Bridge Analysis, Design and Repair
342682 - Corrosion in the Substructure of a Pedestrian Bridge at YSU: A Case Study
Friday, April 20
3:30 PM - 5:00 PM
A pedestrian bridge linking Moser Hall and Cushwa Hall at Youngstown State University (YSU), Youngstown, Ohio, was built almost 40 years ago. Three hammerhead pier caps supporting the bridge deck have experienced severe corrosion at the bottom. This corrosion has led to spalling of concrete at multiple locations in all three pier caps and thereby exposed the rebars to open air facilitating more corrosion. At some locations, corroded steel bars are clearly visible at spalled locations and through larger cracks. Some patches of repair are seen on the site, but the corrosion problem still continues to degrade the substructure.
An investigative study was undertaken to identify the causes of corrosion in these substructures and to propose a viable remedy to this problem. The replacement of the pier caps was not a preferable option, unless no remedial option is available. The researchers studied the construction drawings available at the time of this study, and some important documents containing information on material strength and clear cover were lost in time. This situation created a significant challenge in performing structural analysis of the existing substructure. Therefore, Schmidt Hammer and Profoscope were used to approximate concrete strength, rebar location and clear cover.
Reviews of previous research suggest that substructure corrosion can likely be chloride-induced, sulfate attack, or carbonation-induced. Chloride-induced corrosion is more common in structures subject to deicing salts or saline water. Being a pedestrian bridge between two educational buildings with no access to deicing salts or saline water, chloride-induced phenomenon was ruled out as an active cause of corrosion in the substructure. Sulfate attack was ruled out as well due to no visible sign of white-powder accumulation at the corroded locations. Although carbonation-induced corrosion is a slower process in a dry environment and mostly occurs in older structures, it was predicted that the corrosion in those pier caps was due to carbonation of concrete. Free carbon dioxide in the atmosphere reacts with alkaline hydroxides in the concrete to form carbonic acid. Carbonic acid, unlike other acids, does not attack the concrete paste but it lowers the pH value. Carbonation reaction takes place on the outer surface of concrete and gradually penetrates into the concrete cover resulting in a low pH concrete cover.
A full-scale structural analysis revealed no major reduction in strength of the pier caps. Visual and electrochemical inspections were conducted to ensure no major deficiency exists in the corroded concrete and steel reinforcement. After considering various repair methods, patching with fiber-reinforced concrete at possible locations and cathodic protection were suggested as the best remedial measures to protect the substructure from further deterioration.