Direct Tensile Behavior of Thermally Activated Concrete Reinforced with Short Iron-Based Shape Memory Alloy (Fe-SMA) Fibers: Experimental and Numerical Investigation

Short iron-based shape memory alloy (Fe-SMA) fibers can act as active internal reinforcement after thermal activation. This study examined the direct tensile behavior of concrete containing 2.0% by volume of straight Fe-SMA fibers, geometrically identical steel fibers, or no fibers. ASTM C307 briquettes were tested under ambient conditions and after a 150 min hold at a chamber temperature of 160∘C, followed by cooling. Plain-concrete flexural and compressive tests characterized matrix degradation. Mean tensile strengths were 7.96 and 7.18 MPa for the unheated steel-fiber and Fe-SMA groups, respectively. After heating, these values were 5.89 and 9.41 MPa. The Fe-SMA increase was 31.1%, corresponding to a mean difference of 2.23 MPa with a 95% confidence interval of 1.66–2.80 MPa. A three-dimensional ABAQUS/CAE model with randomly distributed, fully bonded fibers reproduced the main response features using a recovery-stress target adopted from prior same-batch characterization. Separate numerical sweeps predicted higher strength with increasing fiber content and decreasing diameter within the investigated ranges. These results support the feasibility of thermally activated short Fe-SMA reinforcement. The numerical recommendations remain conditional on the idealized interface, adopted activation model, and investigated parameter ranges. Direct core-temperature monitoring and microscopic interface characterization remain necessary for further validation.