Prestressing Systems Freyssinet,Mangal Blaton and Lee Mc Call System
Different arrangements are made to tension the steel wire for the preparation of concrete structure s in pre-stressing .This are called prestressing systems. Following are systems usually adopted :- Freyssinet System
- Mangal Blaton System.
- Lee Mc Call System
Freyssinet system.-
In this system of pre stressing specially prepared cables are used to create tension. These are called freyssinet Cavles. These are wrapped round a helical spring as shown in the figure.An elastic metal cover is placed round these wire. Their ends are kept bars up to the length of 60 to 75 cm (2 to 2.5ft ) so that they can be caught for the purpose of tensioning .This cable is put at the desired place in the form work prepared in concrete member. This properly fitted there. The arrangement for its anchorage has been made already . this arrangement is in the form of two cones. One of the cones is called female while the other the male cone .Females become is made of R.C.C with smooth from inside and rough from outside .This cone is placed at the proper end of at beam or any other member before casting concrete Male cone is equal to the void of female cone. There is metal pipe inside this small cone. This cone is also made of concrete.
The wires of the cable are drawn out when the concrete is hardened . jacks are used or this purpose . the male cone is inserted in the female cone when the desired tension is produced .And the jacks are separated .The cement gout is poured in the hole of the male cone so that the void be filled .Then the extended wires are cut out and their ends are filled with concrete to save the steel wires fro rusting .
MANGEL BLATON SYSTEM .
In this system of pre-stressing duct is left for the cable before casting concrete .A rubber pile is already placed for the place. this pipe is drawn out after hardening the concrete. and cable is inserted at this place this cable consists of 8 wires. A Special steel plates are used for the anchorage of these wires. These plates are called sandwich plates .These pates have two trapezoidal grooves at both of their ends . These plates are placed at the two ends of a concrete member before casting concrete into .in this way these are embedded in the concrete .At the hardening of the concrete the cables are passed through the duct these wires are passed through the grooves of the sand which plates .In this way two wires pass through each groove. After this with the help of the jack. The wires are tensioned to the desired extent and then steel wedges are inserted there. In this way the ends of the wires are caught firmly and the compression is created in the member of the concrete.
LEE MC.CALL SYSTEM
This system of prestressing is also called stress steel system . In this system the Alloy steel rod is used to produce stress . The diameter of the rod is 12 mm (0.5 in.) to 28 mm (1.125 in.) This rod has threads on both sides. In the form work prepared for the preparation of the pre stressed member, this rod is placed at the desired place after wrapping a rubber tube on it .After hardening the concrete this tube is drawn out .Nuts are set out at the ends of the rod by using washer. Hydraulic jacks are fitted at one ends if the length of the member is less and at the both ends if the length is more . Then with the help of these jacks the rod is tension-ed to the desired extent. And the nuts are tightened in the positing .The jacks are them removed .Then the cement grout is filled in the slot with the help of specially designed tube .The grout is also applied at the ends of the rod as well .So that the rod remains safe from corrosion.
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