       Heat Evolution

Compatibility Issues

Pumping of Concrete

Constitutive Relationships

Performance Specs

Special Concretes

Quality Control Issues

NDE of Concrete

Behaviour of concrete under various loads

Concrete very rarely undergoes a crushing or compression failure. Typically, tensile strains generated in the lateral direction (perpendicular to the loading direction) due to Poisson effect are responsible for cracking leading to failure of concrete. Let us consider various cases of loading concrete (in all the cases, the effects of friction at the ends is neglected), as shown in Figure 1. Figure 1. Failure of concrete subjected to various loads

Failure planes in uniaxial compression are the planes of principal tensile strains, which are parallel to the direction of the applied load.
In the case of uniaxial tensile loading, the failure plane is again the plane of maximum principal strain, which in this case is perpendicular to the applied load.

Biaxial compression

In the case of biaxial compression, the failure planes are the planes of maximum principal tensile strains, which are parallel to both the applied principal compressive stresses. Such loading causes an increase in overall strength; this increase is especially high if end restraints also exist. The strength increase can be as high as 27%.

Tension and compression

A pessimum loading case is encountered when a combination of tensile and compressive loads are applied to concrete. The overall strength of concrete is reduced substantially due to the additive effect tensile strains (from Poisson effect and from the tensile principal stress). Failure planes in this case are perpendicular to the maximum principal tensile stress.

Biaxial tension

The behaviour of concrete in biaxial tension is similar to that in uniaxial tension. The change in strength is not significant, and the failure plane is perpendicular to the maximum principal tensile stress.

Biaxial stress interaction diagrams are generated for concretes of different strengths. A typical diagram is presented in Figure 2. From this diagram, it is clearly observed that when both stresses are compressive and equal, the strength of concrete is increased. The strength of concrete in biaxial tension is generally of the order of 1/8 of the uniaxial compressive strength (fc). Figure 2. Biaxial stress interaction diagram

Triaxial compressive loading of concrete causes a drastic increase in the strength of concrete. The tensile strains generated are overcome by the compressive stresses; in effect, the specimen is held together and tensile cracking is prevented. Thus, failure in triaxial loading could occur by a pure case of compression.
The increase in strength is higher for low strength concrete. A Mohr failure envelope (shown in Figure 3) is generated for concretes of different strengths. The maximum shear strain at failure according to this envelope is equal to the half of the difference between the maximum and minimum compressive principal stresses. 