You may have studied many times in your engineering subjects that concrete is assumed to be homogeneous, elastic and isotropic. But we know that in this world no substance will obey these characteristics 100 percent.
Concrete deforms when loads apply, the deformation depends upon the magnitude of the load. You cannot find any simple set of rule.
But you can predict the deformation of your concrete when load applies with the help of stress-strain curve for concrete.
When you come to reinforces concrete, the stress-strain curve not only depends upon concrete but also depends on the properties of steel.
Stress-Strain Curve for Concrete
In the above figure, you can see the variation of strain with the stress or load applied to aggregate, concrete and cement paste.
The first portion of concrete in the above picture is nearly linear. In this part, the concrete will be elastic in nature. When furthermore load is applied, the concrete loses its elastic nature and attains a plastic behavior.
This is due to the formation of microcracks in the concrete.
The microcracks in concrete while loading can be formed when loss of bond between the aggregate and cement paste.
You know that when a load is applied to concrete, the load will be taken by aggregate and cement paste. The aggregate has higher compressive strength than cement paste. So when an excess load acts, the cement paste or mortar will fail ahead of the aggregates.
Hope you can understand the stress-strain curve for concrete in compression. If you have any doubt regarding this section, please do comment below.