Synergetic Influence of Microcrystalline Quartz and Alkali Content in Aggregate on Deterioration of Concrete Railroad Ties Used for 15 Years in High-Speed Railways

This study investigated the deteriorations of precast prestressed concrete (PSC) ties that were used for 15 years in high-speed railways in Korea and its damaging mechanism. The collected PSC ties with longitudinal cracks on sides and map cracks on surfaces exhibited strength degradation. The deteriorations were likely related to alkali-silica reaction (ASR) and delayed ettringite formation (DEF) together, given that the presence of massive ettringite crystals and the decomposition of ASR gel were found from microstructural analyses. Although there were no typical reactive siliceous aggregates for ASR in this study, ASR cracks were generated in the PSC ties. This is because the aggregates in the PSC ties with cracks were potentially reactive, and its high alkali-silica reactivity was likely attributable to the presence of microcrystalline quartz, supplying reactive SiO₂ to trigger ASR. Furthermore, the alkali content in aggregates was associated with the deterioration of the PSC ties. The alkali-bearing minerals in aggregates (i.e., alkali feldspars) likely supplied enough alkalis for ASR. Besides, micas in aggregates could promote ASR due to their porous structure, which helps easy water ingress.