Aperture effect on Rayleigh wave measurements on cement media

Elastic waves are commonly used for the evaluation of concrete structural health. Wave speed is firmly connected to the stiffness and is indicative of strength and damage condition. When access to multiple sides is limited, the evaluation takes place solely from the open surface where all sensors are placed. In this case, the size of the sensor is crucial because of the “aperture effect”. This is basically the phenomenon of wavelengths shorter than the sensor size cancelling each other since both their positive and negative phases act simultaneously on the sensor’s surface. Although this effect has been studied relatively to the amplitude and the frequency content of the surface wave pulses, its influence on velocity has not been similarly studied, even though the velocity value is connected to concrete stiffness, porosity, damage degree and is even empirically used to evaluate the compressive strength. In this study, numerical simulations are conducted with virtual sensors of different sizes to measure the surface wave velocity as well as the dispersion (or its dependence on frequency) in relation to the sensor size on homogeneous and heterogeneous material. The strong effect of sensor size is indicated and suggestions towards rules for reliable measurements on a concrete surface are made. Experimental measurements on cementitious media by sensors of different sizes are also conducted validating the numerical results.