As a decorative concrete contractor, you can obtain mix designs that can help you deal with weather extremes—hot or cold, wind and humidity—that can lead to surface crusting or affect set times. You should design these mixes with the help of your ready-mix producer and keep them up-to-date to produce the results you need so you will have several mixes to use as the need arises.

Hot weather

For some decorative finishes, such as stamped concrete, time is needed to complete the stamping process. But when ready-mix temperatures rise, concrete sets more quickly, and the time needed to finish and stamp patterns decreases. One option is to design a concrete mix with fly ash or slag replacing some of the portland cement. If, for example, you are using a 6-bag mix (564 pounds of portland) and replaced a half sack (47 pounds) with fly ash or slag, an additional half hour could be added to the setting time when the concrete temperature is approximately 70° F. If you replaced one bag of portland (94 pounds) with fly ash or slag, the setting time would be increased by approximately one hour. The color of the concrete will likely be different with the addition of fly ash or slag, so the same mix design should be used for an entire job. A mock-up sample is generally recommended.

You can also add retarding admixtures to a mix to increase the time until initial set. There are two things to keep in mind, however. As the temperature of the concrete increases, the time until initial set decreases, so there is less time available for stamping impressions. You can solve this by adding more retarding admixture to the last half of the load before placing it. So the first half of the load sets before the second half, providing the needed time for stamping. (See Concrete Construction, July 2002, “Stamping Concrete: Managing Initial Set Times,” p. 46.) Prepackaged admixtures by Fritz Pak make this easy to do.

Above: For placing and stamping concrete under winter conditions, special concrete mixes are needed to manage both the cold weather and the excessive bleed conditions that can lead to serious efflorescence problems later. Right: When the air temperature, concrete temperature, wind velocity, and relative humidity are known, the rate of evaporation can be plotted. Surface crusting conditions exist at 0.2 and higher.
Above: For placing and stamping concrete under winter conditions, special concrete mixes are needed to manage both the cold weather and the excessive bleed conditions that can lead to serious efflorescence problems later. Right: When the air temperature, concrete temperature, wind velocity, and relative humidity are known, the rate of evaporation can be plotted. Surface crusting conditions exist at 0.2 and higher.

The problem with using retarding admixtures in hot weather is that more time is available for moisture loss from the surface of the concrete. If conditions are favorable for surface crusting, this may intensify the problem. The use of an evaporation reducer or building wind blocks can help to minimize surface evaporation.

To summarize, consider developing the following mix designs with your ready-mix producer:

Replacing a half sack of portland cement with fly ash (if previously color matched)

Replacing one sack of portland cement with fly ash (if previously-color matched)

Adding a retarding admixture to your mix. A Degussa (Master Builders) admixture commonly used is Pozzolith 100XR. WR Grace's most popular retarding agent is Daratard 17. Adding 2 to 4 oz/cwt (per hundred pounds of cement weight) can add one to three hours to the initial set time depending on temperature. Typically, you should not increase the time from batching to initial set more than 3½ hours.

Cold weather

Adding pozzolans such as fly ash or slag to concrete for cold weather placement will greatly slow down the reaction. Without the use of a special mix design in cold weather conditions, your decorative mix can take all day or longer to set. During that time the concrete can bleed excessively, making the surface wet and sloppy. Accelerating admixtures can prevent this, providing setting times closer to what you would get in summer conditions. Pozzutec 20+ by Degussa and Grace's PolarSet are rated for subfreezing conditions and have water reducers in them to limit bleed. Depending on temperatures, dosage rates typically range from 5 to 60 oz/cwt. It is possible to add them at the jobsite if additional working time is needed.

These admixtures are referred to as “non-chloride accelerators” (NCAs), meaning they do not use chloride and won't contribute to corrosion of the steel reinforcement. Degussa's Pozzolith NC 534 is another example. Dosage rates typically range from 10 to 45 oz/cwt. Chloride-based accelerators are typically not recommended for colored concrete and can promote corrosion of steel in the concrete.

To summarize, consider developing the following mix designs with your ready-mix producer:

  • Remove or reduce pozzolans from your mix design.
  • Use non-chloride accelerator to optimize the setting time of your concrete.
  • Establish mix designs with your ready-mix producer to cover different temperature ranges.
  • Consider both ambient and ready-mix temperatures when you are designing mixes.

Surface crusting

If you know the relative humidity, wind speed, ambient temperature, and concrete temperature, it's possible to predict whether surface crusting will result and how serious the problem will be. You can fog or mist the area around your placement or use evaporative reducers to minimize the moisture loss from the top surface of the concrete, but changes to mix designs can be helpful, too. The addition of accelerators brings about initial set more quickly, limiting the time for moisture loss. It also brings about “bottom set,” giving contractors more time to stamp patterns on the surface—organization is the key, however, because everything happens faster. Garry Culton, a technical specialist for Degussa says the use of Pozzutec 20+ is very common even in warm weather climates, such as Arizona, where the product is used to help control surface crusting problems and achieve faster turnaround.

Using retarding agents may add to the problems when surface crusting conditions exist. But you can add both retarders and accelerators to concrete to assist with placement and bring about initial set more quickly. (See Concrete Construction, Aug. 2000, “Avoiding Surface Crusting,” p. 53.)

For more information about Fritz-Pak jobsite admixtures, Circle 3.