Loss of pre-stress in concrete:

Loss of pre-stress in concrete refers to the reduction in the initial stress applied to the reinforcement before it is subjected to external loads. This loss can occur due to various factors, and it is important to understand them in order to design pre-stressed concrete structures properly.

Factors contributing to the loss of pre-stress:

1. Creep of concrete: Creep is the time-dependent deformation of concrete under sustained load. It causes the concrete to gradually deform and results in a loss of pre-stress over time. This phenomenon occurs due to the slow relaxation of internal stresses in the concrete.

2. Shrinkage of concrete: Shrinkage is the reduction in volume of concrete due to the loss of moisture. As concrete dries and hardens, it undergoes shrinkage, which can cause a loss of pre-stress. The extent of shrinkage depends on factors such as the mix design, environmental conditions, and curing methods.

3. Slipping of steel tendons from concrete: Pre-stressed concrete structures rely on the bond between the steel tendons and the surrounding concrete to transfer the pre-stressing force. If the bond between the steel and concrete is not sufficient, the tendons may slip or become detached from the concrete. This can result in a significant loss of pre-stress.

4. Relaxation of steel tendons: Steel tendons used in pre-stressed concrete are subjected to high initial stress during the pre-stressing process. However, over time, the steel tendons may experience relaxation, which is the gradual reduction in stress. This relaxation can lead to a loss of pre-stress in the structure.

Relation to grade of concrete:

The grade of concrete refers to its compressive strength, which is determined by the ratio of cement, aggregates, and water in the mix. The grade of concrete does not directly influence the loss of pre-stress. Loss of pre-stress is primarily influenced by factors such as creep, shrinkage, slipping of steel tendons, and relaxation of steel tendons.

While the grade of concrete affects the overall strength and durability of the structure, it does not have a direct impact on the loss of pre-stress. The factors mentioned above are more significant in determining the extent of pre-stress loss.

In conclusion, loss of pre-stress in concrete is not directly related to the grade of concrete. It is primarily influenced by factors such as creep, shrinkage, slipping of steel tendons, and relaxation of steel tendons. Understanding these factors is crucial in designing pre-stressed concrete structures that can withstand the anticipated loss of pre-stress over time.