Estimating the alkali contribution from fly ash to expansion due to alkali—aggregate reaction in concrete

This paper reports the findings from an investigation to determine the ‘effective’ alkali contribution from jly ash to the expansion of concrete containing natural reactive UK aggregates. Concrete prism expansion tests were carried out using flint/chert sand (from three different sources), a crushed siltstone and a crushed siliceous limestone; results are also reported for a gre-vwacke aggregate. Fly ash from three commercial sources was used. The results demonstrate that the ‘efective’ alkali contribution from the fly ash, estimated from expansion results, varies depending on the level of replacement and nature of the reactive aggregate. With moderately reactive aggregates, such as flint, 25% fly ash was found to be effective in preventing cracking, regardless of the OPC alkali content. However, with more reactive aggregates (i.e. aggregates that react at lower alkali levels), Fly ash concrete expanded at lower OPC alkali contents than control specimens, indicating an ‘efective’ alkali contribution from the fly ash. Higher levels of fly ash are required to prevent cracking with these aggregates. Effective alkali ‘contributions’ (from fly ash) determined for a particular aggregate are not applicable to concrete containing other reactive aggregates, and specifications need to be cognizant of the need for higher ash replacement levels with more reactive aggregates.