Prestressing of concrete using iron-based shape memory alloy (Fe-SMA) short fibers: Experimental and numerical analysis

Iron-based shape memory alloys (Fe-SMAs) exhibit unique shape recovery and memory effect behavior upon thermal activation, making them advantageous for structural applications such as prestressing. Introducing short Fe-SMA fibers into concrete structures allows for a uniform and localized distribution of prestressing forces within the concrete matrix. In this study, an experimental campaign was conducted to evaluate the efficiency of prestressing concrete using short Fe-SMA fibers. Concrete prism specimens reinforced with randomly dispersed Fe-SMA fibers, steel fibers, and plain concrete reference specimens were tested under three-point bending after exposure to ambient temperature, 160 °C, and 200 °C. All fiber-reinforced specimens contained a targeted 2% volume fraction of fibers with identical geometries featuring end-hooked shapes for enhanced pull-out resistance. At ambient temperature, the Fe-SMA fibers remain in a passive, non-activated state and do not undergo the phase transformation required to generate prestressing forces. As a result, at ambient temperature, specimens with steel fibers show higher flexural strength (22.95 MPa) than those containing Fe-SMA fibers (20.2 MPa). However, when the prisms with Fe-SMA fibers are heated to 160 °C and 200 °C, they recover their pre-defined shape, which applies prestress to the surrounding concrete. This prestress increases the load-bearing capacity of the prism, leading to higher flexural strength in the Fe-SMA specimens (26.65 MPa and 24.39 MPa, respectively) compared to their steel–fiber counterparts (19.46 MPa and 16.67 MPa, respectively). Based on these findings, a numerical model was developed to simulate the behavior of concrete composites reinforced with randomly dispersed Fe-SMA fibers. An algorithm was created to define the random distribution of fibers, and a novel modeling approach accounted for the end-hooked geometry by assigning different contact properties to the modeled straight fiber ends and middle sections. A mesh sensitivity analysis was performed to determine the optimal mesh size, and the model was validated by comparing numerical results with experimental data. In summary, the key finding of this work is that thermally activated Fe-SMA fibers can effectively prestress concrete and enhance its flexural strength beyond that achievable with conventional steel fibers at ambient conditions.