Behaviour of Concrete under compression and tension

The behaviour of concrete is different when it comes to compression and tension. Usually, concrete performs very well under compressive stresses, but when it is subjected to tensile stresses, its performance is considered inferior.
Let us look one by one how concrete behaves under different stresses and what are some terms related to it. 

The behaviour of concrete under uniaxial compression:

IS 456:2000 has stated that to test the compressive strength of concrete specimen of 150 mm cube should be tested after 28 days of curing. Then the testing on the concrete cube is performed. The loading test of the concrete cube is strain-controlled, and the uniform strain rate applied is 0.001 mm/mm/min. 
IS 456:2000 also recommends testing of 3 samples and the average value of maximum stress should be noted as compressive strength. 

Now many people may ask why we perform the strain-controlled test and why not stress-controlled test.
The answer to this question when we perform a strain-controlled test, we can see the post-peak effect on the stress-strain curve. In other words, stress will start decreasing after achieving its maximum value, and thus we will be able to note the compressive strength of concrete. However, in the case of the stress-controlled test, we will not be able to see the post-peak effect. 

One thing to note also is that in the USA, a standard cylinder of 300 mm length and 150 mm diameter is tested. It is experimentally shown that the compressive strength of the cylindrical specimen comes out to be less than the compressive strength of the cube.
This leads us to discuss on Influence of size and shape of the specimen on the strength of concrete.

While testing a concrete block, you will notice that some confinement/platens are provided at the top and bottom of the specimen. Those platens generate shear stresses in the sample. The effect of the shear stress reduces as the thickness of the specimen increases. The shear stress development increases the compressive strength. So the compressive strength of the cubical specimen is more than the cylindrical sample. As in the cubical example, the height to breadth ratio is 1, and in the cylindrical specimen, that ratio is 2.

The behaviour of concrete under tension:
Concrete is usually not designed for tensile load for that we put reinforcement in concrete. However, there are other ways by which tensile stresses are imposed on concrete so we should have knowledge about the tensile strength of concrete too. How tensile stresses can generate are due to temperature, flexure, shear, shrinkage, etc. 
It is tough to test the direct tensile strength of concrete as it requires perfect alignment, and we have to ensure that secondary stresses are not generating. To encounter that we use some indirect methods like flexure test and cylinder splitting test.

Flexure Test:
In this, a simply supported plain concrete beam is subjected to a three-point loading and the stress distribution here is assumed linear. For linear distribution maximum stress will develop at the extreme fibre of the beam. That stress is also known as modulus of rupture.
Modulus of rupture can be calculated as 
               
                    fcr   =  M/Z
Here, M is the moment due to which beam fails.
           Z is section modulus.
Note: Since the stress distribution is not linear in the actual scenario IS 456:2000  recommends a different formula.

                                   fcr   =  0.7 *  ( fck)^0.5
               Here,     fck is characteristic strength of concrete in MPa.


Splitting Tensile Strength of Concrete:

In this test, the standard size of the cylindrical specimen ( length 30 mm and diameter 15 mm) is loaded along its side on the diametrical plane. This loading ensures uniform tensile stress along the plane of loading. 
The splitting tensile strength (fct ) is obtained as :
                      
                                fct  = 2*P / ( 3.14* d * L )
Here, P is the maximum load at which the sample fails.
          d is the diameter of the cylinder
   and L is the length of the cylinder.
           






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