Evaluation of Steel Rebar in Concrete Using Electromagnetic Method

The corrosion of steel reinforcing bar (rebar) is the leading cause of deterioration of concrete. In Japan, many railway bridges were built 40 years ago. It is necessary to develop easy-operation method to evaluate the corrosion of steel rebar. A project about the corrosion evaluation of steel rebar was started in 2015. In this project, we have two objectives: one is to evaluate the depth and the diameter of steel rebar in concrete; another is to evaluate the corrosion of steel rebar in concrete. We developed electromagnetic methods to do nondestructive evaluation of the steel reinforcing bar (rebar) in concrete. Using two probes and lower excitation frequencies of 3.8 and 4.2 kHz, the depth and the diameter of the steel reinforcing bar can be evaluated. Using higher excitation frequency of about 80 kHz, and the X, Y signals of the lock-in amplifier, where the X signal is the same phase signal with the AC excitation magnetic field and Y signal is the 90° phase different signal with the AC excitation magnetic field, we could evaluate the corrosion of steel rebar. A compact system with low power consumption of 0.5 W was developed, and we also did some field experiments using this system.