In prestressed concrete structures, prestress is applied by tensioning steel wires and securing them to the concrete. The prestressing system therefore consists of a means of tensioning the steel and a means of attaching it to the concrete.
Types of Prestressing Systems
1. Pre-Tensioning System
The Hoyer system or long line method is a common technique in pre-tensioning. This method involves installing two bulkheads or anchors firmly anchored to the ground, positioned several meters apart. Steel wires are then stretched between the bulkheads and enclosed in molding forms. Concrete is poured around the wires.
The Hoyer system allows multiple members to be produced in one line, making it an economical solution for pre-tensioning factories. Tensioning is achieved using a hydraulic jack to grip the wires at the bulkheads with split-cone wedges. The wedges & anchor cone are designed with tapered conical pins and a flat surface with serrations to hold the wire securely.
The advantage of pre-tensioning systems is that there is no need for end anchorages or rubber cores or metal sheaths as in post-tensioning systems. The prestressing force is more predictable in pre-tensioning. In post-tensioning, the force depends on the strength of the two end anchorages.
However, pre-tensioning systems have disadvantages, such as the need for strong end abutments, which are only available at precast factories, limiting the size of the members that can be produced. Transporting large members from the factory to the construction site can also be challenging. Additionally, pre-tensioning can result in a greater loss of prestressing force compared to post-tensioning.
2. Post-Tensioning System
In post-tensioning, a metal tube or flexible hose, following the desired profile, is placed inside the mold and concrete is poured. The flexible hose is then removed, leaving a duct within the member. A steel cable is inserted into the duct.
One end of the cable is anchored, and the other end is tensioned using a hydraulic jack. After the cable is stretched, it is anchored at the opposite end. As a result, the post-tensioning system requires end anchorages, Lrpc Strand and hydraulic jacks.
The popular post-tensioning systems are the following:
- Freyssinet system
- Magnel Blaton system
- Gifford-Udall system
- Lee-McCall system
1. Freyssinet System
The Freyssinet system was the first method of post-tensioning introduced by French Engineer Freyssinet. It involves grouping 8 to 24 high-strength steel wires of 5mm or 7mm diameter into a cable, with a helical spring inside to maintain proper spacing between the wires. The cable is then inserted into the duct.
The anchorage device in the Freyssinet system is composed of a concrete cylinder with a concentric conical hole and corrugations on its surface and a conical plug with grooves on its surface, where the steel wires are held at the ends. The concrete cylinder is heavily reinforced.
The members are constructed with the cylinder in place. The Freyssinet double-acting jacks can pull all the wires in the cable simultaneously through the appropriate grooves.
2. Magnel Blaton system
The Freyssinet system stretches several wires at once, while the Magnel Blaton system, developed by Belgian engineer Prof. Magnel, stretches two wires at a time.
The anchorage device in this system consists of a sandwich plate with grooves to hold the wires and grooved wedges. Each plate accommodates eight wires. The spacing between the wires is maintained by spacers. 5mm or 7mm wires are used, and cables consist of multiples of 8 wires, with as many as 64 wires used in special conditions.
3. Gifford Udall System
This system, originating in Britain and widely used in India, is a single-wire system where each wire is prestressed independently using a double-acting jack. Wires can be grouped to form a cable of any number. There are two types of anchorages in this system: tube and plate. The tube anchorage includes a bearing plate, anchor wedges, and anchor grips. The anchor plate, square or circular, has 8-12 tapered holes to hold the prestressing wires, which are secured with anchor wedges.
4. Lee McCall System
This prestressing method uses steel bars with a diameter ranging from 12mm to 28mm. The bars have threads at both ends, which are inserted into precast ducts. After being stretched to the required length, they are tightened with nuts against bearing plates located at the end sections of the structure.
A) Electrical Prestressing
The reinforcing bars are coated with a thermoplastic material like sulfur or low-melting alloy and embedded in the concrete. After the concrete sets, a low-voltage but the high-amperage electric current is passed through the bar, heating it and causing it to elongate. The bar is then tightened against heavy washers using the threads at the other end after reaching the desired elongation. When the bar cools, prestress develops, and the bond is reinforced by solidification of the coating.
B) Chemical Prestressing
Chemical prestressing utilizes expanding cement to apply prestress to steel. By embedding the steel in expanding cement concrete, the concrete's expansion causes the steel to elongate, thereby creating prestress. This, in turn, results in compressive stress in the concrete.
Conclusion
In conclusion, prestressing is a technique used to improve the strength and stability of concrete structures. There are several methods of prestressing, including the Hoyer system, post-tensioning system, Freyssinet system, Magnel Blaton system, and others. Each method has its own advantages and disadvantages, and the choice of method depends on the specific requirements of the project. The use of expanding cement and electric current are also alternative methods of prestressing.
