Shape Memory Alloy Prestressing of Fiber-Reinforced and Ultra-High-Performance Concrete for Bridge Truss Members

This research focused on addressing cracks in concrete trusses in bridge structures using shape memory alloys (SMAs). Shape memory alloys recover from inelastic deformation when exposed to elevated temperatures. This was studied for utilization in two ways. By embedding SMA bars inside concrete and subsequently heating, the concrete is prestressed, and cracks are delayed. Alternatively, heating the SMA after cracking occurred resulted in crack closure. Thirteen small-scale flexural specimens were fabricated from mortar, fiber-reinforced concrete (FRC), and ultra-high-performance concrete (UHPC) to verify these two concepts. The healing approach reduced crack widths by as much as 80%, 90%, and 84% in the mortar, FRC, and UHPC specimens, respectively. This study then applied the SMAs in a larger test specimen. A main advantage of SMA prestressing over conventional prestressing is the elimination of end anchorage equipment. This allows more freedom in placement and makes it appealing for use in concrete truss systems. A large-scale precast truss was fabricated to demonstrate the concept. Voids were introduced into the web of a precast girder to alter its structural behavior to that of a Howe truss. SMA bars were embedded in a vertical member where large tensile forces are expected during loading. The SMA was activated, and an average compressive strain of 301 με was measured. The load corresponding to cracks in the prestressed member was 600% higher than that of the member with only conventional steel.