Prestressed Concrete

Topics: Concrete, Rebar, Tensile strength Pages: 14 (3059 words) Published: October 14, 2013
PRESTRESSED CONCRETE

The development of early cracks in reinforced concrete due to incompatibility in the strains of steel & concrete was perhaps the starting point in the development of new material like “Prestressed Concrete”. Prestressed Concrete is a method for overcoming concrete’s natural weakness in tension. INTRODUCTION

Prestressed Concrete is basically concrete in which internal stresses of suitable magnitude & distribution are introduced, so that the stresses resulting from external loads are counteracted to a desired degree.

MATERIALS USED IN PRESTRESSED CONCRETE

(1) CEMENT –
The cement used shall be any of the following, with the prior Approval of the engineer-in-charge: Ordinary Portland cement conforming to IS : 269-1976”;
Portland slag cement conforming to IS : 4551976t, but with not more than 50 percent slag content;
Rapid-hardening Portland cement conforming to IS : 8041-1978; High strength ordinary Portland cement conforming to IS : 8112- 19765.

( 2) AGGREGATES-
The nominal maximum size of coarse aggregate shall be as large as possible subject to the following:
In-no case greater than one-fourth the minimum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all prestressing tendons and reinforcements And fill the corners of the form. It shall be 5 mm less than the spacing between the cables, strands or sheathings where provided.

Not more than 40 mm; aggregates having a maximum nominal size of 20 mm or smaller are generally considered satisfactory.
Coarse and fine aggregates shall be batched separately.

(3) WATER-

The requirements of water used for mixing and curing shall
conform to the requirements given in IS : 456. However, use of sea water is prohibited.

(4)ADMIXTURES –

Admixtures may be used with the approval of the Engineer-in-charge. However use of any admixture containing chlorides in any form is prohibited. The admixtures shall conform to IS : 9103-1979.

(5) PRESTRESSING STEEL-

The prestressing steel shall be any one of the following:
Plain hard-drawn steel wire conforming to IS : 1785 (Part I )- 1966 and IS : 1785 (Part II)-19675,
Cold-drawn indented wire conforming to IS : 6003-197011,
High tensile steel bar conforming to IS : 2090-19627, and
Uncoated stress relieved strand conforming to IS : 6006-1970

All prestressing steel shall be free from splits, harmful scratches. surface flaws; rough, jagged and imperfect edges and other defects likely to impair its use in prestressed concrete. Slight rust may be permitted provided there is no surface pitting visible to the naked eye. Coupling units and other similar fixtures used in conjunction with the wires or ,bars shall have an ultimate tensile strength of not less than the individual strengths of the wires or bars being joined.

MODULUS OF ELASTICITY - The value of the modulus of elasticity of steel used for the design of prestressed concrete members shall preferably be determined by tests on samples of steel to be used for the construction, For the purposes of this clause, a value given by the manufacturer of the prestressing steel shall be considered as fulfilling the necessary requirements.

Where it is not possible to ascertain the modulus of elasticity by test or from the manufacturer of the steel, the following values may be adopted:

Type of Steel

Modulus of Elasticity E, kN/mm2 Plain cold-drawn wires [conforming to
IS : 1785 (Part I)-1966, IS : 1785
( Part II )-19671_ and IS : 6003-197011

210
High tensile steel bars rolled or heattreated
(conforming to IS : 2090-19621)

200
Strands (conforming to IS : 6006-197011)

195

Untensioned Steel - Reinforcement used as untensioned steel shall be any of the...
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