Mechanical behaviour and durability of self-healing cementitious systems: image analysis of healing agent diffusion correlated with mechanical and durability parameters

Concrete is the most used material in the field of Civil Engineering, due to the considerable flexibility that it offers in design, and also due to its low cost. On the other hand, because of its reduced tensile strength, it is also one of the most prone to cracking materials. For this reason, solutions for damage management have been developed in recent years. However, current researches point to sustainable treatments, both in ecological and economic terms. Therefore, self-healing cement-based materials have been targeted, through the use of different strategies including nonencapsulated and encapsulated technologies, to be embedded inside the matrix during the casting phase. Nowadays, norms or guidelines that regulate their use do not exist so solutions to be used must be sought in relation to various factors such as capsule/sealant/matrix compatibility, environmental conditions, expected extent of damage and so on. In this work, the efficiency of self-healing of cement-based materials was investigated. Mortar specimens were produced by incorporating macro-capsules containing precursors of polyurethane; each healing systems was characterized in terms of composition and coating of the shell, position and dimension of capsules. In order to evaluate the self-healing efficacy, the specimens were pre-cracked through a flexural test and, after the time necessary to allow the chemical stabilization of the sealant, the recovery in terms of durability and mechanical properties was verified, respectively, through water flow test and mechanical reloading. The results were subsequently correlated with the spreading areas of the healing agent over the crack faces, in order to check how the mechanism of healing agent released from the capsules may affect the overall system performance. The individuation and quantification of the covered regions was performed through the segmentation of the images of the fracture surfaces. Comparing the results, it was established that there is not a strong correlation between the percentage of covered area and the recovery of permeability and mechanical characteristics. On the contrary, it was observed that the evaluation of the spatial distribution of the healing agent is more indicative. In fact, the occupied part of the crack surface influences the tortuosity of the water path and the stiffness contribution of the sealant, expressed in terms of moment of inertia. For all the series of specimens, encouraging results were achieved regarding the offered protection against the introduction of aggressive agents from the exterior. In this way, the use of self-healing materials represents an innovative method to increase the service life of the structures, reducing also their environmental impact. Therefore, a more detailed investigation of the properties of these systems must be pursued thanks to future examinations and the definition of Codes to regulate their use has to be promoted.