Determining the best mix 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:
1. the properties of the concrete in its plastic state as it comes out of the truck,
2. the finished concrete’s strength to withstand loads and durability to withstand freeze and-economics — how expensive it is.
3. 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.
In decor uses, the size and the shape of aggregates used in the mix design are important, says Malisch. Each contractor has an opinion about particle size, and many say it depends on what kind of finish will be used.
Here is the conundrum. In decor, texture and pattern are vital, and they are affected by the size aggregate used. The deeper the stamped impression is to be, the smaller the stone needed, but the smaller the stone, the more cement is needed, and thus the more water required.
According to Tom Ralston, president of Ralston Concrete, Santa Cruz, California, using a smaller pea gravel means you have to make up for the reduced solid mass with sand, increasing the need for water and thus reducing structural integrity. While the 3⁄8" inch aggregate promotes clean edges when stamping with rubber or aluminum stamps, he says, the use of a larger angular rock improves strength and the rocks have a better tendency to interlock, reducing cracking. He says the only time a larger aggregate is difficult for stamping is when the aluminum or rubber stamps are used. He suggests using 3⁄4" angular aggregate with all polyurethane texture mat stamps to help overcome some of the shrinkage problems.
Skundrick believes that with stamped applications, a smaller rock, a 3⁄8", should be used, compensating with a higher cement content and a 45% aggregate to 55% sand ratio, just the opposite from a normal mix. He advises using plenty of matrix to get the detail needed on stamped applications.
Idiart also suggests a smaller-sized aggregate be used to help enhance the stamping impression, but mixing it with a larger size to enhance durability and quality.
Round or water tumbled rock is seen only in some areas of the country, and it is considered best in exposed designs.
Admixtures are used to customize just the right mix for a specific job, climate, and site, to control the set and to add durability. Common admixture types are classified in ASTM C494 and include water reducers, accelerators, and retarders.
Water reducers act as the name suggests: they either lower water demand while maintaining slump or they can be used to increase the slump without the need for additional water. Some provide set control in addition to water reduction and act as either accelerators or retarders. Different types of water reducers impart different amounts of water reduction, typically from about 3% to 10%. High-Range water reducing admixtures or superplasticizers offer the highest level of water reducing capability, typically ranging from 12% to about 25%. Ralston notes that superplasticizers increase slump without adding water.
Accelerators as defined by ASTM C494 as “an admixture that accelerates the setting and early strength development of concrete.” They may or may not provide water reduction as well. They primarily allow concrete to maintain an even set when there is a temperature differentiation.
Using these admixtures in combination must be done carefully, as they can offset each other, even from the same manufacturer, so get advice before getting carried away. Says Ralston, “If you use admixtures indiscriminately, using too much retardant for example, it can wreak havoc, delay your setup and create a spongy effect.”
Giving the mix the opposite effect are the retarders. These admixtures slow the rate of setting and are sometimes used to offset the accelerating effect of hot weather or to delay the set, allowing for special finishing processes, which may often times increase strength. Admixtures are usually dosed in fluid ounces for every 100 pounds of cement per cubic yard of concrete.
The design must result in a durable concrete that can stand up to freeze-thaw cycles in wintry climates. This is accomplished through the use of air entrainment, either from using air-entraining cement or from air-entraining admixtures, described in ASTM C260. The microscopic voids provided by air entrainment allow space for frozen water to expand so it won’t fracture the concrete. The need for air entrainment, says Malisch, varies from region to region, running from 3% to 7% of the volume of the concrete.
Skundrick notes that air entrainment additives can make a finish sticky, but the solutions to this lie with the finishing process, not with a change in mix design.
Ralston suggests that contractors look into the use of such materials as acrylic polymers, which help increase a mix’s adherence to old concrete and improve concrete’s flexibility. Such polymers, says Ralston, mean a lower water-to-cement ratio and help reduce shrinkage as the plastic fills these interstitial voids. “As the industry develops,” he says, “modified acrylics and latex additives will become more important in mix designs as we strive for sophisticated applications. Tinkering with these admixtures will mean we will begin to see some awesome things.”
For inhibiting plastic shrinkage and plastic settlement cracking, there are two common types of synthetic fiber, a monofilament nylon fiber and a monofilament fiber or fibrillated tape polypropylene, can be used to provide a three-dimensional secondary reinforcement to decorative concrete. These products are available in pre-weighed degradable bags and are introduced into the ready mix at the rate of from 1.0 to 1.5 pounds per cubic yard.
