BACK TO BASICS ASPHALT PLANT MIX SEGREGATION • The batcher should close on approximately eight inches of mix still in the batcher. This is to prevent a free flow of mix through the batcher. Mix that flows freely through the batcher will tend to segregate. • The batcher gates must be maintained so that they open quickly. The mix in the batcher must fall as a “slug” of material and splatter flat inside the silo. The splatter effect keeps the mix from piling up at its angle of repose, resulting in the larger aggregate tending to roll to the outside of the silo. • The batcher gates must be maintained so that they open together. If only one gate is opening, or the gates are not opening simultaneously, the mix could likely be unusable. • The batcher gates also must be maintained so that no wear will allow the mix to fall through the gates. This can also result in severe segregation. Again, if the plant design does not prevent segregation at the silo batcher, divert plates can be placed to address the issue. Truck loading is the last point where mix segregation can occur in the plant process. It is common knowledge that tandem trucks (20 tons) should be loaded in three drops. The first drop is the most important by far, because the mix comprising a potential end-of-the-load spot will come from this part of the truck. So, this drop must be as close to the cab-end of the truck as possible. In doing so, the corners of the truck bed are filled with a homogenous mix before the larger aggregate can roll off to the corners of the truck bed. The location of the second drop is less critical than the first, but should be as close to the tailgate as possible without overloading the rear axles. Then, place the third drop in the middle of the first two drops. It is the least critical of the three drops. In larger trucks for which there may be more drops, the first and second are the same as on the tandem (20 tons) truck. Finally, if mix samples are taken from the truck for testing, the samples must be representative of the mix being produced. Anytime the human element (i.e., lab personnel obtaining the mix samples from the truck) is introduced into the process, there is an increased likelihood of faulty technique, causing the sample to be segregated. The sample will not represent the mix being sent to the job, which can cause a job or entire plant operation to a halt. The sampling process has much less variability, however, when it is automated. There is also the added benefit of getting the lab personnel out of the back of a truck full of mix for safety reasons. Segregation can be a serious problem, but if the above precautions are taken, the likelihood of encountering segregation will be greatly reduced. Mix segregation can cause: AC content variations, bumpy ride, premature road failure, reduced payment, company reputation damage, new road removal and replacement, plant/job shut-down. VAASPHALT.ORG 23
RkJQdWJsaXNoZXIy Nzc3ODM=