- To calculate the deformation and deflection of structural members, we have to know the relation between stress and strain. The relation between stress and strain is of vital interest over the full range, in structural design. Like many other structural materials, concrete is, to a certain degree, elastic. A material is said to be perfectly elastic if strain appears and disappears immediately on application and removal of load .This doesn’t imply a linear stress-strain relation.
- Concrete behaves nearly elastically when load is first applied. However under sustained loading, concrete exhibits creep i.e. strain increases with time under a constant stress, even at very low stresses and under normal environmental conditions of temperature and humidity. Steel on the other hand, creeps only at very high stresses at normal temperature or even at low stresses at very high temperature, and in both cases a time dependent failure occurs
- In contrast, in concrete subjected to a stress below about 60-70 % of short-term strength, there is no creep rupture or static fatigue. Like concrete, timber also creeps under normal environmental conditions.The importance of creep in structural concrete lies mainly in the fact that creep deformation is of the same order as the elastic deformation. There are also other effects of creep, most of them detrimental but some beneficial.
- Let us first categorize the elastic behavior of concrete in terms of various types of elastic behavior of engineering materials. The definition of pure elasticity is that strain appears and disappears immediately on application and removal of load. The stress-strain curve in the figure below illustrates two categories of pure elasticity:
- ● Case (a) linear and elastic and
- ● Case (b) non-linear and elastic.
- Steel conforms approximately to case (a) whilst some plastics and timber follow case (b).
- Brittle materials such as glass and most rocks are described as linear and non-elastic, case (c) because separate linear curve exists for the loading and unloading branches of the stress-strain diagram and a permanent deformation exists after complete removal of load.
- The fourth category, case (d) can be described as non-linear and non-elastic, a permanent deformation exist after removal of load. The area enclosed by the loading and unloading curves represents the hysteresis. This behaviour is typical of concrete in compression or tension loaded to moderate and high stresses but is not very pronounced at very low stresses.
elasticity of concrete(www.engineersdaily.com)