Mix design optimization for 3D ‐printable ECC reinforced with SMA fiber: Evolution of rheology, printability, strain‐hardening, and microstructure

Extrusion‐based three‐dimensional concrete printing enables automated construction without formwork, yet its structural application is restricted by the brittle behavior and limited tensile capacity of printable cementitious materials. In particular, reconciling fresh‐state printability with tensile strain‐hardening remains a key challenge for engineered cementitious composites (ECC). This study develops and optimizes a shape memory alloy fiber‐reinforced engineered cementitious composite (SMAF‐ECC) for extrusion‐based 3D printing using an integrated material process performance framework. An orthogonal experimental design was employed to optimize ECC mix proportions by jointly evaluating setting time, fluidity, rheology, extrudability, buildability, and hydration heat evolution. An optimal printable ECC mixture was identified and subsequently reinforced with NiTi SMAF of 26 and 32 mm length at 0.35–1.0 vol%. Results show that a 0.60% attapulgite dosage provides balanced early‐age hydration and rheological build‐up, enabling stable extrusion and high buildability (tan θ ≈0.019) without excessive flow resistance. SMAF incorporation slightly reduces flowability but significantly enhances tensile performance. The optimal SMAF‐ECC (32 mm, 1.0%) achieved peak tensile stresses of 5.72 MPa cast and 5.40 MPa 3D‐printed, representing an improvement of approximately 45% over fiber‐free ECC. Ultimate tensile strain increased to 6.8% and 6.35%, respectively, while preserving stable strain‐hardening behavior after printing. Microstructural and porosity analyses confirmed a dense C–S–H‐dominated matrix, refined pore structure, and predominantly mechanical fiber–matrix interaction. The findings demonstrate that SMAF‐ECC enables high‐ductility, damage‐tolerant, and printable smart cementitious composites, offering a promising solution for advanced structural applications in digital construction.