Introduction to alkali-aggregate reaction in concrete

Abstract

This chapter introduces and defines the nature of alkali-aggregate reactions in concretes. Alkali-silica reactivity in concrete is a particular variety of chemical reaction within the fabric of a concrete involving alkali hydroxides, usually derived from the alkalis present in the cement used, and reactive forms of silica present within aggregate particles. This chemical reaction also requires water for it to produce the alkali-silica gel reaction product which swells with the absorption of moisture. The amount of gel and the swelling pressures exerted are very variable depending on reaction temperature, type and proportions of reacting materials, gel composition, and other factors, but they are often sufficiently high to induce the development and propagation of microfractures in the concrete which, in turn, lead to expansion and disruption of the affected concrete structure or element. Typical deleterious features of alkali-silica reaction in concrete structures include cracking, expansion and consequent misalignment of structural elements, spalling of fragments of surface concrete as ‘pop-outs’, and the presence of gel in fractures or associated with aggregate particles within the concrete. The reaction typically takes between 5 and 12 years to develop, though there are many exceptions, and it is most severe where alkali concentrations in the concrete pore fluids are high. A very wide variety of aggregate rock types in structures from many parts of the world have been reported as being alkali-silica reactive. This is consequent on the reactive forms of silica often only forming a minor mineral component of the aggregate such as the cement between mineral grains. This silicious material must be amorphous or cryptocrystalline with a large surface area if it is to react sufficiently to produce deleterious effects in the concrete.