Prestressed concrete refers to concrete members which are preloaded by a compression force prior to application od service loads. Prestressed concrete is achieved by using a combination typical mild steel reinforcement and steel cable (strand) reinforcement. Prestressed concrete elements can be pretensioned, where strands are stressed prior to concrete casting, or post-tensioned, where strands are stressed after concrete has hardened.
Figure 1. Installation of Post-Tensioning Tendons in a Slab.
In general, prestressing force does not result in significantly higher member resistance; however, it improves serviceability aspects such as deflection, both short and long term, and control of cracking. This allows the use of longer spans, without the need of deeper members as compared to conventional reinforced concrete. The use of shallower members reduces the volume of concrete required and can potentially lead to a decrease in the overall cost of the project. In addition, due to crack control, it can potentially provide better durability than conventional reinforcement, potentially decreasing the cost of long-term maintenance of the structure and increasing the service life. Longer spans also provides an aesthetic and cost benefit of allowing for more open floors with less supporting elements.
Figure 2. Stressing process
Prestressed concrete can be applied to a wide range of applications, for both cast-in place and precast elements. Typically, within the building industry, prestressed concrete is used in beams and columns, as well as slabs. Other potential applications for precast members include wall panels and concrete piles.
The use of prestress concrete members provides several beneficial properties; however, it is important to note that it requires special considerations during design and construction. For example, during design, consideration should be given to high bursting forces near the end zone or at anchoring points and it is of high importance that proper detailing is provided to prevent cracking near the anchoring system. The installation, stressing and release of strands along with grouting operations require experienced labor and high-quality control during construction.
Figure 3. PT Slab before Concrete Pouring
Overall, by reducing the volume of concrete required and controlling cracks, prestressed concrete can potentially provide a more cost-effective alternative through the life of the structure when compared to conventional reinforced concrete. However, special considerations are needed during design and construction to ensure high quality products.
Glenda E Diaz