The Effect of Coupling Media on the Pulse Velocity of Concrete

Ultrasonic methods have been widely used in civil engineering for the non-destructive evaluation of the concrete structures. Measurements of the velocity of the ultrasonic wave in concrete can be used to assess the quality of the concrete. As the concrete is a heterogeneous multiphase material which is acoustically inhomogeneous, propagation of ultrasonic waves through it will be a complex process comparing to other solid materials.
This thesis involved testing the hypothesis, suggested by previous studies using ‘non-contact’ apparatus, that a coupling effect might exist between the ultrasound wave and the constituent materials of concrete.The velocity of sound in concrete samples measured by the traditional ultrasonic pulse velocity testing apparatus (PUNDIT), different coupling media of varying acoustic impedance was placed between the transducers and concrete. The coupling effect was evaluated in terms of the couplant used, compressive strength, aggregate content and maximum size of coarse aggregate. Analysis of variances (ANOVA) was performed to determine if there are statistically significant differences between the measurements recorded using a conventional system and a coupled system.
In accordance with the experimental results, coupling materials have an effect on the pulse velocity measured in a given concrete. The effect varies depending on the material used. The UPV measurements with solid coupling were higher than those from the liquid coupling at all strength levels.
For concrete with a specific w/c, the pulse velocity increased as the aggregate content increased. The conventional and rubber tests showed more sensitivity to the changes in aggregate content than the liquids tests. When the aggregate content is constant, concretes with larger MAS generally yielded higher pulse velocities than those with smaller MAS. In the coupling tests, the UPV measurements were more affected by the couplant used than the change in MAS of the mix. While the rubber test showed significant differences between the measurements of the two MAS at each strength level, the propanol test recorded approximately similar values for both MAS at all compressive strength levels.