Diluting or dissolving? The use of relaxation spectrum to assess rejuvenation effects in asphalt recycling

Abstract In asphalt pavement recycling, rejuvenators are often used to improve the engineering properties of asphalt binders in old pavements. A key question interested by practitioners is if the rejuvenators simply dilute the microstructures formed in aged asphalt binders or dissolve them. The latter is clearly preferred in effective asphalt pavement recycling. In this paper, a new approach is developed to assess rejuvenation effectiveness, using relaxation spectra calculated from the oscillation shear test data of asphalt binders. An indicator developed from the relaxation spectra enables the assessment of reduction in the proportion of large microstructures in aged asphalt binder. Experiments using three rejuvenators and three asphalt binders aged in different conditions prove the validity of this approach. Scan tunneling electron microscope (STEM) was also used to examine the possible changes of microstructures in asphalt binders before and after rejuvenation. STEM images indicate that rejuvenators can break down some microstructures formed during asphalt aging, but they cannot totally dissolve them. Observations from the STEM images are consistent with the results shown in the relaxation spectra. It is anticipated that the proposed indicator provides pavement practitioners a convenient way to evaluate the rejuvenation effectiveness.