Tensile Strength Case Study

It can be observed that the compressive strength decreases as the concrete temperature increases. The compressive strength for concrete temperatures between 35-40oC decreased by about 9% compared to the compressive strength for concrete temperatures between 20 to 25oC.

From about 22˚ to 120˚C the concrete compressive strength decrease is attributable to thermal swelling of the physically-bound water that causes disjoint pressures.

From 120˚C to about 300˚C there is a regain of compressive strength that is generally attributed to greater van der Waal’s forces as a result of the cement gel layers moving closer to each other during heating. At temperatures above 300˚C compressive strength losses can become significant probably due to differences.

Concrete is weak in tension, and for normal strength concrete, tensile strength is only 10% of its compressive strength and for High strength concrete tensile strength ratio is further reduced. Thus, tensile strength of concrete is often neglected in strength calculations. However, it is an important property, because cracking in concrete is generally due to tensile stresses and the structural damage of the member in tension is often generated by progression in microcracking in thermal expansion coefficients between the aggregate and cement paste and decomposition of calcium hydroxide. At temperatures above 450˚C concrete compressive strength drops significantly due to loss of bond between the aggregate and cement paste.

Under fire conditions tensile strength of concrete can be even more crucial in cases where fire induced spalling occurs in a concrete structural member. Another property that influences fire resistance is the modulus of elasticity of concrete which decreases with temperature. At high temperature, disintegration of hydrated cement products and breakage of bonds in the microstructure of cement paste reduce elastic modulus and the extent of reduction depends on moisture loss, high temperature creep, and type of

It is used extensively in the analysis of reinforced concrete structures to determine the stresses and deflections in structures. In hot weather concreting, the evaporation from fresh concrete occurs at a faster rate causing drying of the surface and inducing cracks due to breakage of bonds in the cement paste microstructure. An increase in temperature generally leads to decrease in the modulus of elasticity of concrete.

Factors affecting the modulus of elasticity at high temperature are the type of aggregate and the stress due to heating. Due to thermal incompatibility with the cement paste, concretes containing aggregates with low thermal expansion (e.g., limestone) experience a greater reduction in the modulus of elasticity than those with a higher thermal expansion such as gravels and sandstone.

4.2.5 Shrinkage and Creep