Corrosion of steel plate girder bridges and rehabilitation using UHPC

Abstract

Steel plate girder bridges with concrete slab decks have been widely used in the USA highway network. The average age of all US bridges is around 43 years, including steel plate girder bridges. Today, about 26% of the nation’s bridges are structurally deficient. The concrete slabs suffer from delamination and excessive cracking due to corrosion and acid attacks. The steel girders suffer from sectional losses in different locations due to corrosion. This problem negatively influences on the US economy by imposing large funding for rehabilitation as well as imposing limitations on the transport network efficiency due to the reduction of deficient bridges load rating. Therefore, two main aspects have been considered in this work. The first deals with the accurate prediction of corrosion effects on the steel structure. The second deals with offering middle-ground solutions to extend the bridge life without fully replacing it nor reducing its rated loading. Today, the concrete slab is replaced every 40 years on average, and the girder is strengthened multiple times during regular maintenance, then, the bridge rated load is eventually reduced until it is replaced. The goal of this study was to address both aspects by understanding the steel corrosion more clearly and understanding the behavior of reinforced Ultra-High Performance Concrete to propose it as a novel lighter weight replacement for the slab. On the corrosion side, the effects of different environmental exposure conditions on corrosion morphology and the corresponding reduction in strength and apparent ductility are considered. These conditions included: 1) the presence of chloride ions, 2) Temperature variations, and 3) Mill scale effects. The chloride contents were: 0%, 1%, 2%, and 3% by weight relative to de-ionized water. The containers with the designated chloride contents were stored in rooms with different controlled temperatures of 26C, 40C, and 50C. Unlike the previously published steel corrosion experiments, the manufactured dog-bone samples were exposed to the solutions without removing the mill scale. The accelerated corrosion experimental program lasted for 16 months allowing us to reach corrosion levels corresponding to more than 100 years of real service life. An extended experimental program was performed on coupon samples with different surface conditions: with mill scale (as-received samples) and without mill scale (polished samples) to further investigate the effect of mill scale on corrosion initiation and corroded surface morphology. The extended corrosion experiment was done on two chloride concentrations (0% and 1%) at room temperature (26C) and lasted for three months. All dog-bone samples (corroded and reference) were mechanically tested under uniaxial tension tests. Finite element simulations were performed on the corroded and reference steel dog-bone samples, which show a good agreement with the experimental results and explain the origins of apparent ductility degradation. Among major findings was that the mill scale can initiate corrosion even if there are no chloride ions in the solution. Additionally, corrosion morphology highly affects the ductility, which considers all simplified designs of bridges based on average area losses inaccurate in terms of ductility. On the rehabilitation side, replacement of the RC slab using a much lighter UHPC slab is proposed to reduce the slab weight in proportion to the reduction in girder capacity due to corrosion. A case study on a 43 years old corroded non-composite steel plate girder bridge in the State of Pennsylvania was utilized to prove the feasibility of the proposed method where it was shown that the UHPC can reduce the dead load by more than 20% allowing the bridge to still perform at its full live load demand without the need of rehabilitating the steel structure. Next, a new method for casting UHPC slabs with preferential fiber orientation is developed, as this was shown to be a major contributing parameter to the strength of the slab. Finally, in order to better understand the behavior of UHPC sections and due to the limited design considerations available in literature for reinforced UHPC elements, a comprehensive experimental program was conducted to reveal and understand the interplay between fibers content and reinforcement ratio and how that affects the mode of failure, ultimate strength and ductility. The experimental results were compared with the UHPC available codes, and the results showed how current provisions for design are far from accurate predictions. In general, code predictions are too much conservative in shear and are also conservative in bending. On the ductility side, there is a large need to provide a new definition for under-reinforced versus over-reinforced section design because the interplay of fibers and reinforcement can result in lower overall element ductility if high fiber content is provided with low reinforcement ratios. This study serves as a call for developing performance based measures for sectional designs that include the different reinforcing modalities along with element dimensions and loading conditions.