Understanding and Tailoring Intrinsic Self-Healing in Cementitious Materials
- Author(s): Fan, Shuai
- Advisor(s): Li, Mo
- et al.
Damage tolerance and self-healing are naturally occurring phenomena found in many biological systems, optimized through millennia of evolution. Man-made materials, however, generally do not possess such attributes. Concrete is the most heavily used man-made material, but is inherently brittle and inevitably suffers from cracking under various mechanical and environmental loading conditions. Cracking impairs concrete mechanical properties, causing local stress concentration, and stiffness and strength reduction in structural members. Cracking also compromises concrete transport properties, accelerating other deterioration mechanisms such as chloride diffusion, moisture penetration, and reinforcement corrosion. These challenges can be potentially addressed by a new generation of self-healing cementitious materials, which can autogenously regain its transport property as well as mechanical capacity after damage.
While self-healing phenomena has been observed in cementitious materials, questions arise as to whether robust self-healing can occur reliably. The goal of this dissertation is to generate fundamental understandings of the chemical, physical and environmental mechanisms that control the self-healing process in cementitious materials. It is hypothesized that these mechanisms determine what, how and to what extent healing products form along crack walls. Understanding these mechanisms will lead to new material tailoring strategies to achieve robust and intrinsic self-healing in cementitious materials.
In this dissertation, we first establish an experimental framework to accurately probe the level and quality of self-healing within a crack, to analyze the chemical composition of the healing products, and to quantify the recovery of material transport and mechanical properties. Through experiments, we uncover and quantify the nonhomogeneous self-healing ratios and varying chemical compositions along the crack depth. Second, we formulate a coupled transport-thermodynamics model to predict crack profile evolution with time due to self-healing. The model captures three mechanisms that dynamically control the healing process: the advective transport of ions as reactants caused by the flow of the aqueous solution, the molecular diffusion of dissolved reactants due to the concentration gradient, and the kinetics of chemical reactions occurring at the water-material interfaces controlled by thermodynamics. We test and validate the model through a set of designed experiments with well-controlled chemical and physical parameters. Third, we extend the experimental framework and the model to study the effects of different chemical variables (e.g. binder chemistry, age, ion concentration), physical properties (e.g. crack width, geometry, transport properties), and environmental conditions (e.g. wet and dry, flow rate, pH) on the self-healing extent and properties. The results elucidate the healing mechanisms and governing parameters, laying the groundwork for designing robust self-healing into cementitious materials. Fourth, we develop new self-healing cementitious composite materials by satisfying a combination of criteria: (a) the presence of a multitude of essential chemical species or ions that can react with natural actuators upon cracking, (b) distributed damage behavior that sequentially and spatially activates healing reactants, and (c) self-controlled tight crack width that can promote robust self-healing without consuming a large amount of healing reactants. The experiments validate the improved three-dimensional healing extent of the new materials, and the recovery of transport and mechanical properties. Finally, the dissertation explores the multifunctionality of new cementitious materials by coupling the cracking and healing processes with material frequency-dependent electrical response.