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 fly ash to the expansion of conc rete containing natural reactive UK aggregates. Concrete prism expansi on tests were carried out using flint/chert sand (from three different sources), a crushed siltstone and a crushed siliceous limestone; resu lts are also reported from a greywacke aggregate. Fly ash from three c ommercial sources was used. The results demonstrate that the 'effectiv e' alkali contribution from the fly ash, estimated from expansion resu lts, varies depending on the level of replacement and nature of the re active aggregate. With moderately reactive aggregates, such a flint, 2 5% fly ash was found to be effective in preventing cracking, regardles s of the OPC alkali content. However, with more reactive aggregates (i .e. aggregates that react at lower alkali levels), fly ash concrete ex panded at lower OPC alkali contents than control specimens, indicating an 'effective' alkali contribution from the fly ash. Higher levels of fly ash are required to prevent cracking with these aggregates. Effec tive alkali 'contributions' (from fly ash) determined for a particular aggregate are not applicable to concrete containing other reactive ag gregates, and specifications need to be cognizant of the need for high er ash replacement levels with more reactive aggregates.