Employing ultrasonic wave as a novel trigger of microcapsule self-healing cementitious materials

Microcapsule-mediated self-repair of concrete represents an attractive method to improve the durability of concrete. However, it is quite challenging to timely and controllably trigger the microcapsule self-healing materials with the onset of cracks. This study attempts to employ ultrasonic wave as a cost-effective, artificially-controlled, high-efficient and environmentally-friendly trigger to orchestrate microcapsule-induced self-healing activities in concrete. A traditional microcapsule (i.e., the UF/E microcapsule) was synthesized and characterized to study the feasibility and effectiveness of the ultrasonic wave trigger. Cement mortars with and without microcapsules were prepared for ultrasonic/mechanical triggering tests, and the strength repair rates were analyzed. Stereomicroscope and SEM were used to observe the morphologies of the microcapsules in mortars before and after ultrasonic triggering. In addition, MIP was used to analyze the pore structures of mortars subjected to different ultrasonic trigger times. The results showed that the obtained microcapsules (with a particle size range of 100 μm–300 μm) were uniformly distributed in the mortar samples. The enhancement of strength repair rates of mortars triggered by ultrasonic were 2–4 times higher than mechanical-triggered ones. The morphological observations also illustrated that ultrasonic wave delivered a better triggering effect on the microcapsules. It was also found that the optimal ultrasonic time was 10 min under the conditions used in this study (40 kHz and 0.2 w/cm2). Findings from this study demonstrated that ultrasonic wave induced triggering of microcapsules was easily-controllable and cost-effective. Thus, this novel trigger method holds great potential for promoting the application of microcapsules technologies in cementitious materials.