A nondestructive testing system which detects surface defects in steel bars or cables embedded within reinforced or pre-stressed concrete is described. The component under test is energized by an alternating current of sufficient frequency to ensure that current flows takes place on the surface of the steel. Detection coils, in conjunction with a suitable amplification system, are positioned at various alignments with respect to the steel bar or cable, and thereby detect, through mutual induction, both the dominant flow path and the deviated current flow in the vicinity of a defect. The output signals from the multi-coil array are converted and fed to a computer for storage and processing. This system has clear relevance to the civil engineering nondestructive testing industry.