Saturday, November 5, 2011

Prestressing Systems and Techniques

Prestressing Systems
Prestressed concrete construction has been applied to many types of structures including bridges, buildings, dams, tunnels, elevated reservoirs, aircraft runways, floors, roofs, piles and whole variety of precast products. These are the list of prestressing systems that was adopted:

1.       Pre-tensioned Wire/Strands
-       may be either in individual molds or in a long pre-tensioning bed; stretched tendons are held by special grips until the stress is transferred to the concrete where it is maintained by bond.
2.       Macalloy System/ Stresssteel System
-       high tensile steel wires/strands are pre-tensioned and are anchored by means of nut or by wedge grips.
3.       Freyssinet System
-       high tensile steel wires/strands  assembled into cables are pre-tensioned and are secured in special anchorage cones.
4.       PSC and CCL Systems
-       High tensile steel wires/strands are stretched one at a time, and held by special grips against an anchorage plate or in an anchorage tube assembly.
5.       Magnel-Blaron System
-       High tensile steel wires/starnds are pretensioned by pairs and secured by steel wedges in special anchorage plates.
6.       BBR System
-       Fixed lengths of high tensile steel wires or strands are prefixed in common anchorage head by upsetting their ends or by wedging, and are then simultaneously pre-tensioned and anchored by nuts and shins.
7.       VSL System
-       High tensile steel strands are pretensioned and anchored individually by wedge grips in a common anchor block.
8.       Preload System for circular structures

**Each system requires a suitable jack for stretching the cables, wires, strands, or rods; and all post-tensioning systems require that the duct in which the high tensile steel tendon is carried be subsequently grouted up to prevent future corrosion.

Prestressing Techniques

-       post tensioning rods and anchorages are supplied fabricated to detail by the manufacturer, and therefore require no further work on the site except their erection in the forms.
-       post tensioning cables of all types are usually fabricated on the site by passing the wires through a spacer and binding the finished cable with tie wire or plastic tape at suitable intervals. The wwire is supplied from the manufacturer in coils 1.5 to 2 meters in diameter, from which it may be wound off directly.
-        strands are likewise supplied in coils, or on reels, from which it pays off reasonably straight. Tendons of strands may be assmebled into cables, or they may be used individually, depending upon the system of prestressing adopted.
-       tensioning of tendons are carried out with hydraulic jacks, and may be either one or both ends of a tendon.
-       trushing of test cylinders to determine the strength of the concrete before pre-stressing.
-       after prestressing, grouting of the holes from which the tendons pass through is necessary.
-       when the tendons has been stressed and grouted , lengths are cut off the ends.
-       the whole ancchorage is covered with mortar or concrete to protect it and leave a neat external appearance.



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