United States Department of Transportation
i
Blending Efficiency of Asphalt Mixtures Containing Recycled Asphalt Pavement and Shingle
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2017-03-01
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[PDF-9.86 MB]
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Abstract:This study aims to address blending issues in warm mix asphalt (WMA) and hot mix asphalt (HMA) containing recycled asphalt pavement (RAP) and recycled asphalt shingle (RAS). Two fingerprinting methods of asphalt covered in this study are gel permeation chromatography (GPC) and atomic force microscopy (AFM). Laboratory testing methods for determining blending efficiency employed in this study include determining aged binder mobilization rate and staged extraction method. The factors affecting the blending efficiency include mixing time, mixing temperature, aged binder content and effect of WMA technology. Major conclusions from the study are summarized as follows: 1)There existed a strong correlation between the percentage of large molecular size (LMS) and the complex modulus (G*) of asphalt binder based on the comparison of GPC and dynamic shear rheometer (DSR) test results. As mixing time and temperature increased, more blending occurred in the RAP/RAS mixture. The size of virgin aggregate did not affect the blending efficiency of RAS in pavement mixtures. 2)“Blending Charts” could be generated between the RAP/RAS binder content in the blend with the large molecular size (LMS%)defined by molecular weight and the relations were found to be linear. The mobilization rate of RAP binder decreased with the increase of the RAP percentage in the mixture. RAS binder mobilization rate increased with RAS percentage growing from 2.5% to 5%, but decreased with RAS percentage passing 5%. The highest mobilization rate was around 61% and found on 5% RAS mixture while the mobilization rate of mixture containing 10% RAS could be as low as 36%. 3)Trichloroethylene (TCE) was found to be the most effective solvent used in the study for staged extraction. The binder coating on the raw RAP and RAS aggregates was proved to be homogeneous and the layer stripping did occur in a well-controlled composite binder system. Based on well-designed staged extraction and GPC analysis, binder film coating virgin aggregates was approximately homogeneous in RAP mix, while a non-homogeneous binder was generated on RAP aggregates. A potential composite binder system was found coating the virgin aggregates in RAS mix. The diffusion study shows that within the mixture storage time, binder diffusion can be accomplished in both warm and hot mixes containing RAP, indicating old binder mobilization, rather than binder homogeneity, could be more critical in RAP mix. The binder diffusion in RAS mix was captured in a very slow rate. 4)WMA additives yielded higher blending ratio than control mix produced at 135C, but the temperature of 165C still produced the mix with the highest blending ratio value. This indicates that a concern still exists over asphalt blending even if WMA additives are used. Foaming technology yielded a higher blending ratio, indicating foamed WMA may yield a higher blending than regular HMA. It was also found that temperature rather than coating is more critical in RAS blending. 5)According to the observations, AFM proved to be capable of differentiating virgin binder from RAS binder in terms of microstructures. The microstructures of tear-off RAS binder was found to be temperature-dependent, but changed very little within the range from 60C to 180C. Virgin binders selected in this study could not blend through a RAS binder layer of 300 μm within 30minutes at 180C. On the basis of observations on the interfacial zone, RAS binder was found to be “mixing” but not “blending” in a mixing zone of 25 to 30 μm.
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