Post Tensioned Cables


The benefits for using post tensioned cables in concrete our infinite, but the most dramatic is thinner clear spans that would not otherwise be achieved. For a detailed description about what post tensioning is you can follow this link. I will be describing in this article the installation of a post tensioned slab.

When building a concrete slab with post tension system a deck is required as a form for the cables and concrete.

Concrete Deck w/ Electrical Boxes Installed

This large deck shown has a board form finish inlayed on top of the plywood decking so that when the concrete is poured it will mirror the surface. In the picture you can also see that there is hundreds of electrical boxes laid in advance in preparation for laying the post tensioned cables in there planned locations as designated by the structural engineer.










Post Tension Cable Rolls
The cables are brought onto the site in a rolled forms. First the workers layout the locations of the anchors that each cable tendon will require to be connected and then they insert the Anchors at the edge of the forms in their proper locations. The anchors chosen are encapsulated so that they provide an additional form of waterproofing to prevent the cables from rusting. The cables themselves are greased inside a plastic tube that will allow them to be tensioned after the concrete is poured.







Post Tensioning Anchors along the edge of the form








Once the anchors have been placed around the edges of the formwork then the cables are unwound and are placed through the anchors in preparation for the proper arrangements. In this particular slab the cables are placed in a basket weave and must be placed according to a certain order so that they pass correctly over top one another. Also this slab is particularly more complex than most slabs because the depth varies and the cables are placed so that they droop in rise in the proper locations according to the layout by the structural engineer.






Post Tensioned Installation
The Post Tensioning Installers use different sized chairs that position the cables at there proper heights. The cables typically pass higher in the form across the locations of the columns and droop between the supports so the force on the cables pulls with tensioned upwards in the center of the spans. The cables are also individually located at the edge but are grouped as they leave the edges on there journey towards the opposite edge.











Post Tensioning Layout










The final layout of the post tensioning looks like a complex network that includes rebar mats at the top of the column locations.  Now the deck is ready for the electricians to locate all of the electrical conduit through the network of post tension cables before the concrete can be poured. Once the concrete is poured the slab will begin to cure and once it has reach a certain strength the cables can be tensioned. Each cable  is tensioned individually with approximately 30,000 lbs. of force.









Post Tensioning Wedges

The Post Tensioning Wedges are installed together around the cable and inside the cone openings at the edge of the tennis court. If you look closely at the wedges you can see that they have teeth that allow the steel part to grip the cable. The teeth are angled in a way that the cable can easily be pulled pass the wedges but when the cable is released the teeth cinch against the cable and the wedge shaped part is forced strongly against the embedded anchor. The excess cable now can be torched off and the post tensioned force is now fully engaged onto the slab. The force is so strong against the concrete slab that there is measurable shrinking in the slab as well as lift provided by the cables.

Post Tensioning Wedges Installed






The cable at the edge of the slab is marked with spray paint and will be used for measuring the post tensioning when its been stressed. Each cable is calculated on the shop drawing to have a specific amount of pulled length that will be measured by an inspector for approval.







Stressing the Post Tensioned Cables

The hydraulic stressing jack is inserted into the cones and sets the wedges as it grips down around the cable and pulls forcing the wedges forward into the anchor and the cable further out. The Jack is powered by a hydraulic pump and is engaged by the operator until it reaches proper stressing pressure. The jack then returns to its beginning position and the second operator moves to the next cable.








Inspector documenting proper stressing per each tendon.

After the cables are stressed and inspected for proper elongation length and confirmed at approximately 5" per 60' cable strand. The tendon is then cut off close to the back of the wedge so a plastic encapsulating cap packed with grease can be fitted over the leftover tendon and exposed wedge. The cone pocket at the edge is filled with grout with corrosion inhibiter. Lastly a bituminous layer of waterproofing is painted over the entire edge to ensure protection.


Post Tensioned recessed cones filled with concrete grout




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