The main purpose of the work described in this report was to provide information on the alkali-silica reactivity of UK sands and gravels. The study aimed to identify the reactive constituents present and to relate reactivity to certain proportions of reactive constituent within an aggregate. The alkali-silica reactivity of British volcanic rock aggregate was similarly investigated. Alkali-silica reaction can occur in concrete when alkalis normally present in the cement react with certain forms of silica in the aggregate to form an alkali silica gel. The gel rapidly takes up calcium from the cement to produce a calcium alkali silicate gel which imbibes water and expands. This exerts a pressure which can damage particles of aggregate and ultimately crack the concrete itself. The rate of attack of the sodium or potassium hydroxide solution on the reactive silica depends on the hydroxide concentration in the pore solution and it is only in pore solutions of high hydroxyl ion concentration that significant attack on reactive silica occurs. Sands and gravels are the main source of reactive minerals causing damage to bridge structures in the UK. This report concludes a major study with this focus. Sands and gravels from river, beach, marine, glacial and geologically older deposits were examined petrographically and by accelerated expansion tests. The geographical distribution of potentially reactive deposits was mapped. Material from some 45% of locations in England were potentially reactive whereas materials from Scotland and Wales were non-reactive. Chert was a reactive constituent in all reactive mixes, either as the principal reactive component or less frequently in association with other reactive constituents. This is in line with experience from bridge structures where ASR has occurred. The work described in this report was carried out under contract by the Building Research Establishment for the Bridges Division of the Transport and Road Research Laboratory (TRRL).

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