Determine the stage of Cracking moment of pre-stress

 At the stage of transfer of pre-stress in the pre-stressed beam element, only the pre-stressing force and the self weight of the beam may be assumed to act. At this stage, the beam soffit (bottom side) may be assumed to be under compression as the stresses, due to self weight only, are of low magnitude. As the extenal imposed loads are applied on the beam, the compressive stresses (due to pre- stressing) shall decrease due to the tensile stresses generated by external loads. A time comes when these stresses of opposite nature balance each other. At this stage, the soffit of the beam shall be subjected to zero stresses. If extenal loads are further increased, the soffit of the beam shall come under tension. At a particular value of tensile stress the concrete at the location of the soffit of the beam shall crack and visible cracks appear. This tensile stress, at this stage, may be taken to be equal to the modulus of rupture of concrete. The visible cracks have crack widths of the order of 0.01 - 0.02 mm.