Active Thermography Inspection of Surface-whitened Mortars : Measurement of Surface Spectral Absorptivity for Investigation of Efficient Heating Light Wavelengths

The surface spectral absorptivity of surface-whitened mortars due to the occurrence of efflorescence (i.e., mortars whose surface was covered with calcium carbonate) was measured, and the relationship between the spectral absorptivity and inspection capability of active thermography inspection was investigated. The spectral absorptivity of mortars increased significantly at a wavelength of approximately 3000 nm regardless of the presence/absence of the discoloration. Experiments for mortar specimens using optical lights with wavelengths in the visible, short wavelength, and medium/long wavelength ranges showed that the heating efficiency and defect detection capability of active thermography inspection were correlated with the surface spectral absorptivity, and were higher when long wavelength light was used as a heater. Defects in the surface-whitened mortar specimen were detected more efficiently when the specimen was heated using a CO2 laser, whose wavelength is in the long wavelength range, than when using an optical light having a wavelength in the visible/short wavelength range.

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