Preventing Water Stains When Using Curing Blankets
Powdered release agent applied liberally to curing concrete acts as a
water shield against condensation.
Is there any way to cure stamped concrete in cold weather without using blankets? I want to avoid permanent water spots created by condensation that might build up underneath the blankets.
Answer: Using curing blankets is the only accepted method I’m aware of for curing stamped concrete in cold weather. Some contractors use high-early-strength concrete (which has been chemically accelerated) to achieve faster setting times and to avoid the use of blankets. But I don’t recommend these rapid-set mixes for stamped concrete applications because the concrete will begin to set before the stampers can complete their work.
You are right about the downside to blankets. No matter how hard you try to pull them tight and smooth over the concrete surface, you always end up with wrinkles and creases that can trap moisture, resulting in permanent water stains. These stains are barely noticeable on plain gray concrete, but can be major eyesores on stamped and colored concrete. A trick that has worked well on past projects is to use a powdered release agent to create a waterproof shield over the concrete surface. After completing all the stamping and detail work, and right before placing the blankets over the concrete, apply a medium to heavy dusting of release powder over the entire surface. Release powder has hydrophobic properties, so it repels water. However, the powder acts as a one-way water shield: It will allow the warm water vapor rising out of the curing concrete to pass through, but once the vapor condenses into a liquid, it won’t be able to pass back through the layer of release powder. This eliminates or greatly reduces water stains created from curing with blankets (or from using plastic sheets to protect the freshly placed concrete).
If you prefer to stamp with a liquid release agent, you can still use this method. Wait until the liquid release has evaporated, and then liberally dust the surface with a colorless release powder right before putting down the blankets. For best results, remove any condensed water that forms underneath the blankets after 12 to 24 hours. Reapply release powder and blankets as needed until the concrete has cured or the temperature warms up. After a minimum of 72 hours, remove the release powder by scrubbing it off with a broom and a simple soap and water solution. A power washer, at low pressure, can also be used.