Download PDF version (293.1k). The full text of this article is available as a PDF document.

Fine aggregates (minus 4 mesh and smaller) are of little perceived value because they are typically not thought to enhance the performance of concrete, either in its plastic or hardened state. The inferior performance of some manufactured fines, however, is not necessarily due to the material itself, but to the methodology used to characterize the properties of fine aggregates. Sieve-analysis testing is typically flawed, or it gives misleading information about the suitability of fines for a given use.

The main culprit is insufficient attention to three key aggregate properties that affect performance: particle size, shape, and texture.

The thought that "ultra fines," i.e., those of minus 200 mesh size and smaller, increase water demand assumes that specific surface is related to water demand regardless of particle size. Test work by Aggregate Research Industries (ARI), however, shows that the minus 100 mesh materials do not increase water demand.

To assist with compaction, it is desirable to design a concrete mix with a low internal friction coefficient. The internal friction in a concrete mix is the sum of the total friction between the individual particles. The amount of friction exerted by any given particle is directly related to the particle shape, as well as the surface texture and size of the aggregate.

It is generally accepted that cubical aggregates produced by crushing have a surface that allows for a good bond with cement paste while maintaining an acceptable internal friction within a concrete mix. Differences in particle shape and surface texture directly influence the strength of hardened concrete of the same mix proportions.

The article includes tables showing the relationship between minus 100 fines content and water demand, how rough aggregate texture results in greater concrete strength, and the relative effects of aggregate properties on concrete strength.