Seldom do contractors get callbacks regarding the color of their plain gray concrete. Variations in regular concrete are so common that they usually go unnoticed. Add a little pigment into the mix, however, and watch out! Suddenly, discoloration and minor surface defects become a big deal.
Most complaints stem from the fact that the finished slab doesn’t exactly match the manufacturer’s color chart. Despite prominent disclaimers on the charts describing the factors that can affect color, customers still assume their concrete slab will be a solid, consistent color like a painted wall. This is simply not the case.
Pigment manufacturers have strict QC standards assuring uniformity, but when their pigment is shipped to different end-users, any guarantee is out the window. Since no two ready-mix plants produce identical concrete, whatever color is added to that concrete will likewise not be uniform. Different brands of portland cement, fly ash, different aggregates, admixtures, and even different mix water can impact color.
Color differences are also common in loads of concrete batched from the same plant on the same day. Even if the delivered loads are identical in every respect including slump, color differences occur. The following are just a few reasons why.
Condition of the base. Was the concrete poured atop wet or dry fill stone or even dirt or mud? If one pad was poured atop a wetter base than the other, it can affect color.
Temperature and weather conditions. Was the jobsite sunny, shady, hot, cool, damp, or dry? If the weather changed from one pour to the next, the color will likely be mismatched. If it rained on one slab soon after finishing, or the customer’s irrigation system sprayed the concrete overnight, it could result in mismatched colors.
Addition of water to the concrete by the finishing contractor. Did the contractor add water to the load to make placement easier? Did the finishers sprinkle water on it to aid with finishing? Did the broom man dampen or wet his brush while texturing the slab? All of these actions will discolor the concrete. In general, I’ve observed that the drier the concrete is poured and finished, the darker the color remains. Adding water usually causes lighter areas where the color bleaches out a bit.
Finishing operations. This is probably the most overlooked aspect of color problems. All finishing operations should be kept consistent not only between loads, but also within the same load. If the concrete is not textured consistently, the color will likewise be inconsistent. Often, the perceived color difference is no more than how light reflects off the surface. For instance, if one section of a slab is a bit rougher, it will appear darker because there are more shadows created by the texture. Therefore, if one section of a slab is troweled but another part is floated, the texture and color between those sections will be different. On broom-finished concrete, each pass of the texturing brush should be made in the same direction, and when the concrete has reached the same stage of set.
Sometimes just the timing of finishing operations is the culprit, especially on stamped concrete. If finishers wait between bull floating and hand floating the edges, for example, the edges are consequently wetter than the rest of the slab because the bull-floated part has already skinned over. Since the amount of antiquing release that “takes” is directly related to the amount of moisture at the surface, the edges and the field will exhibit different colors. Likewise, if stamping is ended on one slab when the concrete is hard, and begun again on the next slab when the concrete is soft, the colors probably won’t match. Even a very small difference in the amount of antiquing release embedded in the surface can make two slabs look mismatched.
Curing. Color cannot be accurately assessed on concrete less than 28 days old, especially in cool or damp weather. Consider that plain concrete looks dark greenish-gray when it is fresh, but very light gray when it is cured. Colored concrete changes the same way. Color charts represent month-old cured concrete, not concrete that has been down for only a couple of days or weeks.
How to combat color variations
Many contractors use tinted liquids such as sealer, xylene, or mineral spirits to even out mismatched colors. Some opt for an application of powdered antiquing release followed by sealing. Still others go with a pigmented acrylic stain. But even with these methods, obtaining a perfect color match is difficult. I’ve successfully used the following tactics.
Use color hardener. Color differences are most apparent where one slab butts up against the next. So when stamping with integral color, we’d dust a matching color hardener onto the slab at the construction joint where the next slab would eventually connect. We’d apply full coverage along the header board, and gradually fade it into the integral color further out in the slab. We would do the same with the next pour. This way, the pours matched at their connecting points. Adding an antiquing color made the subtle difference between the integral color and color hardener undetectable.
Place borders and ribbons throughout the slab. If two slabs are separated by a contrasting colored band, it eliminates them butting against each other and color differences are not as noticeable.
Thoroughly clean the slab before sealing. Make sure that one slab doesn’t contain more antiquing release than the other. Acid washing with a diluted solution is far more effective than pressure washing, and it is easier to control the amount of release that is removed.
Don’t seal until the end. A large percentage of “color problems” are actually sealer problems. Often, the sealer has simply blushed (clouded) and made one slab appear lighter than another. Problems can occur when different pours are sealed at different times. No part of a stamped slab should be sealed until the entire job can be sealed at once. This allows the contractor to make any needed color adjustments without having to strip the sealer first.
Educate the customer. It would be a minor miracle if a load of concrete ever did achieve an exact match with a color chart. Contractors must let their customers know this in writing before the project begins, pointing out the manufacturer’s disclaimers. In addition, contractors must NEVER allow a customer to choose a color based on an online color chart. Every electronic screen is different and cannot display the color as it is seen on a manufacturer’s professionally printed chart. If the customer is made aware of these facts up front, there will be far fewer problems down the road.
Steve VandeWater is the manager of The Concrete Store in Fishers, Ind. He is a former Indianapolis area contractor and is the creator of the Indiana Decorative Concrete Network website. E-mail firstname.lastname@example.org; also visit www.indecorativeconcrete.com.