The selection of aggregates is very important in quality concrete production because the correct aggregates play a major role in determining the strength, thermal and elastic properties, as well as stability of concrete, says Bryan Perrie, CEO of Cement & Concrete South Africa (CCSA).
“As aggregates make up a large volume of concrete, selecting the correct type of aggregate directly influences the costs involved. Aggregates also affect concrete’s shrinkage levels and help prevent cracking among several other advantages,” Perrie points out.
Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and Portland cement, make up the essential ingredients of a concrete mix. Fine aggregates generally are sand, crushed stone, or crushed slag while coarse aggregates include gravel, pebbles, fragments of broken stone, slag, and other coarse substances.
Gravels and crushed stone are most commonly used as coarse aggregates to combine with sand, water and cement to produce concrete. Natural gravel is usually sourced from a pit, river, lake, or seabed while crushed aggregate is produced by crushing quarry rock, boulders, cobbles, or large-size gravel. “Recycled concrete has become another viable source of coarse aggregate and is being increasingly used in granular subbases, soil-cement, and new concrete,” Perrie adds.
He says some of the characteristics of coarse aggregates to be considered when deciding on the components of a concrete mix include:
* Grading - which focuses on the size of the aggregate particles. Grading affects the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability of concrete.
* The aggregate particle shape and surface texture directly influences the properties of freshly mixed concrete. Rough-textured, angular, and elongated particles need more water to produce workable concrete than smooth compact aggregates. This means the cement content must also be increased to maintain the water-cement ratio, which leads to increased costs. “Generally, selecting spherical shaped aggregates is ideal because if too elongated, the particles could jam and block the voids when pouring or pumping the mix,” Perrie observes.
* The void content between aggregate particles affects the amount of cement paste required for the mix. Angular aggregates increase the void content while larger sizes of well-graded aggregate decrease the void areas.
* The abrasion and skid resistance qualities of aggregates are important when the concrete is likely to be constantly subjected to abrasion, as in heavy-duty floors or pavements. More robust aggregates should be selected for highly abrasive conditions to reduce wear.
“Selecting the appropriate type and volume of sand to combine with the coarse aggregates is naturally also very important in the formulation of a durable and workable concrete mix. Sand adds volume to the concrete by binding the coarse aggregates and consequently increases workability,” Perrie adds.
