Unbonded Post-Tensioning Systems

Unbonded Post-Tensioning System evolved from Grouted Post-Tensioning System before 1950 for Building Construction Application.

In Unbonded Post-Tensioning Systems, tendons are provided by a factory-applied corrosion-inhibiting coating material and an extruded plastic sheath which acts as a bond breaker between the concrete and the prestressing strands. Plastic sheathing and strand coating material serve as a barrier that prevents moisture and chemicals from reaching the strand and also reduce friction between the strand and the sheathing and provides additional corrosion protection. In highly aggressive corrosive environments the tendon can be completely encapsulated, providing additional protection against corrosion.

Plastic sheaths also provide protection against damage by mechanical handling. Anchorages are another critical element, particularly in unbonded systems.

After the concrete has cured and obtained the necessary strength, the wedges are inserted inside the anchor casting and the strand is stressed. When the jack releases the strand, the strand retracts slightly and pulls the wedges into the anchor. This creates a tight lock on the strand. The wedges thus maintain the applied force in the tendon and transfer it to the surrounding concrete. In corrosive environments, the anchorages and exposed strand tails are usually covered with a housing and cap for added protection.

Advantages of Unbonded Post-Tensioning Systems

• The products can be ordered ready for installation in accordance with the project requirements which allows for faster installation
• Long-term Durability (Corrosion Protection by sheathing and anti-corrosive grease coating on cable). Encapsulated Unbonded System for effective protection against corrosion
• Slabs reinforced with Unbonded P-T system have better control over the tensile stresses and deflection
• Grout preparation and tests are creating extra costs and time consuming
• Post-earthquake residual deformation is minimized

For more information on Post-Tensioning Systems and Unbonded Post-Tensioning Systems, please follow the related link on the left, download our Post-Tensioning Systems Brochure or contact us.