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Understanding Colored ConcreteCommon Problems, Why They Occur, and How to Avoid and Fix Them.
Water to cement ratio chart. Courtesy of Lanxes Corp.
Adding additional water during brooming creates inconsistent color streaks.
Becoming proficient at the basics before moving on to more difficult tasks is important in most everything, especially when it comes to decorative concrete. Success in this business is built on a concrete foundation, literally and figuratively. This comes from experience and an in depth understanding of the products and process. Nothing is more basic to architectural or decorative concrete then integrally colored concrete. Becoming familiar with integral color, how it works, factors that affect final color, and methods for fixing colored concrete issues are important steps toward becoming an expert in all aspects of decorative concrete.
Background of Colored Concrete
In the early 1950s, the F.D. Davis Company introduced contractors in Southern California to the idea of adding synthetic iron oxide, at the time a waste product from chemical manufacturing, to their gray concrete mix. By adding this colored powder to traditional gray concrete, it became possible to achieve a wide range of earth tone colors. Fast-forward fifty years, where in 2004 more than 204 million pounds of synthetic iron oxide were used to color cement-based products in North America alone. While we may not repeat that kind of growth over the next fifty years, decorative concrete is forecasted to continue to lead all other segments of concrete construction in growth well into the future. Since iron oxide pigments are used in most every decorative concrete product, and integrally colored concrete is in itself the largest decorative concrete market segment, doesnt it make sense to take some time to understand this popular and key building block of our industry?
How Concrete Gets Colored
Understanding how to avoid and fix problems starts with understanding how the product works, as well as major factors that affect the final outcome. First, we need to use the right terms. Color for concrete is not a dye, stain or paint. They are pigments, either mined from the ground, or most often manufactured in huge chemical plants around the world. They are available in powder, liquid and granular forms, with no one form better then the other. To understand how concrete gets colored, you need only know that iron oxide pigment particles are ten times smaller in size then a particle of cement. When color is added to any cement based mix, the smaller pigment particles cover the larger cement particle. This is why color is dosed based on cement content (sack mix) and nothing else.
Changing cements mid-job will result in a color difference.
Inconsistent cure produces inconsistent color which is much more noticeable in colored concrete versus gray concrete.
Close-up of curing differences.
Half the project was cured with a colored curing compound; the other half was not.
Combine different colors and textures. Break up large, integral color projects with different color and texture bands.
Controlling all the variables can produce large, integrally colored concrete projects.
Colored concrete will often be scrutinized because color magnifies issues that would not be a factor in gray concrete.
When it comes to major factors that affect color, the most critical is water to cement ratio. Controlling the amount of water added to the concrete mix is critical to producing consistent color. The addition of water permanently changes the concrete, typically lightening the final color. For example: The first concrete truck arrives and is placed as is. The second one has 5 gallons of water added. At noon the last truck has 15 gallons added. you now have three different colored slabs! Use slump control admixtures available from ready mix suppliers. Or use the powdered form for use on the jobsite from manufacturers such as Fritz Pack. These products make your job easier while eliminating jobsite added water! This is not only true when additional water is added to the concrete mix, but also when water is added to the surface during the finishing process.
If the surface is drying out, or the weather is hot and windy, dont use water! Instead, use a surface evaporative control agent. Available at most concrete distributors, these surface evaporative control chemicals are a must for anyone placing any type of decorative concrete. While all of these surface evaporative chemicals slow the hydration of concrete in hot windy conditions, some are even designed to help finish wetting out color hardeners if the concrete is drying out to fast.
The role that the gray cement plays in the final color.
The second key factor to consider is the role that the gray cement plays in the final color. Consider that the color you add to the mix has to overpower the gray base color of the concrete. These two colors come together to form the final color we see. This is why colors (in gray cement) are all darker earth tone shades. You can achieve lighter color shades in concrete, but that requires the use of expensive white cement. Another important consideration regarding gray cement is that they are not all the same shade of gray. In a recent comparison study of Portland type I Cement from multiple suppliers in Northern California, color ranged from almost white to battleship gray.
This reinforces the practice of maintaining batch-to-batch consistency. Deal with reputable ready mix supplies that take steps to control these variables, and never switch ready mix suppliers in the middle of a colored concrete project! Make sure your RM supplier understands colored concrete and that they use cement from the same lot (or at least the same cement company!) for the entire job. If a job starts with light cement and then the RM supplier tops off with another color of cement expect color differences. Second only to water related issues, cement is a major culprit of color differences in large pours.
Proper Curing Required
The remaining factors, while not as critical, are important and need to be considered and controlled: The proper curing of concrete is important to reduce surface shrinkage cracking and obtain the proper strength. It is even more important in colored concrete because lack of curing produces inconsistent color. Slight color or shade differences in gray concrete are seldom noticed.
