Ron Mundro has come across his share of challenges in the more than 20 years he has spent with Roughneck Concrete Drilling & Sawing Company, Morton Grove, Ill. The company's crews have for nearly a half century carefully cut and threaded core holes through concrete laced with rebar, pipe, and conduit. Until recently, skill and experience were the cutters' best hopes for not hitting hidden objects, and for dealing with them when they did.
But today, the company uses ground penetrating radar (GPR) to look inside the concrete to find and locate embedded objects, including rebar, tensioning tendons, conduits, pipes, and more. Knowing where and what these embedments are means far fewer are unexpectedly cut or damaged. It also means increased safety for Mundro and other cutters, and fewer headaches for owners and architects.
Concrete tests fall into two broad categories. Some, such as breaking concrete cylinders, are designed to measure strength. But others, including a wide variety of nondestructive tests and some that are only slightly destructive, are principally used to evaluate what's going on inside the concrete. (See sidebar, “ACI on NDT.”) These tests measure such things as relative humidity and help determine the location of rebar and other objects. And among the techniques for locating things inside concrete, GPR is emerging as one of the most popular.
GPR's growing popularity comes in part from technological advances in recent years that have extended its capabilities and accuracy, as well as broadened its applicability. But perhaps more significant are the relative ease of use and economy that GPR offers.
“We purchased our first GPR unit two years ago,” says Bob Kinney, Roughneck's dispatcher. “We knew the demand was growing for scanning concrete because more of our customers were asking for it.” Kinney says that adding a scanning service has been a good fit, but Roughneck remains first and foremost a concrete cutting company.
Ron Mundro has been Roughneck's scanning technician since they purchased the equipment. On a recent job in a Chicago high-rise condominium, he was scanning a concrete slab in preparation for coring holes for a plumbing installation. His first pass, adjacent to a partition wall, showed very little evidence of rebar. Turning the scanner 90 degrees and making another pass gave him a good initial indication of where the top mat was located.
On that pass, Mundro also noticed there was no lower mat on the screen, prompting him to adjust the system settings for a 12-inch slab rather than the 8-inch depth he had originally expected. Mundro says that's one example of how important it is to know how to interpret the GPR data. “It's also important to have a construction background to understand how things are put together.”
Where's the rebar?
Knowing what objects are embedded in concrete slabs, and where they are within those slabs, is becoming increasingly more important. The actual locations of rebar, conduits, and tensioning cables, based on observation rather than just design and as-built drawings, must be known with some certainty to prevent accidentally cutting into them when coring penetrations for utilities or other purposes, or making other cuts. Three test methods in particular, each with it own strengths and limitations, can provide this information.
On one end of the scale lie magnetic and electrical detection systems. ACI 228.2R explains that these covermeters use either eddy currents or magnetic reluctance to locate rebar and other conducting metals within concrete slabs. These devices provide an immediate image and are best suited to detecting metallic objects within the top few inches of the slab surface. Distinguishing between adjacent bars requires that they be far enough apart so there is no electromagnetic interference between them. For similar reasons, this type of system cannot detect a lower mat of rebar. Furthermore, covermeters cannot detect non-metallic objects, such as plastic pipes.
On the high end is radiography, which uses X-rays to provide a very clear and accurate film image of what is inside a concrete slab. This technology can clearly show the size and location of reinforcing steel and the condition of tensioning cables with enough clarity to identify broken tendons.