Imaging Reinforced Concrete

Written by SRI Online on . Posted in Concrete, Ground Penetrating Radar, Utilities

Distinguishing between rebar, conduit and post-tension cables, the three most commonly embedded materials that customers wish to locate in reinforced concrete, requires training and experience.

Even with pre-construction schematics, utilizing Ground-Penetrating Radar (GPR) imaging to know the precise location of each item will help to maintain and protect structural integrity during construction and renovation.

Rebar

rebar foundation

Rebar, or reinforcing bar, is steel bar that is commonly used as a tensioning device in concrete, to increase strength and reduce the risk of cracks and other structural vulnerabilities.

While rebar does act as reinforcement, it can be safe to cut through when necessary; rebar is often cut to maintain a layout concept, especially when an alternative might mean costly redesigns. However, it is still important to know that anytime rebar is cut, the overall strength and tension of the concrete will be somewhat diminished.

Conduit

Smurftube_2

In modern construction, electrical wiring is most often run through plastic conduit tubing – referred to in the industry as “Smurf,” due to the commonly used blue tubing.

Cutting into conduit has obvious risks, including electrical shock if the wiring is live. Depending on the purpose of the wires running through the conduit, the consequences of an accidental cut can vary from a total electrical outage, to a lighting blackout or the loss of communications systems. To help minimize those risks, pilot-testing may be recommended to verify a conduit’s exact location, especially when nearby reinforcement may diminish visibility from the radar signal.

Post-tension Cables

pt cables before tension

Post-tension cables, also known as “PT cables,” are long cords of metal sheathed in greased plastic sleeves. They are used to evenly redistribute the weight and tension of a concrete slab, which allows for concrete decks to be up to 20 percent thinner and use fewer added reinforcement methods such as rebar and beams.

Cutting a PT cable or a set of cables can be dangerous and costly. Due to the extreme tension of the cables – typically about 27,400 lbs/in2 – the force from releasing the tension may break the concrete and send debris flying, putting anyone nearby at risk of injury.

Cables may be embedded in groups so that if one is broken it does not require repair, but in most cases, it is highly recommended that a snapped cable be replaced to maintain the structural integrity of the building. The process is expensive and time-consuming, two factors that can quickly escalate into construction delays and blown budgets.

The Financial Benefits of GPR Services

Using GPR to locate embedded materials prior to cutting into concrete enhances project safety and minimizes financial risk:

  • Replacing or repairing snapped post-tension cables can cost anywhere from $3,000 for a low-grade individual cable to $30,000 for a band of cables
  • The cost to replace or repair conduits can range from several hundred dollars to several thousand dollars
  • Costs and delays from preventable damage may result in loss of business and strained customer relationships

Choosing the right GPR company with properly trained technicians who can discern between types of embedded items can make all the difference in the success and outcome of a construction project. For more information, read our article on the Value of a Great GPR Expert.

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