How to Get Better Stains and Colors in Decorative Concrete
Probably the largest responsibility faced by concrete flooring specialists is to deliver a finished floor that meets their clients’ expectations.
While you, as the contractor, are probably well aware that final colors may vary from the chart that was used for selection, or that batch variations in color are not uncommon, it is just as important that you know why these variations occur so that you can clearly communicate those realities to your customers.
It is also helpful to understand how different stages of the installation can affect the final color so that your process is optimized to produce the best color possible in the finished project.
Let’s look at the stages of installing colored concrete, and how they might compare to a standard concrete slab install.
Before You Pour: Color in the Concrete Batch
Before you even bring concrete to a job site, conditions are being set that might affect the color of your final concrete floor or project. It begins with site preparation.
For example, if the subgrade area does not have adequate drainage, or if a vapor barrier is or is not installed, the potential exists for extra moisture to contact the concrete or for moisture to be drawn from the concrete more quickly in an un-uniform manner. This can cause blotching or uneven color in the finished slab, as well as potential moisture-related problems down the road.
The concrete batch mix specs, too, carry the potential for variations that may impact the final color result.
The Mix Ingredients
Concrete is always a “recipe” of Portland cement, aggregate, admixtures, and water. Be sure that all materials going into the cement are uniform and from the same supplier for the duration of the project. As far as is possible, be sure that the ratios of water to cement to coloring compound remain the same from batch to batch. Any changes in these first steps can cause variations in the color, either from the intended hue or from batch to batch.
Each type of aggregate will also affect final concrete color, either in a subtle or more visible way, depending on the aggregate and its own color. A white sand will lend a different hue to the final concrete than one of the many colored and coarser aggregates available. It is also important to be sure that aggregates are non-reactive and will not interfere with the water absorption and drying process.
It can be tempting to rush a drying time schedule or a set accelerator, but with colored concrete, certain additions like a calcium chloride set accelerator can cause discoloration in the final concrete. Efflorescence, or solid mineral deposits that can appear on the surface during the drying process, can also be even more disfiguring on a colored concrete surface. Don’t let chemistry become your enemy.
There is a wide range of natural and synthetic pigments available for integral concrete coloring, or coloring that is mixed directly into the concrete batch. These can be added to the mix in a dry powder form or in “liquid” slurries. Liquid colorants should always be shaken or stirred thoroughly before adding them to the batch to be sure that the pigments haven’t settled to the bottom during storage.
Depending on the pigment used, the amount of mix water necessary in the batch may be higher or lower from one pigment to the next, or depending on the final color result desired, and should be added specifically following the manufacturer’s instructions. How does water content impact color? Lower water-cement ratios typically have a darker tone, while higher water-cement ratios of the came components will yield a lighter shade of the final color.
As a final consideration, when mixing and placing the concrete, tools and equipment must be free of dirt, grease, oils, and any other possible pollutant that might cause discoloration of the surface. And don’t forget mix time – when color is added to a concrete batch, it is vitally important that the mixture is agitated thoroughly and completely to ensure the pigment is distributed uniformly throughout each batch.
Surface Considerations for Colored Concrete
Once the concrete mix has been determined, there are also important considerations during pouring and finishing a colored concrete slab that will impact the final color.
With all concretes, it is preferable to place each concrete batch as close to its final location as possible on the job site to minimize the time and effort necessary to get the concrete into position. With colored concrete, this becomes even more important as extra vibration or the need for additional mix water can both impact the final color. Placing the concrete as close to the final position as possible can pay off with richer, more uniform color.
Once placed, keeping water additions to a minimum will also ensure richer, truer color. If at all possible, do not wet the slab once it is placed and keep tools (brooms, finishing tools, etc.) as dry as possible. Excess water pales the final color.
Finishing and curing deserve special attention when talking about maintaining truer color on a concrete slab. While some techniques may vary, consistency is the true key to uniform color. Troweling, in particular, must not be overdone or started late because it will increase the risk of discoloration or dark spots.
Some manufacturers recommend skipping troweling if possible and only use a float, screed or broom for a more uniform color finish. It is also worth noting that the overuse of metal tools for flatwork also has a higher risk of “burns” or discoloring the concrete surface.
Curing compounds can help control efflorescence and prevent surface blemishes that will also impact the finished concrete color. (“Sheet” style curing blankets are not recommended.) The benefit for colored concrete is that they can help aid uniform curing and drying, which ultimately leads to more uniform color. Be sure the type of curing compound used is designed for colored concrete.
Moisture Control is Key to Color
Running through the entire process from start to finish is the importance of understanding moisture’s role at each stage of a colored concrete’s production, installation and finish. The more uniformly moisture is distributed through the entire slab, the better the finished color will be.
A final step in monitoring any slab’s drying time is accurate moisture testing. When following ASTM standards, test sites around the job site can help monitor the overall moisture levels of the slab, not only for uniform color development but also for overall slab performance.
Relative humidity testing, like with the Rapid RH®, allows test sensors to be installed within the slab where, once acclimated, moisture content can be monitored quickly and easily as often as necessary. Particularly when a curing compound is used, or if mix ratios have been adjusted for various admixtures or color additions, the final drying schedule can be impacted. Only accurate RH testing can adequately determine when the colored concrete is ready for its final finish to become a strong, beautiful colored concrete floor.
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