CURRENT RESEARCH BY THE COUNTY OF ORANGE, CALIFORNIA, ON CONCRETE DISTRESS FROM SULFATE AND ALKALI-SILICA REACTIONS IN CONCRETE, APPLIED TO MITIGATION OF THIS COSTLY DISTRESS IN FUTURE DESIGNS

Some public works and private concretes in Orange County have been distressed by sulfate reaction and alkali-silica reaction (ASR). These reactions produce petrographically observable secondary minerals. Distress includes fractures, pop-outs, and spalling. Damage in some cases jeopardizes structural integrity, becomes a safety concern, and leads to dismantling.

The common, local sulfate reaction in concrete involves sulfate anions carried by water from the subgrade into the concrete where they combine with calcium cations from the cement paste to form the sulfate mineral gypsum. Gypsum crystals grow in the concrete, causing expansion that leads to fractures, pop-outs, and spalling. The secondary sulfoaluminate mineral ettringite, is usually present but not detrimental.

The ASR involves a chemical reaction of unstable amorphous- to cryptocrystalline-silica, microcrystalline quartz, optically strained quartz, cristobolite, tridymite, chert or chalcedony, opal, highly metamorphic quartz, or microfractured quartz in the aggregate, in the presence of moisture, with alkalis and hydroxyl ions in the cement paste. These silicates are more reactive than others because they are less ordered molecularly, and have weaker bonding and less chemical stability. The County's current aggregate sources are fluvial and alluvial fan deposits containing reactive grains derived from the Bedford Canyon Formation. The ASR produces a hydrous, calcium, alkali-silica gel. The gel absorbs moisture, swells, and fractures the concrete.

The sulfate reaction can be mitigated in areas of high sulfate subgrade by prohibiting water from entering the concrete, placement of an impermeable barrier on the bottom and sides of the concrete, and by using dense Type V concrete. The potential for ASR can be mitigated when the concrete aggregate contains potentially reactive silica, by either (a) using a very low alkali cement (not available in Orange County), (b) mixing the potentially reactive aggregate with a less- or non-reactive aggregate to lower the amount of potentially reactive minerals to acceptable values, or by (c) mixing with cement that contains 15 to 20% substitution of flyash to counter potential reactivity.