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Prestressed Concrete
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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.
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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
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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
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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
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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.
Posted in: Prestressed Concrete
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