Characterization of progressive microcracking in Portland cement mortar using nonlinear ultrasonics

This paper presents the successful application of a nonlinear ultrasonic technique, nonlinear wave modulation spectroscopy (NWMS) to quantitatively track the evolution of microcracks in Portland cement mortar samples. The damage type considered in this study is microcracking due to alkali–silica reaction (ASR), a deleterious reaction occurring in concrete structures around the world. Nonlinear ultrasonic measurements are conducted on six different mortar specimens that are monitored from their initial, intact state up to their fully damaged state. The objective of this research is to determine the sensitivity and suitability of NWMS to quantitatively track this damage state throughout an entire life-cycle and to nondestructively identify the initiation time and the extent of microcracking in these mortar specimens. The nonlinear ultrasonic measurements are made with standard laboratory equipment, and the inherent high attenuation of cement-based materials is overcome with a procedure that uses the sideband energy instead of measuring peak amplitudes. The results show that the NWMS method can track the progressive damage in mortar, demonstrating the feasibility of using this nonlinear ultrasonic technique to quantitatively assess the deterioration of cement-based materials.