Degrading of Aggregate by Attrition in a Concrete Mixer

摘要

All concrete aggregate is exposed to attrition during concrete mixing, and some accompanying change in grading inevitably takes place. The degree to which a mineral aggregate composed of several size fractions will degrade is a question related to the quality of the aggregate, and the severity and duration of mixing. If the attrition forces are fixed, the tendency of the aggregate to degrade will be isolated. A mathematical model is developed to describe aggregate breakdown under fixed mixing conditions in terms of a single variable parameter dependent upon only the physical characteristics of the aggregate. This parameter is particularly suitable for qualitative ratings of the tendency for aggregates to degrade and quantitative approximations of attrition effects on the concrete mix. The attrition process for n size fractions of an aggregate is simulated in terms of an n x n matrix whose elements are functions of the parameterization. Predicted results from the mathematical model compared with experimental data from a laboratory concrete mix agreed within 1% throughout a 2-hr attrition period. The fineness modulus of the 1-1/2-in. maximum-size-aggregate concrete mix changed from 5.76 to 5.35 during the test period. (Author)