Rapid heat-activated post-tensioning of damaged reinforced concrete girders with unbonded near-surface mounted (NSM) NiTiNb shape-memory alloy wires

Abstract

Concrete girders can suffer from serviceability issues due to excessive cracking and deflection. In response to this problem, a novel heat-induced post-tensioning technique utilizing unbonded near-surface mounted nickel–titanium–niobium (NiTiNb) shape-memory alloy (SMA) wires was proposed and evaluated. SMAs are a class of smart materials that can recover seemingly permanent plastic deformation when heated. The post-tensioning forces, thus, can be generated by restrained heat-induced shape recovery of SMA wires. Material characterization tests showed that 3.92-mm diameter NiTiNb wires with 2.5% prestrain can generate recovery stress of approximately 500 MPa when actuated via Ohmic heating in a restrained condition. The proposed post-tensioning system was experimentally evaluated in reinforced concrete girders measuring 2.3 m in length and 23 × 41 cm in cross-sectional dimensions. The girders were initially cracked to simulate typical girder damage. NiTiNb wires were then installed in the bottom cover of the girders and anchored at both ends. Subsequently, the wires were actuated via Ohmic heating, which triggered shape-recovery and generated post-tensioning stresses in the girder. The post-tensioning technique reduced the crack widths by up to 74% (370 μm) and recovered the residual midspan deflection by up to 49% (1.52 mm) in the cracked girders. Following post-tensioning, flexural loading up to failure showed that the cracked stiffness and ultimate moment capacity of the girders had increased by up to 31% and 45%, respectively, with a relatively small NiTiNb reinforcement ratio of up to 0.17%.

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Acknowledgements

The authors gratefully acknowledge financial support from the Louisiana Transportation Research Center (Award No. 18-2TIRE) and the University of Delaware internal funds.

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Jovan Tatar and Nikola Tatar contributed to the study conception and design. Specimen preparation, data collection and analysis were performed by Arkabrata Sinha and Jovan Tatar. The first draft of the manuscript was written by Arkabrata Sinha and Jovan Tatar, and all authors commented and edited previous versions of the manuscript. All authors read and approved the final manuscript.

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Correspondence to Jovan Tatar.

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Sinha, A., Tatar, N. & Tatar, J. Rapid heat-activated post-tensioning of damaged reinforced concrete girders with unbonded near-surface mounted (NSM) NiTiNb shape-memory alloy wires. Mater Struct 53, 88 (2020). https://doi.org/10.1617/s11527-020-01522-8

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Keywords

  • Shape-memory alloys
  • NiTiNb
  • Bridge girder
  • Post-tensioning
  • Concrete
  • Repair