Innovative reinforcement techniques for 3D-printed concrete: The impact of shape memory alloys on flexural strength and crack mitigation

The flexural performance (FP) of 3D Printed Concrete (3DPC) remains a challenge due to its limitations in tensile strength and resistance to cracking. To address these issues, innovative reinforcement materials, such as Shape Memory Alloys (SMAs), which can return to their original shape upon heating or stress removal, thus closing cracks, have been explored recently. The integration of SMAs into conventional concrete has been well-documented, yet their potential in 3D-printed concrete for enhancing flexural strength and enabling crack recovery is still an emerging area of research. This study investigates the crack mitigation and FP of 3DPC beams under various reinforcement conditions. The flexural strength (f b ) was evaluated via three-point bending tests (ASTM C78), while crack mitigation was quantified using Ultrasonic Pulse Velocity (UPV, ASTM C597). The recovery mechanisms were driven by SMA superelasticity (stress-induced strain recovery) and shape memory effect (heat-activated crack closure). Initially, the behavior of unreinforced 3DPC beams was assessed, followed by the addition of fiber reinforcement (FR), and finally, the integration of SMAs. By varying the SMA's content from 0.25 % to 0.5 % by volume, the optimal dosage for enhancing the f b value, displacement recovery, and crack closure was determined. An addition of 0.5 % of SMAs showed a drastic increase in the f b value from 3.61 MPa to 8.67 MPa, which is nearly a 140 % rise. Flexural displacement recovery also improved significantly, attributed to the super elasticity and shape memory effect of SMAs, with FR plus 0.5 % SMAs beams showing a reduction in displacement from 1.97 mm to 1.27 mm, representing a 35.53 % recovery. In comparison, FR with 0.25 % SMAs achieved a 21.66 % recovery. Additionally, SMAs substantially reduced crack width and depth. FR plus 0.5 % SMAs specimens showed a 35.3 % reduction in crack width and a 31.42 % reduction in crack depth, while FR with 0.25 % SMAs showed reductions of 20.80 % in crack width and 12.48 % in crack depth. The outcome of the present study highlights the potential of SMAs to improve the load-bearing capacity, flexural displacement recovery, and crack closure of 3DPC beams, and highlighting their potential as a valuable option for future construction methods.