Prestressing Concrete with Shape Memory Alloy Fibers

Abstract

In partial fulfillment of the requirements for the degree of Abstract Prestressing Concrete with Shape Memory Alloy Fibers Skye Mikaella OrvisConcrete is considerably stronger in compression than it is in tension. When cracks form in concrete members, the flexural stiffness of the member decreases and the deflection increases which increases the overall size of the member. Prestressing concrete remedies this problem by inducing a compressive stress in the concrete thereby reducing the net tension in the member and increasing the load required to crack the member. Traditional prestressing is generally limited to large, straight members. During the last decade, shape memory alloys (SMA) have become more prevalent in engineering and civil engineering applications. The shape memory effect refers to the contraction of the SMA when it is heated to its austenite phase. When a prestrain is induced in the SMA, it can be recovered when it goes through the phase change. Nitinol, a NiTi shape memory alloy was used in this research. Thin, steel cables were also tested to provide a comparison. Two different Nitinol alloys were studied in this research. The alloy M wires were elongated to 8% stain while the alloy X wires were prestrained by the manufacturer. The wires were cast into thin concrete beams and once cured, the beams were heated and a phase change from martensite to austenite occurred in the Nitinol. As a result, the Nitinol contracted and compressed the concrete. The SMA fibers are randomly oriented and allow prestressing to occur along all three axis. This is ideal for thin, curved specimens. Third-point bending tests showed that the SMA fibers prestressed the concrete and upon reheating the cracked specimens, the shape memory effect provides partial crack closure.

On mechanical properties of NiTi SMA wires prestrained by cold rolling

“…This bulging in the cold rolled SMA fibers can increase confining pressure around the fiber which results in strengthening bond resistance against pulling out. To enhance compressive strength of a concrete column, the cold rolled SMA wires are employed as an external jacket resulting in delaying fracture as well as increasing displacement ductility [27][28][29]. The bent part of the SMA wire has been stretched due to the recovery stress (figure 1).…”

Smart Stressing of Concrete Beams with Nitinol Shape Memory Alloy Cables

“…The conducted experiments showed that using SMA fiber in prestressing mortar is feasible. Zhao et al [12] used shape memory alloy plates to strengthen and repair a concrete girder bridge as an alternative to FRP or steel strands which require bulky hydraulic jacking system to apply force. Hong et al [13] used shape memory alloy strip to solve the problem of low workability and reduced ductility of beams by strengthened beams.…”

Multidisciplinary Engineering Student Projects

“…During the process of creating the CNC Engraver, the EE students became interested in another aspect of the concrete canoe design, which was the use of Shape Memory Alloy (SMA) wire as a concrete reinforcement. SMA wire is a material that can be pre-strained, or stretched, with the strain subsequently recovered and the wire returned to its original shape when exposed to heat 2 . In concrete applications, the SMA wire is stretched and cast into the concrete.…”