PRESTRESSING SYSTEMS AND ANCHORAGES
In concrete structures, prestress is introduced by stretching steel wire and anchoring them against concrete. Therefore, the prestressing systems should comprise essentially a method of stretching the steel and a method of anchoring it to the concrete. Different systems are adopted for pre-tensioning and post tensioning.
Hoyer system or long line method is often adopted in pre-tensioning. Two bulk heads or abutments independently anchored to the ground are provided several meters apart, say, 100m. wires are stretched between the bulkheads. Moulds are placed enclosing the wires. Concrete is placed surrounding the wires. With this Hoyer system, several members can be produced along one line. This method is economical and is used in almost all pre-tensioning factories. For tensioning, a hydraulic jack is used. Wires are gripped at the bulkheads, using split-cone wedges. These wedges are made from tapered conical pins. Flat surface of the pin carries serrations to grip the wire (fig. 1 and 2).
There is another pre-tensioning method known as Shorer system. In this system a central tube of high strength steel carries the prestress from surrounding wires and the entire assembly is placed in position and concreted. After the concrete has attained sufficient strength, the tube is removed and the prestress is transferred to concrete through bond. The hole left by the tube is grouted.
The advantage in pre-tensioning system is that there is no expenditure on end anchorages and on rubber core or metal sheath required for post-tensioning system. There is greater certainty about the prestressing force. In post-tensioned members certainty of the force depends on the two end anchorages.
Disadvantages in this system are that the end abutments should be very strong and are provided only in pre-cast factories. This naturally limits the size of the member as large sizes are difficult to transport from factory to the site of construction. Loss is more in pre-tensioned members.
Fig.1: Typical Pre-tensioning Bed
Fig.2: Some Typical Wire Gripping System
A metal tube or a flexible hose following intended profile is placed inside the mould and concrete is laid. Flexible hose is then removed leaving a duct inside the member. Steel cable is inserted in the duct. The cable is anchored at one end of the member and stretched using a hydraulic jack at the other end. After stretching the cable is anchored at the other end also. Therefore post tensioning system consists of end anchorages and jacks. The popular post-tensioning systems are the following:
- Freyssinet system
- Magnel Blaton system
- Gifford-Udall system
- Lee-McCall system
1. Freyssinet System:
Freyssinet system was introduced by the French Engineer Freyssinet and it was the first method to be introduced. High strength steel wires of 5mm or 7mm diameter, numbering 8 or 12 or 16 or 24 are grouped into a cable with a helical spring inside. Spring keeps proper spacing for the wire. Cable is inserted in the duct.
Fig.3: Freyssinet system of Post-tensioning
Anchorage device consists of a concrete cylinder with a concentric conical hole and corrugations on its surface, and a conical plug carrying grooves on its surface (Fig. 3). Steel wires are carried along these grooves at the ends. Concrete cylinder is heavily reinforced. Members are fabricated with the cylinder placed in position. Wires are pulled by Freyssinet double acting jacks which can pull through suitable grooves all the wires in the cable at a time. One end of the wires is anchored and the other end is pulled till the wires are stretched to the required length. An inner piston in the jack then pushes the plug into the cylinder to grip the wires.
2. Magnel Blaton system:
In Freyssinet system several wires are stretched at a time. In Magnel Blaton system, two wires are stretched at a time. This method was introduced by a famous engineer, Prof. Magnel of Belgium. In this system, the anchorage device consists of sandwich plate having grooves to hold the wires and wedges which are also grooved. Each plate carries eight wires. Between the two ends the spacing of the wires is maintained by spacers. Wires of 5mm or 7mm are adopted. Cables consists of wires in multiples of 8 wires. Cables with as much as 64 wires are also used under special conditions. A specially deviced jack pulls two wires at a time and anchors them. The wires with the sandwich plate using tapered wedge is shown in fig.4.
Fig.4: Anchorage of Magnel System
3. Gifford Udall System:
This system originated in Great Britain, is widely used in India. This is a single wire system. Each wire is stressed independently using a double acting jack. Any number of wires can be grouped together to form a cable in this system. There are two types of anchorage device in this system.
a) Tube anchorages
b) Plate anchorages
Tube anchorage consists of a bearing plate, anchor wedges and anchor grips. Anchor plate may be square or circular and have 8 or 12 tapered holes to accommodate the individual prestressing wires. These wires are locked into the tapered holes by means of anchor wedges. In addition, grout entry hole is also provided in the bearing plate for grouting. Anchor wedges are split cone wedges carrying serrations on its flat surface. There is a tube unit which is a fabricated steel component incorporating a thrust plate, a steel tube with a surrounding helix. This unit is attached to the end shutters and form an efficient cast-in component of the anchorage (fig.5).
Fig.5: Plate Anchorage
4. Lee McCall System:
This method is used to prestress steel bars. The diameter of the bar is between 12 and 28mm. bars provided with threads at the ends are inserted in the performed ducts. After stretching the bars to the required length, they are tightened using nuts against bearing plates provided at the end sections of the member (fig.6).
Fig.6: End Anchorage for Lee McCall System
5. Other Methods of Prestressing:
a) Electrical Prestressing:
in this method, reinforcing bars is coated with thermoplastic material such as sulphur or low melting alloy and buried in the concrete. After the concrete is set, electric current of low voltage but high amperage is passed through the bar. Electric current heats the bar and the bar elongates. Bars provided with threads at the other end are tightened against heavy washers, after required elongation is obtained. When the bar cools, prestress develops and the bond is restored by resolidification of the coating.
b) Chemical Prestressing:
Chemical prestressing is done using expanding cement. Prestressing can be applied b embedding steel in concrete made of expanding cement. Steel is elongated by the expansion of the concrete and thus gets prestressed. Steel in turn produces compressive stress in concrete.