Printability assessment of concrete 3D printed elements with recycled fine aggregate

This study investigates the printability of concrete with recycled fine aggregates (RFA) for 3D concrete printing (3DCP). RFA was used to replace conventional fine aggregates - crushed stone sand (CS) at 30 %, 50 %, and 100 % levels. Both rectangular and cylindrical hollow specimens were printed to evaluate mix printability. The methodology combined systematic printing trials, rheological assessments, and advanced computer vision (CV) monitoring. Before printing, rheological assessments were done to check the feasibility of the mix. During printing, inline cameras were used to record layer-wise deformations and surface textures to assess printability. Rheological characterisation indicated that higher RFA content increased initial static yield stress (from 1.69 to 1.86 kPa) and accelerated structural recovery. This enabled the construction of a 1 m cylindrical specimen while reducing deformation from 4 % to 0.8 %. Despite the improved buildability, mechanical property evaluation indicated that a 50 % RFA replacement level optimally balanced rheological performance with compressive strength, achieving 44.6 MPa at 28 days, approximately 3–4 % higher than RFA-30 and markedly superior to RFA-100, where higher replacements significantly reduced compressive strength. Flexural strength increased with delayed printing, correlating with increased surface texture. Additionally, water sorptivity rose with higher RFA content due to increased pore connectivity and aggregate porosity. Compared to the control mix (14.4 × 10⁻⁴ g/mm²·h⁰ . ⁵), sorptivity increased by 21.9 %, 33.8 %, and 72.1 % for RFA-30, RFA-50, and RFA-100, respectively. The integrated approach of inline computer vision and pre-printing rheological assessments produced consistent results, underscoring the reliability of the printability performance of 3DCP.