Specifications: Post-Tension Concrete Cables
Post-tension cables are steel, flexible cables made from several strands of steel wire intertwined with each other in order to form a thicker steel “wire” which we call the Post-Tension cable. The name “Post-Tension” comes from the fact that this cable is stressed with tension forces, after (“post”) a concrete structure has been poured, and has cured for a few days (exact wait time to begin to tension the cables depends on the concrete structures’ specs).
The way Post-Tension cables are installed is as follows.
Lets use a foundation slab as an example if a use case for post-tension cables. First we grade and apply proper sub-grades to the surface where the slab will be built on. Then we form the area where the slab will be poured (“pouring” is the process of laying down the wet ready-mix concrete inside the concrete forms). Next, since we want this slab to be extra strong, we use a type of concrete reinforcement called post-tension concrete, instead of the traditional rebar concrete reinforcement.
Post-Tension cables are installed is as follows:
Lets use a foundation slab as a use case for post-tension cables. First we grade and apply proper sub-grades to the surface where the slab will be built on. Then we form the area where the slab will be poured (“pouring” is the process of laying down the wet ready-mix concrete inside the concrete forms). Next, since we want this slab to be extra strong, we use a type of concrete reinforcement called post-tension concrete, instead of the traditional rebar concrete reinforcement.
Post-tension cables are therefore the next part of the foundation concrete slab that are installed. Post-tension cables are installed by trained and specialized individuals who are certified to install post-tension cables, usually working for a decently “large” company. The post-tension cables are put down on the inside of the concrete formed-area. Each cable is put down from one end of the slab across to the opposite side. The tips of the cables are attached to the forms on each side with special lock-mechanisms. Cables are usually put down on two different directions of the slab, for example some cables running from North-to-South, and other from West-to-East.
That’s a short explanation of how Post-Tension cables are installed and what they are. Below are some images and specs about the post-tension cables that we use for our sport courts:
1/2″ Inch Nominal Diameter
7-Wire Pre-Stressing Steel Strand
One straight center wire is surrounded by six helically wound outer wires.
Rated capacity of 270 KSI
For our applications, these cables are tensioned to about 80% of their tensile strength (~216 KSI)
At a ~1/2″ inch diameter 7-wire steel strand, the 7 twisted wires together have a cross-sectional area of about ~0.153″ inches squared. This cross-sectional area is calculated from the actual cross-sectional area of the 7 wires, not an actual solid 1/2″ inch cross-section which would result in a larger cross-sectional area (0.196″ squared inches). The spaces between the wires and around the circumference of the strand as a whole result in in a smaller cross-sectional area than what a true solid 1/2″ inch diameter cable would have.
Each strand is tensioned with a hydraulic jack with 11 KIPS (11,000 pounds) of force. This results in a strand tensile stress of 71.895 KSI (71,895 PSI). The tensile stress calculation comes from the 11,000 pounds of force from the hydraulic jack, divided by the 0.153″ in² cross-sectional area of the strand, which results in 71.895 KSI (71,895 PSI) of stress that the strand experiences.
A pre/partial stress is commonly done the next day after the concrete has been poured to prevent premature cracking.
Each strand is tensioned with a hydraulic jack with 33 KIPS (33,000 pounds) of force. This results in a strand tensile stress of 216 KSI (216,000 PSI). The tensile stress calculation comes from the 33,000 pounds of force from the hydraulic jack, divided by the 0.153″ in² cross-sectional area of the strand, which results in 215.686 KSI (215,686 PSI) of stress that the strand experiences.
After the cables have been fully stressed, they are cut and the anchor edges are patched.
The final stress is commonly done 5-7 days after the court has been poured.
For all of our projects, we add what is commonly known as “encapsulation” to our post-tension cable system. This is where the anchors/ends of the post tension system, including both “live” and “dead” ends are encapsulated together with the rest of the plastic sheathing that protects the post-tension strand. This creates a fully enclosed environment for the cable to that it is fully protected from the environment.