Engineers Trust Superior’s Foundations
We’ve been creating strong foundations from the start, and post-tension cables are no exception.
Here’s How Post Tensioned Foundations Work:
First, high-grade steel cables in poly sleeves are placed in a grid pattern according to precisely engineered specifications inside the slab. Then, after the foundation is poured, the cables are hydraulically tightened to required pressures to secure the foundation into one mass – eliminating the problems commonly associated with expansive soils here in Mississippi.
There are a wide variety of reasons for utilizing post tension slabs versus conventional slabs. Post tension slabs are a form of pre-stressed slabs which means they are compressed prior to use so that the slabs are able to bear weight or service loads beyond their own natural weight. This is accomplished by pre-stressing cables which run throughout the slab and by compressing the concrete. Thus, the cables and slab are able to handle over 33,000 pounds of tension, making the slab stronger and less likely to crack. The purpose behind this is to avoid the common problems which are associated with typical concrete slab structures. This technique has been around for over 30 years and is the industry standard for pouring slabs in areas with expansive soils.
Post Tension Slab Advantages
– Reduction of shrinkage and cracking in slabs which results in the use of fewer (or even no) joints.
– If cracks do form they are minimal and are held together.
– It enables the possibility of building on a wider variety of soil types which are typically difficult to build slabs on and prevents the associated problems with traditional slab-on-grade.
– The slab concrete can be thinner and the distance between cables wider.
– If a foundation shifts, post tension cables will help prevent the foundation from breaking, which makes repair work much easier.
Post Tension Cables & Rebar
Understanding the method utilized in post tensioning, as well as how the cables are set up helps clarify why post tensioning is so effective and is beneficial over common rebar. The cables (steel bands wrapped together) are set up within the concrete in a grid pattern (usually about 48 inches apart) and are called tendons when they are covered by a plastic outer shell and held by an anchorage. The plastic duct, or outer plastic shell, prevents corrosion and allows the cables to be flexible and move as necessary. The anchorage enables the cable to be pulled during tensioning, while also securing the tendon in place within the concrete. Once the tendons are tensioned (pulled or stressed), they are grouted in the slab.
Benefits of Post-Tension Slabs and Difficult Soil Conditions
Don’t want to have post tension slab failure? Post tension slabs are particularly beneficial on expansive soils and organic soils which are found in the South and Southeastern United States. The expansive, or Yazoo clay soils such as those present here in Mississippi, shrink and swell based on the amount of water content in the soil which makes traditional slab-on-grade slabs crack and become weaker over time. Post-tensioning slabs solve this issue because they are stronger and able to bear the shrink and swell cycle without major cracking. This is particularly helpful when there is a wide variance in the amount of water in the soil.
Avoiding Post-Tension Errors Which Result in Cracking
There are several post tension slab problems which can be avoided. In order to ensure that the post-tension slab used will have the best chance of avoiding cracking, as well as ensuring the longevity of post tension slabs, there are several possible errors to consider. Avoiding post tension errors result in the best chance of a crack-free post tension foundation.
Installing the Slip Sheet Incorrectly
The slip sheet must be installed directly under the slab (between it and the bearing surface) which allows the slab to move smoothly across the ground. Incorrect placement can result in the inability of the slab to close cracks during curing, as well as forcing the slab to exert more force to move and increasing friction.
Inconsistent Slab Thickness
Level and compact the soil and gravel prior to any concrete layering. Slab thickness that varies leads to cracks because it causes the slab to resist movement. If the slab thickness is changing, such as around beams, the change should be gradual and smooth.
Curing Tendons Late
Ensure that the tendons are cured on time in order to avoid the slab from becoming too strong to close up small cracks during the curing process. This should be done within the first 4-10 days as then the concrete will be at the appropriate strength level, barring weather issues.