Rheological behavior of a low crystallinity polyolefin-modified asphalt binder for flexible pavements

An asphalt binder (PG 64−22) is modified with a crystallizable copolymer of polypropylene (PP) and polyethylene (PE) to enhance its ability to resist rutting in flexible pavements during periods of warm weather. After cooling of binders from the mixing temperature at a controlled rate of 3 °C/min, their microstructure and rheological behavior are characterized. The modified asphalts exhibit two-phase morphology with droplets of a maltene-rich polymer phase less than 50 μm in size dispersed in an asphaltene-rich, continuous phase. X-ray diffraction patterns and optical birefringence microscopy indicate that the PP/PE copolymer segregates from the matrix during cooling, forming a network of interconnected, crystalline fibrils. At temperatures of (50–80) °C, at which rutting is an issue in pavements, addition of PP/PE copolymer substantially increases the rutting factor by a factor of 2–6, depending on temperature and concentration. At temperatures of (―20 to 0) °C, addition of 5 wt.% or less of the PP/PE modifier has a minimal impact on performance based on both s-value and m-value. The modified asphalts are s-controlled, with ΔTc less than about 6.4 °C, suggesting that the PP/PE copolymer is an effective modifier for reducing rutting in flexible pavements.