How to Handle Decorative Concrete in Hot Weather
Decorative concrete is still concrete and is subject to all of the same hot weather problems (and solutions) as plain gray. There are, however, a few tricks you can use to get good decorative results under hot weather conditions. Here are a few tips:
- Fritz Pak admixtures can be added to the mixer truck on site are a great way to fine tune your concrete slump and set time. Their hot-weather superplasticizing admixture increases slump by about 7 inches and retards set time by about an hour, although both of these numbers may be lower with hot concrete. Again, be careful of the air content with these admixtures, since it can be increased by up to 2%. Fritz-Pak's delayed set retarders don't increase slump but do effectively delay the set.
- For concrete that is to be stamped, consider using step retardation--adding retarder to the mix after half of the batch or after one-third and two-thirds have been placed. For textured concrete one of the strongest things to do in hot weather is step retardation. "I'll add retarder at least twice," says Brickform's Clark Branum. "Say I have a 9-yard load I'll place the first 3 yards just the way it comes then I'll add 3 or 4 one-yard admixture packets then discharge another 3 yards then throw in one or two retarder packets on the tail. That gives you more time to color and texture and if you're using hardeners or integral colors the retarder won't affect the color either way where water will. You're inducing slump chemically rather than with water."
- Remember that in hot weather, when the effects of a retarder or a superplasticizer wear off, set occurs very quickly. Be ready!
- On hot and dry or windy days, use an evaporation retarder (not a curing compound) that forms a monomolecular film on the surface and will prevent the concrete from drying out, which can lead to plastic shrinkage cracks or crusting. "One thing that I warn guys about," says Branum, "is not to put it under the hardener. Sometimes people spray it on the concrete and trowel it in but that can prohibit bleed water. I recommend that they apply the hardener and float it in and then use it on top of the hardener to keep it moist. You end up working it into the surface a bit but I always install it on top of the hardener." If you are applying a second coat of color hardener, you can use another application of evaporation retarder if the surface seems to be drying out.
- Staining concrete relies on a chemical reaction that happens faster in hot weather. On their web site, L.M. Scofield notes that "Chemicals react faster under heat. Don't stain in 95° to 100° weather; you can ruin a job that way. In a hot climate, stain in the coolest part of the day. Wet the surface down to slow the chemical reaction."
- Most overlay manufacturers recommend that the concrete surface temperature be between 50° and 80° for an overlay installation. Bob Harris, in his book Concrete Overlays & Toppings, notes that "the warmer the air temperature and substrate temperature, the faster the material will set. Installing an overlay in direct sunlight during the hottest part of the day will greatly reduce your working time and jeopardize the quality of the results." Chris Sullivan with QC Products notes that although overlays use polymers, you really need to treat them like regular concrete and target the coolest part of the day for placement. "Some overlay manufacturers offer set retarders," says Sullivan, "so if a contractor is working in hot weather he might want to look into that. Be sure to get the retarder offered or recommended by the manufacturer to make sure it will be compatible."
- In his Q&A post Summer Sealer Basics, Sullivan says that sealers can flash cure if the surface temperature of the concrete or the air temperature is above 90°. "Common sense would dictate that during the hot months, you should avoid sealing during the heat of the day," he says. "Today with the states tightening their VOC requirements, manufacturers have gone to exempt solvents like acetone that tend to flash even faster and we're seeing more problems with sealers blistering and bubbling. Even at 90° you're running a risk."
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