Effect of recycled coarse aggregate replacement on the anisotropic mechanical properties and failure mechanisms of 3D printed concrete

Three-dimensional concrete printing (3DCP) introduces mechanical anisotropy, while recycled coarse aggregate (RCA) generally reduces concrete strength; however, whether these two effects remain separable when acting within the same material remains insufficiently quantified. Cubic specimens sawn from printed elements with RCA replacement ratios of 0%, 50%, and 100% were tested under uniaxial compression along three orthogonal directions (X, Y, and Z), with three replicates for each combination, yielding 27 specimens. Full-field strain and crack evolution were monitored using digital image correlation (DIC), while fracture surfaces were characterized by scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS). The compressive strength consistently ranked Z > Y > X, and the normalized anisotropy coefficients remained nearly unchanged across the three RCA replacement levels, with mean values of 0.749 for X/Z and 0.858 for Y/Z. Two-way ANOVA showed that loading direction and RCA replacement accounted for 73.7% and 20.0% of the total variation in compressive strength, respectively, whereas their interaction accounted for only 0.30% and was not statistically significant (p = 0.92). Increasing RCA reduced the Z-direction strength from 25.02 to 21.52 MPa and increased the peak strain from 0.215% to 0.275%. Under the tested printing conditions, RCA replacement therefore primarily shifted the absolute strength level while preserving the relative directional strength profile, whereas the layered printing geometry governed the directional strength hierarchy and failure mode. DIC revealed direction-dependent failure patterns, while SEM-EDS showed the more heterogeneous and porous ITZ associated with RCA. Directional correction factors are provided for the tested printing conditions.