Alkali-aggregate research in New Brunswick

Abstract

New Brunswick is located within the northeast-southwest trending Appalachian Region. The basement rocks consist largely of metamorphosed sedimentary types with some granitic intrusions and the composition of the natural gravels reflects the bedrock types. Research into alkali-aggregate reaction (AAR) problems in the province was sponsored initially solely by CANMET and more recently in association with the Department of Transportation. The research consisted of (i) petrographic studies of aggregates, (ii) petrographic studies and case histories of existing concrete structures, and (iii) laboratory expansion testing of concrete and mortar bar specimens. Alkali-aggregate reaction has been found to be one of the factors responsible for premature concrete deterioration in New Brunswick. Visual signs of Alkali-aggregate reaction require up to 10 years to appear and the reaction takes up to 30 years to fully develop. Principal reactive rock types are greywacke, argillites, and fine-grained volcanic rocks. The reactive component is thought to be fine-grained quartz less than 100 µm in size. Laboratory expansion test results on concrete specimens are sensitive to alkali levels. Concrete prism tests (CSA A23.2-14A) with 5.5-5.4 kg/m3 Na2O equivalent have been used to predict aggregate performance. By comparison the water soluble alkali contents of field concretes constructed since about 1970 have been found to range from 3.5 to 5 kg/m3 Na2O equivalent. Accelerated mortar bar expansion test results (CSA A23.2-25A) do not correlate well with concrete prism expansion tests and indicate a much higher percentage of deleterious aggregates. Use of supplementary cementing materials such as fly ash and silica fume have been found to be effective in long duration laboratory tests to control aggregate reactions.Key words: concrete, aggregates, alkali-aggregate reaction, standard and accelerated testing for AAR, fly ash, silica fume.