Recently we supplied an integrally colored pea-gravel mix for a job that used textured vertical formwork. The contractor insisted on adding the dissolvable color bags at the jobsite just prior to placement. We dispatched 8-cubic-yard loads in standard 10-cubic-yard rear-discharge mixers.
The driver reported that the contractor instructed him to run the mixer for about 4 to 5 minutes of full-speed drum rotation after throwing in 15 1-pound bags.
Our belt placement operator later reported that he had to stop the conveyor several times to remove large pieces of undissolved paper bags.
While we haven't heard of any problems on the job from the contractor, this doesn't seem to be the way things should be done. What's the correct procedure to eliminate any bag residue in the concrete?
When integral color is properly batched with dissolvable bags, there should be no large pieces of container paper residue. In the case you described, you didn't follow several of the manufacturer's batching recommendations, so it's no wonder your belt operator reported problems.
Many manufacturers encourage producers to insist that color bag additions be made into the plant batching operation, not at the jobsite. Should customers insist on supplying their own coloring pigments, you should require them to drop off the packages at your plant.
There's an easy way to eliminate the chance of any significant leftover bag material when transit-mixing. First, batch about 50% of the load's water in the truck's empty drum. Then place the color bags in the water and wait for at least 2 minutes before adding the aggregate and cement. During this time the water impregnates the bag's outer surface, softening it and starting the dissolving process, allowing the pigment particles to disperse throughout the water.
The agitation from the aggregate batching will distribute the pigment throughout the mix water. The weight and friction from the coarse aggregate hitting the drum's fins further shred the paper. When batching 3/4-inch-diameter coarse aggregate, drivers should add at least 3 minutes of full-speed rotation to their normal loading process. When using pea gravel, drivers should mix at full-rotation speed for an additional 5 minutes.
Producers should consider several mixer-operational factors when determining how much time to allow for mixing with dissolvable bags. Batch operators should allow more time (rotations) when using fully loaded mixers. Additional time is also needed for drums with high fin wear, due to less effective bag shredding. Mixing the bags at the concrete plant mixing also allows the use of large pre-measured pigment bags. Pigment packagers offer several pre-measured bag sizes designed to match batch volume. For example, producers can correctly color-batch an 8-yard load, using either 15 1-pound bags or 1 15-pound bag. With less paper, one large bag significantly reduces the potential for residue from incomplete mixing compared with 15 small bags.
As shown in your experience, producers should let their customers know that they will not be responsible for any quality problems unless the coloring is plant-batched. In addition to paper-residue issues, there can be concerns of inconsistent air-content readings as high-speed mixing just prior to discharge may entrap air. And allowing jobsite coloring may lead to unauthorized changes to the water-cement ratio if workers try to field-match colors from load to load by adding more water.