1. Brief Description :
(a)Post-tensioning is a concrete strengthening technique where steel tendons are tensioned after the concrete structure is cast, applying a compressive stress that offsets tensile stress caused by external loads, thus enhancing the structure's strength and durability. This method involves installing high-strength steel tendons in ducts or sleeves within the concrete, which are then tensioned using hydraulic jacks after the concrete has cured and achieved sufficient strength.
(b)There are many Post-tensioning systems, most of them patented, which are used for stressing the tendons in any Post-tensioned member (e.g. Freyssinet system, DYWIDAG system, BBRV system, Prescon system etc.)
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In post-tensioned members the anchoring devices transfer the prestress to the concrete.
The devices are based on the following principles of anchoring the tendons.
a) Wedge action
b) Direct bearing
c) Looping the wires
- The anchoring device based on wedge action consists of an anchorage block and wedges. The strands are held by frictional grip of the wedges in the anchorage block. Some examples of systems based on the wedge-action are Freyssinet, Gifford-Udall, Anderson and Magnel-Blaton anchorages. The following figures show some patented anchoring devices.
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Fig. 2: Freyssinet "T" system anchorage cones
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Fig. 3: Anchoring Devices
- The rivet or bolt heads or button heads formed at the end of the wires directly bear against a block. The B.B.R.V post-tensioning system and the Prescon system are based on this principle.
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Fig. 4: Anchoring with button heads
(c)The essential devices in any Post-tensioning systems are normally as follows:
1) Casting bed
2) Mould/Shuttering
3) Ducts
4) Anchoring devices
5) Jacks
6) Couplers (optional)
7) Grouting equipment (optional)
There are many patented Post-tensioning systems used for stressing the tendons in any Post-tensioned member
(e.g. Freyssinet system, DYWIDAG system, BBRV system, Prescon system etc.
2. Anchoring Devices
(a) Wedge action