FLATWORK/SLAB
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BY MICHAEL ARVANITIS
What to Know About Ground Penetrating Radar
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oncrete coring and drilling contractors are adopting new technologies that make their day-to-day job easier and safer. A popular and effective solution is the ground-penetrating radar (GPR) that is used to locate rebars, tendons, posttension cables, and other targets inside the concrete. Using GPR not only makes the job easier but also adds value to the portfolio of services offered. Giving your customer the option to choose between a plethora of methods is a huge advantage. HOW GPR WORKS Most popular uses of GPR in concrete are the non-destructive testing and mapping of objects inside the concrete, such as rebars, pipes, ducts, cables, etc.—as well as infrastructure assessment for bridges, roads, and buildings. GPR is a compact device that scans the subsurface in a non-destructive way. It can penetrate the surface from a few centimeters down, to tens or hundreds of meters, based on soil conditions and antenna characteristics. For concrete, we typically use antennas in the range of GHz for better horizontal resolution. GPR emits electromagnetic (EM) waves to the subsurface and calculates the time these waves need to travel through the various subsurface material and get back to the GPR receiver. This two-way travel time, along with a few other parameters such as the dielectric, provides the user an estimate of the target depth. GPR is an extremely useful tool in a variety of applications. Most popular uses of GPR in concrete are the non-destructive testing and mapping of objects inside the concrete, such as
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Th PR rebars, pipes, ducts, cables, etc.—as well as infrastructure assessment for bridges, roads, and buildings. GPR users can also map live electrical cables and see down to the bottom of the slab. Currently, two different GPR devices exist in the market: the pulsed systems and the stepped frequency GPR. Pulsed systems send tiny pulses of EM wave down to the subsurface during periodic intervals, while the stepped frequency devices are continuously sending parcels of wave at different frequencies at the same time. In the past, due to computing limitations, pulsed systems were the main choice in the market. Despite their popularity, they have several drawbacks, like the frequency dilemma. Pulsed systems are limited by the main frequency of the antenna and thus they have specific limitations in terms of depth and resolution. However, we are seeing a switch to the stepped frequency because due to now having the necessary computing power.
A 3D representation of rebar structure below the concrete surface. Screening Eagle Technologies
STEPPED FREQUENCY GPR Stepped frequency is gaining popularity because, with a single device you can sweep through the complete frequency spectrum—from low-to-high frequencies, achieving superb depth clarity and resolution of even small objects at the same time. Scanning data is collected in two ways, either in 2D, which means that you collect independent line scans over the concrete surface, or in 3D, using an area scan over a pre-configured grid. Line scanning is performed by moving the GPR antenna across the surface and this results in targets on your screen. Depending on the direction of your antenna in relation to these targets, you either get linear targets or hyperbolas that correspond to subsurface targets. Based on their amplitude, reflection type, and other characteristics, you can
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