Using admixtures to enhance stamped concrete work

When used in the right proportions, pozzolans and other cement replacements can have positive effects on concrete by improving finishability, reducing permeability, reducing efflorescence and minimizing bleeding of color. Pozzolans can also have an effect on the setting time of the concrete. Granulated blast-furnace slag, for example, can slow setting and extend the working time of the concrete. Based on my experience, however, this retardation effect occurs only at cooler temperatures. When the air temperature exceeds 80 to 85 degrees F, I haven't seen any retardation with granulated slag.

Keep in mind that concrete is influenced by temperature: The warmer the weather, the faster concrete sets. To slow die set, try using a retarding admixture, which can be predosed at the batch plant or added at the jobsite. A retarder can buy you additional stamping time, especially when temperatures heat up.

Conversely, if you are faced with cool conditions, you can use an accelerating admixture to speed setting time and minimize bleeding and segregation. Avoid products that contain calcium chloride or added chloride ions, which can result in discoloration of colored concrete.

To improve the durability of concrete without sacrificing workability, consider using a water reducer. The most commonly used water reducers in the field of stamped concrete generally reduce the water requirement by 7 to 10 percent which can greatly improve concrete strength at all ages and reduce permeability and cracking.

Another admixture that contributes to durability, and one that is essential for concrete exposed to freezing and thawing conditions, is an air-entraining agent. An air-entraining admixture can be added to fresh concrete to cause the development of a system of microscopic air bubbles. This helps to improve the freeze-thaw resistance of hardened concrete. If using a color hardener with a mix design that contains an air-entraining admixture, check with the manufacturer of the color hardener for their recommendation of the maximum air content since this could gready affect how the hardener works.

Prepackaged admixtures

On most decorative stamped concrete projects, especially larger jobs, time is of the essence. But what happens if unexpected delays occur? Or what if the concrete that arrives at the jobsite is already too stiff to place and finish properly or the air content is too low?

Prepackaged admixtures that you can dose on the jobsite will be an important part of your tool kit. They give you greater control over the concrete you roeceive and the amount of time available for decorative stamping. Powdered admixtures, conveniently packaged in water-soluble bags, can simply be tossed into a batch of concrete whenever needed. The premeasured quick-fix powders provide first aid for a number of common problems.

For example, on a recent project that was installed during the heat of summer, we ordered our concrete mix with a small dose of retarder batched at the ready mix plant. We had already predetermined the pour area and broken it up into roughly three equal sections. We poured the first section of the pool deck without altering the concrete. For the next third we added toss-in retarder packs per the suggested dosage rate recommended by the manufacturer. On the last portion we once again added additional retarder packs. This common technique is referred to as step retardation. Had we not used this method, there is no way we would have effectively been able to stamp this complex design.

In addition to retarders, prepackaged admixtures include air entrainers, superplastidzers (or water reducers), and finishing aids that enhance surface finishing characteristics and make stamping and the addition of color hardeners easier.