GPRS continues to see an increase in the use of ground penetrating radar to scan or image concrete slabs. Concrete imaging with ground penetrating radar is a successful process that results that ensure construction, cutting, drilling, etc. can continue without impact to the facility or project. We wanted to take a moment and discuss some common questions with regards to using GPR to scan concrete.
The answer, simply, is yes. This question often comes from someone who is familiar with concrete “X-Raying”. While X-Raying concrete is still a valid technique for determining the location of reinforcing steel it is limited to elevated concrete slabs only. This is because you need to have access to both sides of the concrete to perform an x-ray. One side for the radioactive isotope and the other for the film. Ground penetrating radar is a completely different technology.
GPR utilizes an electromagnetic pulse to determine the reflective values of objects in the concrete. It is a simple send/receive technology. The radar sends an electromagnetic pulse from the surface and the reflections are received, again, at the surface. Thus, it only needs access to one side of the concrete. This fact makes it ideal for scanning concrete slab-on-grade applications.
This is a more complicated answer than the question above. While the technology (ground penetrating radar) does not determine what type of object/anomaly is being located we (the technicians) are very good at determining what type of reinforcing steel or electrical conduit is present. Take the photo below for example…
This photo shows a typical post tension cable mark out. We are able to locate, using the radar, all of the reinforcing steel anomalies. We then mark the findings on the concrete and are quickly able to determine, based on the pattern, what each marking indicates. Most often when something is on an angle through a square layout it will be a conduit. Further, you will have a typical rebar pattern with 12-18” centers. Post tension cables tend to be spaced further apart depending on the design of the building and whether they are banded or uniform tendons.
As you can see, the real answer to the question of whether ground penetrating radar can tell the difference between rebar, post tension cables, electrical conduits, etc. is YES and NO. The radar can’t but an experienced technician can interpret the data the radar discovers to provide you with an accurate representation of what is in the concrete.
We have found that our typical accuracy is +/- ¼” to the center of the object (conduit, post tension cables, rebar) being located. GPR can also inform you of the depth of the object in the concrete. We have found that our typically accuracy when locating the depth of an object embedded in concrete is +/- 10%.
This photo, again, shows a typical elevated concrete slab layout after we finish scanning. You can see that all rebar, post tension cable, and walker duct were marked on this layout. Because the radar’s accuracy is so high we can confidently tell contractors where they are able to drill without the risk of hitting any of the objects shown in the picture. For safety and caution we instruct all contractors to stay 1 inch from any line they do not want to hit. This allows for bar/tendon thickness.
Again, this question often stems from someone who has experience with x-raying concrete. As mentioned, x-ray continues to be a viable and accurate testing method for mapping objects in concrete but it does come with an inherent health risk. Most x-ray companies recommend that you clear the area where the “shot” is taking place.
Because the typical power output of a radar system is less than the cell phone most of us carry in our pocket it is completely SAFE to work around. There is NO concern over health risks of our technicians, your employees/team members, or other tenants in the building. Further, the radar does not emit sound and operates almost silently so there is no noise disruption.
Ground penetrating radar is an extremely efficient and fast technology able to scan large areas in with ease. Our standard layout for a typical core drilling location is 2’x2’ in size. It will likely take us about 10 minutes to scan and mark this area.
This is a GREAT question and one that we are getting more frequently. While the cost of a GPR system may seem inconsequential on a large project there is more to concrete imaging than just having the equipment. We truly believe that half of the job involves knowing how to read the GPR data screen. The other half is being able to determine what the data is telling you.
This comes through experience. Only through experience can we interpret the data on the screen to tell you what anomaly indicates rebar, post tension cables, or electrical conduit. Only through experience and industry knowledge can we take our construction information and apply it to the data that we see after imaging the concrete and provide accurate answers to your questions.
There are many other questions that we are asked with reference to scanning concrete. Those questions involve other aspects of our scanning process including; can you locate voids below concrete? Can you scan walls or ceilings? Can you locate rebar in a CMU Block wall? For answers to these and other questions you can visit our FAQ page on the website or, better, call your local technician who would be happy to answer any and all questions as well as provide you with pricing and availability for service.