Most of the traditional and advanced ceramic materials exhibit a brittle behavior, which stems from the stress concentration at the edges of voids, small defects, or inhomogeneities in the material, and causes the onset and rapid propagation of cracks. Concrete, and cement-based materials in general, are no exception. The presence of cracks in concrete does not necessarily entail a risk of collapse for the entire structure, but is certainly correlated to the acceleration of the material degradation and to the reduction of the structure service life. In fact, cracks act as easy pathways for the ingress of water and water-driven aggressive substances, that can attack the concrete matrix or the steel reinforcement, and impair the overall structural performance. Under the effect of repeated mechanical loads or environmental actions, micro-cracks can grow and coalesce into macro-cracks, thus leading to a further acceleration of the deterioration process.
To improve the durability of reinforced-concrete structures in the presence of cracks, and to extend their service life without the need for extra maintenance, different self-healing systems have been developed recently. Some of the most used ones are based on the incorporation of macro-capsules containing some special healing agents, that must be activated autonomously upon cracking, in order to restore the water-tightness and the mechanical properties of the original cement-based material.
This paper presents an overview on the use of tubular macro-capsules with a cementitious shell in self-healing systems, as developed by the Authors. The use of a cementitious material to manufacture the macro-capsule shell ensures that the capsule itself is well responsive to the same mechanical stimuli that could create cracks in the concrete matrix, thus immediately triggering the capsule breakage and the release of the healing agent at the crack site. Indeed, the ability of these capsules to be autonomously activated upon cracking was demonstrated by means of pre-cracking tests in three-point bending. Their influence on the inherent material strength characteristics was evaluated in comparison with reference materials without capsules. Their capacity to store and effectively release different types of healing agents was studied with reference to organic (polyurethane-based), inorganic (sodium silicate and silane-based), and biological (bacteria-based) agents. The capsule shell formulation, coating, and manufacturing procedure were optimized accordingly. The self-healing effect was investigated in terms of regain of mechanical properties, by means of static reloading tests up to failure, and improvement of water-tightness, by means of different types of permeability and sorptivity tests. The stability of the self-healing performance in time under the effect of repeated mechanical and thermal actions was also studied, in the case of the polyurethane-based healing agents. The results highlighted the versatility of the cementitious capsules and their high potential for self-healing concrete applications. Depending on the healing agent, reasonable values of the load recovery index were achieved, up more than 100% in a few cases. Regarding the durability properties, the cracks were completely or almost completely sealed in the majority of cases, and either water-flow tests or water-absorption tests showed no water ingress through the self-repaired cracks.