What is Post-Tensioning?

My property has poor soil conditions, what can I do?

Homes and small commercial buildings are typically built on soil-supported foundations. Therefore, the characteristics and behavior of the underlying soils have a significant impact on the performance of the soil-supported foundation system.

Soil is a compressible material; the more load applied to it, the more the soil compresses. Building foundation settlement occurs when the building load exceeds the bearing capacity of the supporting soil. Multi-story buildings typically are designed to withstand settlements of several inches. Most homes, are light weight structures which apply relatively small loads. These loads will result in minor settlement in the foundations.

Clay soils are sensitive to moisture fluctuations. During wet weather, water seeping into clay soils causes the soil to expand and create upward pressure under the building’s foundation. During dry weather, the same clay soil becomes dry and the soil will contract, creating voids that leave portions of the building foundations unsupported. The higher the clay content of the soil under the building, the stronger and stiffer the foundation must be to remain undamaged.

Strengthening the slab is effectively accomplished by constructing a waffle-like system of stiffening ribs with reinforcing steel and post tension cables added to the ribs that greatly increases the slab strength. See the photo to your right.

Post-tensioned cables are strands of high-strength steel that surrounded in plastic sheathing, similar to electrical cables. The cables are tensioned with hydraulic jacks and anchored at the slab edges after the concrete has hardened, leaving the slab and grade beams in a state of compression, similar to placing a vise. As long as the concrete is in a state of compression, no cracking is possible in it.

The size of the concrete waffle system and the amount of reinforcement and post tension cables required is determined by the anticipated pressure of the shrinking and swelling cycles of the clay soil. Slab and foundation systems supported on high clay content soils will require more reinforcing than those constructed on lower clay content soils.