Definition of Prestress:
Prestress is defined as a method of applying pre-compression to control the stresses resulting due to external loads below the neutral axis of the beam tension developed due to external load which is more than the permissible limits of the plain concrete. The precompression applied (may be axial or eccentric) will induce the compressive stress below the neutral axis or as a whole of the beam c/s. Resulting either no tension or compression.
Basic Concept
Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree.
Terminology
1. Tendon:
A stretched element used in a concrete member of structure to impart prestress to the concrete.
2. Anchorage:
A device generally used to enable the tendon to impart and maintain prestress in concrete.
3. Pretensioning:
A method of prestressing concrete in which the tendons are tensioned before the concrete is placed. In this method, the concrete is introduced by bond between steel & concrete.
4. Post-tensioning:
A method of prestressing concrete by tensioning the tendons against hardened concrete. In this method, the prestress is imparted to concrete by bearing.
Materials for prestress concrete members:
1. Cement:
The cement used should be any of the following
(a) Ordinary Portland cement conforming to IS269
(b) Portland slag cement conforming to IS455. But the slag content should not be more than 50%.
(c) Rapid hardening Portland cement conforming to IS8041.
(d) High strength ordinary Portland cement conforming to IS8112
2. Concrete:
Prestress concrete requires concrete, which has a high compressive strength reasonably early age with comparatively higher tensile strength than ordinary concrete. The concrete for the members shall be air-entrained concrete composed of Portland cement, fine and coarse aggregates, admixtures and water. The air-entraining feature may be obtained by the use of either air-entraining Portland cement or an approved air-entraining admixture. The entrained air content shall be not less than 4 percent or more than 6 percent.
Minimum cement content of 300 to 360 kg/m3 is prescribed for the durability requirement.
The water content should be as low as possible.
3. Steel:-
High tensile steel , tendons , strands or cables The steel used in prestress shall be any one of the following:- (a) Plain hard-drawn steel wire conforming to IS1785 (Part-I & Part-III)
(b) Cold drawn indented wire conforming to IS6003
(c) High tensile steel wire bar conforming to IS2090
(d) Uncoated stress relived strand conforming to IS6006 High strength steel contains: 0.7 to 0.8% carbons, 0.6% manganese, 0.1% silica
Prestress is defined as a method of applying pre-compression to control the stresses resulting due to external loads below the neutral axis of the beam tension developed due to external load which is more than the permissible limits of the plain concrete. The precompression applied (may be axial or eccentric) will induce the compressive stress below the neutral axis or as a whole of the beam c/s. Resulting either no tension or compression.
Basic Concept
Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from the external loads are counteracted to a desired degree.
Terminology
1. Tendon:
A stretched element used in a concrete member of structure to impart prestress to the concrete.
2. Anchorage:
A device generally used to enable the tendon to impart and maintain prestress in concrete.
3. Pretensioning:
A method of prestressing concrete in which the tendons are tensioned before the concrete is placed. In this method, the concrete is introduced by bond between steel & concrete.
4. Post-tensioning:
A method of prestressing concrete by tensioning the tendons against hardened concrete. In this method, the prestress is imparted to concrete by bearing.
Materials for prestress concrete members:
1. Cement:
The cement used should be any of the following
(a) Ordinary Portland cement conforming to IS269
(b) Portland slag cement conforming to IS455. But the slag content should not be more than 50%.
(c) Rapid hardening Portland cement conforming to IS8041.
(d) High strength ordinary Portland cement conforming to IS8112
2. Concrete:
Prestress concrete requires concrete, which has a high compressive strength reasonably early age with comparatively higher tensile strength than ordinary concrete. The concrete for the members shall be air-entrained concrete composed of Portland cement, fine and coarse aggregates, admixtures and water. The air-entraining feature may be obtained by the use of either air-entraining Portland cement or an approved air-entraining admixture. The entrained air content shall be not less than 4 percent or more than 6 percent.
Minimum cement content of 300 to 360 kg/m3 is prescribed for the durability requirement.
The water content should be as low as possible.
3. Steel:-
High tensile steel , tendons , strands or cables The steel used in prestress shall be any one of the following:- (a) Plain hard-drawn steel wire conforming to IS1785 (Part-I & Part-III)
(b) Cold drawn indented wire conforming to IS6003
(c) High tensile steel wire bar conforming to IS2090
(d) Uncoated stress relived strand conforming to IS6006 High strength steel contains: 0.7 to 0.8% carbons, 0.6% manganese, 0.1% silica
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