Building Materials, Third Edition

ƒ™—2g



2—2g

2g™

 RUQ

A prestressed concrete may thus be defined as a concrete in which stresses of suitable
magnitude and distribution are introduced to counteract, to a desired degree, the stresses
resulting from external loads. The concept of prestressing concrete was first used by Mandl of
France in 1896. In prestressed concrete high strength concrete and steel are desirable. The
former is required because of following:

1. The use of high strength concrete results in smaller cross-section of member and hence
   smaller self weight; longer spans become technically and economically practicable.


2. High bearing stresses are generated in anchorage zones.


3. The shrinkage cracks are reduced, with higher modulus of elasticity and smaller creep
   strain resulting in smaller loss of prestress.
   The loss of prestress at the initial stages is very high and for this reason high strength steel
   is required. High tensile strength wires with ultimate tensile strength up to 2010 N/mm^2 are
   the choice. For prestressed concrete members, the high tensile steel used generally consists of
   coires, bars or strands.
   Prestressing is achieved by either pre-tensioning or post-tensioning. In the former the wires
   or cables are anchored, tensioned and concrete is cast in the moulds. After the concrete has
   gained strength the wires are released. This sets up compression in concrete which counteracts
   tension in concrete because of bending in the member. In the post-tensioning prestressing force
   is applied to the steel bars or cables, after the concrete has hardened sufficiently. After applying
   the full prestress the cable passages are grouted. The minimum 28-day cube compressive
   strength for concrete is 40 N/mm^2 for pre-tensioned members and 30 N/mm^2 for post-tensioned
   members.

e——

1. The cracking of concrete is eliminated enabling the entire cross-section of the member to
   take part in resisting moment.


2. As dead load moments are neutralized and the shear stresses are reduced, the sections
   required are much smaller than those for reinforced concrete. This reduces the dead
   weight of structure.


3. In ordinary reinforced concrete (RCC) the economy is not as pronounced as in pre-
   stressed concrete (PSC). The prestressing force in most cases is computed strictly from
   dead load of the structure; consequently, a weight reduction of 25% results in a substantial
   reduction in the weight of prestressing tendons.
   It is widely used for construction of precast units such as beams, floors, roofing systems,
   bridges, folded plate roofs, marine structures, towers and railway sleepers.

PHU €yv‰wi‚2gyxg‚i„i

The strength of concrete is greatly affected by porosity and attempts to reduce it by vibration,
pressure application, spinning, etc. are of little help in reducing the water voids and the
inherent porosity of gel which is about 28 per cent. The impregnation of monomer and
subsequent polymerisation reduces the inherent porosity of the concrete. Polymers—polyvinyl
acetate, homopolymer emulsions and vinyl acetate copolymer emulsions—are added to increase