Electromagnetic modeling of embedded metallic structures for nondestructive evaluation using time-domain reflectometry

The corrosion of metallic reinforcement is a major threat to the aging infrastructure. The current corrosion detection methods for embedded or encased steel reinforcement suffer from several significant drawbacks. A nondestructive evaluation technique has been developed that is capable of determining the location and severity of corrosion of embedded or encased steel rebar and strands. This technique utilizes time domain reflectometry (TDR), which has been traditionally used to detect electrical discontinuities in a transmission line. By applying a sensor wire alongside of steel reinforcement, an asymmetric two-conductor transmission line is created. Wave propagation in the line has been studied using distributed parameter equivalent circuit. Physical defects of the reinforcement will change the electromagnetic properties of the line and can be detected by TDR. It is also possible to use an existent conductor or a wire applied outside the grout for corrosion detection in existing structures. Both analytical models and small-scale laboratory tests have shown that TDR can be effectively utilized to detect, locate, and identify the extent of damage in steel reinforcement in this manner. Currently, the TDR method is being used as a permanent corrosion monitoring method for Bridge 8F, a prestressed high-performance concrete bridge in Fredrica, Delaware. Differential TDR measurements are used to monitor serious damage due to corrosion of the steel. The nature and repeatability of TDR measurements have demonstrated that the method is viable for actual field use. This nondestructive evaluation technique can also be applied to other steel reinforced structures.