Estimating the Potential Residual Expansion Due to Alkali Silica Reaction (ASR) in In-Service Reinforced Concrete Structures

Abstract

This thesis investigated the extent of deterioration caused by alkali-silica reaction (ASR) in a 78-year-old reinforced concrete (RC) railway bridge in Johannesburg. Five assessments were conducted to characterise and quantify the extent of ASR damage. These included 2D potential residual expansion measurements at a micro-scale level using digital image correlation (DIC) and manual strain readings, focusing on localised expansion at the aggregate and cement paste interface. The study also explored correlations between micro-scale ASR strain and macro scale deformations in the concrete. The results showed that an on-site visual assessment identified varying degrees of ASR damage, leading to the extraction of concrete cores from different locations on the bridge. A modified damage rating approach, using a grid of 2 cm squares and the naked eye, was used to assess the extent of ASR damage based on 2D cylindrical images of each core's surface. Petrography and micro-XRF spectrometry confirmed that ASR was the primary cause of deterioration, characterised by cracks radiating from reactive aggregates and ASR gel presence. With respect to alkali reserves, concrete from the slightly damaged interior region had the highest sodium oxide equivalent content, 3.2 kg/m³. This value falls above the 2.0 kg/m3 limit for Witwatersrand aggregates, thereby validating the susceptibility of the concrete to ASR deterioration. The elastic modulus decreased with increased ASR damage, aligning with existing literature. Compared to slightly damaged concrete 18.0 GPa, moderately and severely damaged concrete had a 22.2% and 31.1% reduction in elastic modulus, respectively. Due to technical challenges, efforts to employ DIC for micro-scale residual expansion strains were unsuccessful. However, manual strain readings indicated that local equivalent normal strains were generally higher, and shear strains were generally lower than their respective bulk strains. The study found that the potential residual expansion of concrete displayed similar magnitudes of bulk expansion despite varying surface damage and alkali availability. Surprisingly, the potential residual micro-expansion displayed a similar trend. The five assessments conducted in this study provided a general basis for quantifying the extent of ASR deterioration in a 78-year-old RC railway bridge. This approach can be used as a basis for the condition assessment of existing structures subject to ASR deterioration. Engineers can use the results to model and understand how ASR-affected concrete impacts the structure's behaviour and its remaining service life. The findings, in particular of the residual potential expansion, emphasised the crucial role of moisture in facilitating future expansion and damage in the concrete due to ASR. Furthermore, owners of RC structures can obtain information about how long their RC structures, subject to ASR deterioration, may last before any repair is envisaged.