Introduction to the Development of Modern Prestressing Technology
Prestressing technology has only been more than half a century since its application in engineering. However, due to its unique advantages, it has developed rapidly and is widely used in various fields, with an increasing number of applications. A representative one is the post-tensioning system for steel strands called VSL (Losingor post-tensioning system), which was successfully researched and used in actual projects by Swiss VSL International in the mid-1950s. Because of the high reliability of the anchorage system, the advantages of simple construction operation, high efficiency, and wide applicability, it has been quickly adopted by various countries and has become one of the main methods widely used in the design and construction of large and medium-sized prestressed structures in the world.
The development of modern prestressing technology can be summarized into the following three aspects:
(1) The use of high-strength and ultra-high-strength prestressed tendons
In the prestressed component, the prestressed tendon itself is in tension. Therefore, the higher the tensile strength and elastic limit, the better. In addition, from the perspective of saving steel, high-strength pre-stressed tendons are also required. High strength, low relaxation, and corrosion resistance are the development direction of modern prestressed steel. The strength of the pre-stressed tendons we use today has been significantly improved compared to the past. In some countries, the strength of rough-rolled rebars can reach 1570 MPa, while steel strands have generally used products with a strength of 1860 MPa in domestic and foreign projects. Some countries are already developing higher-strength prestressed tendons.
In particular, because of its high strength, simple anchoring, and good bonding performance with concrete, steel strands are generally used in European countries, the United States, Japan, and other countries to replace steel wires. In the prestressed tendons, steel strands have more and more advantages. In order to improve the performance of the steel wire, the United Kingdom and Japan have developed a compact prestressed steel wire, and the Japanese Kobelco Steel Ltd has also developed an indented steel strand.
(2) High-efficiency prestressed anchorage system
After decades of development, the technology of the prestressed anchor system has been perfected. The prestress is easy to apply, safe and reliable, the tensile force can reach thousands of tons or even higher, and the anchorage has a high-efficiency coefficient.
(3) Continuous innovation of prestressed construction technology
In the development process of modern prestress, with the continuous improvement of the performance of prestressed tendons and prestressed anchorage systems, the construction technology of prestressed anchorages is also constantly improved and innovated. In summary, it can be expressed in the following aspects:
1. The use of flexible, safe, and reliable serialized jacks makes the prestressing tension very simple.
2. In the reserved ducts of the post-tensioning prestressed tendons, high-strength, high-elastic rubber extraction rods are used, which saves effort and is not easy to damage; improves the turnover rate of the extraction rods, and saves investment.
The use of metal corrugated ducts makes the reserved ducts for curved beams, long beams, and dense beams convenient and reliable, and can completely avoid the phenomenon of broken rods and collapsed holes during drawing and forming.
3. In post-tensioning duct grouting, the grouting technology has made great breakthroughs, especially the improvement of grouting equipment and the adoption of the “secondary grouting” method, which ensured the denseness of the duct grouting.
4. The use of unbonded prestressing technology allows the prestressing tendons to be manufactured in the factory, without the need to reserve ducts in the concrete components. It can be directly buried in concrete like ordinary steel bars, greatly reducing on-site construction procedures, improving product quality, reducing costs, and improving the construction level and construction speed of the structure. In addition, as an external prestressed reinforcement, it not only reduces the friction loss but also reduces the section size of the component, which is more convenient and economical than the former.