Tech-Notes on Coloration Consistency
There are several factors that affect the ultimate color of any colored concrete product. Variations in raw materials, mixing, curing, and finishing can all result in inconsistent color in the finished product. This tech note will deal with raw materials and mixing.
The necessity of consistent raw materials is absolute. You must hold the aggregate, cement , sand, and color constant throughout an entire project. Each ingredient must be added by weight, not by volume. The aggregate must have the correct sizing and screen analysis. The aggregates’ distribu-tion and their relative exposure at the surface must not vary dramatically. If varying amounts of ag-gregates are visible at the surface, the overall color appearance will change. The type and brand of cement must also remain consistent. Changing from Type I to Type III portland cement within one job will cause color variations because Type III portland is a finer grind of cement than Type I. Even though the color changes of the cement would be minimal, it is recommended not to change types of cement.
The final raw material that must remain constant is the coloring. The color itself, obviously, must remain constant and the amount added per batch may not vary. Batch size bags are recommended. This would mean that the actual weighing of the color is taken away from your people and put into the hands of the pigment manufacturer. It is also strongly recommended that if you are using dry pigment, then you only buy pre-blended colors. This would mean that if you do have to weigh the color, it would only be a single color and not two or three colors that would be subsequently blended. Pre-blended, batch size bags are available in any color and for any batch size from DCS. If, for some reason, you do not buy batch size bags, always add the color by weight and not by volume. Pigments have varying densities and therefore can not be accurately added volumetrically.
Once the raw materials are properly controlled, mixing is the next step. Always add the color at the same point in the mixing process. The best sequence is to dry mix the aggregate, sand, cement, and color for a reasonable amount of time, then add the water and continue to mix for an additional fixed amount of time. It is imperative that the length of both dry and final mixing be consistent. The big-gest potential problem with mixing is undermixing, which will result in streaks or blotches of color, especially when only a small quantity of color is added per batch. It takes a long time to evenly dis-perse a small amount of color in a huge mass of aggregate, sand, and cement.
Each factor discussed here is important to color consistency, but variation in moisture content is probably the single most common cause of color consistency problems. A change in the water/cement ratio will always result in color inconsistency problems from batch to batch. Understanding how the water/cement ratio affects color will explain why this ratio must remain constant. On the surface of a piece of concrete are tiny bubbles that form during curing. Light refracts off of these bubbles. The color perceived is directly affected by the number of air spaces on the surface. The more bubbles that are present, the lighter the color appears. The number of bubbles present depends on the water/cement ratio. As the water/cement ratio increases, the number of bubbles that form increases, and therefore the color appears lighter.
The water/cement ratio is determined by the moisture level of all of the ingredients including the amount of water added. What this means is that the water addition may need to be adjusted as the moisture level of the raw materials varies. In order to make the correct adjustments and keep the total moisture level constant, a moisture meter is recommended.
There are several types of moisture meters available. Generally, what they consist of is some type of probe in the mixer that would determine the moisture level of the ingredients and determine the amount of water to be added to achieve a specified water/cement ratio. Many of the metering devices automatically add the proper amount of water after performing this measurement.