Alkali Silica Reactions in Concrete


Concrete Experts International has extensive, world-wide experience with alkali silica reactions (ASR) in concrete structures and with research regarding the nature and effects of ASR. Diagnosing ASR is an integrated part of our petrographic analysis of concrete.

What is Alkali Silica Reaction?

Alkali silica reaction is a heterogeneous chemical reaction which takes place in aggregate particles between the alkaline pore solution of the cement paste and silica in the aggregate particles. Hydroxyl ions penetrate the surface regions of the aggregate and break the silicon-oxygen bonds. Positive sodium, potassium and calcium ions in the pore liquid follow the hydroxyl ions so that electro neutrality is maintained. Water is imbibed into the reaction sites and eventually alkali-calcium silica gel is formed.

The reaction products occupy more space than the original silica so the surface reaction sites are put under pressure. The surface pressure is balanced by tensile stresses in the center of the aggregate particle and in the ambient cement paste.

At a certain point in time the tensile stresses may exceed the tensile strength and brittle cracks propagate. The cracks radiate from the interior of the aggregate out into the surrounding paste.

The cracks are empty (not gel-filled) when formed. Small or large amounts of gel may subsequently exude into the cracks. Small particles may undergo complete reaction without cracking. Formation of the alkali silica gel does not cause expansion of the aggregate. Observation of gel in concrete is therefore no indication for that the aggregate or concrete will crack.

Microscopic appearance

Alkali silica reaction is diagnosed primarily by four main features

  • Presence of alkali silica reactive aggregates

  • Crack pattern

  • Presence of alkali silica gel in cracks and/or voids

  • Ca(OH)2 depleted paste

Please to not hesitate to contact CXI if you have some problems regarding ASR or any other deterioration mechanisms.

Cracked flint aggregate. Fluorescent light.

Gel in air void and cracks. Ordinary polarized light.

Crossed polarized light.

Gypsum plate.

Fluorescent light.
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