Mr. Nicolas Ospitia Profile

Nicolas Ospitia

at Vrije Univ Brussel

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Proceedings Article | 18 April 2022 Presentation + Paper

Aperture effect on Rayleigh wave measurements on cement media

Nicolas Ospitia, Dimitrios Aggelis, Gerlinde Lefever

Proceedings Volume 12047, 120470M (2022) https://doi.org/10.1117/12.2614395

KEYWORDS: Sensors, Dispersion, Cements, Signal attenuation, Numerical simulations, Wave propagation, Velocity measurements, Phase velocity, Wavefronts, Scattering

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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.

Proceedings Article | 22 March 2021 Presentation + Paper

Ultrasonic dispersion and attenuation in bubbly liquids

Nicolas Ospitia, Gerlinde Lefever, Dorian Serafin, Danny Van Hemelrijck, Dimitrios Aggelis

Proceedings Volume 11592, 1159211 (2021) https://doi.org/10.1117/12.2584037

KEYWORDS: Ultrasonics, Signal attenuation, Dispersion, Liquids, Cements, Phase velocity, Visualization

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Ultrasonic monitoring of fresh cement-based materials is important as pulse speed and attenuation are indicative of the increasing stiffness of the medium, and enable characterization of the curing stage and projections to the mechanical strength from an early age. Despite its importance, practical application is not straightforward due to severe heterogeneity and inherent damping. One crucial parameter in the ultrasonic behavior of fresh cement is the air bubbles, which impose a frequency dependent phase velocity and attenuation, as also observed in all bubbly liquids. In this study, ultrasonic experiments take place in fresh mortar as well as in reference media like water and shampoo. Results show that both shampoo and mortar exhibit strong dispersion relatively to water, seen by the dependence of phase velocity on frequency. Gradually and as bubbles are released due to gravitational settlement (in shampoo) or constrained (hardening of cement) the dispersive trend weakens reaching towards a nearly flat dispersion curve like water. The results highlight the influence of cavities which are considered one of the strongest types of scatterers, while quantification of cement ultrasonic dispersion opens the way for more accurate characterization of the curing behavior.

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