Environmental effects on condition assessments of concrete structures with ground penetrating radar

Reflection amplitude from rebar is the most commonly used parameter for condition assessment of concrete structures using Ground-Penetrating Radar (GPR). However, it was reported that such amplitudes might affect by several factors, including the chloride content, rust accumulated on the surface of steel bars due to corrosion, and the environmental conditions at the time of GPR data collection. The objectives of this study were to observe the change of GPR signals over time and to evaluate the effects of those factors on the reflection amplitude of steel bars. A series of six reinforced concrete slabs were fabricated with identical configurations but for different chloride contents. Two testing phases with the multiple GPR measurements were conducted both outside and inside an environmental chamber. In the first testing phase, the development of GPR signals was daily obtained from day one to day 90 in the laboratory conditions. In the second testing phase, the GPR data were collected inside the environmental chamber at an interval of 24 h. Relative Humidity (RH) was set at three levels of 55%, 70%, and 85%, respectively, while the temperature was varied between 5 degrees C and 40 degrees C. The test results from the first testing phase showed that the GPR signals developed at an exponential rate during the first three months. In addition, results from the second testing phase revealed that both the added chloride and the rust on the surface of steel reinforcement reduced the amplitude of rebar reflection. Regarding the effects of temperature, for sound (i.e., non-contaminated) concrete, the reflection amplitude decreased approximately 0.05 dB for every 1 degrees C increments. For the contaminated concrete, such ratio was found to increase proportionally to the chloride levels. Finally, the RH was observed to have insignificant effects on the amplitude of rebar reflection.