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Three requirements must be met for expansive ASR to occur : 1) reactive forms of silica or silicate in the aggregate, 2) sufficient alkali (sodium and potassium ) primarily from the cement, 3) and sufficiently available moisture in the concrete. If any one of the three requirements is not met, expansion due to ASR cannot occur.

In its simplest form, ASR can be visualized as a two-step process :

1. Alkali + SilicaGel Reaction Products

2. Gel Reaction Products + MoistureExpansion

Actual expansion occurs in the second step when the ASR gel reaction product swells as it absorbs moisture. Potentially expansive gel reaction product does not form unless the first step occurs.

As a general rule, two of the three requirements for ASR, cement alkali and reactive silica, are basically fixed components of the concrete and therefore present the potential for expansion, regardless of exposure condition. However, alkali levels can be increased from those in initial mix by external sources such as from salt water and mist in coastal areas or from deicer salts, or the alkalis present can be concentrated in areas of the concrete causing localized reaction. Some of the causes of concentration can be wetting and drying cycles in the concrete or cases where reinforced concrete is being protected by cathodic protection. The third requirement, moisture availability, is a major variable in concrete and has a significant impact on the severity of distress and volume change due to ASR.

Moisture availability in concrete varies significantly with distance from exposed surfaces in most, if not all, highway structures. This is most pronounced under severe atmospheric drying coditions such as those in arid desert-like regions in the sothwestern United States. The resulting crack pattern associated with ASR may thereby become accentuated through shrinkage induced by prolonged, severe drying. It is thus not uncommon for secondary distress mechanisms such as drying shrinkage, freezing and thawing and corrosion of reinforced steel to be accelerated once the integrity of concrete is affected by ASR-induced cracking.

Restraint, due both to abutting concrete and to embedded reinforcing steel, influences the development of cracking associated with ASR. Its effects are observed in highway pavements and bridge structures, as will be seen in the illustrations. Cracking associated with ASR is not uniformly developed throughout concrete members due in part to restraint. Creep is a major factor that tends to relieve ASR-induced stress. Since neither restraint nor creep are uniform in all directions, ASR-related distress is not uniformly developed. For example, abutting pavement slabs offer restraint parallel to the longitudinal direction of the pavement. Cracking therefore tends to be more pronounced in the longitudinal direction; that is, differential movement is greater in the transverse and vertical directions, resulting in the typical cracking illustrated in the photographs in this handbook. Finally, nonuniform cracking patterns can be caused by differences in wetting and drying in portions of a concrete member as by sprinkling or watering systems, or leakage from joints in the deck on bridge substructures.

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