Post-Tension Foundations

Very common here in the Greater Houston area, are Post-Tension Concrete Foundations.

Curious how it all works ?!?

Steel braided structural cables or tendons are encased inside of a plastic sheathing. They are laid out and installed according to strict engineering plans, placing the cables exactly where they are most needed to assist in supporting and tying in separate sections throughout a foundation. Once in place, they are fastened to the outer form boards to keep them in place. Shoes or cable supports are then added throughout to allow the cables to suspend above the compressed foundation pad floor. Intersecting cables are also connected with metal rebar ties.

What’s next …

Once all the rough plumbing, post-tension cables, supports, electrical and any other components needed are in place. Then it’s INSPECTION Time ! After the inspection report indicates present issues / concerns, the builder will remedy these before proceeding to pour the slab.

Boom Truck depositing concrete into slab form…

After concrete is all placed, the surface will be screed / smoothed out and finished with hand and machine trowels. The finished product ends up like…

Generally concrete cures rapidly in the first month, but some believe continues curing or aging throughout its life-span. Concrete reaches approx. 40% of its overall strength in just 3 days, and approx. 99% at just 28 days. So the best practice is to wait at least a week before proceeding to the framing stage. Included below, pretty neat video of how post-tension cables are stretched (Video Link)

Post-tension video

After approx. 7-10 days, a speciality hydraulic jack is utilized to stretch the cables roughly 3-6 inches from their original size. This short distance in turn applies anywhere from 30,000-40,000 PSI on the cables. Once the correct pressure is exerted, a retaining clip is installed to keep the cable secured in place. The remaining portion of exposed cable is then cut off. Then the circular opening is capped off / closed with concrete patch to help prevent rusting and deterioration of the cable ends from weather elements.