Study on the Strain Rate Dependence of Meso-Scale Mechanical Properties of the Concrete Interfacial Transition Zone

To systematically investigate the strain rate dependent mechanical behavior of the concrete interfacial transition zone (ITZ) at the mesoscale, uniaxial compression tests were conducted on mesoscopic concrete specimens under different strain rate conditions by combining optical microscopy and digital image correlation (DIC) techniques. The variations in key mechanical parameters of the ITZ, including thickness, elastic modulus, and Poisson's ratio, were analyzed over a strain rate range of 10⁻⁴ to 10⁻² s⁻¹. The experimental results indicate that, within the investigated strain rate range, the thickness of the ITZ shows no significant variation with strain rate, exhibiting good geometric stability. In contrast, the elastic modulus of the ITZ increases markedly with increasing strain rate, demonstrating a pronounced strain rate strengthening effect, while the Poisson's ratio gradually decreases as the strain rate increases, showing an opposite strain rate dependent response to that of the elastic modulus. Further comparative analysis reveals that the strain rate sensitivity of the mechanical parameters of the ITZ is significantly higher than that of the mortar and aggregate phases, indicating that the interfacial region plays a more prominent regulating role in the overall mechanical response of concrete under varying loading rates. With increasing strain rate, the differences in mechanical properties between the ITZ and the adjacent mortar and aggregate phases gradually diminish. Higher strain rates can effectively suppress the localized weakening effect of the ITZ, thereby promoting a more homogeneous mechanical response of concrete at the mesoscale. The results of this study provide experimental support for mesoscale mechanical modeling and constitutive relationship development of concrete under different loading rate conditions.