Bond–slip characteristics of SMA reinforcing fibers obtained by pull-out tests

• The thickness recovery tests of NiTi SMA fiber are conducted.
• The shape memory effect is used to make the SMA fibers for cement composites.
• The pull-out tests of the SMA fibers are performed.
• The bond stress–slip of the SMA fibers are assessed.

This study was carried out to develop a new concrete reinforcing fibers using the shape memory alloy wires. Conventional steel reinforcing fibers have hooked ends to provide anchoring and induce plastic deformation at the hooked ends to dissipate energy. However, the hooked shape produces negative effects during concrete mixing. For the shape memory alloy fibers, the shape memory effect of martensitic shape memory alloys can be exploited. An elongated shape memory alloy wire in a martensitic state recovers the deformed length upon heating. During this process, the diameter of the wire is also recovered because of Poisson’s effect. The diameter recovery of the shape memory alloy fibers can provide anchoring action for the shape memory alloy fibers in concrete since bulging of the shape memory alloy fibers inside concrete produces confining pressure around the fiber. In this study, martensitic NiTi wires with 1.0 mm diameter were prepared. The wires were elongated by cold drawing until a diameter of 0.93 mm was achieved; this process reduced the cross-sectional area by 13.5%. Two methods were applied to recover the diameter, namely, heating of the shape memory alloy fibers after they were embedded in concrete and recovery of the diameter of both ends of the fibers in air, where the fiber-shape resembles that of a dog-bone. Thus, the following four types of fibers were prepared: (I) as-built fiber of 1.0 mm diameter, (II) cold-drawn fiber of 0.93 mm diameter, (III) cold-drawn fiber in concrete with heating, and (IV) dog-bone-shaped cold-drawn fiber. The cold-drawn shape memory alloy fiber showed higher bond strength than the as-built fiber because of the difference in the stress–strain curve of the cold-drawn fiber. The shape memory alloy fiber in Case III showed increased bond strength compared with the fiber in Case I because of confining pressure around the fiber. The fibers in Case IV showed increased bond strength and energy dissipation capacity compared with the other types of the fibers and are, therefore, the most desirable type.

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