Good concrete depends on good aggregate
Good concrete can only be manufactured with good aggregates. Aggregates that are cubicle in shape, strong, durable, low in absorption and washed. Put simply, hard rock quarries that grade their aggregates effectively through professional screening and washing during the production process, make the aggregates we find in good concrete.
Coarse aggregate can be as much as 75% of the volume in a mix. The best stone for making concrete is hard, durable particles with moderate absorption. The worst aggregate used to make concrete is friable, weakly cemented sandstones with high absorption. Where concrete is exposed to freeze/thaw cycles, lightweight cherts, ochres, shales, and weakly cemented materials such as conglomerate must be avoided. In short limit the amount of deleterious material present, particularly when creating decorative concrete finishes.
When crushing different types and ages of crushing equipment are in use, the most common being jaw crushers, which tend to produce flatter more angular aggregate compared to conical crushers, which produce cubical stone shapes. Cubical shaped aggregate is the best for concrete.
At Concretebroker many of our members including Concrete and Quarries and Wild Quarries and Civil all provide concrete plants with cone crushed aggregates that are predominantly cubicle in nature and ideal for concrete mix designs. Their aggregates are less elongated, obtrusive and sharp in texture, enabling concreters to finish their concrete with less tear, pulling away and inconsistencies upon the surface of their slab.
And washed aggregates are always sought after as clay, dust and silt can ultimately lead to problems in your concrete slab. For in the crushing process, aggregate gets mixed with fine particles (dust), some adhering tenaciously to the rock. This can create problems for good concrete, such as reducing bond to the cement matrix and higher water demand, so all coarse aggregate should be carefully washed. Aggregate containing as little as 0.5% clay fines can cause problems.
But its not just rock that comes into the equation when considering the creation of good concrete. Fine aggregate (sand) is added to a concrete mix to fill the spaces between large aggregate. Fine aggregate is also a blend of several sizes and the most common problems occur when some sizes are missing. Not all sand makes good concrete. For instance, beach sand doesn’t work well because its particles are too smooth and round, making it more difficult to achieve a good bond with cement paste. Sand must also be washed to eliminate dust that can reduce bond and make the concrete “sticky.”
Our concrete broker member Goulburn River Sand and Aggregates supplies sand for many concrete plants who appreciate not only the coarse nature of its composition, but the reduced amount of silt, clay and dust present following the washing process.
And then there’s the question of aggregate gradings. Well-graded mixes have some proportion of aggregate at all sizes from coarse to fine. Well-graded mixes have a higher percentage of aggregate than gap-graded mixes, meaning there is less cement paste and thus less water needed. This helps to produce high-strength concrete with less shrinkage, better workability, and higher durability. If the aggregate can’t shrink, only the paste, then where there is less paste, there’s less shrinkage.
For more information on how to make good concrete, and how to find good concrete contractors, pumpers, suppliers and finishers, stay tuned to concretebroker.com