Successful High RAP Mixes Designed with Balanced Rutting and Cracking Requirements

摘要

This paper presents a comprehensive study focusing on methodologies to improve the performance of high RAP content mixes. Firstly this paper addresses one major concern many agencies have, that of 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. Furthermore, the authors evaluated the impact of RAP on optimum asphalt content (OAC), rutting/moisture resistance and cracking resistance. The findings are that OAC generally increases with more RAP (and RAS) usage, but the increase in OAC is small when the RAP content is below 20%; and increasing RAP content always improves rutting/moisture resistance as measured in the Hamburg wheel tracking test (HWTT). However, in the laboratory it was found that cracking resistance always reduces with increasing RAP content, especially when RAP content is 30% and above and also when RAP/RAS combinations are used. A balanced RAP mix design approach is proposed in this study. Recognizing the challenge of accurately estimating voids in mineral aggregates of the RAP mix, the authors recommend the use of the Overlay Tester (OT) to directly measure cracking resistance of RAP mixes and the HWTT for evaluating rutting/moisture resistance. Additionally, for Texas dense-graded HMA mixes designed using the Texas gyratory compactor, a maximum density of 98% is recommended to avoid over densification and potential bleeding. In the proposed balanced mix design approach the final asphalt content is selected after optimizing the mix density, HWTT, and OT requirements. 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 Interstate Highway (IH) 40 near Amarillo, Texas. More work is needed to develop criteria for different climatic zone and different pavement conditions.