Mismatched Colors In Stamped & Decorative Concrete
Of all the problems encountered by decorative concrete contractors, one of the most frustrating may be mismatched colors between two integrally colored slabs. Color variations are usually unnoticeable on small pours requiring one truck, but can often be seen on multi-load pours. On occasion, the color between slabs poured from separate trucks simply doesn’t appear to match; one slab being a shade or two darker than the other. The different shades aren’t detected while the wet concrete is being placed, but after it cures a bit and is sealed, the differences at the construction joint can be striking. On some occasions it can be like night and day. It’s maddening, and the first instinct is to blame the concrete supplier for an inconsistent mix.
*Note- Concrete color cannot be accurately evaluated until the concrete has fully cured. It is futile to make color complaints until the concrete has attained its final color. In warm, sunny weather, concrete should be nearly its final color after about two weeks. In cooler, damp or overcast weather, the concrete will take at least a month or longer to reach its final color.
After experiencing color variations a few times as a contractor myself, I did some research. I learned that it was a problem suffered by contractors across the industry. The consensus opinion was that all variables need to be constant in order to achieve uniform color. This means using the same concrete mix design, the same water/cement ratio, the same cement and aggregate supplier, etc. If all of these things are consistent, you should get the same color, right? Not necessarily!
There are several other variables which need to be considered, and most of them are beyond the control of the ready mix supplier. These variables include weather, the relative moisture content of the subgrade, finishing practices, and curing. If these factors are not kept consistent, the color between pours is very likely to be different. For example, if it’s hot and sunny during one pour but cloudy during the next, the color can turn out differently. Maybe there was a shade line across the slab caused by the house or trees. Perhaps the subgrade was wet during one pour and dry during the next. Maybe water or a liquid finishing aid was used on one pour and not on the next. It’s very possible and even likely that the concrete was floated, trowelled, broomed, or stamped at different stages of setting. All of these factors can and will affect the final color of the slab.
My most memorable experience of this phenomenon happened when I poured a stamped patio for a local builder. By that time, I knew that everything concerning the mix had to be consistent. Therefore, I hounded the ready mix company about it to the point of annoyance. They assured me that everything would be uniform between loads and they would even send me the same driver with each load. It was a driver with whom I had a good relationship and fully trusted. He and I spoke at length about the need for consistency, and made certain that if any water was added to temper the concrete, the same amount would be added to the next load. The patio was split into two equal pours on the same day.
All went smoothly until I cleaned and sealed the slab. The top half, which had been poured on a cool, shady morning, came out lighter than the bottom half poured in the heat of the sunny afternoon. The only difference between the pours was the weather conditions!
Now that I work for a ready mix company and deal exclusively with decorative concrete contractors, I see what a problem this really is. I’ve looked at several projects recently where the contractor thought he got faulty materials. Luckily, my company has implemented a procedure in which the truck drivers take a small cylinder of concrete from each colored load delivered. The driver takes this sample at the jobsite and labels it. The label contains the contractor’s name, job address, color, yardage, concrete ticket number, and date. It may also contain any notes by the driver such as “poured on muddy base” or “extremely hot weather”. He then returns it to the plant’s Quality Control Lab for storage. In the event of a color discrepancy, the samples are tested for color uniformity. The batch tickets are scrutinized for dosage rates and mix ratios, the driver and contractor are interviewed, and the weather almanac for the days in question is reviewed.
To the left is an example of extreme color variation between pours. The contractor thought he had gotten the wrong color on his second load. However, upon cutting open his cured sample cylinders it was discovered that they matched perfectly. This indicates that the proper material was delivered to the jobsite. Once the concrete was unloaded from the truck however, the ready mix supplier had no more control over it. Something else caused the color variation. Judging by the weather almanac for the days in question, the likely culprit was one slab being poured on a wet base and during cool, damp, overcast conditions. The other slab was poured on a warmer, sunny day and atop a drier base.
Unfortunately, if there’s a color problem just knowing that it’s really nobody’s fault still doesn’t solve the problem. The slab is still two different colors. So how does one cope with such a situation? 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 a reseal. Still others go with a pigmented acrylic stain. Whatever the method, obtaining a perfect color match is difficult. *For more on this topic, read the article entitled “Recoloring Stamped Concrete“.
To keep the situation from happening in the first place, my contracting partner and I began also using color hardener on our integral color jobs. We observed that the color difference is most apparent where one slab butts up against the next, so we decided to eliminate that possibility. To do this, we would dust a matching color hardener onto the slab at the header board (where the next slab would eventually butt up.) We would apply full coverage at 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, applying color hardener where the slabs abutted and again fading it into the integral color farther out. In this way, the pours matched at their connecting point. The subtle difference between the integral color and color hardener would be undetectable. Because we used powdered antiquing release agent, the entire patio achieved a mottled yet cohesive appearance.
Another method we employed to combat color differences at construction joints was to use borders and ribbons throughout the slab. As already discussed, it’s very noticeable when two different shades of a color butt up against each other. However, if you put a contrasting band between them, the color difference is not nearly as noticeable and sometimes disappears altogether. It’s a good idea to consider using bands when making multiple pours on the same job.
Not all color differences between slabs are really color differences at all. Often, the sealer has simply (clouded) and made one slab appear lighter than the other. Blushing happens when moisture becomes trapped between the top of the concrete and the bottom of the sealer. A very large percentage of “color problems” are actually sealer problems. Sometimes, these problems occur because the different pours were sealed at different times. No part of a slab should be sealed until the entire job can be sealed at once. This allows the contractor to make needed color adjustments without having to strip sealer. Perhaps the most common reason for mismatched color on stamped concrete is the fact that differing amounts of antiquing were left on the surface of each slab. If a bit more antiquing is present on one pour than on the next, the slabs can look completely different when sealed. This is demonstrated by the photograph at the top of this page. For more on the topic of proper release removal, please read the article entitled “Too Much Antiquing Release!“.
Educating the customer about potential color issues is a vital step toward avoiding problems. One should make sure that customers know the materials’ limitations and what to expect. The customer should be shown the disclaimers on color charts. Decorative concrete is not a product manufactured in a controlled environment. It is crafted on site from diverse raw materials. Therefore it will have color variations, flaws and defects. If the customer is made aware of these facts up front, there will be far fewer problems down the road.
Mismatched Colors In Stamped & Decorative Concrete