ABSTRACT
Environmental erosion has always been treated as a long-lasting threat to the safety and serviceability of the bridge in the operation stage. Due to the fast development of the cities all around the world, carbonation of RC bridges has received broad concern during the past decades. This is because carbonation is a phenomenon usually occurs in the urban area and bridge structures has vital importance in the city transportation system (Izumi et al. 1986). Carbonation induced corrosion will cause severe loss in the effective cross-section area and mechanical properties of the steel reinforcement. A RC bridge that is originally designed to meet the safety requirements of the design codes may experience sudden structure failure without any significant early warnings due to undergoing corrosion. Fragility analysis could be a powerful tool to illustrate the influence of different hazard level to the safety level of the engineering structure, providing strong supports for the management and decisionmaking of the structure. In this paper, we sets up a complete process to evaluate the fragility of the reinforced concrete bridge when facing the environmental erosion hazard of carbonation induced corrosion. The evaluation process can be treated as a merge of environmental corrosion analysis, time-variant capacity and demand analysis and fragility analysis.
The environmental corrosion process adopted in this paper considers all the corrosion stages including the concrete carbonation, steel reinforcement corrosion and material damage (CECS220: 2007). Concrete carbonation is a chemical reaction process between the diffused carbon dioxide from the environment and the carbonatable substance in the concrete. The corrosion of the steel reinforcement takes place after the passive layer (concrete cover) is destroyed, then steel reinforcement corrosion happens. The corrosion process is divided into two phase: corrosion before concrete cover cracking and after concrete cracking. Different steel reinforcement corrosion rates are considered in each phase. The material damage models also take all the possible influence of the corrosion to the performance of the materials into consideration, including both geometry and strength losses.
The time-variant capacity model is developed by incorporating the corrosion process summarized above into the existing capacity models in the current design code. The capacity of the corroded reinforced concrete bridge is calculated using the existing well developed model and replacing the original structural geometry and material properties with the deteriorated values at the service time considered. In this paper we only consider the bending moment failure mode for demonstration purpose. Other failure modes could be studied in the same way.
Fragility is the conditional probability of attaining or exceeding a predefined limit state for a given set of boundary variables for a bridge structure (Gardoni et al. 2002). The time-variant capacity models developed above are used in a formulation to assess the fragility of corroded reinforced RC bridge. Concrete surface CO2 concentration and service time of the bridge are main boundary values considered to represent the carbonation induced corrosion level.
As a case study, we apply the developed process above to a simple supported reinforced concrete beam subjected to carbonation induced corrosion. Monte Carlo method is used in this paper to calculate the probability of failure.
