Advantages and Limitations of Radiography and Ground Penetrating Radar of Concrete
Abstract: Baker Testing Services has the capability to detect embedded structures and utilities in concrete using Ground Penetrating Radar (GPR) and Gamma Radiography (RT). Each of these inspection methods has advantages and limitations.
Ground Penetrating Radar
Ground Penetrating Radar can detect objects embedded in concrete such as rebar, post-tension cables and metallic conduit with a high degree of accuracy, but with limited ability to differentiate between structure types of similar size. Radar signal reflections are directly imaged as hyperbola ‘targets’ on the GPR screen. An example of typical rebar targets is shown in the image on left.
Accurate Depth. Depth of targets can be accurately determined with this method when concrete thickness is known or when depth of known structures can be measured for calibrations of concrete density. Relatively accurate depth estimates can be provided by setting concrete density/age factors to radar antenna frequency and when the bottoms of concrete slabs provide strong radar signals.
Clear Areas. Clear areas for coring and structures to avoid can be definitively marked out directly on surfaces without the angled projection (parallax) issues of radiography.
Speed of Inspection. In most all cases, GPR can be performed in much less time than radiographic inspection. Although factors such as complexity of structure patterns (multiple or single layers of targets found and orientation of potential utilities) can influence inspection time on a given area, this method can be performed in much less time than radiographic inspection.
Single Surface Access. Since this method only requires access to one side of an inspection surface, slabs on grade that cannot be X-rayed can be inspected with GPR. In areas where sufficient space exists around core sites, scan data can be processed into 3D plan views with depth slice images to accurately mark out complex target patterns. In most cases, embedded structures can be marked out on slabs directly from on-screen data with greater accuracy and less time using GPR than with radiographic inspection.
No Radiation. Ground Penetrating Radar Inspection can be done on any shift. It does not involve radiation. Therefore, GPR does not require areas be vacated during inspection.
Ideal GPR Applications
GPR is an ideal test method to locate clear areas for coring where no metallic targets will be hit. Tracking wheels mounted on the antenna cart and a computer cursor feature provide target depth information and highly accurate spatial marking on relatively smooth surfaces. Very rough or uneven concrete surfaces can cause tracking wheel slippage and antenna drag that could limit accurate target mapping. Typical target marking / mapping is done directly on slab surfaces with crayons, chalk, tape or line paint. Because scan data requires GPR equipment and experience to interpret, target marks and maps are typically recorded with digital photographs or drawings for reports, future reference and meeting purposes.
Limitations of GPR
- Difficulty discerning the difference between target types of similar size
Since radar signal targets are imaged on-screen as hyperbola curves, interpretation of structure types can depend on radar signal patterns, shapes, spacing, depth and strength.
- Lack of penetration through finely spaced mesh
Tightly spaced wire mesh can reflect or attenuate radar penetration and limit the ability to accurately image deeper structures.
- Limited ability to detect targets located in tight corners or directly against walls and obstructions.
GPR tracking antennas must completely pass over targets to detect and image them. Therefore embedded structures that run parallel to walls or other obstructions cannot be detected within a 4-6” area. Structures that run perpendicular to walls can be detected in that zone because radar signals reflect targets that run perpendicular to scan direction.
- Reduced ability to locate targets in contact with metal floor decking and weaker signals from embedded PVC structures.
GPR scanning depends on detection of dielectric property differences between concrete and embedded objects or voids. Metal targets reflect strong dielectric signals. This is an advantage, unless there are targets with lower dielectric signal strength in troughs of metal decking.
- PVC and voids in concrete reflect lower strength signals and provide weaker images. So, they can be difficult to detect in close proximity to metal decking or when they are superimposed by metal structures above them. If slabs on grade vary in thickness, bottom target detection can be difficult because the slab depth is unknown and the bottom edge of concrete may not have enough signal strength to detect and measure.
- Targets below metal decking plates cannot be detected with GPR. Slab thickness limits range up to 16” under ideal conditions with limited layers of metallic reinforcement and 12” for 3D depth slice mapping.
- Targets below grade / slab interfaces, or affixed to ceilings opposite floor slabs cannot be detected by GPR.
- Structure detection in concrete beams and webs may be limited by depth and complexity of targets or by limited size of scan surfaces.
Radiography of concrete floor slabs and walls can also be performed to detect, evaluate, and readily distinguish between embedded targets such as rebar and conduit with a high degree of accuracy. To inspect floor slabs with this method, a gamma ray source is placed above each specific core site and a film holder must be affixed directly below each location (in direct contact with slab) for individual exposures. The largest area that can be imaged per exposure is 14” X 17”. For accuracy and efficiency, each core location/exposure area must be measured and marked out on both sides of slabs by customer prior to inspection. Exposures through 8-12” of concrete typically take between 10 to 25 minutes each - plus time required for set-up and processing of film or digital images.
Advantages. Advantages of radiography over GPR include (1) access to a hard copy radiograph for later reference, (2) the ability to differentiate between rebar, post tension cables, and PVC versus metallic conduit targets; PVC conduit can be distinguished by a hollow structure with metal wire that shows clearly on most radiographs of concrete, and (3) X-ray film or digital images can be processed on-site in one of our mobile darkroom trucks so that targets can be marked out from images that are provided on the same day.
Limitations. Radiography of concrete also has limitations.
- Depth determination can be estimated at best by the relative sharpness of images (an indicator of their proximity to the film or image plate.)
- Radiography requires more time than GPR to complete an inspection project that has large areas to or multiple coring sites to map out.
- Radiation safety concerns. State and Federal laws prohibit exposures greater than 2 milliroentgens per hour to unmonitored personnel. Depending upon available shielding (from concrete walls, etc.) it is often necessary to vacate several floors for distances of 50 to 150 feet around inspection areas for the duration of operations. For the area to be secured and vacant of personnel during radiographic operations, it may be necessary to perform this work on off shifts.
- Since radiography is a through-transmission test method, it is not possible to X-ray through slabs on grade or through thicknesses exceeding 20”.
- Precise centering of locations on top and bottom of slabs is important for accurate images. Due to an effect known as parallax, targets that are off center from the middle of the x-ray beam and those at greater distances from the film side of concrete slabs may be projected at angles away from their actual position or cut off the edge of images. This can cause spatial discrepancies that increase with distance from center or even missed targets at edges of core site images.
Baker Testing works with clients to identify the most accurate and efficient inspection method for each situation. Please call or email us for more information or to provide quotes on specific projects.