Monitoring of corrosion environment for reinforced steel inside the concrete structure was investigated using 3D impedance mapping method. In this method many iron wires in grid alignment were embedded in a test cement block and impedance between two iron wires sequentially chosen from the grid was measured to obtain the resistance map inside the cement block. From the resultant 3D impedance map, corrosion environment for steel inside the concrete such as penetration of water or salt can be speculated.

In the experiment, test cement block of 200 mm x 200 mm x 60 mm in size was made in which iron wires of 99.5% impurity and 0.2mmφ in diameter were embedded in a grid alignment of 8 line with 20 mm spacing in lateral direction and 4 layer with 10 mm spacing in depth, a total number of iron wires was 8 x 4 = 32. Top face of cement block was open to air and the other faces were covered with PVC plates as a casting mold. Impedance was measured for a combination of crossing two wires in two neighbor layers, and a number of combination was 8 x 8 x 3 = 192. To choose the appropriate two wires from 32 embedded wires to connect impedance analyzer, computer controlled analog multiplexer array was used. Impedance analyzer was built in laboratory using one-chip impedance converter (Analog Devices, model AD5933) which integrates components necessary for impedance analysis as direct digital synthesizer (DDS) as AC source, digital signal processor (DSP) engine for FFT (DFT, discrete fourier transformation), operational amplifier as AC output driver and input current / voltage converter, and 12bit analog digital converter. In this study, impedance was measured at a fixed frequency of 270Hz with modulation voltage of 20mV. About 1400 s was required to complete a single impedance mapping.

Example of impedance map inside the test cement block is shown in Fig.1. The value of resistance was in the range from ca. 3kΩ at the central area of deep (30-40 mm) cement block to a several 100kΩ close to the surface (10-20 mm). Relatively low resistance deep inside the cement block indicates that the inside of the cement block was not dried out. On the other hand, not only the surface but also the side faces of cement block were dried because a gap was formed between the PVC mold and side faces of the cement block due to volume shrinking when a cement block had set. Transition of impedance map during drying and penetration of water from the top face were also investigated.