Steel wire is stretched and anchored on concrete to create prestress in prestressed concrete structures. As a result, the prestressing systems should basically consist of a way to secure the steel to the concrete and a way to stretch the steel.

For pre-tensioning and post-tensioning, separate systems are used.

Different Types of Prestressing System

1. Pre-Tensioning Systems

In pre-tensioning, the Hoyer system or long line method are frequently used. There are two independent bulk heads or abutments that are attached to the ground, spaced a few metres apart. Wires, approximately 100 metres long, are stretched between the bulkheads. The wires are enclosed by moulds. The cables are surrounded by concrete.

It is possible to produce several members along a single line with this Hoyer system. Nearly all pre-tensioning factories employ this affordable method.

A hydraulic jack is used to tension. Wires are secured at the bulkheads with wedges made of split cones. Tapered conical pins are used to make these wedges. The pin’s flat surface has serrations on it to help hold onto the wire (figs. 1 and 2).

The Shorer system is a different pre-tensioning technique. This technology places the complete assembly in place and concretes it after a central tube made of high-strength steel takes the prestress from surrounding wires.

The tube is taken out and the prestress is bonded to the concrete once the concrete has reached a sufficient strength. Grout is used to fill the hole the tube left.

The benefit of a pre-tensioning system is that it eliminates the need for end anchorages and the metal sheath or rubber core that are necessary for a post-tensioning system. Regarding the prestressing force, there is more assurance. The two end anchorages in post-tensioned members determine the force’s certainty.

This system’s drawback is that end abutments need to be extremely robust and are only available from precast factories. Due to the difficulty of transporting huge sizes from the manufacturer to the construction site, this automatically restricts the size of the member. Pretensioned members lose more.

Figure-1: Typical Pre-Tensioning Bed

Figure-2: Few Common Wire Gripping Systems

2. Post-Tensioning System

After the mould is filled, concrete is poured inside a metal tube or a flexible hose that fits the specified contour. The flexible hose is then taken out, leaving the member with a duct inside. The duct is filled with steel cable.

A hydraulic jack is used at one end of the member to lengthen the cable after it has been secured there. The cable is fixed at both ends once it has been stretched. Therefore, end anchorages and jacks make up the post tensioning system.

The following are some common post-tensioning systems:

1.Freyssinet System

The first way to be introduced was the Freyssinet system, which was created by French engineer Freyssinet. High-strength steel wires with a diameter of 5 mm or 7 mm, numbered 8, 12, 16, or 24, are arranged into a cable that has an internal helical spring. The spring maintains the wire’s correct spacing. A cable is put within the duct.

Figure 3: The Freyssinet post-tensioning system

The anchorage device is comprised of a concrete cylinder featuring corrugations and a concentric conical hole on its surface, as well as a conical plug with grooves on its surface (Fig. 3). At the ends of these slots, steel wires are carried. The cylinder of concrete is strongly strengthened.

The cylinder is positioned before the members are produced. Freyssinet double acting jacks pull wires; they may pull a whole cable’s worth of wires at once through the appropriate grooves.

The cables are drawn to the desired length by pulling on one end while anchoring the other. The plug is then forced into the cylinder to grasp the wires by an internal piston in the jack.

2. Magnel Blaton system

Multiple wires are stretched simultaneously in the Freyssinet system. The Magnel Blaton system involves stretching two wires simultaneously. Prominent Belgian engineer Prof. Magnel introduced this technique.

The anchorage device in this method is a sandwich plate with grooves to retain the wires and grooved wedges. There are eight wires on each plate.

Spacers keep the wires spaced evenly between the two ends. 5 mm or 7 mm wires are used. Multiples of eight wires make up cables. In certain situations, cables with up to 64 wires are also utilised.

Two wires are pulled and secured at the same time by a specially designed jack. Figure 4 shows the wires connected to the sandwich plate with the tapered wedge.

Figure 4: Magnel System Anchorage

3. Gifford-Udall System

India uses this method extensively; it was developed in Great Britain. This setup just uses one wire. A double acting jack is used to separately tension each wire. In this method, a cable can be made up of any number of wires connected together. In this system, there are two different kinds of anchorage devices.

a) Anchorages for tubes

a) Anchorages for plates

Anchor wedges, anchor grips, and a bearing plate make up tube anchorage. To fit each prestressing wire individually, the anchor plate can have eight or twelve tapering holes in a square or circular shape. Anchor wedges are used to secure these wires into the tapering holes.

Furthermore, the bearing plate has a grout entrance hole for grouting. Split cone wedges with serrations on their flat surfaces are known as anchor wedges.

A steel tube with a helix encircling it and a thrust plate are part of a tube unit that is manufactured. This device forms an effective cast-in element of the anchorage and is fastened to the end shutters (fig. 5).

Figure- 5: Plate Anchorage

4.The Lee-McCall system

Steel bars are prestressed using this process. The bar’s diameter ranges from 12 to 28 mm. The completed conduits are filled with bars that have threads on the ends. The bars are tightened using nuts against bearing plates that are provided at the member’s end portions once they have been stretched to the necessary length (fig. 6).

Fig. 6: Lee McCall System End Anchorage