Steel plates were centrally embedded in concrete prisms made from a potentially reactive siliceous aggregate but with an alkali content just below the threshold level required to induce significant expansion due to alkali-silica reaction (ASR). The aggregate contained 10% of calcined flint (a proportion close to the 'pessimum value' for that material) and the alkali content of the concrete was 2.5kg/cu m (expressed in terms of Na2O equivalent). The steel plates were subjected to cathodic protection applied either potentiostatically (steel potential held at -700mV versus saturated calomel electrode) or galvanostatically (cathodic current density maintained at 20mA/sq m) for periods extending to approximately three and a half years. Strain measurements showed that significant expansion and cracking of the concrete around the steel cathodes occurred in four out of the six prisms in which potentiostatic polarisation (-700mV, SCE scale) was applied. These effects were confined to the material in the vicinity of the cathodes, with ASR products and an accumulation of alkali metal ions being found by petrography and chemical analysis in a region extending to about 15mm from the steel plates. Very little expansion and ASR were detected for the prisms in which constant current cathodic polarisation (20mA/sq m) was applied, even after times when the total charge passed was equivalent to that for the potentiostatically controlled specimens. The inference of this is that, for the type of concrete studies, the fact that constant potential polarisation necessitated the application of high cathodic current densities (>100mA/sq m) during the early stages had a considerable effect in promoting alkali enhancement and ASR around the steel. The results of the present investigations, which constitute the second stage of a two-part programme of research, are related to the earlier findings reported in TRL Contractor Report No 310 (see IRRD 847792). Practical implications for the application of cathodic protection and related electrochemical rehabilitation techniques to concrete structures containing ASR-susceptible aggregates are discussed. (A)

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