Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/536721
EquipmentWorld.com | July 2015 51 A nd while use of RAP now requires pro- cessing, testing and separate stockpiling to a degree not done when it first came to market – add- ing costs for the asphalt producer – the enhanced control over RAP's composition means it can be used reliably in larger amounts in mixes with predictable results. It wasn't always this way. Begin- ning in 1970s, when extensive use of RAP came into use, skyrocketing crude oil prices first drove accep- tance of RAP as a way to lower the per-ton cost of hot mix asphalt. At that time, the price of hot mix asphalt soared during the Arab oil embargo, while sophisticated cold milling equipment came on the mar- ket. The aggregates contained within the new flow of RAP were permit- ted, shot, crushed, screened, tested and transported, saving the ultimate user enormous amounts of money in the cost of virgin aggregates. Also, up to that point, demolition RAP from construction projects had been landfilled, incurring tipping costs and using up landfill space better suited for refuse; now RAP could be reused as an industrial waste material. "Over the years, contractors generally stuck with having one stockpile of recycled material and feeding anywhere from 10 to 25 percent RAP into the mix," says the Asphalt Pavement Association of Michigan in an analysis. "This pro- duced significant cost savings, and the industry was content to remain at this level of recycling." A big bonus But there was an added bonus to RAP used as virgin aggregate re- placement: the RAP also contained a residual amount of asphalt envel- oping the aggregate particles. A controversy then followed over whether RAP was just a "black rock," or whether there was value locked into that bituminous coating. The characteristics of this residual asphalt were studied and quanti- fied, and soon RAP was assigned a value. That included the degree to which the residual asphalt allowed a reduction of liquid binder added to an asphalt mix. However, the quantification of residual binder now required that RAP be tested, and asphalt produc- ers got into the game of processing RAP into consistent sizes and testing those gradations for asphalt con- tent. Gone are the days of RAP as just a recycled aggregate; now, RAP producers create separate stockpiles containing particular particle sizes, and blend the RAP in stockpiles to ensure a consistency that can be dialed into an asphalt mix design. Shingles enter market As RAP use increased, reclaimed asphalt shingles (RAS) were also found to be beneficial. Like RAP, RAS contains asphalt and mineral components – with the added ben- efit of fibers – that can be used in pavement mixes. But the native asphalt in RAS is a much stiffer variety: the asphalt is "air blown," or purposely oxidized to increase its softening point and decrease its penetration. This is done to keep the asphalt stiff and solid on a hot roof. That may be good for roofing materials, but it is much stiffer than liquid asphalt binder and has to be accounted for in the design when RAS is added to an asphalt mix. While an asphalt producer or contractor may receive, store and process RAP in his yard using crushing and screening equipment in-house and thus saving money, RAS nearly always is processed by an independent supplier. This supplier reduces the shingles to a small gradation that will react more efficiently in a mix. The supplier certifies that the material meets state specs and also sorts, grinds and tests the RAS to make sure it does not contain asbestos, wood scraps or metal and is kept separate from pre-consumer (manufacturer waste) shingles. Valuable commodity That these recycled materials have great value is beyond question. The 4th Annual Asphalt Pavement Industry Survey, released October 2014 by the National Asphalt Pave- ment Association and sponsored by the Federal Highway Adminis- tration, shows significant growth End users of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt mixes – both contractors and government agencies – are benefiting from the lower raw material prices that the bi- tuminous and mineral reclaimed products bring to the table. Those components – which are 100 percent recyclable – replace virgin materials used in asphalt mixes, whether hot, warm or cold.