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Post-Tensioned

Concrete
Text Box: As our cities have grown and extended outside of traditional borders, builders are faced with more expansive soil conditions.  Engineers must design foundation systems that are able to withstand larger soil induced shrink/swell movements in order to protect the supported structure from the consequential damage that can be caused by these soil movements while at the same time maintaining an economic solution.  Basically, these expansive soil conditions have increased the complexity of designs and have driven the need for economical post-tension foundation solutions. Text Box: WHAT IS POST TENSIONING? Post-tensioning is the method of achieving pre-stressing after the concrete has hardened and takes advantage of concrete's inherent compressive strength.  Concrete is exceptionally strong in compression, but generally weak when subjected to tension forces or forces that pull it apart.  These tension forces can be created by concrete shrinkage caused during curing or by flexural bending when the foundation is subjected to design loads (dead and live loads from the structure and/or expansive soil induced loads).  This tension can result in cracking which can lead to large deflections that can cause distress in the building's structure.  The application of an external force into the concrete, recompressing it before it is subjected to the design loads, makes the foundation less likely to crack.  Unlike traditional reinforcing steel or wire mesh, pre-stressing is an active form of reinforcement that applies an external load before the design loads are applied. HOW DOES IT WORK? As in any ground-supported application, the design of a slab-on-ground foundation is dependant upon the soil conditions at the project site.  A geotechnical engineer will provide a report containing information about the soil characteristics of the site as such as the swell/shrink potential of any expansive clays, bearing capacity, moisture content, depth and compaction of any existing fill material, and other characteristics of the region that will influence how the soil will react.  These parameters are then used to determine what type of foundation can be used.  Post-tension reinforced foundation systems fit most all soil conditions from stable sites to highly expansive sites and have several major advantages when used in slab on ground foundation applications.                                                    The amount of post-tensioning is dependent upon the thickness of the concrete used for the foundation.  Typically the tendons will be spaced from 2 to 4 foot centers, depending on the slab thickness, and tendons will be placed in most down turned ribs.  After stressing, the post-tensioning will apply a particular amount of psi compression onto the concrete.  Post-tensioning tendons are placed so they are straight between end anchor points so that the force that is applied "compresses" the concrete together.     Information from: Post-tensioning Institute      www.post-tensioning.org Suncoast Post-Tension        www.suncoast-pt.com