The prestressing force is the most crucial factor in applications involving prestressed concrete. Early on, it was noted that the prestressing force fluctuates over time rather than remaining constant. even when the tendons are being prestressed and the prestress is being transferred to the concrete member, the prestressing force has decreased from the value that was reported in the jack gauge. The term “loss of prestress” refers to various reductions in prestressing power.
Losses can be broadly divided into two categories:
Instantaneous and time-dependent. The immediate losses take place when the tendons are prestressed and when the prestress is transferred to the concrete part. During the prestressed member’s service life, the time-dependent losses take place. Losses resulting from the member’s elastic shortening, friction at the tendon-concrete interface, and anchorage slip occur immediately
The losses due to the shrinkage and creep of the concrete and relaxation of the steel are the time-dependent losses. The causes of the various losses in prestress are shown in the following chart
Elastic shortening:
Members with pretension
The concrete immediately shortens as a result of the prestress when the tendons are severed and the prestressing force is applied to the member. The same degree of tendon shortening also results in prestress being lost.
Members with post-tension
There is no loss if there is only one tendon because the applied prestress is recorded following the elasticity in the member’s shortening If multiple tendons are affected by stretching. A tendon gradually deteriorates when the other tendons are stretched after it. The decrease in prestress (fp) in a tendon caused by the elastic shortening loss is measured. Alteration in the tendon’s strain (p) it is believed that the tendon’s shift in strain is equivalent to the concrete’s strain at the tendon level because of the prestresing force.
This assumption is called strain compatibility between concrete and steel. The strain in concrete at the level of the tendon is calculated from the stress in concrete (fc) at the same level due to the prestressing force. A linear elastic relationship is used to calculate the strain from the stress.
For ease of calculation, the stress in concrete at the level of CGS can be used to determine the loss in all tendons. When tendons are stretched one after the other in a post-tensioned member, this simplification cannot be applied. The calculation is shown separately for the following member categories.
- Axial Members That Are Pre-Tensioned
- Members with Pre-tensioned Bending
- Axial Members with Post-tension
- A member that bends under tension
