The development of self-healing concrete has witnessed several strides in the last three decades. Among these is the understanding of the mechanisms of self-healing concrete, as well as the realization of new technologies of enhancing crack self-healing in concrete. Previous research suggests that crack closing mechanism occurs due to mainly 1) Hydration of anhydrous cement, 2) Formation and crystallization of Calcium Carbonate, 3) Swelling of cement matrix and 4) Sedimentation of particles in the crack interstices. In this research however, after investigating them one by one it was found that these mechanisms do not explain the drastic water flow recovery that occurs in the early stages of water permeation through cracked concrete. It then prompted direct visual observation of water flow through crack narrow space; and with this an interesting finding was observed-the air bubble effect. It was observed that water flow through the narrow crack openings creates air bubbles by various water action mechanisms. The created air bubbles narrow water flow paths in the crack causing significant water flow reduction in the initial and later stages of water supply condition.

Keywords: Self-healing concrete, Water flow, Crack, Water permeation, Air bubble