Most mill-and-fill projects will see the haul truck receive millings, drive those millings to the plant, dump them at a designated area, load with mix for the job, and return to the job to charge the hopper. There are some additional steps the haul truck driver needs to take to ensure the milling fines (the number 200s) don’t contaminate subsequent loads and mar the mat.
Here’s the problem: Not all truck drivers take the time to clean out the bed after dumping the millings. This leaves stray millings and dust from the fines melted to the truck bed, which was still hot from unloading HMA. When he drives under the silo gates, new HMA loads on top of cooled chunks and fines. When he delivers mix to the paver, those 200s look like dust on top of the new mat we’re paving.
Here’s the solution: When the truck drivers get to the plant, have a designated wash-out area and release agent spray station before the loadout scales. For a project in Alabama, we introduced a backhoe to scrape out the bed, ensuring the melted fines and millings were all cleaned out before the truck went under the release agent spray station. It’s just not feasible to let a truck sit at a cleanout area on the side of the road, cooling for 30 minutes before getting in front of a milling machine, so the next best thing is to engineer quality control at the plant.