Ductile Fiber-reinforced Cementitious Composite for Corrosion Mitigation in Reinforced Concrete Structures
Ravi Ranade, Cemal Basaran, and Hamidreza Fakhri
Abstract (for presentation and technical paper) Naval reinforced concrete infrastructure consisting of piers, docks, sea-defenses, submarine weapons storage, and other facilities require frequent maintenance and repair. The main causes of this problem are the highly corrosive chloride-rich environments of these structures and the inherent porosity and brittleness of concrete cover (that is supposed to protect the steel reinforcement). Corrosion of a rebar embedded in a reinforced concrete structure initiates after a critical amount of chloride ions diffuses through the porous concrete cover and breaks the protective passive layer around the rebar. Due to expansionary pressures of the rust products, the concrete cover cracks and spalls owing to its low tensile strength and brittleness. All the conventional methods (e.g. corrosion inhibitors, less porous concretes incorporating high volumes of mineral admixtures such as fly ash, epoxy coating on rebars, etc.) to address rebar corrosion mainly focus on delaying the initiation phase of corrosion and are effective only when the concrete cover or epoxy coating on the rebar surface is intact without any cracks or defects. In practice, due to restrained shrinkage, thermal deformations, mechanical loads and poor construction practices, the concrete cover unavoidably cracks. At the same time, defects in the epoxy coating (which can be introduced during manufacturing, transportation, or worksite handling) and adhesion loss between epoxy and rebar can significantly accelerate the rebar corrosion. Thus, the effectiveness of existing methods for addressing rebar corrosion is significantly reduced in presence of cracks due to non-corrosion-related mechanisms. This research addresses the fundamental problem of concrete brittleness and cracking by replacing the cover concrete with a ductile fiber-reinforced cementitious (FRCC) composite that has a strain-hardening behavior under tension. In this study, potentiostatic accelerated corrosion tests are used to investigate the performance of ductile FRCC covers; results show about 50% reduction in corrosion rates with ductile FRCC covers as compared to conventional concrete covers. The ductile FRCC, similar to high performance concretes with mineral admixtures, has low porosity and diffusivity prior to cracking. More significantly, unlike normal concrete, ductile FRCC maintains low diffusivity even in the post-elastic (stage) stage as the self-controlled tight crack widths in ductile FRCC continue to limit the diffusion of chloride ions and access to water and oxygen. This is the main reason behind such significant reduction in corrosion rates. Detailed analytical and micro-scale investigations to further understand the influence of ductile FRCC on the nature of corrosion products and their formation rates are ongoing. In their excellent review, Imanian and Modarres  report that common practices for corrosion fatigue structural integrity assessment in metals and alloys have isolated and evaluated the damage characteristics at different life stages. As such, the corrosion fatigue damage process is divided into four regimes: the pit nucleation step, the nucleated pit to grow stage, surface crack initiation and growth into a through-crack phase, and through-crack growth to a prescribed critical length step. For each of these stages, the literature offers empirical models for life estimation of metal and alloys. The lifetime of a component subject to corrosion fatigue is estimated by the sum of the lifetime obtained from empirical models developed for each stage. However, we are able to predict corrosion process without any need for empirical models. Irreversible entropy based universal degradation theory was proposed by Basaran (an author of this paper) in 1998 (funded by ONR YIP- Dr. Roshdy Barsoum). This theory was proven experimentally for mass transport, chemical diffusion problems like, electromigration, thermomigration, creep, thermo-mechanical degradation, phase change, fatigue tribology and many other loading scenarios. The theory was also proven mathematically by Sosnovskiy and Sherbakov in 2016. According to the universal theory by Basaran, corrosion process is modeled purely based on principles of chemistry and laws of thermodynamics. Corrosion is an irreversible entropy generating chemical process. According to second law of thermodynamics, irreversible entropy generation increases and irreversible entropy generation rate decreases as system reaches near failure (end of corrosion). Finally, the system reaches the final state and then irreversible entropy generation rate becomes zero. We calculate the irreversible entropy generation and its rate to predict the progress of corrosion process. In this research, we will present the fundamental mathematical and experimental proof of this theory.
- Imanian, A. and Modarres, M A  “Thermodynamic Entropy Approach to Reliability Assessment with Applications to Corrosion Fatigue” Entropy 2015, 17, 6995-7020; doi:10.3390/e17106995
- Leonid A. Sosnovskiy and Sergei S. Sherbakov  “Mechanothermodynamic Entropy and Analysis of Damage State of Complex Systems” Entropy 2016, 18, 268; doi:10.3390/e18070268 "