New insights into autogenous self-healing with NMR tests

Concrete is a brittle composite cementitious material that easily fractures under tensile loading. Microcracks can appear throughout the concrete prior to application of any load because of temperature-induced strain and autogenous and drying shrinkage. There is no doubt that these cracks provide preferential access for aggressive agents to penetrate into the concrete, probably causing corrosion of reinforcement steel and degradation of concrete. As a result, the service life of reinforced concrete structures is shortened. Fortunately, concrete has the ability to heal the crack itself without manual efforts when water is present in cracks. This ability is defined as autogenous self-healing. However, the effect of migration of water from cracks into the bulk paste on autogenous self-healing is not clear yet. The aim of this study is to investigate the effect of water migration from cracks into the bulk paste on autogenous self-healing. Nuclear magnetic resonance (NMR) technique was utilized to monitor water migration from cracks into the bulk paste during the process of autogenous self-healing. NMR results show that in the beginning of autogenous self-healing the water in the crack migrates into the bulk paste and the water content of the bulk paste increases significantly. However, after 5-hour autogenous self-healing period the amount of non-chemically bound water in the bulk paste (adjacent to the crack surfaces) determined by NMR decreases instead. It indicates that some of the water coming from the crack was used by additional hydration of unhydrated cement particles in the bulk paste during the process of autogenous self-healing. NMR results reflect that most of the reaction products of additional hydration are formed in the bulk paste adjacent to the crack surfaces, rather than in the crack. Because of the additional hydration caused by the water from the crack, the capillary porosity of the bulk paste adjacent to the crack surfaces decreases significantly. Before this work the filling of cracks is the main concern in term of autogenous self-healing. The densification of the cement paste adjacent to the crack surfaces, which was observed for the first time in this study, will also decrease the ingress of aggressive agents into the bulk concrete matrix and prolong the service life of concrete structures. This observation provides a new insight into autogenous self-healing in cement paste.