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%.

中文翻译：

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使用无粘结近表面安装 (NSM) NiTiNb 形状记忆合金线对受损钢筋混凝土梁进行快速热激活后张拉

由于过度的开裂和挠度，混凝土梁可能会遇到适用性问题。针对这一问题，提出并评估了一种利用未结合的近表面安装镍钛铌 (NiTiNb) 形状记忆合金 (SMA) 线材的新型热诱导后张技术。SMA 是一类智能材料，可在加热时恢复看似永久的塑性变形。因此，可以通过 SMA 线的受限制的热诱导形状恢复来产生后张紧力。材料特性测试表明，当在受限条件下通过欧姆加热驱动时，具有 2.5% 预应变的 3.92 毫米直径 NiTiNb 线可以产生大约 500 MPa 的恢复应力。建议的后张系统在钢筋混凝土梁中进行了实验评估，测量值为 2。长 3 m，横截面尺寸 23 × 41 cm。梁最初开裂以模拟典型的梁损坏。然后将 NiTiNb 线安装在大梁的底盖中并固定在两端。随后，通过欧姆加热驱动钢丝，这会触发形状恢复并在梁中产生后张应力。后张技术将裂纹宽度减少了多达 74% (370 μm)，并将裂纹梁中的残余跨中挠度恢复了多达 49% (1.52 mm)。在后张后，弯曲载荷直至破坏表明梁的开裂刚度和极限弯矩承载力分别增加了 31% 和 45%，而 NiTiNb 增强率相对较小，最高可达 0.17%。梁最初开裂以模拟典型的梁损坏。然后将 NiTiNb 线安装在大梁的底盖中并固定在两端。随后，通过欧姆加热驱动钢丝，这会触发形状恢复并在梁中产生后张应力。后张技术将裂纹宽度减少了多达 74% (370 μm)，并将裂纹梁中的残余跨中挠度恢复了多达 49% (1.52 mm)。在后张后，弯曲载荷直至破坏表明梁的开裂刚度和极限弯矩承载力分别增加了 31% 和 45%，而 NiTiNb 增强率相对较小，最高可达 0.17%。梁最初开裂以模拟典型的梁损坏。然后将 NiTiNb 线安装在大梁的底盖中并固定在两端。随后，通过欧姆加热驱动钢丝，这会触发形状恢复并在梁中产生后张应力。后张技术将裂纹宽度减少了多达 74% (370 μm)，并将裂纹梁中的残余跨中挠度恢复了多达 49% (1.52 mm)。在后张后，弯曲载荷直至破坏表明梁的开裂刚度和极限弯矩承载力分别增加了 31% 和 45%，而 NiTiNb 增强率相对较小，最高可达 0.17%。随后，通过欧姆加热驱动钢丝，这会触发形状恢复并在梁中产生后张应力。后张技术将裂纹宽度减少了多达 74% (370 μm)，并将裂纹梁中的残余跨中挠度恢复了多达 49% (1.52 mm)。后张后，弯曲载荷直至破坏表明梁的开裂刚度和极限弯矩承载力分别增加了 31% 和 45%，而 NiTiNb 增强率相对较小，最高可达 0.17%。随后，通过欧姆加热驱动钢丝，这会触发形状恢复并在梁中产生后张应力。后张技术将裂纹宽度减少了多达 74% (370 μm)，并将裂纹梁中的残余跨中挠度恢复了多达 49% (1.52 mm)。在后张后，弯曲载荷直至破坏表明梁的开裂刚度和极限弯矩承载力分别增加了 31% 和 45%，而 NiTiNb 增强率相对较小，最高可达 0.17%。52 毫米）在破裂的大梁中。在后张后，弯曲载荷直至破坏表明梁的开裂刚度和极限弯矩承载力分别增加了 31% 和 45%，而 NiTiNb 增强率相对较小，最高可达 0.17%。52 毫米）在破裂的大梁中。在后张后，弯曲载荷直至破坏表明梁的开裂刚度和极限弯矩承载力分别增加了 31% 和 45%，而 NiTiNb 增强率相对较小，最高可达 0.17%。