High-Resolution Ground-Penetrating Radar (GPR) Equipment Designed Specifically to Improve Concrete Structure Inspection and Evaluation
In recent years, the medical field has introduced improved imaging techniques to non-destructively probe the human body, replacing potentially hazardous x-ray imaging that was once a frequent and routine practice.
Similarly, breakthroughs in high-resolution imaging techniques using ground-penetrating radar (GPR) have begun to make a significant impact in imaging reinforced concrete structures. No other non-destructive technique technology can be used to provide rapid, accurate, high-resolution images into concrete when the backside of the concrete is inaccessible. Slabs poured on grade, backfilled tunnel walls, airport runways, roadways and parking lots—all these structures are easily imaged and surveyed with GPR when concerns and problems arise.
This real-time NDT technique quickly locates the position and depth of post-tension cables, rebar, and electrical or fiber optic conduits embedded in concrete, eliminating dangers associated with cutting, drilling or coring and the high costs required for their repair if cut or damaged.
In cases where the backside of concrete can be accessed, the considerable costs and hazards associated with using x-ray imaging to inspect concrete are eliminated. GPR imaging is the best technology in every situation. It saves time and money, and has no safety hazards associated with its use.
Some actual jobs using GPR:
(1) Concern: At Denver International Airport (DIA), concrete posts are needed as barricades to protect fuel pumps while planes refuel at the gates. Eight-inch holes through the reinforced concrete pavement are required. Electrical conduits had previously been cut because coring was done blind. Other techniques are not available for imaging into a thick concrete pavement for embedded targets. Only a rapid survey technique is permitted so airport operations are not impacted.
Solution: GPR easily locates all embedded reinforcement and conduits in the concrete pavement, allowing the concrete posts to be installed without incident. The job is completed without impacting airline schedules.
(2) Concern: In a second job at Denver International Airport (DIA), electrical and communication equipment at Delta Airlines gates require upgrading. A substantial number of holes need to be cored into existing concrete for routing new electrical lines and conduits. Critical communications links between gate areas already exist in the concrete and cannot be severed during cutting or coring operations. Access to both sides of existing structures is not always available; relocating airline employees and the general public so that x-raying could take place is not an option.
Solution: GPR is used to find the PVC pipe that contains the critical fiber optic cables. Coring, cutting and drilling proceeds without interrupting existing utilities or communications links. The operations are carried out quickly and safely without interruptions to operations or inconvenience to the traveling public.
(3) Concern: Install anchor bolts for rail system used to support moveable shelving units into a 9-inch thick concrete slab located in a commercial office building. Post-tension cables cannot be hit during the installation. The pad is 35’ x 50’ and holes 1 5/8” deep are needed for anchorage of the rails.
Solution: GPR survey identified shallow cables, redirected some drilling locations, and quickly assured the contractor that most of the floor was clear. A complete x-ray survey of the same floor was estimated to require up to 1000 shots, ten days to complete, and nighttime operations so that building occupants would not be exposed to radiation.
(4) Concern: Remodel a home constructed with post tension cables in the floor made with concrete floor poured directly on the ground. Post-tension cable routes and depths must be accurately determined so that concrete can be cut to install new floor drains and anchor bolts can be sunk into the concrete to support new load-bearing walls. Cutting or drilling into post-tension cables requires expensive repair (over $10,000 per cable replacement) and can result in structural damage or injury to personnel if the concrete collapses or explodes. X-ray technology cannot be used because access to the backside of the floor for placement of film is impossible.
Solution: GPR quickly and accurately maps the post tension cables. A long path was cut through concrete floor and a large number of anchor bolt holes were drilled without severing post-tension cables or embedded conduit. New floor drains and load-bearing walls are installed without incident.
(5) Concern: A contractor charges for specified 4” thick asphalt pavement in a commercial parking lot. You are convinced that less material was placed within the pavement than what was specified. Furthermore, you are concerned that the job quality (uniformity of pavement thickness) is highly variable, which would shorten the life of the product. Short of coring the pavement into Swiss cheese to verify your suspicions, what alternative do you have but to pay the contractor and wait for eventual repairs?
Solution: Radar quickly and easily provides real-time profiles of the thickness of the pavement, thoroughly covering ten thousand square feet of pavement in a few hours. Measurement accuracy of better than ¼” ensures that calculated pavement volume (cubic feet existing in the pavement) matches the reported quantities delivered to the jobsite. Uniformity of pavement thickness can be verified, or substandard work can be addressed. These results can be used to refuse or limit payment for substandard work.
(6) Concern: Hotel balconies made of reinforced concrete, particularly along coastal areas and in tropical climates, undergo salt water corrosion and deterioration—a problem which must be addressed to maintain their structural integrity. If allowed to corrode, the reinforcement will lose a significant amount of its load-carrying capacity, a significant safety concern. Some balconies are repaired based solely on visual inspection of obvious flaws, but most corrosion remains hidden.
Solution: A survey of the balcony, with survey lines spaced a foot apart, is performed in a few minutes. Signal variations along the balcony—at the critical reinforcement depth—are compared. A map is made detailing areas of concern and is used to guide repairs. This technique has been demonstrated to outperform traditional evaluation methods.
(7) Concern: Floods in the past few years, along the Red River in North Dakota and elsewhere, have undercut and washed out supporting fill and sediment beneath pavement structures, warehouses, and other facilities that must support heavy equipment or heavy traffic loads. Occasionally, small collapses signify that a problem exists; larger collapses cause considerable damage and raise safety concerns, but how can these areas be assessed? Access to the underside of an extensive floor or pavement for visual examination or x-ray imaging is impossible.
Solution: Radar is used to identify the locations of subsurface voids beneath concrete slabs, even slabs thicker than several feet. Rough estimates of the volume of the voids are often obtained from the GPR data, as well. The same technique is used to detect voids behind tunnel walls or sinkholes beneath pavements, allowing preventative measures to be taken to fill the voids with grout or aggregate slurries.
In summary, accurate target location within a concrete slab-on-grade, wall, or supported slab can be achieved more quickly, safely, and economically with GPR instead of other existing techniques. Because of these features, interruption of operations in commercial office buildings, at airports, or in hospitals can be eliminated or minimized. Furthermore, a continuous stream of data is sampled, information about the quality of concrete structures can be obtained to assure residents or owners that the integrity of the property is adequate.