High RAP mixes design methodology with balanced performance.
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High RAP mixes design methodology with balanced performance.

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    "The use of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) can significantly reduce the increasing cost of hot-mix asphalt paving, conserve energy, and protect the environment. This report presents a comprehensive study focusing on methodologies to improve the performance of high RAP content mixes. Firstly this report addresses one major concern—RAP variability. It was found that in Texas the RAP materials are consistent and have low variability in terms of aggregate gradation and asphalt content, within individual stockpiles. The authors evaluated the impact of RAP on optimum asphalt content (OAC), rutting/moisture resistance, and cracking resistance. OAC generally increases with more RAP (and RAS) usage, but the increase in OAC is small when the RAP content is below 20 percent; increasing RAP content always improves rutting/moisture resistance as measured in the Hamburg wheel tracking test (HWTT). However, in the laboratory cracking resistance always reduces with increasing RAP content, especially when RAP content is 30 percent and above and also when RAP/RAS combinations are used. Additionally, the use of 10–15 percent RAP, without lowering virgin binder PG grade has a small impact on rutting/moisture and cracking resistance, but the influence, especially on cracking resistance, is much more significant when higher levels area used.

    A balanced RAP mix design approach is proposed in this study. In the proposed balanced mix design approach the final asphalt content is selected after optimizing the mix density, HWTT, and Overlay Test (OT) requirements. RAP handling in the process of mix design is critical, especially the mixing and compaction temperatures for high RAP mixes. It was recommended that the mixing and compaction temperatures used for the virgin binder be used for the RAP mix as well. Finally, the balanced RAP mix design procedure is demonstrated and validated through the construction of field test sections containing different levels of RAP. One of the interesting findings is that cracking requirement in terms of OT cycles should vary, depending at least on climate (cold vs. hot), traffic (heavy vs. light), and existing pavement condition (overlay over cracked pavements vs. new construction). For asphalt overlays over severely cracked pavements, a minimum OT requirement of 300 cycles previously proposed was further validated with performance data from the RAP sections on IH40 near Amarillo, Texas. More work is needed to develop criteria for different climatic zone and different pavement conditions."

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