Evaluation of the effects of asphalt binder aging degree on the curing, compatibility, and mechanical behaviors of epoxy asphalt binders

The application of epoxy asphalt binder in hot mix plant recycling for reclaimed asphalt pavement (RAP) can significantly increase the utilization rate of waste material on the premise of ensuring the service quality of the asphalt pavement. However, the development of epoxy asphalt binder modified RAP is hindered by the limited understanding of the effect of aging degree and types variability of asphalt binder in RAP on the properties of epoxy asphalt binder. Therefore, short-, long- and ultra-long- term aging tests of base asphalt and SBS modified asphalt binder were conducted through rolling thin film oven (RTFO), pressure aging vessel (20 h and 40 h) tests to obtain the asphalt binders with different aging extent, and then the binders were mixed with two epoxy systems to prepare epoxy asphalt binders, respectively. Next, the effects of asphalt binders aging degree on curing, compatibility, and mechanical behaviors characteristics of epoxy asphalt binders were investigated from chemical, microscopic, mechanical, and rheological perspectives based on Brookfield viscosity, Fourier transform infrared (FTIR), laser confocal fluorescence microscopy (LSCM), Shore hardness, direct tensile, strain sweep, and frequency sweep tests. The viscosity results indicated that a higher asphalt binder aging degree brought about a faster increase in the viscosity of epoxy asphalt binders. The construction allowable time of type-A epoxy asphalt binder of at least 3.5 h was higher than that of type-B. The conversion degree model constructed with the 917 cm−1 peak area as a parameter showed that the epoxy asphalt binder prepared by the asphalt binder with a more severe aging degree presented a higher conversion degree within the same curing time since the reaction space between epoxy and curing agent reactive groups was compressed by aggregated aged asphalt binder. The LSCM and Shore hardness results suggested that the aging asphalt binders induced the phase separation between asphalt and epoxy resin. With the asphalt binders aging, the distribution of asphalt phase in the epoxy system changed from black dots (12–15 um) to island-shaped structures (50–98 um), and eventually transferred a neuron-shaped network structure to connect island-shaped synaptosomes to block the continuity of three-dimensional network structure of epoxy resin. The rheological tests showed that the linear viscoelastic (LEV) region of the epoxy asphalt binders changed with the asphalt binders aging degree, and a shear strain of 0.01% ensured that specimens was within the LEV region. The effects of asphalt binders aging degree on the complex modulus and phase angle master curves of type-A epoxy asphalt binders mainly reflected the low frequency region (10−4 to 10−1 Hz), while the effects on type-B epoxy asphalt binders were in the whole frequency range (10−4 to 103 Hz).