26 SPRING/SUMMER 2023 BACK TO BASICS Best Practices: Stockpile Management T.J. Young, President, T2ASCO LLC Over 95% of asphalt mix for federal, state, and local governments is produced through “drum-mix” or continuous mix facilities. Since gradation is controlled at the feed bins, managing the integrity and consistency of stone stockpiles is critical for final asphalt mix success. Add to this the common use of blended aggregates, wherein rock sizes are not “pure” or uniform in size—unlike our European cousins who employ fractionating stone sizes—and the need for proper stockpile management is even more acute. Across millions of tons of mix production in every state, blended aggregates have been proven to make amazingly consistent asphalt at a lower overall cost because production is quicker and more cost-efficient, using fewer feed bins. However, this ultimately relies on a clear understanding and managing the makeup of stockpiles to • Eliminate contamination from the surface where aggregates are stored • Eliminate cross-contamination between stockpiles, and • Minimize/eliminate segregation caused by stockpiling techniques. The most common stockpiling techniques are horizontal stockpiling, stockpiling with radial stackers, and bunker stockpiling. Depending on the stockpiling procedure, the asphalt producer should employ best management practices (BMP) to ensure consistent mix production. One BMP common across all stockpiling techniques is ensuring the consistency of the gradation coming from the quarry. As we have established, most asphalt plant operations in the United States do not use “same size” aggregates but use blended aggregates such as AASHTO #8, #10 or #68 to produce a mix. However, within these AASHTO-designated sizes, large tolerances exist for each size stone. Therefore, it is imperative that the gradation of aggregates used in the mix design is consistently delivered from the quarry and does not vary from being too fine to too coarse while still meeting the AASHTO specification. Another typical BMP is preparing the surface for the placement of the aggregate. Again, the aggregates must not be contaminated by the underlying surface. Therefore, placing the aggregates on a sloped surface— and, if possible, on a paved surface—is best. This will mitigate contamination and promote moisture drainage from the stockpile. For every 1% moisture change in the aggregates, the typical plant’s fuel consumption is reduced by 10% and production capability increased by 13%.1 In addition to the fuel savings and production increases, the global warming potential for the mixture is reduced and thus the material’s carbon footprint. A third BMP is “lifting and placing” aggregate versus “pushing” aggregate when forming or replenishing a stockpile. In Virginia, dump trucks bring materials from quarry stockpiles on-site or off-site to feed asphalt plants at most locations. When the truck arrives at the plant, the aggregate is dumped and moved to the stockpile. Aggregate tends to separate when pushed, and aggregate segregation will certainly occur
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