Characterization and best use of recycled asphalt shingles in hot-mix asphalt.
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Characterization and best use of recycled asphalt shingles in hot-mix asphalt.

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      Recycled asphalt shingles (RAS) often containing more than 20 percent asphalt binder has become another black gold in the asphalt industry. There are two basic types of RAS scraps in the market: tear-off asphalt shingles (TOAS) and manufacture waste asphalt shingles (MWAS). Both TOAS and MWAS have been used in hot-mix asphalt (HMA) paving in Texas. Since RAS binder is very stiff, lots of concerns have been raised on using RAS in HMA. The researchers conducted a comprehensive investigation on HMA mixes containing RAS, including RAS binder characterization and blending charts for virgin/RAS binders, impact of RAS content on optimum asphalt content (OAC) and engineering properties of RAS mixes, and approaches for improving cracking resistance of RAS mixes. Furthermore, a variety of RAS field test sections were constructed to validate the approaches for improving cracking resistance of RAS mixes. Additionally, this report discusses the environmental and economic benefit of using RAS in HMA. Based on the research performed, it was found that TOAS binders with an average of high temperature grade of 175°C are much stiffer than MWAS binders, which have an average of high temperature grade of 131°C. Impact of MWAS binder on the high and low PG temperatures of virgin binder is different from that of TOAS binders. Thus it is important to consider differentiating MWAS from TOAS when used in asphalt mixes. The use of RAS has no significant influence on dynamic moduli of HMA mixes, but improves their rutting/moisture damage. However, RAS mixes have very poor cracking resistance, compared with the 0 percent RAS mixes with PG64-22 or PG70-22, even though the RAS mixes have higher OAC. Two approaches for improving cracking resistance of RAS mixes were explored in the laboratory. The test results clearly indicated that both using soft binder and increasing design density can improve cracking resistance of RAS mixes. When considering rutting/moisture damage of RAS mixes, using soft binder is superior to increasing design density. Furthermore, a series of field test sections have been constructed around Texas. The observed field performance of the test sections indicated that RAS mixes can be successfully used as surface layer of new construction with strong foundation. It also has been observed that the use of extra virgin asphalt binder (or increasing design density) can effectively improve cracking performance of RAS mixes. Apparently, more work is also needed to investigate the performance of RAS/RAP mixes produced at the warm mix temperatures.
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