Concrete Aggregate - Not Just Any Old Rock

By James W. Schmitt, Principal and President of Schmitt Technical Services: http://schmitttechnicalservices.com

Learning aggregate basics improves product quality and saves money.

By selecting the right aggregate, you’ll improve product quality at an economical price. A poor quality or improperly specified aggregate can reduce efficiency in labor during placement, decrease service life and increase maintenance. All add up to additional, but avoidable, expenses.

Aggregate, the granular filler in Portland cement concrete, occupies 60% to 75% of most concrete volume. As concrete’s main component, it influences the way fresh concrete behaves and how hardened concrete performs. High quality aggregates used in a mix result in an economical, workable, durable and enhanced concrete.

Basic Considerations

Aggregate can be naturally occurring rocks and minerals that make up sand and gravel, crushed stone, or manufactured sand. It also can be man-made, such as air-cooled blast furnace slag or recycled concrete. Local aggregate and ready mixed concrete producers who stock various aggregates should be a knowledgeable source of technical information and aggregate project applicability. Be prepared to answer questions about the aggregate in your yard.

Generally, desirable aggregate used in concrete is:

• Square or triangular shaped – provides a workable plastic concrete

• Hard – adds strength and reduces creep and shrinkage

• Dense – minimizes absorption of mix water

• Well-graded – allows concrete to flow freely, yet occupy the greatest volume for maximum economy

• Clean – ensures a tight bond between the aggregate and the cement paste matrix, minimizes introducing additional dust or fines (from loose coatings) into the concrete mixture, and reduces the possibility of introducing soluble, deleterious coatings, or impurities into the concrete mixture

• Inert – prevents deleterious chemical reactions from occurring between aggregate particles and the cement paste matrix

• Durable – reduces exposure to freezing, thawing and sulfates

These properties are evaluated through sound laboratory testing and then comparing those results for compliance with industry guidelines and specifications. The aggregate or concrete supplier should provide test results on the aggregate and on concrete made with the aggregate under consideration. Additional independent testing of the aggregates by a reputable laboratory provides verification of specification compliance.