|Concrete Under Compression Testing Machine|
The short term properties of concrete such as elastic modulus, shear strength, tensile, compressive strength and the stress-strain characteristics are expressed in terms of the uniaxial cube compressive strength of a 15cm x 15cm cube, moist cured for 28 days. This compressive strength is used as the design basis.
The strength under uniaxial compression can be determined by the compression testing machine, as per IS 516: 1959. The loading is strain - controlled and generally applied at a uniform rate of 0.001mm.mm per minute in a standard test.
The Maximum stress that is attained during the loading process is called as the cube strength of concrete.
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Concrete Strength under Uniaxial Compression Stress
By conducting tests on the cube, only strength parameters can be obtained. By conducting tests on the cylinder of size 150mm diameter and 300mm height, compressive strength, as well as stress-strain properties, can be obtained.
|Fig.1: Stress-Strain Characteristics of Hardened Concrete Under Uniaxial Compression|
The figure-1 above shows the stress-strain characteristics of concrete under uniaxial compression. The normal - strength of concrete reaches the peak stress, called as the Maximum Compressive Strength of Concrete.
The portion of stress - strain curve beyond the maximum stress(fc') and up to the failure load is known as the falling branch.
The tensile strength of concrete is low and it is susceptible to sudden failure under tension. Hence while designing structures, the tensile strength is ignored.
In structures, concrete is rarely subjected to uniaxial stress. In deep beam and walls, it is subjected to biaxial stress. Under compression tension, the compressive strength decreases linearly with the increasing tension. Under biaxial tension, the strength is independent of biaxial bending.