Aggregate is essential in making concrete; it is the largest single component in the mix. But aggregate can also be placed into concrete for other more specific applications, including adding color and sparkle to decorative applications, and strength and durability to industrial flooring. These special purpose aggregates can help contractors achieve specific, desirable results, whether for visual aesthetics or increased performance.
Special purpose aggregate comes in many forms. It can be integrated within the concrete mix, but in the majority of applications it is used in smaller quantities on the surface of the concrete. Two important types are decorative or colored aggregates, and mineral or metallic aggregates.
Colored aggregates commonly can be purchased from your local aggregate producers, and the available colors often depend on the local geology. Grain size and shape are important to consider when looking at aggregate. Exposed aggregate surfaces often use rounded aggregates, but angular aggregates can be revealed through polishing the concrete.
Other aggregates are treated with oxide pigments to provide color. Two such products come from 3M. Its Colorquartz aggregate is bonded with an exterior ceramic coating that contains inorganic pigments. The colors resist bleeding or fading, even in constant sunlight. Because it is commonly used in pools, it also resists strong chemicals. For flooring, it can be combined with other materials and applied over a variety of substrates. It also offers texture, durability, and slip resistance.
Accent Stone is also ceramically coated, except the material is a diabasic rock instead of quartz. It is commonly used in pools or as exposed aggregate.
Other types of decorative aggregates include recycled glass. There is a wide array of color options in distinctive translucent blues, greens, reds, and browns. Recycled glass typically comes as graded, broken fragments, though more rounded varieties can also be purchased. Several companies—American Specialty Glass and Heritage Glass—offer recycled glass for use in concrete and terrazzo.
Metallic and mineral aggregate
Contractors can use metallic or mineral aggregate dry-shake hardeners to increase the surface durability of the concrete. Many improve the surface compressive strength to more than 10,000 psi. Dry shake hardeners consist of dry cement and metallic or mineral aggregates that are worked into the surface of fresh concrete. They offer the convenience of pre-blended packaging and are available from numerous manufacturers. For a description of dry shake hardeners and how to use them, see “Armor Floor Surfaces with Metallic Aggregates” (Concrete Construction, July 1994).
Industrial flooring must stand up to forklift traffic, particularly solid-wheeled or steel-wheeled traffic, which can be hard on concrete. Sudden impact loads can also shorten the lifespan of industrial floors. Because the aggregate resists abrasion better than cement paste, increasing the amount of aggregate in the surface improves the wear resistance. In trying to determine how much is enough, contractors can refer to ACI 302.1R-04, “Guide for Concrete Floor and Slab Construction.”
ACI 302 has classified concrete flooring based upon the floor's anticipated traffic, use, special considerations, and final finish. Class 6 and 7 floors are industrial floors that must withstand hard wheels and heavy wheel loads. This is where mineral or metallic aggregates come in.
Metallic aggregate is a term frequently tossed around, but it means different things to different people. Eldon Tipping, former chairman of ACI 302, with Structural Services Inc., describes metallic aggregates, particularly those used in dry shake hardeners, as metal filings, typically iron filings.
Keith Kennedy, product engineering and technical services manager for 3M's Industrial Mineral Products Division, refers to metallic aggregate as a naturally occurring mineral containing a cation—such as magnesium, calcium, or potassium—that does not readily oxidize, resulting in the degradation or discoloration of the mineral or the cementitious coating. “A mineral with a high unbound iron content may result in oxidation, discoloration, softening, and loss of the mineral over time,” he added.
Whether you are referring to metallic, mineral, steel, or iron ore aggregates, it is important to remember that they can be used to strengthen industrial floor surfaces. The material is broadcast onto and worked into the surface of wet concrete. One thing that separates them from dry shake hardeners is grain size. Dry shakes are well graded, and generally smaller than these aggregates. The coarseness, up to pea gravel or larger, adds to their durability.
Trap rock is a familiar example of mineral or metallic aggregate. It is usually a basaltic or diabasic aggregate known for being dense and wear resistant. It tends to be usually locally derived. Iron ore aggregate is also dense and wear resistant, and is available in the north central part of the United States. Colorquartz Accent Stone by 3M and its Indag product would also qualify as metallic aggregate. These heavier (or denser) aggregates are similar to those used in heavyweight concrete or radiation shielding applications. For a description of how to use trap rock (and other mineral or metallic aggregates) see “Trap Rock Aggregate for Floor Construction” (Concrete Construction, October 1988).