Performance evaluation and potential prediction of the self-healing concrete based on composite capsules

Encapsulating minerals into composite capsules represents an effective strategy to prevent minerals from reacting prematurely before cracking. In this study, the healing characteristics of composite capsules containing MgO expansive agent (MEA) were first investigated. Then the expansive and mechanical properties of the cement-based materials incorporating composite capsules were studied. Afterwards, the ultrasonic, electric flux and water permeability tests were used to quantitatively evaluate the effects of cracking age and capsule dosage on the self-healing efficiency of concrete. By revealing the relationship between healable crack width and initial crack length at different capsule dosages, the prediction model of self-healing potential was finally established. Results showed that composite capsules had significant healing characteristics. The intact capsules could maintain stability in an alkaline environment, and the reaction rate of MEA inside ruptured capsules reached 91% after curing in wet-dry cycles for 30 days. The concrete incorporating composite capsules exhibited great volume stability and obvious self-healing ability. When the cracking age extended from 3 days to 60 days, the recovery rate of ultrasonic pulse velocity maintained at 73%-82% and the reduction rate of electric flux remained at 76%-86%. With the increase of capsule dosage, the reduction rate of water permeability coefficient and electric flux of concrete significantly improved. The self-healing of crack was mainly attributed to the formation and deposition of calcite, nesquehonite and hydromagnesite/dypingite. The self-healing potential of concrete was predicted by healable crack width, which decreased with the increase of initial crack length but rose obviously with the increase of capsule dosage.