The Effect of Stone Content on Concrete Expansion Due to ASR

During an investigation of the potential of quartz rich aggregate to cause alkali-silica reaction, a number of concrete mixtures were made with different stone contents. One set was made at the proportion of 70% coarse aggregate defined in the ASTM C1293 version of the test. A second set was made with 52% coarse aggregate which was the concrete intended to be used on the job site. Other than the stone content the mixture proportions and subsequent storage regimes were identical. The concrete made with 70% coarse aggregate did not expand significantly and showed no sign of deleterious alkali-silica reaction. The concrete made with 52% coarse aggregate gave an average expansion of 0.083% at one year and 0.13% at two years, exceeding the usual limit of a maximum of 0.040% at one year. The 70% coarse aggregate mixture gave an average expansion of 0.038% expansion at two years. Petrographic examination showed alkali-silica gel and associated cracking in the deleteriously expansive prisms made with 52% coarse aggregate. Gel and cracks were not found in the prisms made with 70% coarse aggregate.

This observation appears to be associated with a pessimum effect. This indicates that concrete prism testing is best done at stone contents likely to be those used in actual practice and that testing at unusual aggregate proportions may be misleading.