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Concrete Durability

It's in the Mix

I won't nag you any more about keeping the water content low—I'll let Doug Bannister do it for me: "We modify slump with superplasticizer so we maintain a good water-cement ratio and still have the concrete placeable at the end of the discharge hose. We would never order more than a 4-inch slump but we would increase it with a super so that if it was 6 at the pump we knew we would get 5 at the hose. There's never a need to place concrete at a slump greater than 5 inches."

Harris demurs somewhat from this, because the vast majority of his stamping jobs use color hardener. "I like color hardeners for the abrasion resistance and the range of colors. We specify a 4 ½ inch slump. Specifying agencies are concerned about performance and strength and they will specify a 3-inch or 3 ½-inch slump but we are stamping with color hardeners and they need some moisture on the surface, especially in windy conditions. You're going to fight like hell to get enough bleed water to get the color hardener working at the lower slump."

The other issue in durability is entrained air. For exterior concrete you simply must have proper entrained air in the concrete when it is placed. Even with a color hardened surface, water will penetrate into the concrete and if there is not a good air bubble matrix, when it freezes the concrete will begin to break up. Concrete's strong, but it's no match for the force created by freezing water.

Here again, there is some disagreement. "With a color hardener I won't go over 4% air," says Harris, "so that we get some bleeding. With integral I would go up to the 5% to 6% range." Bannister, on the other hand, says, "We keep the air at 5% to 7%. I know some guys using color hardeners will reduce the air a little so they would get more bleed water. I think that's a big mistake."

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