This chapter provides a comprehensive procedure for the time-dependant structural performance evaluation and life-cycle cost analysis of reinforced concrete highway bridges located in extreme chloride-laden environments. The penetration of chloride ions into the concrete is simulated through a finite difference approach, which takes into account all the parameters that can affect the corrosion process. From simulation results, the corrosion initiation time is predicted and the extent of structural degradation is calculated over the entire life of bridge. A group of detailed bridge models with various structural attributes are developed to evaluate the changes in the structural capacity and seismic response of corroded bridges. For the purpose of the probabilistic seismic risk assessment of bridges, the seismic fragility curves are generated and updated at regular time intervals. The time-dependent fragility parameters are employed to investigate the life-cycle cost of bridges by introducing a performance index which combines the effects of probable seismic events and chloride-induced corrosion. The proposed approach provides a multi-hazard framework, which leads to more realistic performance and cost estimates. It also indicates the inspection and maintenance intervals in a way that the inspection and maintenance costs are optimized, while the safety of bridge is ensured.
Top1. Introduction
From a long-term point of view, the durability of reinforced concrete (RC) highway bridges is significantly impacted by the deterioration of their structural members. When investigating the damaged bridges, the deterioration caused by the corrosion of reinforced concrete members is usually found to be one of the main sources of structural degradation which may eventually result in the serviceability failure of bridges under service or extreme loading conditions. An accurate estimation of the extent of degradation during the structure's life-cycle provides both engineers and decision-makers with valuable information which helps to ensure the safety of bridges while reducing the associated costs. Towards this goal, the current chapter focuses on the corrosion process caused by the chloride ions attack and evaluates its effects on the life-cycle performance and cost of RC bridges.
Chloride-induced corrosion is one of the deterioration mechanisms caused by the rapid intrusion of chloride ions into the concrete. This mode of corrosion is expected when the bridge is exposed to aggressive environments (e.g., coastal environments or the application of deicing salts). The penetration profile of chloride ions in a reinforced concrete member demonstrates the highest chloride content near the surface with a decreasing trend towards the depth of the member. The chloride transport mechanism in concrete is a complex phenomenon that may occur in several forms, such as ionic diffusion, capillary suction, and permeation. When the concentration of chloride ions in the pore solution within the vicinity of reinforcing bars becomes high enough to depassivate the protection film of the reinforcement, the layers of rust start to form on the reinforcing bar surface and the corrosion of steel begins.
In this chapter, an integrated computational methodology is developed to simulate the penetration of chloride ions into the reinforced concrete members. Through a comprehensive study, the effects of various influential parameters, such as water-to-cement ratio, ambient temperature, relative humidity, concrete age, free chloride content, and binding capacity, are considered to obtain a precise prediction of the chloride content in different depths of RC members with the progression of time. By comparing the chloride content values with certain critical thresholds suggested in the literature, the corrosion initiation time is estimated. After corrosion initiation, the time-dependent characteristics of corroded bridges are identified through the extent of the cracking and spalling of the concrete cover, reduction of the steel bar cross-section area, and decrease in the yield strength of reinforcing bars. Based on that, the probabilistic life time fragility parameters of a group of RC bridges with different structural attributes are evaluated over the time using fragility analysis procedure.
Furthermore, the life-cycle cost of RC bridges under corrosion attack is studied in this chapter. The total life-cycle cost of the bridge is calculated from the present value of the construction cost, inspection and maintenance costs, serviceability and earthquake-induced failure costs, and finally user costs associated with them. These costs are reviewed in detail and the relevant assumptions are discussed to provide a more realistic estimation of the total cost. Among the mentioned costs, a special attention is paid to the serviceability and earthquake-induced failure cost. The serviceability failure cost is incurred from necessary repair and replacement actions due to the concrete cover spalling and steel rebar corrosion while the earthquake-induced failure cost is due to the repair and rehabilitation actions after a specific seismic event and is dependent on the occurrence probability of different damage states. This cost is estimated here from the results of probabilistic life-time fragility analysis by introducing a performance index which represents the expected performance of a corroded bridge under a particular seismic hazard risk. This index is updated regularly over the time and takes into account the combined effects of seismic hazard and chloride-induced corrosion in the calculation of life-cycle cost of RC bridges.