Strength of Concrete

TENSILE STRENGTH:-

Although concrete is not normally designed to resist direct tension, the knowledge of tensile strength is used to estimate the load under which cracking will develop. This is due to its influence on the formation of cracks and its propagation to the tension side of reinforced concrete flexural member. Shear, torsion and other actions also produce tensile stresses to the particular section of concrete member. In most cases member behavior changes upon cracking. So tension strength of concrete is also considered in proportioning concrete member. This strength is of interest in designing of highway and airfield slabs as shear strength and resistance to cracking are very important to sustain such loading. The tensile strength of concrete is relatively low, about 10 to 15% of the compressive, occasionally 20%.

Test for tensile strength:-

The tensile strength of concrete can be measured by radically different tests, namely flexure, direct tension and splitting, and the resulting values of strength are not the same. A direct application of a pure tension force, free from eccentricity, is very difficult. A direct tension test, using bond end plates, is prescribed by the U.S Bureau of Reclamation. In flexural strength tests, a plain (unreinforced) concrete beam is subjected to flexure using symmetrical two-point loading until failure occurs. British Standard BS 1881: part 118:1993 and ASTM C78-94 prescribed third point loading on 150 by 150 by 750 mm beams supported over a span of 450 mm. The modulus of rupture is determined from this test. In splitting tension test, a concrete cylinder, of the type used for compression tests, is placed with its axis horizontal between the platens of a testing machine, and the load is increased until failure by indirect tension in the form of splitting along the vertical axis takes place.

As stated above, the tensile strengths of concrete measured by different tests produce results of varying value. Incidentally, the value of the compressive strength is also not unique but is affected by the shape of the test specimen. So the numerical value of the ratio of the tensile strength to the compressive strength is not the same. For these reasons, in expressing the ratio of the tensile to compressive strengths, the test method must be explicitly stated. If the value of flexural strength is of interest, a factor relating the splitting strength to flexural strength needs to be applied.