Mix Design for Stamped Concrete
Harris stresses the need for fines and cement paste to get a crisp stamped pattern. "I like a mix that has lots of paste—that's fatty." He recommends a 5 ½- to 6-sack mix. As mentioned in the section on materials, aggregate size is important in controlling shrinkage and the larger the aggregate the better. For stamped concrete though, large aggregate interferes with the stamping. Try some sample panels to determine the largest aggregate size that allows you to get a good stamped pattern. Harris recommends ¾ inch if possible but notes that for deeper patterns you might need to reduce that to ½ inch. "It's rare that we go less than ½ inch because of the shrinkage concern."
The Stamp Store's Doug Bannister, though, advocates using bigger aggregate to reduce shrinkage—up to 1 inch) and working around the problems. "I like to use the largest rock possible," he says, "because I understand that the more aggregate sizes I get, the less paste I need and the fewer shrinkage problems. Other stampers like to have small rock so they don't have to work so hard and they aren't pushing big rocks around. When we use bigger rocks it shortens the window because we have to catch the material when it's malleable, but I know it results in a better product. My standard is ¾-inch even with a deep stamp."
Fly ash is good to include in a stamping mix to improve the concrete's durability and because the added fines improve finishability. Some will say that it can reduce color intensity, but Bannister disagrees. "There's a myth about fly ash that you can't add integral color, but my experience is that it doesn't change the color. On a multiple pour project, you just have to make sure you stick with the same mix design in every batch to avoid color variations."
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