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Patio Refinishing: Uncovering the Chemistry
Recently, we decided to refinish our stamped concrete patio. While the concrete itself was in great shape with no pitting or cracking – thanks to the durability from using recycled fly ash in the concrete mix – the coloration on the patio had worn off in many sections. The patio is 10 years old, and had seen many years of chair scraping, table moving, drink spilling, and feet pounding (particularly little feet).
The finish was dull, but the areas where the color had flaked off revealed the uncolored concrete underneath:
We called several contractors to get quotes for refinishing the patio. I was shocked at the virtual ineptitude of most contractors in explaining the process of color restoration, despite repeated questioning. After all, I’m a geek. Explain the chemistry to me. Most of the contractors insisted that all we would need is a clear coat sealer. Even with no formal training in concrete preparation, I was reasonably confident that a clear sealant on bare concrete color would do little to improve the appearance. We had two data points that supported this: 1 – when it rained, those areas slightly darkened but still lacked color, and 2 – we had done the clear coat ourselves several years ago and it did not help.
What I also remember is when the patio was installed, the contractor first sprinkled powdered colorant over the poured lightly-colored concrete and then stamped the pattern into it. The colorant was used to “antique” the patio (give it variation in color to look more like natural stone). After curing, the patio was sealed. I found two contractors that said they would do color correction by using “release” to color it. When I asked what release was, the explanation given was that the release is the color which “mingles with the concrete” and “no one is quite sure how it works but it does.”
So off I went to do research on what “release” was, why the color had flaked, and how to fix it. Release is so named because it is a bond breaker that prevents the stamping tools from sticking to the concrete. Powdered releases are made with hydrophobic stearates (fatty acids) which allow polymer-based stamps to cleanly lift from the concrete. Some of the release becomes embedded into the surface of the unhardened concrete. Areas of depression and joints in the stamping pattern get more embedded release, while the high spots exhibit more of the base concrete color. This imparts the natural gradation of color.
Excess release is supposed to be scrubbed off after the concrete hardens. Even power washing may not remove enough of the release. Otherwise, when the sealer is applied, it is the equivalent of putting sealer on polymer-repelling dirt or dust, and it does not chemically bond with the concrete. Sealers form a protective layer which impedes moisture penetration into the concrete. In areas where the sealer has not bonded with the concrete, normal wear can cause the sealer to delaminate, or peel off taking the release color with it. There are a number of different types of sealers, some are penetrating and some are coating. Needless to say, our patio had a coating sealer that lifted up in areas with excess release. This is apparently a very common problem with stamped concrete.
After discussing all this with the contractor we eventually chose, the solution was to re-antique the bare spots with a cement-based agent that adheres to bare concrete, and then reseal. Excess colorant was brushed and washed off prior to applying a tinted sealant (slightly darkening the patio color) to prevent the sealer delaminating problem from recurring.