Opal-Coated Aggregates – Investigation and Processing for USE

    Published: Jan 1990

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    Petrographic examinations of gravel aggregate for use in the reconstruction of a dam revealed the presence of opal in coatings on aggregate particles. It was considered likely that the aggregate crushing and classifying operations would reduce most of the coatings. However, the removed opal needed tracking to insure that it was not in the fine aggregate.

    Petrographic examinations were used to check the processed aggregate for opal. ASTM C227 tests, modified for use of the as-graded aggregate, was utilized to provide information on the potential for concrete expansion.

    The data indicated that concrete made with cements having alkali contents of 0.36, 0.55, and 0.92 percent did not have expansions considered deleterious.

    Because of the availability of low-alkali cement, the concrete was made using a cement having an alkali content of 0.4 percent. After over five years of service, the dam concrete does not contain outward evidence of alkali-silica aggregate reactions.


    Aggregate, opal, alkali-silica, coatings, dam, concrete, low-alkali cement

    Author Information:

    Van Epps, RJ
    ConsultantPresident, Petrographer, The Erlin Company, NeedhamLatrobe, MAPA

    Erlin, B
    ConsultantPresident, Petrographer, The Erlin Company, NeedhamLatrobe, MAPA

    Committee/Subcommittee: C09.65

    DOI: 10.1520/STP23456S

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