This document discusses different systems used for prestressing steel, which are grouped into four categories: mechanical, hydraulic, electrical/thermal, and chemical. It provides details on common tensioning devices within each category. Mechanical devices use weights, pulleys, and screw jacks. Hydraulic jacks ranging from 5-600 tonnes are widely used. Electrical/thermal heating of wires before concreting is another option. Chemical devices use expanding cement. The document also describes several popular prestressing systems including Freyssinet, Gifford Udall, Lee-McCall, Magnel Blaton, BBRV, and Baur Leonhardt.

1. UNIT – I
INTRODUCTION – THEORY AND BEHAVIOUR

2.  Pre tensioning system
 Post tensioning system

8.  The various types of devices used for tensioning steel
are grouped under four principal catagories.
◦ Mechanical
◦ Hydraulic
◦ Electrical or Thermal
◦ Chemical

9. Mechanical devices Hydraulic devices
 It includes weights with or
without lever transmission.
 Geared transmission in
conjunction with pulley
blocks.
 Screw jacks with or without
gear drives.
 In simple it is wire binding
machines.
 Hydraulic jacks are the
simplest means of producing
large prestressed forces or
extensively used as tensioning
devices.
 This systems are used for the
ranges of 5 to 100tonnes
 Large hydraulic jacks for
forces in the range of 200 to
600tonnes have also been
developed by baur-leonhardt.

10. Electrical or Thermal devices Chemical devices
 It have been used
successfully in 1958 for
tensioning of steel wires and
deformed bars.
 The steel wires are
electrically heated and
anchored before placing of
concrete in the moulds.
 It is also called as thermo-
electric prestressing.
 Expanding cements are used
and the degree of expansion
is controlled by varying
curing conditions.
 Since the expansive action
of cement by sitting in
restrained it induce tensile
forces in tendon and
compressive forces in
concrete.

11.  Freyssinet anchorage system
 Gifford Udall systems
 Lee-McCall systems
 Magnel blaton system
 BBRV system
 Baur Leonhardt system

13.  This system is widely used in Europe and India
 It consists of a cylinder with a conical interior through
which the high tensile wires pass and against the walls
of which the wedged by a conical plug lined
longitudinally with grooves to house the wires.
 The main advantage of this system is that a large
number of wires or strands can be simultaneously
tensioned using double acting hydraulic jacks.

15.  It is developed in UK.
 It consists of steel split cone and cylindrical female
cone anchorages to house the high tensile wires bearing
against steel plates.
 Each wire is tensioned separately and anchored by
forcing a sleeve wedge into a cylindrical grip resting
against a bearing plate.
 The ducts are generally formed by metal sheaths cast
into the concrete member.

17.  In this method, the tendons comprise high tensile bars
of diameter varying from 12 to 40mm which are
threaded at the ends.
 After tensioning, each bar is anchored by screwing a
nut and washer tightly against the end plates.
 In this system the forces are transmitted by the bearing
at the end blocks.
 While the system eliminates the loss of stress due to
anchorage slip, it has a disadvantage in that curved
tendons cannot be used.

19.  This system adopts metallic sandwich plates, flat
wedges, and a distribution plate for anchoring the
wires.
 Each sandwich plate can house up to four pairs of
wires.
 The distribution plate may be cast into the member at
the desired location.
 The number of wires in the magnel cable varies from 2
to 64.

21.  This system is well suited for transmitting large forces.
 A BBRV tendon consists of several parallel lengths of
high tensile wires, with each end terminating in cold
formed button head with a machined anchorage fixture.
 In the case of tendons formed by strands, they are
anchored to the machined fixture by split cone sleeves.