Preparation and application of nested capsules for secondary self-healing of concrete cracks

Crack initiation and propagation significantly impair the durability and service life of cement-based materials, particularly under repeated loading conditions. Although microcapsule-based self-healing systems offer a promising solution, their performance is often constrained by insufficient rupture efficiency and limited healing capability after crack reoccurrence. In this study, a novel nested capsule system was developed to enhance secondary crack-healing performance in concrete. Isophorone diisocyanate (IPDI) was encapsulated within polypropylene microcapsules, which were further embedded in a ceresine wax matrix to form a hierarchical structure. Upon crack initiation, the outer wax shell preferentially ruptures to release the healing agent, while the inner capsules remain intact and are redistributed within the matrix. These residual capsules can subsequently rupture when cracks reoccur, thereby enabling a second healing event. The morphology, chemical composition, particle size distribution, and mechanical properties of the nested capsules were systematically characterized using SEM, FTIR, laser particle size analysis, and nanoindentation. The effects of capsule dosage on mechanical performance, pore structure, and impermeability of concrete were also investigated. Furthermore, repeated preloading–healing tests combined with X-ray computed tomography were conducted to evaluate crack closure and recovery behavior during two successive damage–healing cycles. Results show that a representative capsule dosage of 3 wt% (by cement and fly ash mass) achieves the best overall performance. After secondary loading, the compressive strength recovery ratio reached 87.5%, and the chloride diffusion recovery ratio reached 71.6%. Cracks with an initial width of 0.25 mm were effectively sealed during the second healing cycle. These findings demonstrate that the proposed nested capsule system provides an effective strategy for enhancing secondary crack healing and functional recovery in cement-based materials subjected to repeated cracking.