Q. We are trying to use regular transit mixers to produce roller compacted concrete (RRC). Most of our markets do not have a central mixer available. We have developed a mix that discharges fairly well from a non-paving mixer. It has sand (39%), #67 limestone (10%), #8 pea gravel (48%), and Type C fly ash as a mineral filler (2%). The mix also has 300 pounds of cement, 100 pounds of Type C fly ash, and 18.5 gallons of water with a hydration stabilizer in the mix. The problem is how to prevent excessive buildup of this very dry mix. We are considering washing out the units every hour of production and then coating the mixer blades and drum with a hydration stabilizer. Any other suggestions on ways to minimize buildup on the mixer blades when dealing with this very dry mix?
A. The problem you describe is not uncommon with this type of mix. Batching order can make a difference. Batching the large aggregate first can help to clean off any of the previous batch that stuck to the blades. Follow that with some of the batch water and then the sand and cement, or you can add the water last. The key would be to have the dry aggregate do its work first.
The hydration stabilizer you are including in the mix should help, but perhaps you need to increase the dosage. If neither of those methods improves the situation, you might try batching bone-dry sand.
In the end, though, you might be trying to force a specialized tool (the transit mixer) to do a job it is just not well suited to do. One of the advantages of RCC is its simplicity--that it can be delivered with regular highway trucks and placed with asphalt paving equipment and relatively simple lay down skills. To take advantage of that you might look into using a mobile central mix plant.