An earlier survey of concrete road pavements in Great Britain identified the Stokenchurch section of the M40, Oxfordshire, as one of the sites where alkali-silica reaction had occurred. Therfore, a more detailed systematic coring programme of one of the slabs was carried out in order to identify the degree and location of the reaction. It was found that the alkali-silica reaction was associated with the joints; these were poorly sealed and the concrete in the vicinity was already heavily cracked due to other physical factors. This allowed water, laden with de-icing salt, to penetrate into the interior of the concrete. This initiated the reaction in the concrete, which under normal conditions and alkali levels would have been regarded as stable. It is probable that without this ingress of water and salt into the interior of the concrete, no reaction would have occurred. The reactive aggregate was identified as the highly porous white flint in the Thames Valley river gravel, other flints and silica rocks in the gravel were considered to be unreactive. Advice is given for avoiding the problem of alkali-silica reaction in present and future concrete roads: it includes ensuring that joints are properly constructed, sealed and maintained. (A)

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