Q: We placed, colored, and stamped the concrete in the photo two years ago. It looked great after the first winter, but then during the second winter, the surface started breaking down. We specified only 3% entrained air in the concrete because I was told not to use too much air or else the concrete wouldn’t bleed enough to wet out the color hardener. And besides, I thought the color hardener would create an impervious surface to protect from freeze/thaw damage. What happened?
A: The only real protection exterior concrete has from freeze/thaw damage is a proper entrained-air system. That means 6% entrained air or higher with smaller aggregate. We’ll assume overfinishing isn’t a problem because you were planning to stamp it. You’re right, though, that some people will say not to use this high an air content, because it won’t bleed enough and air-entrained concrete is stickier and more difficult to finish. But if you see concrete that looks like this, where the spall is beneath the color hardener, then it is certainly due to freeze/thaw damage. The dense color-hardened surface, and even a good sealer won’t absolutely protect the surface. You could do the most beautiful stamping job in the world, but if the underlying concrete isn’t sound, you’re still going to get freeze/thaw damage when it gets saturated and freezes. And besides, minimizing your bleed water is the best way to prevent efflorescence, which is caused by water coming through the concrete and bringing dissolved alkaline salts that then precipitate on the surface as a hazy white layer. Use air-entrained concrete.