Multi-scale analysis of 3D printable high-strength engineered cementitious composite with carbon and polyethylene fibres: rheology, printability and hydration kinetics in structural components

Current standards for evaluating the three-dimensional (3D) printability of concrete rely heavily on observational methods, lacking well-defined criteria. In this work, quantitative printability criteria were established by investigating the impact of carbon fibre (CF) volume fraction (0–0.60%) on the rheology, early-age strength and hydration kinetics of 3D printable high-strength engineered cementitious composites, validated through practical printing tests. Key results demonstrate that increasing the CF content to 0.45% significantly enhanced the rheological properties, increasing the static yield stress by 20.89%, the dynamic yield stress by 41.19% and plastic viscosity by 44.35%. Furthermore, the early-age mechanical strength exhibited a substantial five-fold increase with CF content up to 0.60%, achieving a peak strength of 176.03 kPa. This improvement in strength and rheology correlated with accelerated hydration as the CF fraction was increased from 0 to 0.60%, evidenced by a reduction in the termination point of the accelerated reaction phase from 28.4 h to 19.8 h. Practical printing verification confirmed that optimal printability, defined as the ability to exceed 25 layers, occurred within specific rheological and strength thresholds (static yield stress of 800–980 Pa, dynamic yield stress of 200–320 Pa and early-age strength of 30–120 kPa). These findings establish validated quantitative criteria for assessing the 3D printability of concrete.