Rapid heat-activated post-tensioning of damaged reinforced concrete girders with unbonded near-surface mounted (NSM) NiT
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ORIGINAL ARTICLE
Rapid heat-activated post-tensioning of damaged reinforced concrete girders with unbonded near-surface mounted (NSM) NiTiNb shape-memory alloy wires Arkabrata Sinha . Nikola Tatar . Jovan Tatar
Received: 4 December 2019 / Accepted: 1 July 2020 RILEM 2020
Abstract Concrete girders can suffer from serviceability issues due to excessive cracking and deflection. In response to this problem, a novel heat-induced posttensioning technique utilizing unbonded near-surface mounted nickel–titanium–niobium (NiTiNb) shapememory 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 9 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. A. Sinha J. Tatar (&) Department of Civil and Environmental Engineering, University of Delaware, Newark, DE, USA e-mail: [email protected] A. Sinha e-mail: [email protected] N. Tatar Tractebel Engineering, Brussels, Belgium e-mail: [email protected]
Subsequently, the wires were actuated via Ohmic heating, which triggered shape-recovery and generated post-tensioning stresses in the girder. The posttensioning technique reduced the crack widths by up to 74% (370 lm) 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%. Keywords Shape-memory alloys NiTiNb Bridge girder Post-tensioning Concrete Repair
1 Introduction Structural components, carrying flexural stresses, can suffer from serviceability issues due to excessive cracking and deflection. Traditional post-tensioning techniques used to repair such damage utilizing steel [1] and fiber-reinforced polymer (FRP) composite tendons [2] require heavy hydraulic jacking equipment and intricate anchorage which results in long construction times with implications to structure’s functionality. To overcome the shortcomings of the traditional post-tensioning techniques, a novel posttensioning technique using unbonded near-surface
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mounted (NSM) nickel–titanium–niobium (NiTiNb) shape-memory alloy (SMA) wires is proposed and evaluated in this study. SMAs are a class of alloys that can recove
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