Ultrasound frequency analysis for identification of aggregates and cement paste in concrete

The aim of the present study was to identify specific frequencies related to aggregates and cement paste during concrete hydration, by performing a Fourier analysis of the ultrasonic response of concrete specimens to different excitation frequencies. This identification will reduce the high influence of aggregates in the ultrasound signal analysis, enabling a better assessment of changes occurring in the cement paste. Thirty-five cylindrical specimens with a diameter of 100 mm and a length of 200 mm were cast with a water to cement ratio = 0.60. Thirty specimens were destructively tested at 1, 3, 5, 7, 14 and 28 days for their compressive strength. The remaining five specimens were non-destructively tested at 1, 3, 5, 7, 14, 28 and 56 days using longitudinal and transversal ultrasonic wave transducers with frequencies from 50 kHz to 500 kHz. Analysis of the evolution in frequency observed in the specimens identified variations related to progressive hydration of the cement paste, in contrast with the invariant behavior of the inert aggregates. Results show that it is possible to distinguish the behavior of cement paste and aggregates in the frequency domain. As a consequence, it should be possible in future research to evaluate more efficiently different phenomena that affect only the cement paste.