Friday, November 4, 2011

Pre-Stressed Concrete

Prestressed Concrete

-       the basic principle of prestressed concrete is that the reinforcing steel is under tension, while the concrete is under compression.
-       prestressing is applied to steel or concrete for 2 purposes; to include desirable stresses and strain in the structure and to conterbalance undesirable strains and stresses.
-       the steel reinforcement is prestretched to avoid later excessive lengthening under severe loads and the concrete is precompressed to prevent later cracking under tensile stress.

Methods of Prestressing

1.       Pre-tensioning method
– steel wire cables are stretched between two abutments to a predetermined stress. Concrete is then placed around the steel wire cables and is allowed to harden. Tensile forces in the steel are transferred to the concrete by  bonding between the steel and concrete. After the concrete has reached a predetermined strength level, the cables are cut off at the ends of the concrete members.

2.       Post Tensioning method
 – concrete is placed with ducts left in the concrete for the steel wire cables, after the concrete has reached a predetermined strength level, the steel wire cables are threaded through the ducts left in the concrete member, then, stretched to a predetermined tension stress with hydraulic jack and finally, attached anchorages on the end of the member. Stress is transferred to the concrete by the end anchorages. Bending between the cable and concrete may also be prevented by greasing or wrapping the cables which permits post tensioning.
3.        Thermal Prestressing
– the steel is preheated by means of electric power which is anchored against the opposite end of the concrete beam. The cooling process produces prestress force through restrained contraction.
4.       Volumetric Expansion
 – is the use of expanding cement restrained by the steel strand or by fixed abutments producing prestressed force.

Concrete for Prestressing

-       prestressed construction specify a compressive strength of concrete between 4000 to 6000 psi because of the following advantages that it offers.

A.      High strength concrete has a higher modulus of elasticity. It minimizes the reduction of pretsress loss.

B.      Increasing the compressive strength of concrete meets the pproblem of high bearing stresses at the ends of post and beam where the prestressing force is transferred from the tendon to anchorage dowels which directly bears against the concrete

C.      High strength concrete develops stronger bond prestresses to pretensioning construction.

D.      High strength concrete gives higher strength to precast construction when curing is carefully controlled.



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