Designers use post-tensioning as a way to reinforce concrete by prestressing it. In prestressed members, compressive stresses are introduced into the concrete to reduce tensile stresses resulting from applied loads including the self weight of the member (dead load). Prestressing steel, such as strands, bars or wires, is used to impart compressive stresses to the concrete. Pre-tensioning is a method of prestressing in which the tendons are tensioned before concrete is placed and the prestressing force is primarily transferred to the concrete through bond. Post-tensioning is a method of prestressing in which the tendons are tensioned after the concrete has hardened and the prestressing force is primarily transferred to the concrete through the end anchorages
Unlike pre-tensioning, which can only be done at a precast manufacturing facility, post-tensioning is performed on the jobsite in cast-in-place applications. The concrete component is cast with steel reinforcing strands installed in a way that protects them from bonding with the concrete. This practice gives designers the flexibility to further optimize material use by creating thinner concrete members.
The materials used to post-tension concrete members are ultra-high-strength steel strands and bars. Horizontal applications (like beams, slabs, bridges, and foundations) typically employ strands. Walls, columns, and other vertical applications usually utilize bars. Steel strands used for post-tensioning typically have a tensile strength of 270,000 psi, are about a half-inch in diameter, and are stressed to a force of 33,000 pounds.
While concrete is strong in compression, it is weak in tension. Steel is strong under forces of tension, so combining the two elements results in the creation of very strong concrete components. Post-tensioning can help create innovative concrete components that are thinner, longer, and stronger than ever before.
Many of today’s “high-performance” concrete structures, including many landmark bridges and buildings, employ some type of prestressing. Parking garages, high-rise residential towers, and many other kinds of structures also employ post-tensioning techniques.