In the early fifties, post-tensioning systems for the pre-stressing of concrete was developed by specialists, such as VSL in Switzerland.
This technique consists in introducing an internal stress state into a concrete element, with the installation and stressing of post-tensioning tendons. This comes to counteract stresses due to external loading, and results in reduced deformation and cracking of concrete.
Post-tensioning has revolutionized the design and construction of concrete structures, in particular bridges: It has permitted to extend the application of concrete for larger spans, and has enabled the development of new construction methods, making construction simpler and faster.
VSL post-tensioning technology includes several systems that are specifically designed for different applications and requirements. The choice of a suitable system can be made by considering three key criteria:
- Type of structural element: the depth of the structural element influences the type of system to be used:
- Multistrand tendons are usually used for structural elements with medium to large depths (bridge girders, beams in buildings…)
- Slab tendons with flat ducts are generally used for thin structural elements (slabs)
- Structural design: taking account of structural design requirements
- An internal or external post-tensioning system, or a combination of both, is adopted for multistrand tendons
- For slab tendons, a bonded or unbonded post-tensioning system may be chosen
- Required Protection Level (PL) to protect the tendon from corrosion attacks: the tendon encapsulation is chosen depending on the required protection level – PL1, PL2, PL3 as defined by the FIB Bulletin 33.
As an experienced specialist contractor, VSL carries out high quality installation using trained personnel and reliable equipment, in accordance with well-proven procedures.
One of the key procedures is about grouting, which comes to protect the tendon from corrosion attacks.
VSL offers various accessories and services for structural monitoring such as:
- Grout Void sensors and monitoring guarantee that the tendon has been properly filled with grout and confirm the grout properties,
- Electrically Isolated Tendons (EIT) allow for long-term monitoring of tendon encapsulation and detection of the exact location of any possible defect,
- In-service monitoring for special structures such as nuclear power plants.
