Chloride Induced Pitting Corrosion-Fatigue in Reinforced Concrete Structures
Zhu, Ming 1984-
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There is currently a general agreement that the long-term performance of many civil infrastructure facilities, such as bridges, tunnels, harbor facilities, nuclear power plants etc. is strongly dependent, not only on mechanical loading, but also on the harshness of their service environment. CorrosionFatigue (CF) is a damage mechanism commonly found in many Reinforced Concrete (RC) structures (bridges, harbor structures, oil platforms, etc.) exposed to variable loading and a corrosive environment. CF is a synergetic phenomenon between corrosion and fatigue wherein the damage caused by the simultaneous action of both mechanisms is usually greater than when either corrosion or fatigue is acting alone. Recognizing that aging of these structures may adversely impact their ability to fulfill their intended function in the future, some strategies have been developed for predicting their service life and for mitigating the effects of aging on their performance. Despite its practical importance, only a relatively small number of studies have been carried out on the CF of reinforced concrete structures. Indeed, most durability studies in RC focus on either corrosion alone or fatigue without corrosion. In the few instances where CF was considered, oftentimes only RC structural elements, such as beams, were tested. Although providing important information about the overall CF behavior of this kind of structure, such tests also make it virtually impossible to extrapolate the application of those results to other structures or loading/exposure conditions. This study proposes a new approach for assessing the CF of reinforced concrete structures that relies on a realistic constitutive characterization of CF of steel reinforcement in simulated concrete pore solutions. The novelty of the proposed approach resides in its ability to accelerate both corrosion and fatigue loading simultaneously and independently so that a given field condition can be represented without favoring one mechanism over the other. Given the difficulty in accelerating corrosion to the required values to match the acceleration of fatigue loading, oftentimes the acceleration factor for fatigue is much higher than the one used for corrosion, making the test results not very representative of the field conditions. The research objectives of this study were divided into two main sequential stages. In stage one, the corrosion-fatigue behavior of carbon steel reinforcement is characterized as a material operating in a chloride-laden simulated concrete pore solution in a way that is compatible with two widely used fatigue analysis approaches (S-N curves and Fracture Mechanics). A combination of pore solution chemistry and an electrochemical method is used in this study to develop a novel corrosion fatigue cell that can accelerate, independently, both corrosion and fatigue so that the degradation rates are representative of typical in-service conditions. The simulated pore solution chemistry is chosen so that it is representative of typical concrete environments that favor pitting corrosion in the presence of chloride ions. The electrochemical method is used to overcome the limitations of the maximum corrosion rates that can possibly be achieved through the chemical composition of the simulated pore solution alone, so that CF tests (representative of field conditions) can be carried out within a reasonable time frame. In stage two, the ability of the two constitutive models, developed in stage one, to predict the fatigue life of reinforced concrete beams in a corrosive environment is assessed, as an example of a structural component. The results of the model predictions, for both approaches (S-N curves and Fracture Mechanics), were compared with the experimental results from an independent set of RC beams tested under corrosion-fatigue. The results show that both methods can be used to estimate the service life of a RC structure subjected to corrosion-fatigue. It is worth mentioning that the S-N curve approach provided more precise estimations than those provided by fracture mechanics, with standard errors ranging from 9.0% to 23.0%, instead of ranges between 26.7% and 54.2%, respectively. However, although the estimation of the fracture mechanics approach shows a higher error range, this method provides more insight into the evolution of damage in the rebar over time. The fracture mechanics model accounts for the four main stages of the metal degradation process: pit nucleation and growth, pit-to-crack transition, crack growth state, and ultimate fracture failure. The results indicate that the pit nucleation and growth stage occupies over 79.9% of the total service life in the prediction of the tested RC beams under CF. This result suggests that unstable crack propagation would not take place before the occurrence of extensive corrosion in the material. In other words, corrosion plays a more significant role than usually reported in the corrosion fatigue life of RC under realistic conditions.
DegreeDoctor of Philosophy (Ph.D.)
DepartmentCivil and Geological Engineering
CommitteeElwood, David; Sparling, Bruce; Odeshi, Akindele; Soliman, Haithem
Copyright DateAugust 2017