HOW AGGREGATE PROPERTIES INFLUENCE THE PROPERTIES OF FRESH CONCRETE

GRADING

In case of sands, it has been found that materials having a large deficiency or excess of any size fraction adversely affect the workability of concrete. The most important size fraction in a concrete sand are those passing the 0.30, 0.15 and 0.075 mm sieves. If these fractions are too low the concrete tends to be prone to segregation. A rule of thumb guideline for optimum percentage is 30%, 15% and 7.5% respectively passing the 0.30 mm, 0.15 mm and 0.075 mm sieves. The percentage passing the 0.075 mm sieve may be used as a first estimate of the concrete’s tendency to bleed -the greater this fraction the lower the bleeding.

BULK DENSITY

Bulk density is a measure of aggregate packing capacity and therefore influences water requirement and mix proportioning. It does not have a direct influence on workability, provided the mix is correctly proportioned.

PARTICLE SPECIFIC GRAVITY

Because bleeding and settlement are driven by the force of gravity, the higher the specific gravity the greater is the tendency for aggregate particles to settle. The effect is marginal with the range of conventional aggregate (specific gravity from 2.6 to 2.95) but can be significant with high density aggregates such as metallic ores.

PARTICLE SHAPE

Particle shape, especially of sand, has a marked effect on the water requirement of a concrete mix. The closer the shape is to spherical, the lower the water requirement.

Particle shape affects the workability of concrete. Rounded particles make concrete with good workability; flat and elongated particles tend to make harsh concrete. Particle shape of sand has a greater effect than that of stone. Good mix proportioning can to a large extent overcome the adverse effect of poor stone shape.

SURFACE TEXTURE

The effects of surface texture are similar to those of shape, although less pronounced; smooth textures reduce water requirement and improve workability.

STRENGTH

Provided that aggregates are stronger than the paste and strong enough to not break down or abrade in the mixing process, crushing strength has no effect on the properties of fresh concrete.

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