Enabling recycled coarse aggregates in ductile ECC: A synergistic enhancement of mechanical and sustainability properties

To meet the micro-mechanical design guidelines for achieving strain hardening and steady-state cracking behavior, the coarse aggregates are conventionally excluded from engineered cementitious composite materials. The absence of coarse aggregates lead to challenges, including enormous carbon emissions, elevated drying shrinkage, and reduced compressive strength. To overcome these limitations, this study explored the development of a novel ultra-high ductile concrete (UHDC) incorporating coarse aggregates, with the particular focus on the recycled coarse aggregates. The effects of coarse aggregate content (16%, 20% and 24%) on the workability, drying shrinkage, and basic mechanical properties of UHDC were systematically investigated through the physical and mechanical tests. The results demonstrated that all the UHDC mixtures, both with natural and recycled coarse aggregates, consistently exhibited strain-hardening behavior and multiple micro-cracking characteristics. All the mixtures achieved the average tensile strains exceeding 5%, while maintaining crack width below 150 μm. As the coarse aggregate content increased, the workability, drying shrinkage, tensile strength, and tensile strain exhibited progressive reduction, whereas the compressive strength demonstrated significant enhancement. Notably, compared to the mixture without coarse aggregates, UHDC containing 24% coarse aggregates markedly reduced the 28-day shrinkage strain by 65.83% to 450.56 µε, while simultaneously enhancing the compressive strength by 28.4%, reaching 38.9 MPa. Furthermore, scanning electron microscopy and X-ray computed tomography (CT) tests were conducted to analyze the micro-structural and pore structure characterization, respectively. In addition, the study demonstrated that compared to the mixture without coarse aggregates, UHDC containing 24% recycled coarse aggregates significantly reduced the energy consumption and carbon emissions by 18.38% and 25.25%, respectively. It was preliminary verified that the incorporation of coarse aggregate effectively enhanced the mechanical performance and promoted sustainability through waste concrete utilization.