Microcapsules containing rejuvenator are particles used to increase the self-healing capability of asphalt. To date, no reports focus on studying their behaviors in asphalt binders between the aggregates. The purpose of this work was to directly observe the states of self-healing microcapsules in asphalt binders. Asphalt samples were prepared by mixing bitumen and various weight contents of microcapsules. Experimental tests were carried out to observe the morphology, integrity, distribution, thermal stability, interface bonding and triggered rupture of microcapsules in asphalt binders. Fluorescence microscope morphologies and X-ray computed tomography images showed that microcapsules survived in asphalt resisting high temperature and strong agitation without premature damage. At the same time, microcapsules were homogenously dispersed in asphalt binders avoiding particles aggregation and adhesion. A circular heating-cooling process was used to simulate temperature changes in the natural environment. It was found that microcapsules still kept a stable state after an extreme temperature change. In addition, interface debonding phenomenon did not appear. Microscopic observation results reflected that microcapsules could be pierced by microcracks in the asphalt binder and the encapsulated rejuvenator flowed out under the force of capillary action. All the above conclusions indicate that microcapsules containing rejuvenator meet the application conditions and play the role of self-healing material in asphalt binders.

The aging problem of bitumen leads to pavement failure after years of usage. Microcapsules containing rejuvenator is a promising chemical product applied to improve the self-healing ability of bitumen. The aim of this work was to fabricate and characterize the self-healing microcapsules containing bituminous rejuvenator with nano-inorganic/organic hybrid shells. The shell had a two-layer structure: the inside layer material was the cross-linked methanol modified melamine-formaldehyde (MMF) resin and the outside materials was composed of methanol modified MMF resin and nano-particles of calcium carbonate (nano-CaCO₃). The forming mechanism of the two-layer structure was described based on a twice-condensation process. Fourier transform infrared spectroscopy (FT-IR) and Energy Dispersive Spectroscopy (EDS) results confirmed the nano-inorganic/organic hybrid structure of shells. The ideal content of nano-CaCO₃ particles was optimized through the morphologies observation. The addition of nano-CaCO₃ particles did not greatly influence the mean size of microcapsules. On the contrary, the nano-CaCO₃ particles increased the shell thickness of microcapsules owing to the loosely composite structure of shells. Thermal stability tests showed that the microcapsules could survive in the bitumen with a temperature of 200 °C. Moreover, the microcapsules could resist a violent temperature change process without destruction attributing to the protection of nano-CaCO₃ particles. The nano-particles on microcapsules decreased the deformation possibility of shells tested by nanoindentation.