Curing Concrete with Plastic? Be Careful…

A few months ago, a buddy of mine who I play hockey with had a problem: His newly-poured concrete porch and patio had pattern stains in the finished product. He asked me to take a look at it to see what I thought.

The basis of the story is a familiar one to those of us who work in northern regions of the US: The hot & dry weather of August & September seems to turn cold & wet in October & November overnight. Work needs to get done to beat the coming winter weather, and extra efforts are usually required to install work, sometimes in less-than favorable conditions.

Go or No-Go

My buddy’s concrete subcontractor was in the fall weather predicament: Cold evening temperatures and a chance of overnight precipitation. The contractor made the decision to move forward and pour the front and rear patios. When the concrete crew was done with the pour, they covered the concrete with plastic sheeting (known in the industry as “visquene”).

Covering the concrete is the correct course-of-action for cold and/or wet conditions: Visquene both protects the concrete from the cold by helping to keep the heat it is generating during cement hydration as well as provides curing protection by preventing the concrete surface from drying out. However, problems can arise when the plastic is applied too early, when finishing & late bleed water are still present on the surface. When the visquene makes localized contact with a “still wet” surface, the moisture coagulates with the contact surfaces of the visquene, but if there are adjacent areas where the visquene is not in full-contact, air pockets will exist. Because the moisture is causing the surface to dry at different rates, what results is a mottled finish, where contact and non-contact surfaces have, essentially, two different finished surfaces resulting in two distinct, visible shading variations.

While there is no likely quality issue in either of the finishes, the aesthetics are very noticeable:

Certainly, in a parking lot or roadway/highway/bridge scenario where function might be more of the priority, this condition might be acceptable. But, when your concrete serves both a function AND aesthetics, this is not something you want in your finished product.

Possible Solutions

Unfortunately, there isn’t any real after-market solutions that can be implemented to get rid of the mottled finish:

• Because the concrete has some broom-dragged texture to it, grinding or polishing the surface is difficult to do to maintain an even look, and the grinding will remove the texture & slip-resistance that it was originally installed for.
• Stains & sealers may not be a good solution either since the stain will not be absorbed evenly into the concrete due to the differences in the density/porosity in the surface where the visquene made, or did not make, contact with the wet concrete surface.
• A pigmented stain can be used, but the aggressiveness of color variation required to cover the mottled finish will require a significant altering of the concrete color and might not be what you want aesthetically in your finished product.

I stumbled across an excellent post that tells the same story:

Beware Covering with Plastic! | In Decorative Concrete

Plastic Sheeting Can Permanently Discolor Concrete Adverse weather conditions such as rain, snow, or freezing temperatures often make it necessary to protect

And if you’re interested in reading an article on the differences between dyes & stains, this is a good one:

Understand the Differences Between Concrete Dyes, Stains and Pigments

When it comes to imparting color on a concrete surface, understanding the product options is critical in order to achieve the finished look your customer desires

After Actions

Bottom Line: If you are installing concrete that has an aesthetic quality to it, make sure you talk to your contractor about how he will be finishing, curing and covering/protecting it, especially if less than desirable weather is expected. Sometimes, the best course-of-action is to wait until more-favorable weather conditions are available. Although that might not be the best scenario for your job schedule, it may be the right call to give you the finished product you desire.