Mix Recipes for Decorative Concrete
by Cindy Grawl
design for decorative concrete is like creating a recipe for a gourmet treat — it should take advantage of the best qualities of the various ingredients in it to produce the specific product for your needs. When it comes right down to it, according to Kelly Idiart, vice president customer service, Central Concrete, San Jose, California, good quality concrete is the same for both decor and nondecor applications. Much is dependent on job site conditions, with their specific moisture levels, subgrades, finishes and cure practices.
More specifically, mix design for decor will vary depending on the region of the country, the site, and the type of finish the project will receive, says Ward Malisch, Senior Managing Director, Technical Operations, the American Concrete Institute (ACI), Farmington Hills, Michigan. On general principles, he says, there is no one-size-fits-all mix design. He suggests that a good mix take into account three elements:
- the properties of the concrete in its plastic state as it comes out of the truck,
- the finished concrete’s strength to withstand loads and durability to withstand freeze and thaw cycles, and
- economics — how expensive it is. If it is possible, he says, reduce the amount of raw materials used as much as possible while retaining strength and durability.
It all begins with proportion, says Malisch, getting the right ratios of each ingredient. Concrete is made of cement and water to form a binding agent, and then sand and aggregate that provide its substance. Admixtures are used to provide special qualities, and these can include either water reducers of varying capabilities or set modifiers. Finally, air entrainment products and fibers add strength and durability, while color gives a special aesthetic. Each one plays a role in making the concrete both beautiful and long-lasting. Here’s how:
The first ingredient in concrete is portland cement, which is typically 10% to 15% of the mix by volume. ASTM C150 lists eight types of cement that offer a variety of performance benefits to concrete. Three of the most common types are Type I, for normal concrete, Type II, offering moderate sulfate resistance, and Type III, for fast setting and high early strength applications. Portland cements now come in blends that can meet more than one ASTM specification.
Fly ash, one of the mineral admixtures, can replace from 18% to 20% of the cement in the mix, resulting in a slower set. Fly ash also tends to decrease early age compressive strength, but it typically increases late age strength. One main advantage of fly ash is that it lowers concrete cost because it is cheaper than cement. According to Don Skundrick, Liniger’s Tru Mix, head of operations, Medford, Oregon, fly ash in decor mixes reacts with lime to hold down leaching, which can result in unsightly deposits. It is important to note that not all fly ash is suitable for concrete. Improper usage can even cause surface flaking on color hardened concrete applications.
Water is the catalyst that hydrates the cement and makes the mixture workable. For complete cement hydration, a water-to-cement ratio of 0.19 to 0.22 is needed. However, to make the concrete workable, the w/c ratio ranges from 0.40 to 0.60. Concrete consistency, as it relates to its fluidity or mobility, is determined by its slump. Slump is not a measure of the workability of concrete. Rather, the higher the slump value, the more mobile the mixture. The method for determining slump is described in ASTM C143. Slump should be maintained throughout the pour, and admixtures can be used to do this.
When determining the mix proportions for a cubic yard of concrete, the w/c ratio (which is related to the durability compressive strength) and the maximum size of the aggregate first need to be determined. In the absence of historical performance with a given set of materials, ACI 211 provides relational tables and additional information for proportioning a concrete mixture.
“Keeping the water-to-cement ratio right is the number one rule for mix design,” says Skundrick. “Use as little water as possible, as this affects the quality of the concrete.” The excess of water will cause excessive bleed water as the concrete begins to cure, he says, and thus the substance will shrink. Using too much water also risks the surface color. Superplasticizers and other admixtures can be used in place of water to reduce the amount used.
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