Case Study: Mitigation of Alkali Silica Reaction in a Signature Stay-Cable Bridge

Also known as the Great River Bridge, the US-34 Burlington Bridge is a signature stay-cable structure spanning over the Mississippi River between Gulf Port, Illinois, and Burlington, Iowa. Opened to traffic in 1993, the main river crossing span is supported by stay cables connected to two 305-foot tall reinforced concrete pylons. In 1992, prior to the bridge being opened to traffic, vertical hairline cracks were observed on the interior and exterior faces of the concrete pylons. The width of the cracks continued to increase slowly and consistently over time. The size and location of the cracks suggested that they may be structural in nature. Extensive investigations performed between 1999 and 2004 could not completely identify the cause or significance of the cracking. A more recent investigation was performed after the crack widths had grown to 1/4 inch wide. This follow-up study utilized instrumentation, material sampling, laboratory testing, and computer analyses, which determined the cause of the crack growth to be due to an alkali-silica reaction (ASR) in the concrete pylons. This case study will discuss the results of the investigation and why the cause of the crack was so difficult to identify. In addition, potential approaches to mitigate ASR deterioration in existing concrete structures will be presented.