In this paper, active thermography NDT method with active heating is developed for concrete structures. Active heating is effectively employed when temperature fluctuation on structure is small. As an estimation method of defect depth, the Fourier analysis of temperature descent curve after heating is employed. As the results, by using the phase delay distribution obtained from the Fourier analysis, the influence of the irregularity heating was reduced in comparison with the original temperature distribution. The phase delay image was also useful for determination of defect depth. Values of phase difference between delamination area and sound area show peaks at certain values of data processing period. These peaks can be directly related to defect depth. It is shown that three-dimensional shape of delamination defect can be accurately evaluated by the proposed technique.
A new quantitative nondestructive testing technique for delamination defects in concrete structures was developed based on the phase delay measurement using a lock-in infrared thermography under the application of periodical heating. The lock-in thermography technique was developed based on the thermal insulation method thermographic NDT. Experimental studies were made on the applicability to the detection of artificial delamination defects in concrete blocks. Concrete blocks were periodically heated by quartz lamps combined with the light dimmer controller. The controller was operated by the same reference signal for the lock-in thermography. It was found that the delamination defects were detected by the localized contrast change in the phase delay images. It was also found that the location and size of the delamination defects can be estimated by the area of contrast change in the phase delay images which was clearly observed compared with conventional thermography techniques. The relationship between the values of phase delay and heating period was examined for several defect depths and several heating periods. It was found that the phase delay curve for certain defect depth shows the peak of the contrast in phase delay image at certain heating period and the depth of the delamination defects can be estimated from this relationship. Finally, the proposed lock-in thermographic NDT technique was applied for the quantitative measurement of the actual delamination defects found under railway bridge. It was fund that the depths of the delamination defects can be estimated using the master curve of the relationship between the values of phase delay and heating period made by the experiments for artificial delamination defects.