No abstract

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.

Greywacke as a rock group has been implicated in promoting alkali–aggregate reaction (AAR) in a number of structures throughout the world. The Maentwrog Dam is believed to be the first major case recorded in the U K. Petrographic examination of concrete cores taken from the dam showed that the structure is affected by AAR, notwithstanding any other contributory causes of distress. SEM examination revealed three main AAR gel types having morphological, situational and chemical characteristics consistent with those in flint aggregate concrete promoting the alkali–silica reaction. X-ray diffraction studies showed that matrix phyllosilicates did not exhibit exfoliations diagnostic of the controversial alkali–silicate mechanism, corroborating the petrographic examination evidence that an alkali–silica mechanism is the most likely explanation for distress. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies showed that the microcrystalline quartz, acting as a matrix cement in the rock, has local dislocation densities of 1014 m−2, and this is believed to be the reactive constituent. Similarities between the Maentwrog greywacke and a Canadian greywacke, which has promoted AAR in the Malay Falls Dam (Nova Scotia), are shown. This confirms that greywackes are vulnerable to the alkali–silica reaction rather than alkali–silicate reactions.