PRE STRESSED CONCRETE

A creation of internal stresses in a structure in order to improve its performance. Such stresses are designed to counter-act stresses induced by external loads. Concrete is strong and ductile in compression, it is weak and brittle in tension, and hence its response to external loads is improved by pre-compression. Prestressed concrete is a type of Reinforced Concrete in which steel has been tensioned against the concrete.

Prestressing Methods

Two different procedures for prestressing concrete were developed:

(a). Pretensioned Concrete

In this method, the prestressing tendons are initially tensioned between fixed abutments and anchored. With the formwork in place, the concrete is cast around the highly stressed steel tendons and cured. When the concrete has reached its required strength, the wires are cut or otherwise released from the abutments. As the highly stressed steel attempts to contract, the concrete is compressed. The stress is imparted via bond between the steel and the concrete.

Pretensioned concrete members are often precast in pretensioning beds long enough to accommodate many identical units simultaneously.

(b). Post-tensioned concrete

In this method, the concrete is cast around hollow ducts which are fixed to any. The steel tendons are usually in place, unstressed in the ducts during the concrete pour. When the concrete has reached its required strength, the tendons are tensioned. Tendons may be stressed from one end with the other end anchored or may be stressed from both ends. The tendons are then anchored at each stressing end.

The concrete is compressed during the stressing operation and the prestress is maintained after the tendons are anchored by bearing of the end anchorage plates onto the concrete.

LOSSES IN PRESTRESSING

The losses are broadly classified into two groups, immediate and time-dependent. The

immediate losses occur during prestressing of the tendons and the transfer of prestress

to the concrete member. The time-dependent losses occur during the service life of the

prestressed member. The losses due to elastic shortening of the member, friction at the

tendon-concrete interface and slip of the anchorage are the immediate losses. The

losses due to the shrinkage and creep of the concrete and relaxation of the steel are the

time-dependent losses.