Fracture of the reinforcing bars and deterioration mechanism due to Alkali Silica Reaction

通过对碱-集料反应的试验和分析，得出以下结论：(1)碱-硅酸反应(ASR)影响结构芯样的膨胀特性研究对21个外观严重劣化的ASR影响桥墩进行了分析。在21个桥墩中，有6个桥墩的核心扩展超过1000×106，其严重破坏超过了1m/m^2以上的累积裂缝密度。因此，1000×10~(-6)的芯层膨胀量可以作为ASR影响结构劣化管理和评估的参考值。(2)ASR对实际结构损伤的定量评估从受ASR影响的实际结构中得到了芯层开裂行为与抗压强度之间的关系。模拟碱集料反应引起的钢筋断裂试验采用模拟实际结构的1/8比例试件，试件内部含有膨胀混凝土，进行了模拟ASR膨胀的试验。钢筋的断裂具有很好的重现性，断裂面表现出与实际结构断裂面相似的脆性断裂。研究还发现，钢筋的肋骨形状对钢筋的裂纹扩展有显著影响。根据不同肋形的现筋(8φ)和旧筋(2φ)的对比，在有筋的情况下，混凝土膨胀引起的裂缝扩展不到钢筋直径的5.0%，而旧筋的裂缝扩展到直径的80%以上。

The following conclusions were drawn from the experiment and analysis focusing on the alkali aggregate reaction :(1) Research on the expansion characteristics of cores sampled from alkali silica reaction (ASR)-affected structuresAnalysis was conducted on 21ASR-affected piers having seriously-deteriorated appearances. Of the 21 piers, six had a core expansion of over 1000×10^6. They showed serious damage exceeding a cumulative crack density of 1m/m^2 or more. Therefore, the core expansion of 1000×10^6 can be used as the reference value for the management and evaluation of deterioration of ASR-affected structures.(2) Quantitative evaluation of ASR damage to actual structuresThe relationship between cracking behavior and compressive strength of cores was obtained from ASR-affected actual structures. It was found that the crack density increased up to 8.0 m/m2, but about 80% of the design compressive strength was retained.(3) Experiment simulating fracture of reinforcing bars due to alkali aggregate reactionsAn experiment simulating ASR expansion was conducted using 1/8 scale specimens modeling actual structures and containing expansion concrete within the specimens. Fracturing of reinforcing bars was well reproduced and the fractured planes showed brittle fracturing similar to that observed in the fractured planes of actual structures. It was also found that the rib shape of reinforcing bars has a significant effect on the crack propagation of those bars. According to the comparison of present bars (8φ) and the former bars (2φ) having different rib shape, cracks caused by concrete expansion propagated for less than 5.0% of the bar diameter in the case of present-reinforcing bars, but that of former reinforcing bars was as large as 80%.