At every stage handling aggregate and asphalt, there is a chance for material segregation to occur. At the quarry, improper stockpiling techniques and loader handling can compromise spec product gradations. At the HMA plant, again how the aggregate is stored and the use of improperly designed self-relieving cold feed bins can increase the chances of particle segregation. Then, after the asphalt is mixed, silo storage and truck loading practices at the plant can further segregation problems.
En route to the site, the asphalt will undoubtedly develop some degree of thermal segregation. Invisible to the naked eye, temperature differentiations will exist mainly with the asphalt at the top and sides of the truck. Thermally segregated material makes the asphalt more difficult to compact and affects mat densities, which will ultimately affect mat life.
With numerous opportunities for segregation to occur even before the asphalt reaches the jobsite, it does not seem quite fair that the paving crew is now challenged to lay a high-quality, smooth mat void of segregation. Fair or not, however, it is a fact of life. Luckily there are some paving techniques and equipment adjustments and options, which allow paving contractors to deal with and minimize segregation issues.
Education and paving practices
Minimizing segregated material exiting the screed can be accomplished in several ways before adding equipment. First and foremost, the paving crews must be thoroughly educated in the correct paving techniques and how to properly use each piece of equipment on the site. Crews should be trained on the equipment they will be using in the field and know the capabilities and limitations of each piece. No amount of paving equipment will end segregation unless the crew operates it in the correct manner.
There are a number of quality paving practices that will help to reduce segregation in the mix. Beginning at the plant, using the three-dump method for loading trucks - dumping two batches at each truck end and a final batch in the middle - will reduce larger particle run-off to the sides and end of the truck, reducing particle segregation.
Once at the jobsite, the paving crew can help reduce segregation by employing the following practices:
Controlled hopper wing cycling - The wings are where the large, cooler stone tends to collect if not properly re-introduced back to the mix. Regular cycling, where allowed by spec, will reduce large build-ups of this segregated material.
Keep the hopper full - Leave as much surge as possible between truck exchanges and do not run the hopper empty. This will minimize cyclical segregation by allowing hot, uniform material from the next truck to blend with mix from the previous dump.
Keep a constant head of material at the spreading augers - A consistent flow of material to the spreading augers will prevent them from spinning too fast or too slow, which can cause longitudinal segregation.
Properly timing the conveying and spreading systems - Ensure the ratio pots or flow gates are set to deliver enough, but not too much, material to the spread augers so they run continuously.
The correct lead crown setting and proper strike off adjustment - Equipment fine-tuning issues that will help eliminate longitudinal segregation.
Having the correct spread auger length - This will ensure the proper amount of material conveyed to the screed end plates.
For more on the story, read the August issue of ROADS&BRIDGES.