What happened to the good old days of making it hot and black? Customers have become more sophisticated; consequently, hot mix suppliers and equipment manufacturers continue to strive to meet their requirements.
Department of Transportation agencies across the country are specifying mix designs with ingredients difficult to control, like hot aggregate dust, fly ash, hydrated lime and fiber. These ingredients may not be required often, and feeder systems can be rented as opposed to being purchased.
The choices are many and confusing when you take into consideration the ingredient equipment, the asphalt mixing process, the DOT introduction spec, how often you may make this specific mix, the ingredient’s availability, MSDS and handling characteristics.
These ingredients vary in density and flowability depending on aeration, compaction and bridging characteristics, making measurement and control difficult. This problem makes volumetric control unacceptable and gravimetric control a necessity. Just because you know vane feeder speed does not mean you know the amount of material being introduced into the mix. When it comes to stone matrix asphalt (SMA) mixes in particular, precise control of the minus 200s is critical to a successful mix.
A vane feeder is much like a cold feed bin in that both are volumetric feeders. The aggregate in cold feed bins is uniform in density. The aggregate in cold feed bins flows easily without mechanical stimulation. One revolution of a cold feed bin might yield 1,000 pounds of aggregate if the bin is not bridging or running empty. If the bin runs empty, it does not take long for the loader operator to visually notice or the aggregate belt flow scale to sense the difference.
The success of the cold feed bin and the vane feeder as reliable volumetric feeders differs sharply. Mineral filler in a silo may be quite uniform in density when the material is flowing through the silo. That changes when material is being aerated over time, or the amount of aeration or vibration changes, or if the silo is being filled at the time. During vane feeder volumetric calibration, one revolution of a vane feeder may yield 10 pounds if the silo is not bridging or running empty. If the silo runs empty, it may take a long time to notice. The vane feeder is totally enclosed and impossible for any plant personnel to see that it is running empty. Unless a gravimetric flow scale is measuring flow down stream, there is nothing to sense the difference. Meanwhile, thousands of tons of mix has been produced, trucked, laid, milled up, trucked back and then recycled. The cost in all this waste is considerably more than the cost of adding a flow measurement device.
Flow measurement devices
The process of blending these materials into mix requires a continuous weighing device that provides an electronic signal like you would get from a belt scale. Belt scales do not work with powders and fibers because of the ease of these materials becoming airborne. Therefore, the scales must be totally enclosed.
• Nuclear. Expensive to own and operate.
• Silo on-load cells depletion. Good for inventory control, bad for immediate accurate control.
• Weigh depletion hoppers. Good for flow control of dust, fly ash, calcium carbonate, fiber, but not as good for hydrated lime because the silo hydrated lime flow starts and stops when charging the weigh hopper; the repetitive silo flow interruption sometimes encourages bridging. Aeration is necessary for the pod as well as the silo. Long 60-degree cones are necessary for maintaining flow.
• Continuous flow scale. Good for flow control of hot aggregate dust when temperature is compensated; good for fly ash, calcium carbonate, fiber and hydrated lime. Flow scales are the least expensive to buy and operate.