Effectiveness of Silane in Mitigating Alkali-Silica Reaction (ASR) in the Bibb Graves Bridge

Project Details









Darren K Johnson, Robert L Warnock




Alkali silica reactions

Project description

Alkali-silica reaction (ASR) is a detrimental reaction in concrete that can cause severe expansion and cracking in structures. The Bibb Graves Bridge is a reinforced concrete bridge that was constructed in 1931, and is located in Wetumpka, Alabama. Both arches of span 5 have severe cracking and surface deposits caused by ASR. A silane-based, ASR mitigation was applied to spans 4 and 5 of the Bibb Graves Bridge during October and November of 2010. The goal of this mitigation procedure was to lower the internal relative humidity of the ASR-affected concrete to below 80 percent so that continued ASR-related expansions do not occur. After the application of the mitigation procedure, monitoring of the internal relative humidity, concrete expansion, and new crack development in the bridge was performed for 35 months to evaluate the effectiveness of the mitigation procedure. Analysis of the 35 months of data revealed that there were few signs of decreasing relative humidity or slowed expansion rates in the ASR-affected concrete. Analysis of the in-situ concrete strain data indicates that the ASR expansion in span 5 is continuing at the same pace as it was before mitigation. This was shown by the onset of new cracking and strong, linear trends with high expansion rates in a majority of the span. Some of the highest expansion rates in the two arches of span 5 range from 344 to 546 microstrains per year. It is concluded that the silane sealer was ineffective, and alternative mitigation options should be considered. It is recommended to continue in-situ monitoring to quantify the effects of ASR in this bridge.