A comparative study on ultrasonic immersion and laser-vibrometer tests for characterization of cement based materials

Concrete's heterogeneous structure makes it a highly dispersive medium for wave propagation, affecting ultrasonic test measurements. Aggregate grain size is a major source of heterogeneity, influencing wave characteristics. To have a clear understand of the effect of grain size on wave properties, it is essential to eliminate variability from aggregate-mortar interface and concrete mixture design; this requires examining specimens made of identical aggregates. Coupling also influences wave measurements significantly. While ultrasonic immersion testing provides consistent coupling, it isn't suitable for in-situ testing. Laser vibrometers (LV) have the potential to offer a reliable non-contact alternative for in-situ measurements. This study investigates the influences of aggregate content and coupling on ultrasonic measurements. Immersion and LV tests are performed on homogeneous (acrylic, nylon) and heterogeneous (concrete, mortar with varying glass bead content) specimens. Material properties, including wave velocities, acoustic impedance, elastic moduli, and attenuation, are determined using both methods. Wave signals are analyzed in time and frequency domains to identify optimal material characterization method. Results indicate that attenuation primarily originates from the irregular shape of aggregates compared to the smooth surface of glass beads. Furthermore, LV measurements of P-wave velocity and attenuation strongly agree with immersion tests, suggesting LV as a viable alternative.