The advantages of using synthetic fibers are directly related to concrete durability, according to Bob Zellers, PE, vice president of technology and engineering at NyCon, Westerly, Rhode Island. Synthetic fibers control plastic shrinkage cracking, increase toughness and impact resistance, and reduce water migration. None of these benefits could earlier be obtained from the older approach with wire mesh, a secondary reinforcement.
Zellers says that synthetic fibers keep shrinkage at bay and improve durability by helping the finish stand up to surface abrasion. “Every contractor has a preference for nylon or polypropylene,” he says. “It’s like Fords vs. Chevys.” One thing is certain however, the shorter micron-fiber is most suitable for decorative concrete. Longer fibers will generally stick up on the surface of the finished concrete but can be easily burned off with a torch to maintain the aesthetic appearance with colorful textured or stamped concrete finishes.
In addition to texture, it is color that sets the hallmark for decor applications. “With color, concrete becomes a piece of art,” says Ralston. “But it is the trickiest, most difficult part of the mix.” Clients, he says, expect the finish to look like painted substrate, and they must be taught that it will have a natural, variegated effect caused by the many elements in the mix.
There are two methods for adding color to concrete. One method of coloration is through the use of integral color or iron oxide pigments added to concrete, never more than 10% by weight. The second method uses color only on the top layer, adding some labor cost but generally decreasing the cost of color throughout.
Frank Piccolo, of the Decorative Concrete Council and Natchitoches, Lousianna’s Artcrete, Inc., says that different concrete contractors prefer different kinds of coloration. “Opinions are like belly buttons,” he says. “Everybody has one.” Some contractors use only integral color, and others prefer a dry shake, thinking it gives a richer color without fading out. Still others will even use a combination of both integral and topical coloring applications.
Malisch says that integral color is expensive and liable to vary from pour to pour, but Skundrick suggests that the cost is higher only for reds, greens, blues and darker colors, that integral earth tones are generally a better value.
“Integral colors were the hardest to use,” says Ralston, but many companies have now gone to liquid systems over the dry powders, added by computer rather than an individual driver, so consistency is now greatly enhanced.
Adds Idiart, the new liquid systems are “the wave of the future,” offering ease of application as well as consistency.
The industry standard for comprehensive strength of concrete is measurement performed at 28 days. Some 3,500-4,000 psi is considered acceptable for decor uses.
According to Piccolo, top applied color hardeners, which include wetting agents, were developed to allow a higher specification for PSI in industrial uses, creating abrasion resistance at lower cost as they can strengthen concrete surfaces two to three times higher than the standard compressive strength. However, he says, nowadays color hardeners are used in the decorative concrete to enhance aesthetics.
“PSI is not as important in decor,” adds Skundrick. “You strive to get a high quality surface, and color hardeners simply give concrete that unique natural stone appearance. Because of color hardeners strengthening qualities however, it does make the top of the concrete very resilient to foot or wheel traffic and repairing damage to color hardened concrete is quicker and easier to blend with the existing surface.”
What it all comes down to is that each contractor must construct his own basic recipe for a mix design to meet his climate, site and aesthetic needs. Frank Rusk, a Las Vegas-based contractor turned consultant, says that he came up with his own mix design, adapting it with the help of a ready-mix supplier. He starts with a Type V cement as it is resistant to the attack from minerals in soil and water, and does not use fly ash, as it causes splotching.
For stamping applications, he advises a 5.5 to 6.5 bag per cubic yard ratio, with a 50/50 sand and aggregate ratio, and a 3⁄4-inch aggregate. In exposed aggregate applications, he advises a 70/30 or 60/40 sand-to-aggregate ratio, with a 3⁄8-inch rounded aggregate used. He uses integral color, sprayed with a nationally recognized surface etching product. And because sun and shade will cause the concrete to cure irregularly, colors can also develop differently. Therefore, he suggests covering the surface with a 6-mil black plastic overnight then returning the next morning and pressure washing the surface several times. For stamping applications that require a more production oriented environment he suggests running lights and adding color hardeners at night to achieve a more consistent color.
Decorative concrete contractors can work with ready mix producers by describing what they want to happen with the mix and then working together to find the best way to achieve it. Ultimately, Malisch advises doing a preliminary mix and then tweaking it to come up with the right proportions for that site application.
Piccolo adds that help is available on-line, The American Society of Concrete Contractors, he says, which the Decorative Concrete Council is part of, has a hotline for members that will answer technical questions about mix design. Contractors can also get help from such resources as at the Scofield Institute, ACI, Portland Cement Association and the National Ready Mix Concrete Association.
In short, mix design, though it is based on scientific properties, is as much a matter of art as of science. Like a great chef, each contractor will bring his own expertise and insight into the recipe.