Stamping Concrete: Hot Weather Stamping

The Concrete
Concrete is the most important variable in hot weather stamping. When hot weather arrives, most ready mix suppliers start adding retarders to the mix. But how much retarder is a good thing, and what do they bring to the party? Are there better alternatives?

Your objective in hot weather is to get the concrete placed as quickly as possible. Let’s face it, that last “lump” of concrete out of the truck after sitting onsite for an hour is NOT the most user-friendly concrete in the world!

Later, you are faced with the daunting prospect of determining when the concrete is ready to stamp. A false “set” or “crust” often means that stamping operations begin too soon. The top seems ready but the body of the concrete is still like Jell-O! This leads to sloppy impressions, “squeeze” (concrete oozing out at the edge of the tools), and poor release from the slab, in effect pulling the surface up with the mat or skin. Set controlled retarders are available, but what are some other methods for controlling slump and set time?

Let’s take a look at a load of concrete batched out of the plant at a nominal 4" slump with retarder added. By the time it arrives at the site and/or passes through the pump line, it may be closer to a 3" slump! What are your options? Unfortunately, most of you are raising your hands to give the truck driver the thumb-in-the-mouth “More Water!” signal! On commercial projects, jobsite-added water, or water of convenience, is tightly monitored to prevent low concrete strength and excessive shrinkage cracking. But I believe water is just as important on decorative concrete projects — for the same reasons and a few others.