Concrete Basics

What is Concrete Made of

Pouring Concrete

Concrete Mix Designs

Aggregates in Ready-Mix Concrete

Building Concrete Slabs

Concrete Testing

Seasonal Pouring Tips

Repairing Concrete

Concrete Repair

Fixing Decorative Concrete

Applications

Staining Concrete

Stamped Concrete

Concrete Countertops

Concrete Patios

Concrete Floors

Commercial Concrete

Quality Counts

Make sure that your concrete producer purchases good-quality aggregate as verified by regular aggregate test results in compliance with ASTM C 33, "Standard Specifications for Concrete Aggregates." A history of good performance of a local aggregate also provides an indication of how well the material performs in service.

Good-quality aggregate must be clean, hard, strong, have durable particles, and be free of absorbed harmful chemicals, coatings of clay, or other contaminates that can affect hydration of cement or reduce the paste-aggregate bond. Aggregates to avoid include:

• Those that are friable or susceptible to being split.

• Those with significant quantities of soft and porous materials.

• Certain kinds of "chert," since they may have very low resistance to weathering and can lead to surface defects, called popouts (Photos 2 and 3).

Photo 2 – Surface of concrete flatwork where a popout occurred over a chert particle. Though not a structural concern, popouts are a nagging surface defect that can be a source of aesthetic complaints, and if excessive, may compromise cover quality in reinforced slabs. (Photo courtesy of the Portland Cement Association)

Photo 3 – Cross section of a core sliced longitudinally exhibits the mechanism of a popout. Some chert particles are more absorptive than higher-quality aggregates. When located near a saturated surface, they can absorb water and upon freezing, expand and spall the concrete immediately above it, along with a portion of the aggregate. (Photo courtesy of PCA)

As a naturally occurring material, aggregate will sometimes include weathered or unstable particles in the delivered product. The allowable percentage of deleterious substances for both fine and coarse aggregate are listed in ASTM C 33 Tables 1 and 3, respectively. Some projects might require even more-stringent limits. To avoid post-placement problems in concrete that are difficult to rectify, verify that these limits are not exceeded at the time of material submittals.

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