Dispersion of ultrasonic guided surface wave by honeycomb in early-aged concrete

A pair of 50 kHz P-wave ultrasonic sensors was used in this research to study the effects of concrete containing honeycomb on ultrasonic guided surface wave propagation shortly after the initial few hours of mixing and after 28 days. Honeycomb was made inside the 100 mm thick concrete specimens by adjusting dosage of superplasticizer. The pair of sensors were accommodated 100 mm apart from each other, at the soffit and on the same side of the formwork. The ultrasonic guided surface wave signals were continuously measured and post-processed by short time Fourier transform (STFT) to evaluate the frequency spectra along the time axis. Over the ultrasonic time window, the frequency spectra were very dispersed for the specimens with honeycombs. The results suggested that during the early hydration of concrete, the added water filled the honeycombs and led to incoherent surface wave that significantly dispersed the frequency spectra. It is because the size of the honeycomb is in the same order as the wavelength the scattered surface wave. With this method, the presence of honeycomb immediately after casting of concrete can be detected non-destructively.