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Calcium alginate capsules can release internal rejuvenator under cyclic loading to repair cracks and rejuvenate aged asphalt, thus being a promising preventive maintenance technology for asphalt pavement. However, this cyclic loading induced oil release pattern is passive and cannot ensure timely release of rejuvenator from the capsules at due time. In view of this, this paper prepared a kind of novel calcium alginate capsules that could release internal healing agent rapidly under microwave excitation. Calcium alginate/nano-Fe3O4 composite capsules were synthesized by sharp hole coagulation bath method. The morphology, mechanical strength, thermal stability and rejuvenator content of the capsules were characterized by a series of tests. Meanwhile, the surface temperature distribution and oil release ratio of the capsules under microwave irradiation were quantitatively analyzed. The results showed that the calcium alginate/nano-Fe3O4 composite capsules had obvious multi-chambers structure with rejuvenator encapsulated inside. The mechanical strength and thermal stability of the capsules meet the requirements of mixing and compaction of asphalt mixtures. Most importantly, the capsules could release internal rejuvenator under short time irradiation of low-frequency microwave. In short, the introduction of nano-Fe3O4 provided calcium alginate capsules an effective oil release capacity under microwave irradiation, which is expected to improve the self-healing ability of asphalt pavement containing capsules actively.

Self-healing capability of the asphalt mixtures containing encapsulated healing agent can be affected by different water components as results of their exposure to aggressive environmental conditions such as the acid rain or saline-alkali solutions in coastal areas. In view of this, this paper aims to study the effect of harmful water environments on the self-healing capability of asphalt mixtures containing a kind of novel composite fibers. The Ca-alginate/SiO2 composite fibers encapsulating healing agent was synthesized by using a microfluidic device. Several tests were conducted to characterize the micromorphology, thermal stability and mechanical properties of the fibers. Effectiveness of the composite fibers in asphalt was observed by fluorescence microscopy. Additionally, three-point bending test was performed to measure the effects of different water environments (including acid, alkali and salt-water solutions) on the self-healing properties of asphalt mixtures with, and without fibers. Furthermore, the mechanism of the effect of different water environments on the self-healing performance of the asphalt mixture was studied by Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) analysis. Main results showed that the composite fiber encapsulated 61.9 wt% of healing agent, and the fibers could meet the temperature and mechanical requirements during the manufacturing process of asphalt mixture. The composite fiber worked and enhanced self-healing capability of asphalt. Harmful water environments did not weaken the effect of the composite fiber. Compared with the drying condition, the self-healing property of asphalt mixtures containing the composite fibers was further improved after being immersed in alkali and salt-water environment. The results indicated that the sodium ions in the alkaline solution and the salt solution could undergo a de-crosslinking chemical reaction with calcium alginate in the fiber wall, as a result of which more healing agent leaked out to soften and rejuvenate asphalt. In short, the composite fibers encapsulating healing agent worked, and these were successfully applicable into asphalt mixture under different water environments.