Placing concrete in the heat - Fine Homebuilding Question & Answer
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Placing concrete in the heat

Q: I just tried putting in a concrete sidewalk for the new home I am building. It was a hot, dry day with temperatures in the mid-90s. The pour took over an hour, and because I’m more of a carpenter than a concrete person, the finish work took longer that I anticipated. After about 1-3/4 hours, the concrete was unworkable even though I sprayed water on the work constantly. Can I salvage the work that I’ve already done, or should I tear it all out and start over? Is it possible to use a thin concrete/epoxy coating over the whole thing for the finished surface?

A: Rocky Geans, a concrete contractor in Mishawaka, Indiana, replies: I recommend tearing out the walk and starting over. You mentioned that you constantly applied water on the surface of the concrete while it was curing, which dramatically increases the water-tocement ratio and weakens the top surface of the concrete. Anything you apply over this weakened concrete probably won’t adhere properly. If you try the skim coat of epoxy/concrete mix and it doesn’t hold up, you’re back where you started, with even more money down the drain.

There are many things you can do to make sure you don’t have a replay of your disaster. After you’ve torn out the old concrete and reset your forms, set the concrete delivery for early in the morning so that you’re not fighting the heat of the day. Order the concrete in a 5-in. slump, which is a fairly wet mix and will extend the working time.

Water down the area that the concrete will be poured over thoroughly to prevent the subgrade from absorbing moisture from the concrete. If a warm day is forecast, you can ask the concrete company to add retarder to the mix. A retarder slows the concrete’s curing time to give you time to get it in shape.

To make the pour go more smoothly, lay out all the contraction joints, and make sure you have all your tools ready well ahead of time. Also, grab a few friends to help you during the critical time of pouring and screeding. Then you’ll have more time to work on getting the finish just the way you want it. If you’re working alone, you might consider ordering the concrete in smaller batches. You’ll probably pay a little bit more for the concrete, but the extra money will buy you a lot of valuable time.

From Fine Homebuilding 112, pp. 18 November 1, 1997

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