A geological assessment of selected South African concrete aggregates susceptible to alkali-silica reaction

Abstract

Concrete deterioration is one of the biggest concerns in civil and environmental engineering, as it affects the service life of concrete structures. While the sources of concrete deterioration vary, this study focuses on alkali-silica reaction (ASR). ASR is a chemical reaction that can lead to cracking and loss of material integrity. Given the nature of concrete manufacturing and the mechanism of ASR, studying rocks as aggregate is critical for understanding aggregate behaviour in concrete. In South Africa, many concrete structures have been reported to be affected by ASR; however, the reasons behind these effects are not well understood. Additionally, there is a scarcity of geological assessments of South African concrete aggregates concerning ASR. Therefore, the potential impact of petrographic studies on our understanding of ASR in South Africa has led to this research investigation. This study focuses on seven selected aggregate samples used in South African concrete. The materials were sampled from known quarries around South Africa due to suspected or reported ASR (to varying degrees) in the use of the aggregates. A geological assessment of these aggregates and their constituent minerals at a microscopic level was performed using optical microscopy, X-ray diffraction (XRD), and Tescan Integrated Mineral Analyzer (TIMA). The results of these assessments were compared with the expansion behaviours exhibited by the aggregates during the Accelerated Mortar Bar Test (SANS 6245). The findings reveal that rocks containing high silica concentrations, an abundance of microcrystalline and strained polycrystalline quartz, the presence of microcracking, and intense undulatory extinction of quartz crystals were more expansive. Specifically, the Witwatersrand quartzite and metashale, the Dwyka tillite, and the Malmesbury feldspathic metawacke exhibited expansions greater than 0.1% on day fourteen, indicating potential alkali reactivity. In contrast, the Natal arkosic sandstone and Cape Granite Suite monzogranite, which showed expansions less than 0.1% on day fourteen, were defined by non-reactive silica and medium-grained quartz crystals with minimal microcracking and no deformation features. Although undulatory extinction is globally recognised as an important indicator of ASR susceptibility, this study suggests it cannot be solely relied upon. Instead, a combination of high crystalline lattice distortion, high reactive silica concentrations, the presence of microcracking, and increased surface area of quartz grains is necessary for the occurrence of ASR. The study emphasises the importance of careful selection of concrete aggregates to avoid using potentially alkali-reactive rocks. These results provide valuable insights for engineers and geologists on suitable aggregates for concrete production. If the use of potentially alkali-reactive rocks is unavoidable, mitigation strategies such as using supplementary cementitious materials, a less alkali-rich cement, and a low water/cement ratio should be implemented. In conclusion, integrating geological techniques such as optical microscopy, XRD, and TIMA is essential for a comprehensive understanding of aggregate behaviour in concrete and accurate ASR susceptibility and expansion prognosis. This research contributes to the limited geological assessments of South African concrete aggregates and the approaches employed make it a first-of-its-kind investigation regarding their susceptibility to ASR.