Stochastic service life prediction of RC girders subjected to chloride induced corrosion – shear limit state

<p>Reinforced concrete (RC) girders are prone to corrosion of reinforcement due to chloride ingress into concrete cover; this is considered to be the main cause of capacity degradation of reinforced concrete bridge girders located in marine environment. Also, recent experimental studies on strength and behaviour of corroded beams indicate that corrosion of stirrups can change the failure mode from flexure to shear. Thus, the failure mode of reinforced concrete girders can change to brittle shear failure mode when exposed to marine environment. Keeping in view of these, the aim of the work presented in this thesis is service life prediction of corroded reinforced concrete girders, which forms an important part of durability design. The scope of this work is limited to shear limit state considering the chloride induced corrosion of reinforcement as the degradation mechanism. The review of literature shows that the service life estimation of reinforced concrete bridges exposed to marine environment is an active area of research.</p> <p>To achieve the aim of this thesis, first the shear capacity of uncorroded reinforced concrete girders, with different shear span to effective depth ratios, are predicted using identified mechanics-based models. The modelling error associated with the shear capacity models are determined using the shear capacity database created in this study. The distributions of shear capacities are obtained using Monte Carlo simulation technique. While in this case, the deterministic values of shear capacity are determined using mechanics-based models, the randomness associated with it is quantified using a brute force technique. Hence, the concepts of thermodynamics, within the information theoretic framework, is used as an alternate mechanics-based approach for generating the shear capacity distribution. The applicability of mechanics-based shear capacity models is then examined for reinforced concrete beams with corroded reinforcement using experimental data available in literature. The probabilistic analysis of shear capacity of reinforced concrete girders, subjected to chloride induced pitting corrosion of both stirrup and longitudinal reinforcement, is carried out to determine the shear capacities at different times. The analyses account for reduction in reinforcement diameter and yield strength of steel, spatial and temporal variability of basic variables due to corrosion, variation of corrosion currents along the length of the girder and ductile and brittle behaviour of stirrups. The reliability analyses against limit state of shear are performed to estimate the service life of girder. Also, a new non-stationary Markov Chain model is proposed for modelling the performance degradation. The proposed stochastic model is capable of predicting the service life of the girders and also helps in making engineering decisions regarding maintenance. Although, the service life in this study is predicted with respect to ultimate limit state, it is also important to consider the performance of reinforced concrete girder with respect to serviceability limit state in durability design. Therefore, an attempt is made to identify a method for predicting the shear crack widths of reinforced concrete beams with shear span to effective depth ratios greater than or equal to 2.5. The probabilistic analysis of shear crack width under service load is carried out and characteristic equation is proposed to carry out serviceability checks.</p> <p>The methodology presented in this thesis for ensuring safety against ultimate shear capacity and serviceability against shear crack width, is demonstrated for a typical reinforced concrete girder spanning about 10m. The results show that the reinforced concrete girder designed against flexural limit state, reinforced with adequate shear reinforcement and exposed to severe environment is governed by the shear limit state during its service life. The estimated design service life is shorter than the intended life of the girder. However, the shear crack widths are within the permissible limits. The corrosion initiation time in this example is estimated using a model based on Fick's second law of diffusion with time variant diffusion coefficient. However, a more realistic prediction of corrosion initiation time is an important step in durability design. Therefore, a sub-diffusion model is proposed to the chloride ingress into concrete cover that accounts for the fractal nature of concrete pore-structure and heterogenous nature of concrete. The numerical solution for sub-diffusion equation is obtained using explicit finite difference method along with Grunwald-Letnikov discretization scheme. The application of the proposed model in predicting the corrosion initiation time of existing structures is also demonstrated. However, for incorporation of sub-diffusion model into the developed methodology, further studies are required for quantifying the associated randomness. The contributions from this work are expected to result in a more rational methodology for durability design that involves service life prediction of corroded girders against shear limit state.</p>