Pyramid Geophysical Services completed a ground penetrating radar (GPR) concrete inspection survey in a municipal parking garage in North Carolina. The purpose of the survey was to locate rebar and post tension cables within the concrete floor slab of one of the parking decks. Geotechnical engineers were planning to core through the concrete slab to install parking barriers, and wanted to avoid encountering or damaging any reinforcement structures present within the concrete. For this survey, Pyramid utilized a hand-held, high frequency GPR antenna. GPR can 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.
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.
The slab of this parking deck was approximately 12 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. For this project we utilized a GSSI StructureScan Mini unit with a 1600 MHz antenna.
The survey was able to accurately locate all rebar and post-tension cables at multiple locations across the slab. Chalk was used to initially mark out the locations of the reinforcement, followed by more permanent marking using utility locating marking paint. The entire deck level was surveyed in one day, allowing the concrete coring to be performed the subsequent day.