A Microstructural Basis for Diffuse Ultrasound in Concrete

Abstract

Ultrasound has been the basis for nondestructive evaluation of concrete for decades. Its success is founded upon the fact that the characteristics of the propagating elastic wave depend on the same micro and macrostructural features that dictate the material’s structural properties. However, decoding the relationship between ultrasonic wave propagation and material properties is a difficult problem. Heterogeneities in the concrete exist over a wide range of length scales, leading to a strong frequency dependence in the elastic wave. Thus, for practical applications we tend to rely on simplified empirical relationships between the ultrasonic signal characteristics and the material properties of interest. In an effort to improve our understanding of the relationships between microstructural features and ultrasonic signal behavior and experimental program was set up in such a way that concrete specimens of varying composition (water to cement ratio) were both imaged using x-ray computed tomography, and interrogated with through-transmission ultrasound. From the CT images, measurements were made of porosity and pore size distribution, cement paste density, and the distribution of interfaces. All these features were clearly attributable to characteristics of concrete composition. Ultrasonic scans of the same specimens were done at varying frequencies. In addition to pulse velocity and phase velocities, the ultrasonic signals were fit to a diffusion model to separate absorption attenuation from scattering attenuation. The former being represented in the diffusion model as dissipation rate, while the latter being represented by diffusivity. The hypothesis to be tested was that scattering attenuation would be directly linked to internal interfaces, while absorption attenuation would be linked to cement paste density. The results showed that at the frequencies tested, ultrasonic dissipation rate correlated weakly with paste density, while diffusivity correlated well with number of interfaces, but only if entrained air is considered separately. Cement paste density was found to be very well predicted by diffusivity, leading to a clear power-law relationship between diffusivity and compressive strength.