Why an increase in the shear strength of concrete is observed when it is subjected to compressive force?
Concrete usually fails in tension so ideally we only want compressive forces. Shear failures are usually in tension as well, so the idea is to reduce the experienced effect of shear stress. One way to reduce the effect is by pre-stressing the concrete with compressive forces which will in essence reduce the effect of shear and tensile stresses. This occurs because of the principle of superposition which allows the combined effect of loading to negate or balance out other loads. There is an initial compressive stress before loading which takes a greater load to balance out before the beam starts to go into tension.
