A Novel Retrofitting Strategy for Crack Mitigation Using NiTi SMA Wire-Reinforced High-Strength Cementitious Composite

Shape memory alloys (SMAs) constitute a distinct class of smart materials characterized by their remarkable deformation capability and the ability to recover their original geometry upon exposure to an appropriate stimulus. Among these, nickel–titanium (NiTi) alloys exhibit superior superelasticity and shape memory effect (SME), making them promising candidates for advanced structural rehabilitation and retrofitting applications. The present study investigates the integration of NiTi SMA wires with an average ultimate tensile strength of 900 MPa in the austenitic phase within high-strength cementitious composites (HSCC) as a retrofitting strategy, focusing on enhancing the self-centering capacity, i.e., the ability to restore its original shape after unloading or deformation owing to the recovery stresses in structural elements. NiTi SMA wires of 0.8 mm diameter, embedded in bundles (consisting of 1 to 5 wires) within the tensile zone of HSCC prisms, were evaluated under monotonic three-point bending tests. The influence of the number of SMA wire bundles on key performance indicators—namely, maximum midspan deflection, crack initiation and propagation, and energy dissipation—was systematically assessed. Furthermore, the crack-closing efficiency of the embedded NiTi SMA was examined under controlled thermal activation, with image processing. Experimental observation and quantitative evaluation using ImageJ demonstrated that two-strand HSCC samples (S2) consistently achieved more than 90% crack-width recovery in the tensile zone, whereas three-strand samples (S3) showed mixed performance (in the range of 65–78%). The S4 and S5 samples relatively exhibited poor recovery with less than 25% crack-width closure. Crack closure in S2 specimens was observed to accelerate in the range of 2.4–3.2 V and in a temperature zone of 55–75°C, consistent with the reported austenitic phase ( A f ) temperature of 68°C for the NiTi SMA wire. These findings underscore the feasibility of NiTi SMA–reinforced HSCC as a ready-to-deploy, high-performance retrofitting solution for extending the service life and resilience of structural components.