Influence of surface layer bitumen on the performance evolution of base layer over service time

In conjunction with the aggregates, bitumen is commonly applied for surface and base layers of asphalt pavements. A large number of studies concentrated more on the surface layer performance influenced by different types of bitumen and service time. However, the performance evolution of base layer bitumen properties influenced by surface layer bitumen over time remains understudied. To contribute to this gap, this study aims to investigate the influence of surface layer bitumen type on base layer performance with varying service time. Four road segments with two-layer structure (surface layer and base layer) serviced for 4, 6, 7 and 9 years were studied. The surface layer was paved with neat bitumen with Pen 70/100 and polymer modified bitumen (PMB), and the base layer was applied with soft bitumen with Pen 160/220. Physical properties, chemical structure, intermediate-temperature rheological properties, permanent deformation resistance, anti-rutting ability and recovery properties of the field bitumen were analysed and compared with the reference bitumen. The temperature- and frequency-dependent characteristics of asphaltic core samples, as well as the reference mixtures, were characterised. The statistical analyses were performed to quantitatively evaluate the significance of service time and bitumen type of surface layer to base layer performance using mean absolute percentage errors (MAPEs), Pearson correlation coefficient and eta squared. The results showed that the performance of the surface layer was greatly related to service time, and PMB presented better durability and anti-ageing properties than 70/100. Notably, the bitumen type of the surface layer played an important role in base layer performance except for service time, especially in penetration, phase angle and Jnr with eta squared of 0.925, 0.976 and 0.988. Moreover, the ageing speed of the base layer was highly determined by the bitumen type of the surface layer. The findings imply that a better bitumen type of surface layer can enhance the durability of both surface and base layers, resulting in significant improvement in lifespan of asphalt pavement.