The correct answer is C. Prestress loss due to friction occurs in both post-tensioned and pre-tensioned beams.
Prestress loss is the reduction in the effective prestress in a concrete member after the initial stressing of the tendons. There are several causes of prestress loss, including friction, creep, shrinkage, and relaxation.
Friction loss is caused by the movement of the tendons through the concrete. As the tendons are stressed, they pull on the concrete, causing it to deform. This deformation creates friction between the tendons and the concrete, which resists the movement of the tendons. This resistance causes the tendons to lose some of their prestress.
Friction loss is a major cause of prestress loss in post-tensioned beams. In post-tensioned beams, the tendons are stressed after the concrete has been placed and cured. This means that the tendons must be pulled through the concrete, which causes significant friction loss.
Friction loss is also a cause of prestress loss in pre-tensioned beams. However, the amount of friction loss in pre-tensioned beams is typically much less than the amount of friction loss in post-tensioned beams. This is because the tendons are stressed before the concrete is placed, so there is less movement of the tendons through the concrete.
In conclusion, prestress loss due to friction occurs in both post-tensioned and pre-tensioned beams. However, the amount of friction loss is typically much less in pre-tensioned beams than in post-tensioned beams.