Self-healing cement based materials: 5 years of research experience at Politecnico di Milano

Worldwide increasing consciousness for sustainable use of natural resources has made “overcoming the apparent contradictory requirements of low cost and high performance a challenging task” and a major concern. The importance of sustainability as a requisite which has to inform structure concept and design has been also highlighted in Model Code 2010. Self-healing technologies, by repairing “early-stage cracks in concrete structures”, could prevent “permeation of driving factors for deterioration”, and, in case, even provide partial recovery of engineering properties relevant to the application, thus extending the structure service life. The author’s research group has undertaken a comprehensive research project, focusing on both experimental characterization and numerical modelling of the self-healing capacity of a broad category of cementitious composites, including normal strength concrete and high performance cementitious composites reinforced with either steel or natural fibres. Both autogenous and engineered healing have been considered. Dedicated experimental methodologies have been employed to characterize the healing capacity of the different investigated materials, based on comparative evaluation, in a pre-cracking and a post-conditioning stage, of the mechanical performance. Influence of exposure conditions and duration has been considered. The healing capacity has been quantified by means of “healing indices”, based on the recovery of the load bearing capacity, stiffness, ductility, toughness, correlated to the amount of crack closure, measured by means of optical microscopy and also “estimated” through a suitable indirect methodology. As a further step a predictive modelling approach, based on modified micro-plane model, has been formulated. The approach incorporates the self-healing effects, in particular, the delayed cement hydration, as well as the effects of cracking on the diffusivity and the opposite repairing effect of the self-healing on the micro-plane model constitutive laws. The work represents a comprehensive and solid step towards the reliable and consistent incorporation of self-healing concepts into a durability-based design framework.