Seismic Risk Assessment of Reinforced Concrete Bridges with Ageing Effects

21st May, 2019
    Francesco PugliesePhD Student

Exposure to aggressive environments is one of the main causes of reinforced concrete (RC) civil infrastructure degradation and damage, which can become critical if such structures are unprotected, as in the case of bridges, and even more crucial if subjected to seismic loading. Particularly, post-earthquake surveys have shown that the losses generated by the failure of bridges can be relevant and the limitation for the transitability of important highways can erode the resilience of large communities.

It is thus deemed necessary to provide an efficient and reliable tool to analysize the earthquake performance of RC bridges and to quantify their seismic risk, considering also the ageing effects, especially for the concrete and the steel reinforcement bars.

Collapsed Morandi bridge (Genoa, Italy). Investigators find corrosion on main stay cables. Source

Despite numerous changes in Standards and Codes over the years, small concrete-cover thickness and low-strength concrete remain prevalent. As a result of this, aggressive agents can easily penetrate through the concrete and de-passivate the layer of film lying uniformly on the surface of steel rebars. Chemical and physical processes cause various damage processes and alter the mechanical properties of both the steel reinforcement bars and the concrete. Accordingly, mass loss and cross-section reduction of steel reinforcement bars and concrete cover cracking, as well as reduced concrete compressive strength and effectiveness of confined concrete summarise the main consequences of corrosion. The broad context of environmental deterioration and environmental risk assessment is directly related to structural performance and its serviceability. Many unprotected civil infrastructures, such as bridges, even in nonseismic zonas (i.e. UK), show deficient conditions due to ageing and, often poor or non-existent inspection-ratings are available to assess and plan risk mitigation. When inspection-ratings are available, they usually yield an overestimation of repair costs. An investigation carried out by RAC Foundation (The Royal Automobile Club Foundation) with the help of the National Bridges Group of ADEPT (The Association of Directors of Environment, Economy, Planning and Transport) found that 2434 out of 50561 (5%) bridges in the UK are in poor conditions and cannot carry the loading of more than one lorry driving through. As a result, all these bridges are categorised as “substandard”. Therefore, actions are required to recover those bridges, and further investigations have revealed that bills will hit more than £6.7 billion.

The primary aim of this project is to formulate an accurate finite element model that can simulate the fundamental response quantities of a bridge system, which can experience also ageing effects. The structural model should be refined and versatile so that local phenomena, such as cracking, bond, etc. can be reliably implemented in a structural analysis computer program to perform comprehensive non-linear dynamic analyses.

West Virginia Historical Society Quarterly Vol. NV, No. 4 October, 2001. The Collapse of the Silver Bridge Source

A robust model that can characterise the time-dependent material properties, account for the uncertainty of the strength and stiffness of the material, the type of the boundary conditions, should be formulated to perform robust fragility analysis, therefore I will be performing some experimental campaigns on corroded smooth rebars between University of Liverpool and Xi'an Jiaotong-Liverpool University (XJTLU). A combined use of structural analysis programs, such as Opensees and/or Abaqus, and the open-source simulation analysis tool COSSAN-X should be considered to quantify the risk of the selected bridges structures.

To evaluate the condition of corroded RC structures, the actual performance has to be determined. New corrosion-related inspection-ratings have to be established to plan maintenance, new cost-effective strategies and risk mitigation. To meet all these goals, this study proposes a new method based on the crosssectional analysis of RC elements subjected to different degrees of corrosion. Accordingly, numerical validation based on experimental results will be performed to demonstrate the effectiveness of the method.

Part of this work has been published at the SECED London Conference 2019, and an abstract been accepted for the annual conference of Earthquake Engineering in Ascoli Piceno (Italy-ANIDIS 2019). I am currently writing a journal paper for the Academic Journal called “SOIL DYNAMICS AND EARTHQUAKE ENGINEERING - JOURNAL – ELSEVIER”, while the final work will be submitted to the “STRUCTURAL SAFETY ACADEMIC JOURNAL ELSEVIER”.