GPRS’ precision concrete scanning & imaging services saved a contractor from coring through banded post tension cable – preventing tens of thousands of dollars in damage and keeping everyone on site safe in the process.
GPRS Senior Project Manager Brad Oberklaus was called out to a multi-story bank building in downtown Spokane, Washington, where the client was tasked with coring through a post-tensioned concrete slab to run plumbing and electrical.
“Rebar was less of a concern, but they wanted to avoid it, if possible,” Oberklaus said.
What is Post-Tensioned Concrete?
Post-tensioned systems consist of a series of high-strength steel cables embedded within concrete slabs that, once stressed, support the loads imposed on the slabs. The cables are commonly called “tendons.”
Post-tensioned cable systems reinforce concrete, allowing structures to require fewer beams, longer clear spans, more slender elements, and thinner slabs. This helps to reduce the amount of concrete required for construction, also reducing the weight and height of the building. Lower building weight and height can contribute to savings in terms of facade and mechanical system costs. There will also be a lesser load on the foundation.
Post tension cable location, spacing, and depth are determined by an engineer, and are specific to each project.
Post-Tensioned Concrete Cutting Safety
Drilling, cutting, or coring into a post-tension slab can be dangerous. It is important to avoid hitting the cables when cutting or drilling for construction or renovation. Each tendon pulls 24,000 to 33,000 lbs. of pressure. Damaging a cable when cutting, coring, or drilling into a slab can cause a tendon to burst with enough force to cause severe injury or death, and impact the structural integrity of the slab.
This is why it is important to locate post-tension cable systems when finishing or repairing buildings or structures. An accurate layout of post-tension cables protects tendons from damage, prevents injury and mistakes, and protects the integrity of the building.
Our client had initially hired a different concrete scanning company to clear coring locations at the bank building. But when Oberklaus demonstrated to them the level of accuracy inherent in GPRS’ concrete scanning process, they requested that he re-scan the areas which had been cleared by the other company.
“The first two locations I scanned were directly on top of banded cables, which were missed by the other scanner,” Oberklaus explained. “While I was on site, the customer called the previous scanner to get a walkthrough of the findings from their scans and he was told he was clear to drill in the locations I detected cables. This was enough to convince the customer to expand my scope and re-scan everything.”
GPRS Project Managers have achieved and maintain an industry-leading 99.8%+ rate of accuracy when locating utilities and scanning concrete slabs. This is thanks to our innovative, industry-leading training program, centered around the Subsurface Investigation Methodology (SIM).
Subsurface Investigation Methodology Explained
SIM is the first comprehensive specification for professional utility locators and concrete scanning technicians. It aims to raise the quality of subsurface investigations by combining the requirements of experienced-based training, tested technologies, and proven application methods to create an industry standard for these vital practices.
All GPRS Project Managers are required to achieve SIM certification, a process that involves completing a minimum of 320 hours of field training and 80 hours of classroom training. SIM also requires that all utility locates and concrete scans involve the use of multiple technologies, ensuring that the results are repeatable and accurate.
Oberklaus deployed a ground penetrating radar (GPR) concrete scanning antenna and electromagnetic (EM) locator to accurately locate all PT cable and conduits embedded within the Spokane bank’s slabs.
GPR scanners emit radio waves into the ground or a surface such as concrete, then detect the interactions between those waves and any buried objects. These interactions are displayed in a GPR readout as a series of hyperbolas, which GPRS Project Managers can interpret to determine what type of objects are located and the approximate depth of those obstructions.
EM locators detect the electromagnetic signals radiating from metallic pipes and cables. These signals can be created by the locator’s transmitter applying current to the pipe, from current flow in a live electrical cable, or from a conductive pipe acting as an antenna and re-radiating signals from stray electrical fields and communications transmissions.
GPR scanning and EM locating act as perfect compliments to each other when locating buried utilities and performing precision concrete scanning, achieving the redundancy and repeatable results required by SIM. Using these technologies, Oberklaus was able to locate and map the banded PT cable the other locating company missed.
“I was able to mitigate significant damage and threats to safety,” he said.
The Green Box Guarantee
GPRS is so confident in the accuracy of the concrete scanning services provided by our SIM-certified Project Managers that we introduced the Green Box Guarantee.
When we place a Green Box within a layout prior to you anchoring or coring that concrete, we guarantee that the area within the box will be free of obstructions.
If we’re wrong, we agree to pay the material cost of the damage.
Concrete Sawing & Drilling Safety Week Returns January 27-31
GPRS’ commitment to you and your team’s safety is what sparked us to create and sponsor Concrete Sawing & Drilling Safety Week, which will return January 27-31, 2025.
During this week, our safety experts will travel across the country to deliver free safety programs designed to give you and your team the tools necessary tools to stay safe when working with and around concrete.
Click here to schedule your free CSDSW safety talk today!
Whether we’re aiding you with subsurface damage prevention, providing existing conditions documentation, or assisting you with your construction project and/or facilities management, GPRS Intelligently Visualizes The Built World® to keep you on time, on budget, and safe.
What can we help you visualize?
Frequently Asked Questions
Can GPR determine the difference between rebar and electrical conduit?
Ground penetrating radar (GPR) can accurately differentiate between rebar and electrical conduit in most cases. We have an extremely high success rate in identifying electrical lines in supported slabs or slabs-on-grade before saw cutting or core drilling.
Additionally, GPRS can use electromagnetic (EM) locators to determine the location of conduits in the concrete. If we can transmit a signal onto the metal conduit, we can locate it with pinpoint accuracy. We can also find the conduit passively if a live electrical current runs through it.
The combined use of GPR and EM locating allows us to provide one of the most comprehensive and accurate conduits locating services available.
How is GPR used to identify tendons vs. rebar in a post-tensioned slab?
In post-tensioned structures, we typically find one mat of support rebar near the base of the slab. This mat is generally consistently spaced and remains at a constant elevation. Post-tension cables are generally found above this support mat and “draped” throughout the rest of the structure. The elevation of the cable is usually high near the beams and column lines and drapes lower through the span between beams and column lines. Knowledge of these structural differences allows us to accurately differentiate between components. Our Project Managers will leave you feeling confident in our findings and in your ability to drill or cut without issue.
