With the growing diversity of concrete materials, developing an interface mechanics theory that can universally describe and regulate multi-system concrete materials has become a key path to advance the field from empirical practice to theoretical formulation. The multiple interfacial transition zones (MITZ) in recycled aggregate concrete (RAC, hereafter recycled concrete) are significantly more complex than the single interfacial transition zone (ITZ) in ordinary concrete in terms of spatial morphology, microstructures, and formation mechanisms. Therefore, MITZ can serve as an ideal research subject for building the interface mechanics theory of RAC. The ultimate goal of this theory is to establish a unified mathematical framework capable of systematically predicting the overall mechanical behavior of RAC. To achieve this goal, three fundamental scientific issues should be systematically addressed: anisotropy, multi-scale characteristics, and time-dependent behavior. Anisotropy arises from the significant differences in the mechanical properties of the ITZ in three axial directions, leading to scattered characterization data and hindering theoretical modeling. The main research challenge lies in the complex spatial morphology of the ITZ and the fact that part of the interface is hidden between the aggregate and the mortar, making accurate testing of the three axial surfaces difficult. For this purpose, this paper proposes the development of ITZ deconstruction technology. This involves designing regular aggregates to standardize the geometry of the ITZ and using aggregate-mortar separation techniques to enable direct characterization of its three axial surfaces, thereby overcoming the experimental bottleneck in anisotropy research. Multi-scale characteristics refer to the fact that the mechanical behavior of the ITZ is jointly influenced by mechanisms across multiple scales, from molecular and micro to meso and macro scales. There is a need to systematically establish quantitative mathematical models that link these scales. In view of the difficulty in directly deriving inter-scale relationships through theoretical deduction, this paper suggests integrating big data and machine learning technologies. This approach involves systematically acquiring multi-scale experimental data and building performance mapping models across scales to achieve accurate prediction of the ITZ’s cross-scale mechanical behavior. The key to this path lies in the quality control and integrated analysis of multi-source data, requiring collaborative efforts from materials, informatics, and other disciplines. Time-dependent behavior reflects the evolution of the ITZ’s chemical composition and microstructures over time, leading to changes in its mechanical performance. To enhance the universality and forward-looking nature of the interface mechanics theory, it is essential to reveal its time-varying laws. This paper proposes building a multi-period, multi-parameter collaborative analysis framework. This includes developing in-situ monitoring techniques at the experimental level to track the evolution of the ITZ, introducing aging and damage state variables at the simulation level, and establishing performance prediction models that incorporate time factors at the theoretical level. The combination of these three aspects is expected to form a systematic predictive capability for the time-dependent behavior of MITZ. In summary, based on a systematic review of the current research status and shortcomings of MITZ, this paper clearly proposes the concept of a unified “interface mechanics theory of RAC” for the first time, identifies its three fundamental scientific issues, and proposes feasible technical pathways to address the research difficulties of each basic problem. This work not only provides a systematic theoretical framework and methodological support for ITZ research but also lays an important scientific foundation for achieving unified design and performance regulation and optimization of concrete materials. It has foundational significance for promoting the interdisciplinary integration of materials science and civil engineering.
Xiao et al. (Thu,) studied this question.