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Retarded Setting

ASCC Position Statement #28

ACI 318-08, “Building Code Requirements for Structural Concrete,” does not limit the amount of fly ash or other pozzolans that can be used in concrete not exposed to deicing chemicals.

The recent emphasis on reducing cement contents to reduce greenhouse gas emissions has resulted in project specifications that require replacing up to 50% of the portland cement with fly ash or other pozzolans. Concrete with 50% fly ash normally sets more slowly than concrete containing lower amounts.

A concurrent trend also affects setting. Concrete containing large amounts of fly ash may also contain up to three different admixtures that retard setting to varying degrees: ASTM Type A waterreducing admixtures, ASTM Type F high-range water-reducing admixtures, and hydration stabilizers.

These admixture combinations are often used in concretes with water to cementitious materials ratios at or below 0.45, and magnify the effects of the fly ash on setting time. The effect on setting time can be extreme if the admixtures are dosed on the basis of total cementitious material in the concrete instead of basing the dosage only on the amount of portland cement used.

When used in concretes with as-delivered temperatures of 60F or less, and at jobsite ambient temperatures of 50F or less, effects of the fly ash and admixtures are greatly magnified. 

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