Concrete is not normally designed to resist direct tension. However, tensile stresses develops in concrete members as a result of flexure, shrinkage and temperature changes. Principal tensile stresses may also result from multi-axial states of stress.
Normally cracking in concrete occurs when tensile strength exceeds its limiting value. Pure shear in concrete causes tension on diagonal planes, thus the value of direct tensile strength of concrete is useful in estimating the shear strength of beams with unreinforced webs, etc. Also, the flexural tensile strength of concrete is required for estimation of the moment at first crack required for the computation of deflections and crack widths in flexural members.
Concrete is very weak in tension, the direct tensile strength is only about 7 to 15 percent of its compressive strength. It is difficult to perform a direct tension test on a concrete specimen, as it requires a purely axial tensile force to be applied, free of any misalignment and secondary stress in the specimen at the grips of the testing machine. Hence, indirect tension tests are conducted to determine direct tensile strength of concrete, usually by the flexure test or the cylinder splitting test.
Stress-Strain Curve of Concrete in Tension
Concrete has a low failure strain in uniaxial tension. It is found to be in the range of 0.0001 to 0.0002. The stress-strain curve in tension is generally approximated as a straight line from the origin to the failure point. The modulus of elasticity in tension is taken to be the same as that in compression. As the tensile strength of concrete is very low, and often ignored in design, the tensile stress-strain relation is of little practical value.