The transition from 1st stage to 2nd stage occur gradually and in a stable manner.The extend of fracture size depends on the type of loading ie; sudden loading or sustained loading.

3) 3rd stage under load cracking system . The member become unstable and release of strain energy is sufficient to further propogate the crackeven without laoding or further increase of stress.This stage starts at about 70- 90 percentage of ultimate stress.