Advanced ultrasonic technology for freezing damage prevention of concrete pavement

Freezing of concrete can lead to the development of internal crystalline pressure due to the volume expansion of pore liquid. Crack will initialize when such internal pressure exceeds the fracture strength of concrete. Therefore, ensuring the existence of sufficient pore volume to accommodate the volume expansion due to the icing process is important for the prevention of freezing damages to concrete. The volume of air voids of fresh concrete can be measured with standard methods such as ASTM C231. The results, however, do not necessarily indicate the actual pore volume in the hardened concrete, as the pore space continue to evolve as the results of cement hydration. Therefore, determinination of the air voids in hardened concrete pavement typically has to involve destructive coring and sophisticated laboratory procedures, i.e., petrological analyses. This paper describes the development of an ultrasonic method for measuring the pore size distribution in hardened concrete specimens. The theoretical basis of the method is developed based on advanced ultrasonic wave scattering model. The model accounts for the attenuation effects of multiple sized air voids. The air voids are treated as either elastic scatterers or cavities, two extreme conditions for their actual physical behaviors. The total attenuations are derived by superimposing the wave attenuation by air void scatterers, coarse aggregates and viscoelastic matrix. The pore size distribution is estimated by a model based inversion analyses procedure. The assumptions of elastic wave scatter and cavity provide the upper and lower bound of the actual air void distribution. The results are encouraging. Factors causing the differences between experimental and theoretical attenuation curves are discussed, which will serve as the thrusts to further develop this technology.