Ultrasonic wave dispersion and attenuation in fresh mortar: Fid-Bau Portal

Ultrasonic wave dispersion and attenuation in fresh mortar

Aggelis, D.G. / Philippidis, T.P.

The objective of this paper is two-fold: the study of wave propagation in fresh mortar followed by an investigation on the possibility of quality estimation through composition control. Results from the tone-burst and sine-sweep experiments indicated the dispersive and attenuative nature of fresh cementitious material. The dominant role of the inclusion (sand) content in wave propagation affecting both velocity and attenuation was highlighted. However, the contribution of other sources of inhomogeneity as the entrapped air bubbles, always present in fresh mortar, should not be neglected, since paste specimens containing no aggregates exhibit strong attenuation at low frequencies. Remarkable differences, especially in attenuation, are observed where sand rich mixes exhibit much higher attenuation than paste for frequencies above 300 kHz, while the situation is reversed for frequencies lower than 200 kHz. Also the larger grain size results in higher attenuation than fine sand leading to the assumption that scattering is the dominant attenuation mechanism. The effect of water content was not clearly observed; however, a methodology based on the determination of sand content allows for estimates of w=c with encouraging accuracy. The use of a more sophisticated pattern recognition approach has the potential to improve the results while theoretical investigation using multiple scattering theory, currently undertaken, could lead to an understanding of the exhibited complicated behavior. Testing of fresh mortar is a step towards concrete examination. The practical advantage of mortar is the low volume of material required. In order to test concrete, where the maximum aggregate size is of the order of 40 mm, the experimental set-up should be drastically modified both in geometry, to contain an indicative volume of the material and probably in equipment to compensate for the reduced amplitude of the wave after several centimeters of propagation through concrete. However, considering the difficulties of ultrasound measurements in fresh concrete another alternative can be examined; the assessment of mortar sieved from the given concrete. This procedure is described in the ASTM codes for measuring the setting time of concrete.