Multiscale mechanical characteristics of ultra-high performance concrete incorporating different particle size ranges of recycled fine aggregate

The influence of the particle size range of recycled fine aggregate (RFA) on the microstructural characteristics and mechanical properties of ultra-high performance concrete (UHPC) was thoroughly investigated by conducting the nano-, micro-, and macro-scale characterization tests. The designed UHPC using RFA exhibited higher compressive strength compared to the reference group incorporating quartz sand (QS). The uniaxial tensile strength and strain capacity of UHPC-1 (using 0–1 mm RFA), being 10.44 MPa and 0.56%, were, respectively, 18% and 23% higher than those of the control group. The improvement of mechanical performance could be attributed to the second hydration effect of the remained anhydrous phases in RFA. The incorporation of 0–1 mm RFA may also favorably lead to the optimized gradation of solid constituents in UHPC-1, which could contribute to the densification of the cementitious matrix and the refinement of micropores. The pore structure investigation confirmed that the volume percentage of pores larger than 100 nm was the lowest in UHPC-1 matrix while the corresponding proportion was the highest in UHPC-QS matrix. The results of nanoindentation test unveiled that an efficient bond between RFA and matrix was formed. When the recycled aggregate boundary is approached, no lowering of the mechanical properties of the paste matrix was observed. The interfacial transition zone (ITZ) was densely packed with compact C-S-H gels. Thus, the qualified UHPC incorporating 100% RFA could be achieved by appropriate tailoring of the constituted ingredients.