- Post-tensioned slabs-on-ground provide a cost-efficient, high-performance solution for problems associated with ground-supported residential foundations on shrink-swell soils. The compressive stresses resist the anticipated tension stresses induced by the soil movements, enhancing the performance over a non-prestressed foundation. Cost benefits are achieved by reductions in quantities of concrete, steel, and excavation, which in turn reduce labor costs.
- In less expansive soils, a uniform thickness foundation is utilized. Typical thickness ranges from 7.5 to 12 inches, with increases in material quantities offset by reductions in labor and equipment costs. The elimination of stiffening ribs allows for rapid construction, reducing labor and equipment costs related to digging and disposing of excavations. This is particularly beneficial in sandy soils requiring shoring. Post-tensioned foundations are also effective in stable soils for reducing cracking, eliminating control joints, increasing flexural capacity, and improving constructability. Fewer control joints enhance serviceability and durability.
Advantages of Using Post-Tensioning in Slab-on-Ground Projects
- •Stronger and More Efficient: Less concrete and steel are needed for the same structural capacity, and the slab stiffness is increased to better resist bending caused by differential soil movements.
- •Minimizes and Controls Cracking: Post-tensioning reduces cracking and keeps any cracks that might form tight, preventing the entry of insects and reducing water penetration, which can damage flooring and cause mold problems.
- •Controls Deflections: The added stiffness of a post-tensioned foundation reduces slab bending under load.
- •Faster Installation: With fewer pieces to handle and less concrete to place, a post-tensioned slab can often be installed more quickly than a comparable rebar- or wire mesh-reinforced slab.
- •More Reliable: Post-tensioning is an engineered solution designed to exacting standards and code requirements, with an excellent performance record and increased reliability.
- •Economical: Cost benefits are achieved by reducing quantities of concrete, steel, and excavation, which in turn lowers labor costs. Smaller beams and reduced slab thickness lead to savings in excavation and site preparation.
Construction of post-tensioned slabs on grade is similar to using reinforcing steel, with the addition of a tensioning step. Cables are arranged as specified by the engineer and chaired to run through the center of the slab. Residential construction typically uses tendons at 48 inches on center, while commercial foundations use more steel. Tendons can be routed around obstructions.
A residential post-tensioned concrete slab is usually 8 inches thick and uses 3000 psi concrete. Once the concrete reaches 2000 psi, typically within 3 to 10 days, the tendons are stressed.
Tendons consist of seven high-strength steel wires wound together and placed inside a plastic duct. At each end, a PT anchor is located, embedded into the slab edge. When stressed, the wires stretch.
After stressing, the tendon is cut off, and the pocket containing the anchors is filled with grout to protect them from corrosion.
About Post-Tensioning
- Post-tensioning is a method of reinforcing concrete. High-strength steel tendons are positioned in ducts or sleeves before the concrete is placed. Once the concrete gains strength, tension is applied, pulling the tendons and anchoring them against the outer edges of the concrete before service loads are applied.
- The PT technique is a form of pre-stressing, which differs slightly from post-tensioning. In pre-stressing, steel tendons are tensioned before the concrete is placed. The tendons are stretched between strong bulkheads that withstand external forces, and the concrete is poured around them.