The risk of alkali–silica reaction in concrete made with non-conforming cement

Abstract

A cement plant had supplied large quantities of cement into the market and the actual alkali contents were in excess of the manufacturer's declared mean. When the correct cement alkali levels were used to recalculate the amount of alkali, it was found that in some concretes the total alkali contents were above industry-recommended limits designed to minimise the risk of deleterious alkali–silica reaction (ASR). Given the large number of projects in which the concrete alkali content did not comply with the available guidance, it was necessary to develop a quick initial method of assessing the risk that concrete performance would be unacceptable. The unusual aspect of the investigations was that although the concretes did not conform to the recommended limits for minimising ASR risk, they had not shown any external symptoms or signs of damage. However, better information than is normal was available on the materials used, structural details and exposure of the concrete. The project-specific information was considered in the initial assessments using a severity ranking classification developed for the purpose. Subsequently, some of the projects were subject to more detailed physical investigation to establish more definitive conclusions. The investigations concluded that there was a general conservatism in the current guidance on minimising risk of ASR, but found greater degrees of uncertainty than expected in the base data used to underpin normal industry guidance. In addition, the level of record keeping on a number of sites was not sufficient to identify and locate concrete that was subsequently determined to be non-conformant. This work will help those investigating structures potentially affected by ASR and inform the future consideration of guidance for minimising the risk of ASR.