Investigating Mechanisms of Porous Structure of Steel Slag Affecting Low-temperature Crack Resistance of Asphalt Mixture

The macroscopic and microscopic mechanisms of porous structure of steel slag aggregates affecting low-temperature crack resistance of asphalt mixtures remain unexplored, hindering the effective maintenance of long-term performance of asphalt pavements incorporating steel slags. To address this deficiency, based on the results of laboratory low-temperature beam bending tests on asphalt mixtures incorporating steel slags (AMISS), a refined discrete element method (DEM) model of AMISS was established characterizing the porous structure of steel slag aggregates. DEM simulations of low-temperature beam bending tests were performed under different levels of surface pores of steel slags and absorbed asphalt by surface pores to quantitatively analyze the influence patterns and mechanisms of steel slag aggregates' surface pore structure on the fracture strength and micro-crack evolution characteristics of the specimens. Laboratory test results indicate that porous steel slag aggregates can only replace coarse aggregates larger than 2.36 mm, and that controlling the heating temperature of steel slags and extending the wet mixing time can ensure adequate absorption of asphalt binder by surface pores of steel slag aggregates. Numerical simulation results show that stress concentration occurs near the open surface pores of steel slag aggregates with micro-cracks propagating through the open surface pores. As the surface porosity of steel slag aggregates increases, the peak stress decreases, and micro-cracks are more likely to initiate around the open pores. The peak stress significantly decreases with decreasing level of absorbed asphalt by the open pores. The absorption of asphalt binder by the surface pores of steel slag aggregates significantly influences the low-temperature crack resistance of AMISS, especially micro-cracks tend to initiate from the open pores. This crack extension feature is more obvious when the open pores are barely filled by asphalt binder. The surface pores enhance the slag-asphalt interface bonding. However, larger open pores lead to stress concentration, making it easier for cracks to initiate in their vicinities. Therefore, it is recommended to control the content of steel slag aggregates with large open surface pores. In pavement applications, the amount of asphalt binder added into the drum drying the steel slags and the wet mixing time should be well controlled based on the natural moisture content of the steel slag aggregates to ensure low-temperature crack resistance of AMISS.