Stress strain curve curve for concrete under uniaxial compression is some what linear in very early phase of loading & non-linear starts when level exceeds (1/2-1/3) of max stress. The max stress us reached at a strain of approx 0.002. For all grades of concrete, strain at failure in range (0.003-0.005). The higher is the grade of concrete, steeper is curve in initial portion & peak is sharper & failure strain is less. When stress value reaches (0.7-0.9) of the maximum stress, internal microcracks are developed in the direction of loading due to which stress starts to reduce with increase in strain & poison’s ratio also starts increase.
It is noted that the value of maximum compressive stress of concrete in cylinder is taken as 0.85times of compressive strength of cube. When we tested in lap, the value may more than the actual value, so it is multiplied by 0.8 to get actual value.
The failure strain of concrete under compression is taken as 0.002 and under flexural is taken as 0.0035.
