Alkali Silica Reaction - Mechanism

The Reaction

Alkali—sodium, potassium, and lithium—are generally introduced into concrete weakly bonded to hydroxyl ions, (OH)^-, by the cement. However, alkali also may be present in pozzolans, aggregate, admixtures, and mix water used to make the concrete. The weak bonds allow the compounds to dissolve quickly into solution becoming free ions. As the compounds dissolve, the (OH)^- concentration and subsequently the solution’s pH increase. A low-alkali cement produces concrete with a pH of 12.7 to 13.1 compared to a pH between 13.5 and 13.9 for concrete made with high-alkali cement (Hobbs, 1988). The increasingly basic alkali solution reacts with the acidic reactive silica aggregate producing an alkali-metal-ion hydrous silicate gel (Hobbs, 1988) in aggregate cracks (Farny and Kosmatka, 1997). Once the gel is formed, it osmotically imbibes water and expands into available spaces. The water, unable to escape, causes the gel to grow and eventually reaches the concrete surface if proper conditions exist.

The alkali-silica reaction depends on the alkali concentration and the aggregate’s reactivity. A sufficient amount of alkali must be present in the pore solution to drive the concrete’s pH high enough for the reaction to occur. The silica’s reactivity level defines the pH threshold for initial reaction. Low reactivity demands a higher pH than a silica with a higher level of reactivity. Once ASR has begun, it will continue until the reactive silica has been depleted, the solution pH has been reduced below the threshold required, or the concrete mass becomes depleted of the required moisture. (Farny and Kosmatka, 1997)

Condition for Reaction

Available moisture must exist for ASR to affect concrete’s structural integrity. As a concrete member expands with increasing temperature, the pore structure expands as well. Alternatively, stress applied to the concrete element produces fractures or opens previously formed cracks. If water is available, the gel will grow into the newly opened areas. When the concrete contracts because of cooling or stress relief, it can not return to its original position because alkali-silica gel has taken its place. The incompressible gel responds to the forces exerted by the contracting concrete with an equal and opposite hydraulic force. This response causes stress concentrations in the concrete’s microstructure producing new cracks and/or propagating existing cracks. If this condition persists, the gel will expand as space becomes available and eventually destroy the integrity of the concrete.