Ground penetrating radar (GPR) has a multitude of uses, many of which we have discussed in past newsletters. Applications include the detection of underground storage tanks (USTs) and drums, geologic mapping, void detection, archeological investigations, and subsurface utility locating. In addition to these uses, GPR can also be used to examine concrete structures, both horizontal slabs and vertical walls, for the presence of reinforcement such as rebar and post tension cables and even the integrity and moisture content of the concrete itself. This month we will examine these higher frequency applications of GPR.
The frequency of the GPR antenna that is used during a survey is the primary factor in the depth of penetration and imaging that will result from the scans. Lower frequency antennae (i.e. 200-400 MHz) will penetrate deeper, and are used for imaging items such as buried utilities, USTs, geologic strata, and karst. While the depth of penetration may be greater, the data resolution is lower, making very small, shallow targets difficulty to identify. In contrast, higher frequency antenna provide increased data resolution that allows smaller diameter objects such as rebar and even mesh reinforcement to be resolved, but depth of penetration is lowered. The proper antenna must be chosen for a GPR survey based on the target that needs to be identified.
For concrete inspections, floor slabs and concrete walls are typically anywhere from 4 to 24 inches thick. This provides a situation where very high resolution GPR antennae work well. Antenna frequencies can typically range from 1600 MHz to 2600 MHz depending on the thickness of the slab to be surveyed. Geophysical Survey Systems, Inc. (GSSI) produces a variety of antennae and systems for this type of work. The nice thing about concrete inspections and GPR is that the higher the frequency of the antenna, the smaller it is in size. This allows for the construction of compact, hand-held GPR systems that work well in concrete environments that often have limited space around corners and near walls.
Specific applications of high resolution GPR include:
- Concrete inspections
- Locate rebar, conduit, and post tension cables
- Determine concrete thickness
- Void detection
- Relative moisture content investigation
- Road inspections
- Examine asphalt and sub-base thickness/depth
- Void detection
- Utility/conduit locating
- Bridge Inspections
- Bridge deck condition assessment
- Slab thickness, void detection
- Locate rebar, conduit, cables
Most of the high frequency GPR systems available today allow the user to view data in real time, making the identification of structures such as rebar and conduit a rapid and accurate process. Some systems also include the option to map out an area in 3-D, generating a more comprehensive understanding of the orientation and depth of structures within concrete for complex situations.