Achieving uniform color appearance with integrally colored concrete isn't as easy as you might think. Ambient conditions, the ready-mix producer, and concrete contractors must all work together for the best results.
The intensity of a color depends on how light reflects from a colored surface. For example, color appears more intense on a hard-troweled slab and duller on broom-finished concrete. If you don't want to see a contrast difference, every placement should be finished the same.
The total amount of water in a mix is important as well. Water beyond what is needed for hydration purposes, sometimes referred to as “water of convenience,” forms canals in fresh concrete and works its way to the surface and evaporates. With different amounts of water between batches or between the first part of a load and the last, the porosity of the finished surface changes, causing light to reflect differently.
Conditions causing problems
If you place concrete in the warmest time of the year and then place adjoining concrete when ambient temps are cold, the concrete at lower temperatures will bleed for a longer period of time before initial set occurs, thus more porous concrete results.
You also should be aware that concrete that bleeds more produces more efflorescence, giving finished work a whitish appearance that can be difficult to remove afterward.
Mike Clokey, manager of technical services, Nevada Readymix, Las Vegas, says that a common problem in his area happens when contractors place colored concrete by wheelbarrow or buggy—colored concrete frequently is placed in hard to reach areas. Long concrete placements requiring retempering with water, can result in the end of a load being more porous than the beginning. Adding a superplasticizer instead of water to the last half of a load can be helpful so long as you don't add too much. Adding a retarding admixture can cause problems if surface crusting conditions exist. Retarding admixtures also react differently when mixed with concrete that is already starting to set.
What to do
The most important step for matching color is to control the amount of water in your concrete. The total water should be the same for each load delivered. Water-cement ratios in the lower ranges are better than ratios at the higher end because less bleed will occur and the concrete will be more impermeable.
Arrange with your ready-mix producer to use the same mix ingredients in each load. Different portland cements or sand colors can change the color of your concrete.
If you know that your placements will take a long time, it's better to order smaller loads—small enough to place without adding water.
Place concrete at the same striking slump. When the slump starts to decrease, add a little superplasticizer instead of water. For more on water, see Measuring Water in Concrete.
Matching color to older installations
If you are placing colored concrete next to an older installation and have to match the color, you often an rely on the experience of your color manufacturer to help match the color. Some will provide its service to their customers for any size job. But it might also come down to you as the contractor making the decision. When this happens, saw off a piece of the existing slab and match color to the sawed surface. This color will most represent the original color. Using liquid dispensed color provides the opportunity to choose between a wide range of shades.