Research On The Microstructure And Rheology Performance Of Asphalt Under The Salt Freezing Cycles

With the rapid development of transportation industry, the asphalt concrete pavement was widely used. It has more rain and snow in winter in the north region and leads to snow and ice cover on asphalt pavement easily. The de-icing salt was often used for melting snow and ice. But it was prone to freeze-thaw cycles in large temperature different areas, further destory the pavement structure. Some damage phenomenon such as cracking and loose appeared, further more the asphalt pavement service life was reduced. Asphalt mixture is a multiphase dispersed system composed of mineral aggregate and asphalt. Its mechanical properties and road performance depend largely on the micro or nano level material properties. Especially as a cementing material, the performance of asphalt is particularly important to ensure the durability of the pavement.Matrix asphalt and two modified asphalt(SBS modified asphalt and rubber powder modified asphalt) widely used in the northwest region were analyzed. The scanning electron microscope, atomic force microscopy observations and dynamic shear rheological test, repeated creep recovery test and bending beam test were used to observe the microstructure and study the rheology performance of asphalt, and provided selection reference for pavement materials in cold region.SEM and AFM observation results complement each other can better understand micro and nano structure change of asphalt under different state. SEM results showed that all kinds of asphalt occurred different degrees of water aging after freeze-thaw cycles; especially after salt freezing, salt grains appeared in base asphalt and SBS modified asphalt, asphalt film was destroyed; while in rubber modified asphalt, rubber powder particles could prevent the growth of salt grain, but it can still maintain a better state after repeated of freezing and thawing. AFM results indicate that there were some “bee” structures in base asphalt and SBS modified asphalt, and the quality and size of “bee” structures changed after freeze-thaw cycles; but there was not “bee” structures in rubber modified asphalt, at the same time, there was little change in microstructure before and after freeze-thaw cycles.The DSR results showed that the high temperature performance of rubber powder modified asphalt was better than other asphalt. After freeze-thaw cycles, G*/sinδ of asphalt had very little difference from the original asphalt but δ changed relatively obvious. Grey relation entropy analysis of the rheological parameters before and after freeze-thaw cycles found that the biggest factor influence on the G* was de-icing salt solution concentration, followed by ductility at 10 ℃. The biggest factor influence on δ was the equivalent softening point, followed by the PI.The repeated creep recovery results showed that rubber powder modified asphalt had better high temperature performance. The εP/εL of SBS modified asphalt and rubber powder modified asphalt was increased at the beginning of the loading and then reach a steady state. The εP/εL of SBS modified asphalt reduced and the εP/εL of rubber powder modified asphalt unchanged after freeze-thaw cycles. Burgers model was utilized to fitting the parameter Gv. The Gv of SBS modified asphalt decreased and the Gv of rubber powder modified asphalt increased after freeze-thaw cycles. And the Gv of asphalt after water freezing had little different from that after salt freezing.The BBR results showed that the low temperature performance of SBS modified asphalt and rubber powder modified asphalt was better than matrix asphalt basically. And low temperature performance of different kinds of asphalt had different temperature adaptation range. The low temperature cracks resistance of asphalt reduced after freeze-thaw cycles. Compared with the matrix asphalt, the low temperature crack resistance of modified asphalt was relatively stable. The rheological parameters of asphalt before and after freeze-thaw cycles were analysed by grey entropy analysis. The results showed that the biggest factor influence on the S was de-icing salt solution concentration, followed by ductility at 10 ℃. The biggest factor influence on the m was test temperature, followed by the freezing temperature.The research results showed that the rubber powder modified asphalt had better adaptability to salt freeze-thaw cycles. De-icing salt solution concentration had a greater influence on the solubility of the high and low temperature performance of asphalt, followed by the asphalt performance itself. So we should strictly control the de-icing salt amount and choose appropriate asphalt materials in cold region.