Monolithic Post Tension Concrete Slab
The owner builder may be required to have a monolithic post tension concrete foundation. It is becoming more common in areas subject to siesmic activity or with expansive soils.
The standard slab on grade is a foundation only in areas where there is a footing. With a monolithic post tensioned slab, the entire slab assembly acts like a foundation. Footings are still in place under load bearing walls, however, the post tensioning acts to lift the rest of the slab and increase the overall slab PSI strength.
Post Tension Vs. Standard Concrete
Concrete on its own has a high compression strength but a very low tensile strength. For decades, high rise builders have been adding tensile strength to their lightweight concrete by using post tension. The post tension cables are tightened. While unsupported concrete would naturally sag, high tension cables lift the concrete assembly. This same concept is incorporated in the monolithic post tension slab.
Monolithic slabs are also much thicker than the standard 4-6" slab. 9,10 even 12 inches is not uncommon. The combination of a thicker slab and post tensioning allows slab foundations in areas with expansive soils and the possibility of earthquakes.
There are several things the owner builder should look for prior to a preslab inspection.
-All cables should be on chairs and off the subgrade.
-All footings and areas to be poured should be clear of debris.
-Pipes running through the slab should be wrapped with foam to prevent direct contact with concrete.
-The concrete contractor can verify the proper placement of all hurricane ties, straps and anchor bolts.
Monolithic Post Tension Preslab Video
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It is important to remember that we learn more about the process so we can manage the process, not perform the work. You will get more bang for your buck when you focus on management. Sometimes that means hiring a consultant or construction manager to assist with your project.