Longitudinal guided waves for monitoring corrosion in reinforced mortar

Guided longitudinal modes in both low ( < 200 kHz) and high (2-8 MHz) frequency ranges were invoked to monitor damage in reinforced mortar specimens undergoing accelerated uniform corrosion. The fundamental longitudinal mode, i.e. L(0, 1), and the L(0, 9) mode were invoked for low- and high-frequency testing, respectively. Because of the significant amount of axial displacement at the steel/mortar interface, the L(0, 1) mode was so appreciably attenuated for the particular specimen size used that it is was not detected until after corrosion had initiated and corrosion product accumulation caused mortar cracking. Once detected, the L(0, 1) mode was sensitive to the combined effects of bond deterioration and mortar stiffness reduction. The L(0, 9) mode has negligible radial and axial displacement at the steel/mortar interface. As a result, the L(0, 9) mode is relatively insensitive to the surrounding interface conditions at high frequencies. This allows for changes in the steel cross-sectional area and bar topography to be isolated and monitored from the onset of corrosion up to severe pitting.