Federal Highway Administration
Iowa Department of Transportation
About the research
The Iowa Department of Transportation (DOT) funded a three-phase research project focusing on rapid bridge deck joint repair. Phase I focused on the documentation of current means and methods of bridge expansion joint maintenance and replacement. Under Phase II, a workshop with Iowa DOT personnel, engineers, and researchers identified possible improvements to traditional expansion joint options. From the workshop, a deck over backwall detail was developed that moved the expansion joint away from the bridge deck and instead placed it on the approach slab. This would not only minimize the concrete removal needed for a rehabilitation but would, in turn, prevent deicing chemicals from leaking through the deck joints and damaging the bridge’s substructure. In Phase III, the research team was tasked with the further development of this deck over backwall concept.
Full-scale finite element (FE) models of two different bridges were developed to analyze the impact of the deck over backwall concept. Both models were validated using the original drawing plans and American Association of State Highway and Transportation Officials (AASHTO) specifications.
Through experimental testing and the development of FE models, two reinforcing options within the approach slab and diaphragm sections were considered. The results showed that, when both the top and bottom longitudinal reinforcing was kept continuous through the approach slab and diaphragm section, negative moment was transferred to the bridge deck. This transfer of stress through the top reinforcing caused cracking to occur on the top of the bridge deck, which could lead to harmful chemicals leaking onto the substructure. Conversely, experimental testing showed that these stresses could be eliminated if the top longitudinal reinforcing and concrete cover were saw cut.
A plan for construction observation and post-construction testing was developed that included an instrumentation plan and various real-world truck loading cases to be correlated with the FE models; an initial cost estimate was also developed. Implementation of the deck over backwall concept and the post-construction plan is expected to be conducted in a future Iowa DOT construction season.