When work involves placing concrete, bull floating, and adding a broom finish, the risk due to plastic shrinkage is minimal because little time is required to place and finish. There also is little manipulation to the surface. But be sure to place curing membranes quickly when evaporation rates are high. The risk for slabs that receive trowel finishes—either hand or machine applied—is much higher because the surface can be distorted under the weight of manpower and machines. It also can lead to very risky outcomes when a decorative stamped finish is desired. The worst condition of all for a decorative concrete contractor happens when they try to stamp impressions on surface crusted concrete.

There are several things you can do when jobsite weather information points toward rapid surface drying conditions. The key is to have enough lead time to make decisions and to adequately prepare. When evaporation rates are excessively high, or above 0.2 pounds/square foot/hour, the safest decision is to cancel plans to place concrete. If you decide to place concrete under less than desirable conditions, several things make a difference, including using evaporative retarders, windscreens, fogging the area of the placement, and using accelerating admixtures. The most common method for controlling surface evaporation is to spray evaporative retarders on the concrete just after it is struck off and immediately after each finishing operation.

Changing any of the weather variables (relative humidity, air temperature, and wind speed) that affect evaporation rates is another way to manage surface evaporation rates. You can fog the air above the concrete to increase the relative humidity; however, don't use a garden hose to accomplish this because too much water pools on the concrete. Water must be atomized by special fogging nozzles or pressure washers, increasing the relative humidity above the concrete.

Erecting windscreens to reduce the wind speed over fresh concrete reduces evaporation rates as well. This method commonly is used for building construction. Manufacturers of above-grade floor forming systems also sell windscreen systems to protect floors when they are being placed.

It's also common practice to raise ready mix temperatures when ambient temperatures drop. Aside from decreasing labor costs, it also decreases the amount of time water can be lost from the concrete due to evaporation.

Many contractors add accelerating admixtures to their concrete when they know that conditions are present that cause rapid surface evaporation rates. This causes initial set to occur more rapidly, reducing the amount of time that high rates of surface evaporation can occur. It also causes “bottom set,” making it possible to start finishing operations sooner—before surface crusting can cause problems. Another advantage is that the matrix of the concrete remains stable while finishing operations are in progress. Surfaces stay flatter and there is less potential for scaling. Many contractors take advantage of this when they install lightweight aggregate floors. If you use accelerators, be sure you are organized to handle each finishing operation earlier.

At 90° F the initial setting time for concrete occurs approximately twice as fast as it does at 50° F. When concrete, especially dark colored concrete, is placed under hot sunlight, surface temperatures can surpass 160° F. This difference in temperature easily can lead to surface crusting conditions because of early surface initial setting problems as well as the increased drying conditions on the surface. In the case of early spring and late fall when ground temperatures are low in many parts of the country, concrete is further cooled and this helps to increase the temperature difference between the concrete and its surface temperature. Monitoring these temperatures can be important.

As a contractor, you can never learn too much about concrete. Issues involving surface shrinkage and especially surface crusting cause serious problems for contractors. Being proactive is the best way to prevent them from occurring. And using weather instruments and IR thermometers to plot accurate jobsite information can help you make decisions.