Résumé
Previous studies on concretes based on reactive chert aggregates recognised that for a fixed amount of alkali, there is a reactive silica content that induces maximal expansion due to the alkali-silica reaction (ASR). Beyond this critical point called the ``pessimum'', the ASR expansion decreases. The various theories proposed to explain this phenomenon are not all in agreement and do not explore sufficiently the possibility of using the pessimum effect for the implementation of concretes with low ASR expansion. The objective of the present work was to consider the reduction of ASR expansion according to a pessimum effect and its relationship with the evolution of local alkali contents in silica grain. An accelerated expansion test was performed at 60 degrees C and 100% relative humidity (RH) on the concrete mixture based on reactive chert aggregates. The relative longitudinal variations of the prisms (DL/L) were measured over 24 weeks and microstructural studies were conducted on the samples tested. The results showed that the increase in reactive aggregate proportion allowed the reduction of the alkali-silica reaction (ASR) expansion. This expansion reduction, attributed to a pessimum effect, could be used to prevent risk of ASR expansion in the concretes made with potentially reactive aggregates. The phenomenon of reduction of the expansion was explained in this paper by the capacity of reactive siliceous phases in the aggregates to fix alkali ions up to a certain threshold without being degraded. This neutralising capacity was identified thanks to EDS analyses by monitoring the increasing of the height of alkali peaks according to the microstructural degradation. Indeed, at the microstructural scale, two area types' were identified to be major in the less expansive mixture: 1/undegraded areas which predominate, showed no detection of sodium, potassium and calcium as it appeared for the raw materials (degree 0). 2/areas in which signs of low degradation appeared were characterised by a slight increasing of alkali signals (degree 1). In contrast, in expansive mixture the alkali signal intensities were higher for the most degraded zones (degrees 2 and 3). The neutralisation process, induced by mixing concrete with both coarse and fine chert aggregates, is so dominant that the ``local'' alkali threshold causing the reaction is rarely exceeded, and expansions are below the standard limit. In terms of application, knowledge of the alkali threshold may help to verify the effectiveness of fixing harmful alkalis in the granular skeleton and could allow optimising the future concrete mixtures.