Using soybean-derived materials to rejuvenate reclaimed asphalt pavement (RAP) binders and mixtures

Elkashef, Mohamed
Major Professor
R. Christopher Williams
Committee Member
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Civil, Construction, and Environmental Engineering

Over the past few years, the use of reclaimed asphalt pavement (RAP) has been growing consistently from 15% in 2009 to 20.3% in 2015. The desire to use higher amounts of RAP is inspired by the need to lower costs, conserve energy, and preserve the environment. Increasing asphalt prices, and limited supply of higher quality virgin aggregates, are strong motivations to use RAP as a replacement for the more expensive virgin asphalt and aggregates.

The main obstacle from using higher amounts of RAP is the aged and deteriorated properties of the RAP binder. With aging, asphalt binders suffer from oxidation which results in the conversion of part of the maltenes fraction to asphaltenes. Asphaltenes are primarily responsible for increasing the asphalt stiffness. The use of rejuvenators help restore the balance between the asphaltenes and maltenes, by adding more maltenes and/or improving the dispersion of asphaltenes.

Current rejuvenators that are available in the market are based on several materials including petroleum-based aromatic extracts, distilled tall oil, and other natural oils (i.e., organic oils). Bio-based rejuvenators have proven to be a better and safer alternative to petroleum-based rejuvenators containing aromatic compounds.

This research introduces a soybean-derived rejuvenator which is used to enhance the low temperature and fatigue properties of asphalt binders. During the first phase of the research, the effect of the rejuvenator is assessed by blending it with a neat PG58-28 and a polymer modified PG64-28 binders. Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR) tests are conducted to characterize the rheological properties of the rejuvenated binders. Temperature-frequency sweeps are conducted and complex shear modulus curves are constructed to compare between the control and the rejuvenated binders.

Dynamic modulus specimens are made using the rejuvenated PG58-28 and PG64-28 binders. The impact of the rejuvenator on both the dynamic modulus and phase angles is studied using master curves. A comprehensive statistical analysis using split-plot repeated measures (SPRM) is conducted to reveal statistical differences between the performance of the rejuvenator in both types of binders. The preliminary results indicate that the soybean-derived rejuvenator was successful at lowering both the high and low critical temperatures of both types of binders. The statistical analysis revealed that the extent of modification brought about by the rejuvenator was dependent on the binder type. The results of the dynamic modulus testing showed a consistent reduction in the dynamic modulus values and an increase in the phase angles with the use of the rejuvenator. A Fourier-transform Infrared study (FTIR) performed on the rejuvenated binders indicated that their aging behavior was similar to that of the control binders, indicating that the rejuvenator did not adversely impact the durability of the binders.

In the second phase of this research, a rejuvenated PG58-28 binders was blended with an extracted reclaimed asphalt pavement (RAP) binder. The fatigue behavior of the rejuvenated RAP binder is evaluated using linear amplitude sweep (LAS) testing. A significant increase in the fatigue life, particularly at low temperatures and increasing shear rate, is noted with the use of the rejuvenator. The rejuvenator was successful in lowering the performance grade of the stiff aged RAP binder to acceptable ranges. 100% RAP mixtures made and compacted into dynamic modulus and disk-compact tension (DCT) specimens were made using the neat PG58-28 and rejuvenated PG58-28 binders. The DCT specimens containing the rejuvenator showed higher fracture energy at a test temperature of -6oC which indicates better thermal cracking resistance. To assess the effect of blending efficiency, additional DCT specimens were prepared using extracted RAP binder blended with the rejuvenated PG58-28 binder. The RAP/rejuvenated PG58-28 blend was then remixed with the extracted RAP aggregate to simulate full blending. The DCT specimens prepared as such yielded even higher fracture energies indicating the significance of proper blending.

The thermal stability of the rejuvenated RAP binder was verified using thermogravimetric analysis (TGA). The mass loss due to thermal decomposition of the rejuvenated RAP binder was similar to that of the control binder. A study of the evolved gases using FTIR showed that the rate of mass loss of the rejuvenator can be inferred by comparing the FTIR spectra at different times.