ABSTRACT
When the beam is prestressed, the large value of the prestress makes the beam hog up. Assuming that deflection is positive downward, at zero load the deflection is negative. At this stage, the stress distribution in concrete is compressive at the bottom fibre and tensile at the top fibre as shown in Fig. 7.2(a). As the load is increased, the compressive stress at the soffit begins to decrease so that as shown in Fig. 7.2(b), at a certain stage it becomes zero. However, the opposite happens at the top face where the tensile stress reverses and becomes compressive. On further increase in load, at some stage the stress at the soffit becomes tensile and the compressive stress at the top face continues to increase as shown in Fig. 7.2 (c). On increasing the load further, cracks appear at the soffit and the stress for a
certain distance from the soffit becomes zero as shown in Fig. 7.2(d). However, the compressive stress and strain at the top face continue to increase. With further increase in load, cracks at the soffit continue to travel towards the top face. The compressive strain at the top face keeps increasing and the stress distribution in the compressive zone becomes non-linear as shown in Fig. 7.2(e). On further increase in load, the compressive strain in the top face continues to increase till it reaches the maximum value permissible, equal to εcu3. The maximum value of the permissible strain is smaller for higher strength concrete compared with the ‘normal’ strength concrete. At this stage, concrete has reached its maximum resistance and the concrete begins to crumble. This represents the ultimate capacity of the section in bending.
