The mechanisms governing ASR are quite complex. ASR kinetics depends on three variables: (1) sufficient pH and alkalies from cement; (2) reactive silica from aggregates and (3) sufficient moisture/water. Each of these criteria must be met for the reaction to occur . The alkali-silica reaction takes place between silica and hydroxides. Alkalies (namely K+ and Na+) coming from cement ultimately reside in the concrete pore solution. Cement reacts with mixing water and thus concrete pore solution are associated with hydroxyl ion (OH-) which produce high pH in the pore solution. Reactive silica in aggregate tends to breakdown due to the high alkalinity of pore solution in concrete and subsequently react with alkali-hydroxides to form ASR gel. Not all aggregates containing silica are susceptible to react with hydroxides. There may be two forms of silica in natural aggregates: crystalline or non-crystalline. In a crystalline silicate structure like quartz (SiO2) there are no complete tetrahedron formed on the surface and structural impermeability of such structures allow it to react only on the surface, and thus, they are less reactive. Unlike to crystalline silicate, non-crystalline silicate structures are formed by irregular tetrahedral arrangements which make it more porous with greater surface area and thus, they are more susceptible to reaction. Hence, aggregate's reactivity to alkalis are greatly influenced by its irregular crystallinity. Sufficient moisture is important for potential ASR reaction. Moisture facilitates the flow of ions in the porous media and allows hygroscopic gel to swell when it absorbs water. Expansion is directly related to the percentage of relative humidity in concrete. Many researchers have reported increasing ASR expansions as the relative humidity tends to increase. A minimum relative humidity of 80 percent is required to set out alkali-silica reaction and cause significant expansion. Moreover, increased temperature accelerates the alkali-silica reactions in some extent . Many research have been conducted to figure out ASR problems and develop numerical models to predict expansion caused by alkali-silica reaction. Existing research shows versatility in the field of ASR analysis and modeling.
ASR proceeds in the following stages
(i) formation of reaction rim, exudation
(ii) rimming/exudation of sol/gel on the reacted aggregate partially filling microporosity in cement paste,
(iii) cracking within reacted aggregate accompanied by gel-filling,
(iv) propagation of radial gel-filled expansion cracks fromthe reacted aggregate into surrounding cement paste, and
(v) precipitation of ASR gel into air voids along cracks distant from the reacted aggregate.
Chemical Reaction
≡ Si −O − Si ≡ + H2O → ≡ Si − OH + OH − Si ≡ (1)
≡ Si − OH + ROH →≡ Si − O – R + H2O (2)
≡ Si − O − R + nH2O →≡ Si − O – R. nH2O (3)
K = A(T – Tref)n e-Ea/RgT (4)
Reference:
1. Rahman, Md. Asif, Yang Lu, “A Meso-Scale Multiphysics Model for Predicting Concrete Pavement Performance Subject to ASR Degradation,” Transportation Research Board (TRB) Annual Meeting, 2019, Manuscript No. 19-03839R1. DOI