Selecting a Concrete GPR Vendor Part 1
Determining Technology Required
There are many factors that affect the decision-making process when selecting a company to scan concrete for core drilling, saw cutting or other general demolition/destructive services. Anytime the construction process requires penetrating a concrete slab with unknown contents the potential for major damages and/or injuries is present. Structural items such as Beams, Columns, Post-Tension Cables, and rebar as well as non-structural items like rigid conduit, “smurf” tubing, PVC piping and even plumbing (yes, we see sanitary lines, water lines and a myriad of other items within the slab on regular occurrences) all come into effect when performing coring, cutting or drilling. Many times, contractors don’t account for the many variables that can be present when performing these destructive services and don’t properly vet a contractor that completes the scanning services prior.
This article will be the first in a three-part series discussing the process a contractor might go through in trying to determine how to choose the appropriate vendor for their project. The purpose of this article will be to discuss the first decision in the process: “Which technology will I need to use on this project?” There are many factors to outline, and this article will detail them in order while addressing some of the most common concerns with each.
Choosing Your Technology
The first step is to decide (and is sometimes specified by the owner) if your project will require X-ray or GPR (Ground Penetrating Radar). X-ray is just as it sounds, a technology that uses a radioactive isotope to project through the slab and catch on a film on the bottom side of the concrete to complete a picture of the findings within the concrete. While an image is formed of the contents of the concrete, there are many downfalls to this technology.
Concerns with Using Concrete X-Ray
- Radioactive: Due to the isotope burst the nearby areas can be exposed to health hazards and must be quarantined. To use x-ray, the area around the location must be cleared of people to assure no one is affected by the radioactive materials.
- Image: While the image is superior to GPR in that you can see the definitive items within the concrete, there is an inherent downfall is well. It is extremely difficult to place the image perfectly on the concrete the way that it was taken. Many times, core drilling locations still hit reinforcement because the image was backwards or misaligned and the items in the concrete are still damaged.
- Size of Scan: Generally, X-rays are only taken in 14”x14” locations and to take images of a larger area, multiple x-rays are required. To put together the image of a large area, multiple x-rays, closely placed must be taken and processed to get an image of everything present in the larger area.
- Efficiency: X-ray is slow-paced and very time consuming. Due to the quarantine area and post-processing system, only a few x-rays can be taken per day. When you compound that factor with quick moving schedules and the balancing act of working multiple subs in one area, this is a major factor when deciding what technology to use.
The Benefits of Using GPR to Scan Concrete
GPR has limitations of its own, but we will contrast those with that of X-ray technology. The following are the limitations/differences that GPR poses as compared to X-ray:
- Radioactive: GPR contains no radioactive isotope, nor does it contain any waves/bursts that are harmful to the human body. This means that the technology can be used at the same time as other subcontractors in the area or even in the ICU of a hospital with other patience present. GPR’s lack of health hazard is its main advantage over X-Ray.
- Image: GPR doesn’t provide a picture of what is in the slab. Data is collected and interpreted live in the field and marked directly on the concrete. While you don’t get a picture of the PT/rebar and other items present, there is no mistaking the location and layout of what is in the concrete.
- Size of Scan: Due to the fact GPR constantly pulses and collects data while in use, it allows the user to collect as much or as little data as possible. GPR can complete scans of 6”x6” or entire rooms, depending on the needs of the client without changing the size or number of antennas.
- Efficiency: GPR efficiency is much higher than x-ray in that you can complete as many locations per day as the skillset of the technician allows. The process of scanning (later in the article) does take time, but an extremely efficient and well-trained scan technician in average conditions can complete 40-60 locations per day. A location is generally described as a 24”x24” area.
The following table provides a quick reference guide for GPR vs X-ray technologies.
The following images are included to help understand the difference in output of GPR vs X-ray. As previously discussed, X-ray provides an image that can be used to place on the slab and understand the contents; Ground penetrating radar results are marked directly on the concrete to help the user understand the precise location of the contents within the slab.
The first image is of a GPRS Project Manager scanning and marking a concrete sheer wall containing rebar. The findings are marked on the surface in a fashion that is legible, understandable and accurate. The second image below is a concrete X-ray result from a PT box girder; the image contains a photo of the items within the concrete as previously discussed, but can be difficult to read without additional interpretation. Source for second photo: USDOT Federal Highway Administration Research and Technology.