Encapsulated polyurethane for self-healing concrete applications: prototyping, mechanical and durability characterization

Concrete is the most widely used material in civil and building engineer thanks to its highly cost-effective production and installation, compressive strength and durability properties, but one of the major problems that affects this type of material is the emergence of cracks which can impair its durability and mechanical characteristics, possibly leading to premature collapse of the structure. The research in this field is moving fast to improve the maintenance strategies, with the development of novel high-efficiency repairing products and the definition of innovative application techniques, in order to both guarantee and extend the structure lifetime and limit the need for demolition and production of new concrete, thus reducing the overall environmental impact of these activities. A growing attention has been addressed to the development of solutions focused on self-healing properties in cementitious materials, such as incorporated active methods that act in order to repair the cracks directly from the inside, not affecting the inner properties of the material. The purpose of this thesis is to check the feasibility and effectiveness of an autonomous self-healing strategy based on the encapsulation of a highly moisture-reactive healing agent, in order to improve both the durability and mechanical properties after cracking due to self-repair. The first stage of the process was the production of capsules and the subsequent filling with an expansive polyurethane precursor. In a second stage, prismatic cement mortar specimens were manufactured as prototypes of the proposed self-healing system. The capsules previously produced were placed inside some of them, while other were left capsule-less for the sake of comparison. The second third phase stage was the pre-cracking, through which a single crack with pre-defined characteristics was introduced in each specimen, in a controlled and repeatable way. Finally, the fourth and last stage consisted in testing the mortar prisms from the mechanical and durability points of view, in order to evaluate the performance recovery of the system after the autonomous repair has been completed. The results show that the self-healing mechanism introduced via the encapsulated polyurethane generates a significant improvement in the post-crack behavior in comparison with the standard mortar. The flexural strength was recovered almost entirely, and the same was observed for the durability, as tested in a water permeability set-up. The potentialities of the system in terms of long-term effectiveness were also preliminarily investigated by means of mechanical testing under cyclic loading conditions and subsequent re-evaluation of the water permeability, until final failure of the specimen. Promising results were obtained, opening the way for future developments of the proposed self-healing system in view of its application in real field conditions.