Engineering Self Healing capacity of cement based materials through crystalline admixtures

ID No:180 ABSTRACT In this paper aims a thorough characterization will be performed of the effects of crystalline admixtures, currently employed as porosity reducing admixtures, on the self-healing capacity of the cementitious composites, i.e. their capacity to completely or partially re-seal cracks and, in case, also exhibit recovery of mechanical properties.In this paper aims a thorough characterization will be performed of the effects of crystalline admixtures, currently employed as porosity reducing admixtures, on the self-healing capacity of the cementitious composites, i.e. their capacity to completely or partially re-seal cracks and, in case, also exhibit recovery of mechanical properties. The problem will be investigated with reference to both a Normal Strength Concrete (NSC) and a High Performance Fiber Reinforced Cementitious Composite (HPFRCC). In the latter case the influence of flow-induced fiber alignment will also be considered in the experimental investigation. With reference to either 3-point (for NSC) or 4-point (for HPFRCC) bending tests performed up to controlled crack opening and up to failure, respectively before and after exposure/conditioning, the recovery of stiffness and stress bearing capacity will be evaluated to assess the self-healing capacity. Moreover, in a durability-based design framework, suitable self-healing indices to quantify the recovery of mechanical properties will be defined. INTRODUCTION Efforts to reduce the permeability of concrete and improve its resistance to ingress and movement of water and other water born or transported aggressive substances, thereby improving its durability, has led to the development of a class of “admixtures” referred to as Permeability Reducing Admixtures (PRAs). Crystalline admixtures are a category of PRAs, consisting of proprietary active chemicals provided in a carrier of cement and sand, which, because of their hydrophilic nature, react with water and cement particles in the concrete to form calcium silicate hydrates, increasing the density of the CSH phase, and/or pore-blocking precipitates in the existing microcracks and capillaries. The mechanism is analogous to the formation of CSH and the resulting crystals become integrally bound with the hydrated cement paste, thus contributing to a significantly increased resistance to water penetration under pressure. It can be furthermore argued that, as hairline cracks form over the life of concrete, crystalline admixtures continue to activate in the presence of moisture and seal additional gaps, even if cracks may still develop that exceed the self sealing capacity of the concrete. The focus of this paper will be on the effectiveness of a crystalline admixture on the self sealing capacity of cementitious composites. A comprehensive experimental programme has been going on at the author’s home institution to investigate the effects of crystalline admixture on the recovery, if any, of residual load bearing capacity of different categories of cementitious composites, also addressing the beneficial effects of a synergy between the same admixture and a dispersed fibre reinforcement, as in the case of HPFRCCs. A dedicated experimental methodology has been conceived to the purpose, and the reliability of the aforementioned concept has been shown, also through the definition of suitably defined “self-healing indices”, which are instrumental to quantify the outcomes of the self-healing phenomenon. This opens challenging perspectives to the use of cement based materials intrinsically able to recover their pristine durability levels, thus guaranteeing a longer service life of the designed applications and a performance less sensitive to environmental induced degradation. (a) (b) (c) Figure 1: SEM magnification (a,b) and EDS analysis (c) of admixture particles.