Study on uniaxial compressive properties and microfracture toughness of interface transition zones in recycled sand concrete

This study investigates the uniaxial compression behavior of recycled sand concrete (RSC) prism and cube with different recycled sand contents and strength grades. The characteristic parameters of the stress-strain curves are analyzed, and a constitutive model is developed based on the energy evolution relationship of RSC during loading. The microfracture toughness of the interfacial transition zone (ITZ) in RSC is evaluated using the nanoscratch method. Classical size effect theory is used to characterise the strength of RSC with different height to thickness ratios in combination with finite element simulation and experiment. The results show that the peak stress and modulus of the C20 RSC increase with the RS content. Conversely, the C40 and C60 RSCs are observed to be highest when the RS content is 50 %. The Poisson's ratio of RSC exhibits a range of 0.192–0.388, which exceeds the typical value of 0.2 for ordinary concrete. This highlights the significant transverse deformation characteristic of RSC. The microfracture toughness of ITZ shows a consistent trend in accordance with the variation of the peak stress. It can be observed that an increased microfracture toughness serves to improve the macromechanical properties of RSC. The total strain energy and dissipation energy can be increased by the addition of RS in a manner dependent on the water/binder ratio, which in turn leads to improved ductility and toughness. The compressive constitutive relationship of RSC, established by combining dissipation energy with statistical damage theory, shows a high degree of correlation with experimental values, thus allowing the description of the constitutive behaviour of RSC. The strength of RSC is consistent with size effect laws, including Weibull size effect law (WSEL), Bazant size effect law (BSEL) and Carpinteri size effect law (CSEL). In particular, CSEL has a correlation coefficient of more than 0.95 when compared with experimental values.