Question: We're supplying integrally colored concrete for a large architectural concrete project. We've supplied similar concrete previously but have always had trouble maintaining color uniformity. What steps can we take at the plant to minimize color variation?
Answer: Here's a list of tips we picked up from manufacturers of integral coloring materials:
- If you're using bagged color-conditioning admixtures or pigments, always use a whole number of bags per truck. Don't try to batch partial bags.
- Use the same batch size for each load delivered to the jobsite. This is especially important if color is batched from large bags. For example, if you're using three 50-pound bags for a 10-yard load, switching to an 8-yard load will require using a partial bag of color pigment or admixture. Imprecise batching of color pigments almost always causes color variations in the concrete.
- Don't change cement brands in the middle of the job. Cements from different sources are different shades of gray. Changes in cement color cause changes in concrete color. Also avoid changing aggregate or coloring-agent sources.
- Prior to batching, check each truck's drum closely for excessive residue or cleanout water.
- Hold slump within as tight a range as possible, and don't let the slump exceed 5 inches. Failing to adjust for cleanout water in the truck or changes in aggregate surface-moisture content causes water content and slump variation, which causes color variation.
- Don't add the color first. First batch some of the coarse and fine aggregate and water. With the mixer running, add the color and mix for at least a minute before adding the balance of materials. Have the driver turn the drum for three to five more minutes before the truck leaves the yard.
- Avoid admixtures that contain calcium chloride since it can cause discoloration.
- Work with the contractor to schedule truck arrivals without excessive waiting time.
- If it's a pumped job, supply color-matched slurry mixes to prime the pump line.
- When using mix designs that include pea gravel, extend initial mixing time by about 40%.
- If the project is to have either a sandblasted or exposed-aggregate finish, use smaller disposable bag sizes, or open each bag and pour the pigments into the mixer.
Despite your best efforts, color variations are still possible if the contractor doesn't use good finished and curing practices. For best results, the same finishing crew should finish all of the colored concrete flatwork. The crew should avoid adding water during finishing and avoid making too many troweling passes. Rotary, dry-broom or rough finishes usually cure with a more even-colored appearance than smooth-troweled finishes. The contractor should use a curing compound that is color-matched to the pigment or color-conditioning admixture.
- In addition, the contractor should have the mixer driver quickly view the jobsite before unloading. The subgrade should be well-drained and uniformly compacted. It's also important that the area is moistened evenly, with no puddles, standing water or muddy areas.