ABSTRACT
Alkali reactivity of aggregates in concrete is basically a chemical reaction. In alkali silica reaction (ASR), the alkalies of primarily Na and K react with the siliceous aggregate to
produce a silica gel (Hobbs, 1988). The gel fills available pores. Two main theories have been proposed to explain the expansion of the concrete: the pressure of the gel as it fills the pores (Vivian, 1950), and pressure due to osmotic difference between the water in the gel and outside of the gel (Hobbs, 1979). In the latter, the gel imbibes water, swells, and creates expansive stress on the pore walls. The concrete expands, and eventually fails in tension by microcracking. The larger the surface area of the reactive silica or silicate and greater its surface activity, the greater the production of the gel and expansion (assuming sufficient alkalies).
