Intérêt de la minéralogie des granulats dans la connaissance des risques de l'alcali-réaction

Résumé.

Dans les ouvrages en béton apparaissent parfois des désordres sous forme d'un réseau de fissures multidirectionnelles dont le nombre et l'épaisseur s'accroissent au cours du temps. La cause en est une réaction chimique entre la silice des granulats partiellement dissoute et les alcalins du ciment contenus dans la solution interstitielle à pH très élevé, réaction génératrice d'un gel souvent expansif. Les irrégularités que présentent les structures du quartz, de la calcédoine et de l'opale favorisent l'introduction d'eau et d'ions divers qui joueront un rôle essentiel dans la mise en solution de la silice.La fragmentation tectonique de grands cristaux de quartz en une multitude de petits cristallites augmente les surfaces de contact quartz/solution interstitielle, donc les risques de réaction alcaline. La prévention des alcali-réactions passe par une connaissance des éléments minéralogiques et pétrographiques qui constituent les granulats. L'emploi du microscope polarisant et du compteur de points permet une bonne approche du pourcentage de minéraux réactifs; des tests de qualification (test dimensionnel, test cinétique...) conforteront le diagnostic. On distinguera alors trois classes de granulats, non réactifs (NR), potentiellement réactifs (PR) et potentiellement réactifs à effet de pessimum (PRP). Un exemple d'application est donné pour le bassin de la Garonne sur la relation entre l'évolution de la composition minéralogique des alluvions du fleuve en fonction de l'apport des affluents successifs et leur degré de réactivité.

Abstract.

Alkali aggregate reaction (AAR) is responsible for the degradation of a number of concrete structures. Its occurrence depends on the interstitial voids, the presence of soluble silica in the aggregate and the presence of water as a dissolving agent and medium for chemical exchange. AAR results from the dissolution of soluble silica (reactive silica) present in the aggregate, such as quartz and chalcedony, by highly alkaline interstitial fluids with a pH in the order of 13. The reaction leads to the formation of an expansive alkali silica gel. This creates a crack pattern in the concrete and, once started, the cracks propagate into other areas of the concrete mass, permitting further water percolation. To prevent such reaction, it is essential to limit the alkali content of the cement or ensure that the aggregate does not contain too much reactive silica.

It is important, therefore, to understand the significance of the silica within a concrete aggregate and the nature of the more reactive silica, such as quartz, chalcedony and opal. In quartz, it is the OH-ions that cause the dissolution of silica. Where the quartz has been subjected to stress, the large crystals are more prone to fracturing. Chalcedony has a similar structure to quartz but being composed of many smaller crystals, water and other ions are able to penetrate the mineral more easily. Although opal was in the past considered to be amorphous, it is now known to be at least partially crystallised as low-temperature cristobalite and tridymite. Again, the high percentage of small particles facilitates water percolation and thus reaction to any dissolved silica. Indeed, AAR is known to occur with the presence of only 4% opal. Volcanic glass or the glassy fraction from hyalopohitic lavas are always alkali and silica rich. Hydrolysis of such silicates (e.g. feldspars) may free the silica and alkali ions which then recombine in formations susceptible to AAR.

On the basis of the reactive silica content, aggregates are classified as: non-reactive (NR) – where the reactive silica content is insufficient to allow significant expansion in the concrete; potentially reactive (PR) – where there are a sufficient number of reactive particles in a sufficiently alkali-rich medium for expansion of the concrete to occur in the presence of water; and potentially reactive with a pessimum effect (PRP) – where an upper threshold value of flint, chalcedony or opal (the pessimum) is exceeded, expansion of the concrete does not occur. This process is not yet well understood and the aggregates in which it occurs represent a low proportion of the world's gravel deposits, although they may be of regional importance, e.g. the Thames valley in the UK, the Paris basin in France.

In order to determine sensibility to AAR, the aggregate should be assessed both petrographically and mineralogically . Mortar or concrete bar tests can be used to measure the expansion, which should be below the threshold value for an NR classification. However, such tests may take several months and it may then not be possible to distinguish between NR and PRP. "Autoclave" and "microbar" tests give a more rapid result as the expansion of the mortar bar is greatly enhanced by the use of temperature and an alkali-rich brine, but the results may be excessively pessimistic. The chemical kinetic test directly measures the soluble silica content of the sample, but the interpretation of the results is sometimes difficult.

The paper uses the case of the Garonne River basin to discuss the way in which a petrographic knowledge of the gravel deposits may assist in mapping the reactivity of the aggregates along the course of the river.