Using the 3-D Velocities Contour Plot of Surface Waves to Evaluate the Fire-Damaged Concrete Slabs

The strength of concrete near the surface would be greatly reduced subjected to high temperature, usually accompanied by surface cracking and spalling. In this study, two concrete slabs heated to 600 degrees C and 800 degrees C on one surface were tested using a small hammer and a displacement receiver placed on the two ends of a test line. The dispersion curve was obtained by performing Short-Time Fourier Transform and amplitude reassignment technique on a received displacement waveform. Combining 10 dispersion curves obtained from multiple grid lines, a three-dimensional surface wave velocity contour map is constructed. Concrete cores were taken after NDT testing and compared with the contour map of the location where the core was taken. The test results show that both the concrete strength and the crack depth affect the surface wave velocity. This technique allows rapid assessment of the cross-sectional wave velocity distribution of a surface, even on rough, spalled surfaces.