What is Post -Tension Cabling in Concrete Slabs?
When it comes to ensuring the durability and longevity of concrete slabs, post-tension cabling plays a crucial role. Here’s a breakdown of how this process works and why it’s essential in certain applications.
The Process
In traditional concrete pouring, you typically have stagnant pieces of steel rebar (reinforcing bar) that provides structural stability in the slab. This reinforcement is essential in all concrete as it hold the concrete intact, preventing breaking and separation. Click to Learn More About Concrete Reinforcement. While rebar is stagnant, in post-tension cabling we use robust half-inch cables that provid
e superior strength and flexibility. Post tension cables increase the compressive strength of the concrete by 75% and in large scale application become less expensive than traditional rebar reinforcement.
Before post-tension cables can be used, it’s important to determine which sides of the site will be the Dead Man’s Side and which will be the Live End Side.
- The Dead Man side is where the cables terminate into an anchor plate embedded in the concrete. These plates are completely encased in concrete, providing a solid anchor point for the cables.
- The Live End Side is where the cables exit the concrete slab and where the stressing machine does its work. It’s essential to have 2 to 3 feet of clearance on this side for the stressing machine to access and pull the cables. Click to Learn More About Post Tension Slab Procedures
Pouring and Tensioning
- Once the concrete is poured and sets around the anchor plates, the real magic happens. The cables are pulled on the live ends with 30,000 psi of force effectively squeezing the slab together. This process ensures that the slab is held tightly, reducing the risk of substantial separation.
Does post tension produce a crack free slab?
Unfortunately, no. It’s a well-known fact in the construction industry that all concrete cracks. All concrete slabs, no matter how well it’s poured, will develop cracks due to shrinkage as it cures and dries. As quoted by a concrete contractor, “there’s two facts about concrete… no one steals it and it always cracks.”
The primary benefit of post-tension cabling is its ability to minimize these shrinkage cracks. By pulling the cables in both directions, the slab is kept under compression, which helps to mitigate the formation of cracks. This process ensures that the cracks that do form are smaller and more controlled, maintaining the slab’s structural integrity.
What are the downsides of post- tension cabling?
Precision- Due to the intense pressure that post tension cables put on the concrete slab it requires a much more precise concrete installation. The cables must be exactly in the center of the slab and the perimeter beam must be designed correctly to withstand the force. If not, the slab may fail and break apart.
No DIY- Post-tension should never be completed by uncertified installers/ companies. Do not try this at home and verify the contractor you’ve hired to do your work is certified by the Post Tension institute – https://www.post-tensioning.org/. Improper installations can at best produce failure of the slab and at worst result in death for the installer.
Cutting, coring, drilling – Because the cables maintain high elasticity and pressure, penetration to them can cause a cable to snap. In bad cases of this the cable can lift up through the concrete causing severe injury to the installer and damage to the slab. If you need to do any of these activities, reach out to a local cutting/ coring company for them to scan and mark the cables.
Conclusion
Post-tension cabling is a sophisticated method that significantly enhances the performance and durability of concrete slabs. By understanding and implementing this technique, we can ensure that our concrete structures are more resilient, durable, and longer-lasting. Whether you’re working on a large commercial project or a residential build, post-tension cabling is a game-changer in the construction industry.
