Prestressed Concrete

Loss of Prestress (1)

Instructor:

Dr. Sawsan Alkhawaldeh

Department of Civil Engineering

Introduction Definition Loss of prestress is the progressive process of

reduction over time of the initial prestressing force applied to the concrete element.

Classification Loss of prestress can be classified into two

categories:

- Immediate elastic loss during construction process (Elastic shortening of concrete, anchorage losses, frictional losses).

- Time-dependent losses (Creep, shrinkage, loss due temperature and steel relaxation).

Causes of prestress losses

Sources of prestress loss

Estimation of prestress loss

An exact determination of theses losses is not feasible since they depend on many factors.

Empirical methods of estimating losses differ with the different code of practice or recommendations (ACI-ASCE, AASHTO, CEB, FIB).

The difference between these methods depends on the approach chosen and the accepted practice of record.

The loss estimations provided by some codes include elastic shortening, steel relaxation, shrinkage and creep, and they applicable for routine, standard conditions of loading; normal concrete, quality control, construction procedures, environmental conditions, the importance and magnitude of the system.

Loss Estimation

Total loss in Prestress

Pretensioned Members

Computations of steel relaxation loss have to be performed for the time interval t1 through t2 of the respective loading stages.

Post-tensioned Members

Loss due to Elastic Shortening The difference in length of the prism before and after release

induces a stress change in the tendon, which is equal to its elastic shortening loss.

It should be determined using the modulus of elasticity of the concrete at the time of prestress transfer.

The calculations depend on whether pre-tensioning or post-tensioning is used.

Pretensioned systems Pretensioned members

The magnitude of shortening is:

The stress in the concrete due to prestressing is:

The concrete stress is:

Example (1) Elastic shortening loss of pretensioned member

A prestensioned prestressed beam has a span of 50 ft (15.2 m).

Post-tensioned systems In Post-tensioning beams, the elastic shortening varies from zero if all tendons are jacked simultaneously to half the value calculated in Pretensioned case if several sequential jacking steps are used, such as jacking two tendons at a time. If n is the number of tendons or pairs of tendons sequentially tensioned, the elastic loss is:

j is the number of jacking operations.

Note:

The tendon that was tensioned first suffers from the maximum amount of loss, while the one that was tensioned first does not suffer any losses due to elastic shrinkage.

Example (2) Elastic shortening loss of post-tensioned member

If the previous beam is post-tensioned and the prestressing operation is such that: