Q: I recently did a stamping job on a long driveway. Part of the job went fine, but in some areas the concrete had already started setting, so we had trouble getting it to take the same textures. How can I overcome that problem on my next project?
A: Every time concrete contractors imprint concrete with patterns and textures there is the potential for problems achieving uniform—depth impressions and surface textures. Overcoming these problems takes experience and having the job well planned—and having a little luck from time to time. One technique for helping to solve uniformity problems in your stamping program is “step retardation.”
First, you must know the temperature of the concrete being placed. Using a concrete thermometer, take a concrete sample as soon as it arrives on the jobsite. If the temperature is 70° F, it's fairly easy to stamp 640 square feet (8 cubic yards of concrete at 4 inches thick) of pattern before the concrete gets too hard. But at 80° F or higher, stamping 8 yards of concrete is nearly impossible.
Step retardation involves adding retarding agents (available from your ready mix supplier) at the jobsite to regulate the rate of initial set. If you know, for instance, that half an hour will be required to place the concrete, with additional time for coloring and finishing before the stamping process begins, mixing a retarding admixture into the entire load can provide the additional time needed. When concrete temperatures are 75° F and higher, you can “step retard” by adding retarder to portions of the truck-load of concrete, a third or half, in order to provide the time needed to complete the stamping process.
In liquid form, the quantity of retarding admixture is measured as liquid ounces per hundredweight of cement in the mix. For example, if you want to add 1 ounce of retarder per 100 pounds of cement, and are using a 6-bag mix, the amount of retarding admixture needed for 1 cubic yard of concrete is 5.6 fluid ounces. If the temperature of the concrete is 75° F, the initial set would be delayed approximately 30 minutes—2 ounces per 100 pounds of cement would delay it one hour.