How Deep Are Post -Tension Cables?

by Tendon, on Sep 21, 2021 12:19:34 PM

Post tension cables bring unmatched strength to concrete slabs in all kinds of structures. They save money on both the initial costs of materials and the eventual costs of restoration and repairs. But post-tension cables come with some challenges as well.

When they are cut or damaged, they can not only compromise the integrity of the foundation, parking garage, office building, or other structure, but they can also cause serious injuries. That’s because the immense lateral force they are under can cause them to burst out of the slab when they are cut. 

Where the post-tension cables are located in a slab is must-know information before you drill into, cut, core, or otherwise modify a post-tension slab. But how deep are post-tension cables? How far can you safely enter a slab before you risk compromising a tendon? We answer those questions and more in the post below.

For help with all aspects of post-tensioning and concrete work, reach out to the concrete experts at Tendon Systems, LLC. We will put our years of experience to work for you to make sure your job is done safely and to the highest possible standards.

How to Find Tendons in a Concrete Slab

To understand how deep post-tension cables are in a concrete slab, you will need to be able to see them — all of them. A single slab can contain dozens of tendons in a grid pattern. How are you supposed to see the cables before you drill? Through ground penetrating radar (GPR).

GPR’s applications go well beyond concrete work, but this technology is well-suited to finding tendons and determining how deep the post-tension cables are. Here’s how it works: 

  • A GPR device emits radio waves that penetrate the surface of the slab.
  • When the radio waves hit objects embedded in the concrete, they reflect off of them.
  • The reflection of the waves disrupts the typical wave pattern.
  • A receiving antenna on the GPR device detects these disrupted wave patterns.
  • The unit uses the wave pattern to generate an image of the inside of the slab.

 A GPR professional then analyzes the image to map out where the post-tension cables are in the slab. GPR will also detect rebar, plumbing, electrical conduits, and similar elements.

How Deep Are the Cables in a Post-Tension Slab?

Because post-tension cables are meant to strengthen concrete slabs, they are usually embedded somewhat far into the slab to add strength throughout. However, that does not mean that cables are always vertically centered in a slab. How deep the cables in a post-tension slab are will depend on several factors:

  • The overall depth of the slab
  • When the slab was poured
  • The standards used by the original contractor

If your answer to the question, “How deep are post-tension cables?” is a guess or estimate, you have the wrong answer. There is no room for error, so it’s best to use the tried and true method of concrete scanning through GPR.

Ferroscan vs. GPR for Detecting Tendon Depth

GPR is not the only concrete scanning method. Many companies use other scanning methods like Ferroscan to detect and measure metal elements in concrete slabs. The important distinction here is that GPR is the concrete scanning method that can detect post-tension cable depth. Ferroscan is great for determining rebar diameter and similar measurements, but for post-tension cable depth, GPR is the proven scanning method.

How Far Apart Are Tendons in a Post-Tension Slab?

Understanding how deep the cables are in a post-tension slab is an important first step, but you also need to understand other aspects of the tendon placement inside any slab you are repairing, surveying or restoring. In particular, you will need to know how far apart the tendons are.

Like post-tension cable depth, cable spacing is typically determined by an engineer and is specific to the project. With that being said, tendons in floor slabs can be as far apart as five feet and as close together as a few inches. Also, keep in mind that tendons are sometimes placed in bundles of cables in one place.

In the end, the safest way to determine exactly how far apart tendons are in a post-tension slab is to scan the slab using GPR.

The Risks of Damaging Post Tension Cables

Even if you feel confident in your concrete restoration or construction abilities, it’s important to understand the risks you run if you cut into post-tension concrete without knowing the exact depth and spacing of the cables. Here are the risks of compromising a single cable:

  • Severe injuries. The post-tensioning process puts cables under immense pressure. Picture a rubber band snapping — it moves with lightning speed as it releases the tension that was in the band. The same thing happens when post-tension cables break, and the result can be a tendon bursting from the concrete with enough force to cause severe injuries or even death.
  • Expensive repairs. Even if an injury doesn’t happen, a broken or weakened tendon needs to be repaired. Replacing a single cable can cost more than $10,000.
  • Slab and structure failure. Post tension slabs are strong because of the cables inside of them. When one or more of those cables fails due to damage, they aren’t as strong as they once were. This can cause slab or structure failure that can cost an obscene amount of money to repair, and it can be dangerous.

Call in the Post Tension Concrete Experts

How deep are post-tension cables? It depends, but you definitely don’t want to take a chance by guessing. Instead, you want an exact picture of where the cables are. That’s exactly the kind of picture a GPR scan can provide.

At Tendon Systems, LLC, we are concrete scanning and construction professionals who put safety and our customers’ budgets first. If you’re working with post-tension concrete, you can trust us to deliver safe, efficient, and high-quality results. To work with us, give us a call at (678) 835-1100 or contact us online today.


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