Those same slight color differences can more often then not hold up your payment or even result in concrete removal and re-pour. To help avoid this potential problem, the use of a matching colored curing compound or colorwax is always recommended when pouring colored concrete.
In fact some manufacturers color cards and colored samples are based on a finish with a colored curing compound. Several Cure and Seal (meets ASTM 1315) products are available on the market as well. Make sure they say they are non yellowing,blush resistant or for decorative concrete. Regular C-309 cures will discolor colored concrete, and non C309 cures (i.e. penetrating cures) will not hold in the moisture uniformly. Do not use polyethylene sheeting. Wrinkles not in contact with the slab will cure (color) differently than areas in solid contact.
When Placing Large Areas of Colored Concrete
The last of the key factors that affect color are sub base preparation, placement, finish and maintenance. As with most issues in architectural concrete, color tends to magnify the above issues that are overlooked or unnoticed in gray concrete. When placing large areas of colored concrete over days, weeks or months, take into consideration the ability to maintain color consistency.
Consider breaking up the large areas with bands of different color and or texture. For large pours of one color, it might be worthwhile considering the use of dry shake color hardener that takes most color related surface variables out of the picture.
Offer maintenance services for the colored concrete you install.
The issue of maintenance in regard to colored concrete often never comes up when the product is sold or promoted. This comes from the fact that gray concrete is considered a maintenance free material. Once again, add color to concrete and you can expect the people writing the check to scrutinize the final product from the day it is poured and for years to come. Without periodic cleaning and resealing, the color will change. Notice how I said change, not fade. If you use pure iron oxide pigments, which most reputable suppliers do, these colors will not fade. Years of surface affects such as efflorescence, pollution, dirt and traffic take their toll on the concrete surface giving the appearance of fade. Remember how we discussed how color surrounds particles of cement in order to overpower the gray and impart the chosen color? As these color-coated particles of cement are worn away you start to see bits of sand and other small non-colored aggregate in the concrete. Those natural colors will make the color look faded. Typically a good cleaning and sealing brings back the original color even after years of neglect and lack of maintenance. In an effort to avoid future callbacks and client discontent, clever applicators around the country have begun to offer maintenance services for the colored concrete they install. Each year, or as needed, they charge an agreed upon fee to return to reseal their work. This not only keeps their name fresh in clients minds for future work, but also generates a nice steam of revenue on repeat basis.
Techniques that may save a slab with inconsistent color
Unfortunately I tend to get a lot of calls after the concrete has been poured where one or more of the above-mentioned factors are at play. When inconsistent color becomes a problem on your project, one or more of the following techniques may save the slab from having to be ripped out and replaced.
Surface effects can produce inconsistent color.
After it is cleaned and sealed, the proper color in the concrete comes to life.
The most common and cost effective method for dealing with blotchy, inconsistent, or a wrong color is hiding it with a tinted sealer. Adding color to sealers has been used for years to cover color problems. What most applicators are not aware of is that tinted sealers are available in water and solvent bases, as well as multiple levels of gloss and opacity. You can buy a pre mixed colored sealer, or add tint concentrates to clear sealers on site. Key factors that need to be considered in the decision making process of what type of tinted sealer to use are based on the surface to be coated (sealed or un-sealed) level of hide or opacity, and type of gloss desired. While piling on multiple coats of any colored sealer will eventually create a totally opaque coating, this is not recommended or desired. Typically one or two coats are sufficient to provide the desired effect. The solids content and type of carrier tend to determine the translucency (how much you can see through the tinted sealer) of the sealer. Higher solids and water-based sealers tend to be more opaque then lower solids solvent-based sealers. Also solvent-based sealers are always higher in gloss then water based sealers. I recommend that you do some research and testing as well talk with your local distributor or manufacturers representative about their offering of tinted sealers. Depending on type of tinted sealer, typical material cost per square foot runs $0.07 to $0.25.
The last, and most costly method for fixing inconsistent color is the use of polymer modified thin section toppings. These micro thin toppings have come a long way in the last few years in regard to adhesion, strength, freeze thaw resistance, and durability. They are available in both interior and exterior grades, in virtually any color, and can be finished to look exactly like concrete. Most require minimal surface preparation (over clean, sound concrete) and last for years with minimal maintenance. Depending on the type of overlay, material cost per square foot will run $0.70 to $1.50.
Integrally colored concrete remains a staple in the world of decorative concrete. While the technology has changed little in the last 50 years, its uses and popularity continue to grow. While Sun Belt regions of the country have known the benefits and money making potential of colored concrete for years, other areas are just now discovering its many uses and aesthetic value. Whether you are an established decorative concrete professional, or thinking about getting into architectural concrete, understanding the basics of integrally colored concrete, and how to troubleshoot common problems is a key building block to becoming an expert!
Chris Sullivan writes feature articles for The Concrete Network. He is the Vice President of Sales and Marketing for ChemSystems Inc.