Subject matter

Concrete, primarily composed of cement, is the second most consumed material globally after water. Since its invention over two centuries ago, it has been widely used in the construction of various buildings and civil engineering infrastructures. Among the key regions in concrete, the interfacial transition zone (ITZ)—located between the aggregate and the mortar paste—has long been recognized as its weakest region. Consequently, the properties of the ITZ have attracted increasing attention from researchers. More than two decades ago, RILEM TC 159-ETC (Engineering of the Interfacial Transition Zone in Cementitious Composites) and 163-TPZ (Interfacial Transition Zone and Properties of Transfer Committees) conducted pioneering studies on the ITZ. The ITZ forms through complex chemical and physical mechanisms, including the hydration process, and plays a critical role in influencing concrete’s strength, deformation, and durability.

In the context of global warming and the increasing need to reduce carbon emissions, concrete—as a major contributor to carbon emissions—faces continuously evolving performance requirements. Recycled concrete, recognized as a green and low-carbon material, plays a significant role in reducing environmental impact. However, the variability in the age and spatial distribution of the parent concrete, along with micro-cracks induced during the crushing process in the aggregate, old mortar, and old interfacial transition zones, leads to the complex formation of multiple ITZs (MITZ). These include the new mortar–old mortar ITZ, new mortar–original aggregate ITZ, and old mortar–original aggregate ITZ, making recycled concrete more intricate than natural aggregate concrete (NAC). Given this complexity, ITZs play a more critical role in regulating the overall performance of recycled concrete than in NAC.

Furthermore, insights from our previous TC 273-RAC (Structural Behavior and Innovation of Recycled Aggregate Concrete), indicate that ITZ also affect the structural performance, including mechanical properties and durability. Additionally, due to the diverse sources of recycled aggregates, recycled concrete may contain a mix of recycled concrete aggregates, recycled brick aggregates, and other materials, further contributing to the presence of MITZ and increasing the complexity of ITZs in recycled concrete. Therefore, further in-depth research and analysis of ITZs are essential for advancing the understanding of their behavior and impact on mechanical properties, long-term performance, durability, and functional aspects such as thermal and acoustic properties in recycled concrete.

However, existing studies on ITZs in recycled concrete are fragmented and often limited by:

• Localized micro-area analysis constraints, requiring extensive sample preparation and suffering from roughness-induced errors.
• Overly simplified experimental loading conditions, leading to incomplete mechanical characterization.
• Finite element modeling limitations, where the heterogeneity, anisotropy, and randomness of ITZs are inadequately considered. Existing numerical simulations typically use mortar-based constitutive models for ITZs, which are not derived from multi-scale experimental constitutive relationships.
• Qualitative assessments, lacking robust quantitative frameworks for ITZ evolution and material performance prediction.
• Lack of quantitative correlation models for the mechanical properties of ITZs across different scales (molecular, micro, meso, and macro scales).

To address these gaps, advanced characterization methods and theoretical models must be developed to achieve a deeper understanding of the microstructure and mechanical properties of ITZs. This will enable a quantitative analysis of its formation and evolution, providing a comprehensive view of ITZ heterogeneity and anisotropy. Multi-scale characterization techniques should be implemented across molecular, nanoscopic, microscopic, mesoscopic, and macroscopic levels to achieve accurate multi-scale performance control of ITZ.

Simultaneously, finite element simulation methods should be improved to better capture materialz randomness, time-dependency, cross-scale effects, and complex mechanical behaviors. Integrating experimental data with theoretical models will provide a scientific foundation for optimizing recycled concrete mix designs and construction techniques, ultimately improving the overall performance and durability of recycled concrete. Understanding the evolution of MITZ properties from a mechanical perspective is essential for effectively enhancing the properties of recycled concrete.

To achieve a better understanding of ITZs in recycled concrete, this Technical Committee (TC) aims to:

• Investigate the formation mechanisms and damage evolution of MITZ in recycled concrete.
• Characterize the anisotropic and heterogeneous characteristics of MITZ across different scales.
• Develop advanced multi-characterization techniques, integrating experimental and numerical methods.
• Explore the effects of aggregate particle size (fine recycled aggregate, coarse recycled aggregate, large-size coarse recycled aggregate for hydraulic engineering, etc.), aggregate type (recycled concrete aggregate, recycled brick aggregate, etc.), curing age, admixtures, environmental conditions, and other key variables on ITZ performance in recycled concrete.
• Quantify the influence of MITZ on the macro properties of recycled concrete, including mechanical properties, long-term performance, durability, and functional performance.
• Propose innovative technologies to strengthen MITZ properties and improve the overall performance of recycled concrete.
• Establish quantitative models for MITZ evolution, incorporating time-dependent effects and multi-scale interactions.

It is important to note that this TC focuses exclusively on MITZ within recycled concrete, and this study will be limited to recycled concrete materials only.

Terms of reference

• The proposed timeline for the proposed TC is 5 years.
• The TC members will be selected from experts in academia, industry, and research institutions, based on their expertise in cementitious materials, recycled concrete and MITZ.
• The research will include a thorough literature review, the publication of a state-of-the-art report, with potentially the development of new equipment and techniques for MITZ characterization. This will be followed by a detailed evaluation of the test/simulation results and the formulation of recommendations.

The study of this TC is relevant across various fields, including civil engineering, materials science, nanotechnology, chemical and environmental engineering.

Detailed working programme

Year 1

• Conduct a start-up meeting to introduce members, provide an overview of their skills and experience, plan activities, identify key areas requiring further research, and organize the initial workshop.
• Perform literature review and preliminary studies on MITZ formation, properties, and its impact on recycled concrete structures.

Year 2

• Undertake experimental investigation of MITZ characteristics, including microscopic mechanical properties (nanoindentation and nanoscratch tests), macroscopic interfacial mechanical properties (interfacial tensile strength, interfacial shear strength, and interfacial fracture properties), and microstructural analysis using SEM, XRD, etc.
• Conduct Round Robin tests to ensure the reliability and reproducibility of experiment
• Propose new methodologies for characterising MITZ.
• Prepare the state-of-the-art report and new workplan will be agreed upon.
• Plan regional seminars in Asia, Europe and North America.

Year 3

• Evaluate initial results, reporting progress on the development of new testing methods, and outline the next steps.
• Develop a multiscale numerical model to capture the behavior of MITZ and its impact on the macroscopic performance of recycled concrete structures.

Year 4

• Discuss the results obtained, followed by adjustments to the test methods and predictive models based on these findings.
• Explore and develope new technologies and methods for improving the properties of MITZ.
• Prepare of international workshops.

Year 5

• Finalize all results and drafting of recommendations.
• Propose a standardized test method and a prediction model for MITZ properties.
• Prepare and publish guidelines for characterisation and regulation of MITZ.