Practical Research Work on Diagnosis and Strengthening for ASR Deteriorated Bridge Piers with Fracture of Steel Reinforcement

本研究取得的研究成果归纳如下：（1）对受ASR影响的结构中的钢筋断裂的调查研究工作表明，在石川县的桥梁中，由于ASR引起的混凝土过度膨胀，在桥墩和桥台的枕梁或基础中发生了钢筋弯曲角处的脆化断裂，导致混凝土元件的结构完整性的显著损失。（2）ASR钢筋的断裂机理在桥墩的枕梁或基础中，大量的预应力钢筋在弯角处断裂，而钢筋没有发生伸长。钢筋断口平整光滑，为典型的脆性断裂。讨论了钢筋断裂是否与应力腐蚀开裂和/或氢脆断裂有关。（3）ASR恶化桥梁的资产管理对于位于被认为是日本ASR最严重的石川县的所有桥墩，通过目视检查裂缝来调查由于ASR导致的桥墩恶化程度。针对各桥墩的ASR劣化程度分级，提出了相应的维修加固方法和维护程序。（4）ASR破坏桥墩的加固与监测以石川县鹿岛大桥枕梁和德田2号大桥基础为例，对ASR破坏桥墩的程度进行了无损检测。在对各构件ASR劣化程度进行分级的基础上，采用了最合适的加固方法。在这些桥梁的维修中，裂缝传感器的监测得到了成功的应用，以确保这些桥梁的日常安全。

Research results obtained in this study are summarized as follows ;(1)Investigation on fracture of steel bars in ASR-affected structuresResearch works revealed that the embrittle fracture of steel bars at the bending corner of stirrup or bent-up steel bars, due to excessive expansion of concrete due to ASR, occurred in the pillow beam or footing of piers and the abutment in bridges located in Ishikawa Prefecture, leading to the significant loss of structural integrity of concrete elements.(2)Mechanisms of fracture of steel bar due to ASRIn the pillow beam or foundation of bridge pier, a lot of stirrup steel bars fractured at the bending corners, without any elongation of steel bar. The fracture surface of steel bar seemed to be very flat and smooth, which indicated the typical embrittlement fracture of steel bar. Whether or not the fracture of steel bars may be associated with the stress corrosion cracking and/or the hydrogen embrittlement fracture was discussed.(3)Asset management for ASR-deteriorated bridgesFor all bridge piers located in Ishikawa prefecture, which is considered to be the most serious ASR case in Japan, the degree of deterioration of bridge pier due to ASR was investigated by the visual inspection of cracking. Corresponding to the classification of ASR deterioration level of each bridge pier, the repair/strengthening method and maintenance procedure were proposed.(4)Strengthening and monitoring of ASR-deteriorated bridge piersIn the case study of the pillow beam of Kashima Bridge and the foundation of Tokuda No.2 Bridge in Ishikawa Prefecture, the degree of deterioration of bridge pier due to ASR was also investigated by the non-destructive inspection. Based on the classification of ASR deterioration level of each element, the most appropriate strengthening methods were adopted for them. In the maintenance of these bridges, the monitoring using crack sensors was successfully applied in order to ensure the daily safety of these bridges.