Enhancing self-sealing performance of concrete using novel water-absorbent capsules: Effects of capsule properties, molding methods, and crack-filling mechanisms

Concrete, a widely utilized construction material, is prone to micro-cracking due to external loads and environmental factors, leading to water infiltration and structural deterioration. Conventional repair methods are time-consuming, whereas self-healing mechanisms typically necessitate prolonged durations to become effective. This study investigates the self-sealing performance of concrete blended with novel water-absorbent capsules containing superabsorbent polymers (SAP). The capsules were designed to prevent premature SAP expansion during the mixing and curing stages, thereby enhancing the material's self-sealing capability upon crack formation. The research evaluated the capsules' waterproofing performance, expansion characteristics, and dispersion, followed by permeability tests on concrete specimens with through-cracks. The effects of multiple factors, including capsule dosage, SAP content, water-cement ratio (w/c), and different molding methods (cast-molded and press-molded), were examined. The results demonstrated that double-walled capsules containing 5 % SAP achieved the optimal balance of waterproofing and swelling behavior. Compared to other mixtures, press-molded concrete incorporating 5 % capsules and 7 % SAP showed the most robust self-sealing capacity, reducing permeability by up to 55.2 % over 60 min and 95.7 % over 540 min separately, significantly outperforming cast-molded specimens, which showed a maximum reduction of 65.1 %. X-ray Computed Tomography (XCT) analysis confirmed that the SAP within the capsules absorbed water and expanded, filling cracks and reducing crack volume by 12.6 % when exposed to a 5 % CsCl solution. These findings highlight the potential of water-absorbent capsules as an effective strategy for autonomous crack sealing in concrete structures. The press-molded method, in particular, provides better control over capsule breakage and SAP release, leading to more effective and prolonged sealing performance.