According to the American Society of Civil Engineers, the average bridge in the United States is 43 years old.
With an intended design service life of 50 years, a large number of the nation’s bridges are already considered to be structurally deficient or functionally obsolete.
And Iowa is no exception. Of its approximate 25,000 bridges, 20 percent are considered deficient or posted with a weight restriction. In short, Iowa is one of the top three states in the nation with the most deficient bridges.
“And that’s why the work we are doing with the Iowa DOT and FHWA is so important,” says Sri Sritharan, the principal investigator on a $127,500 research project looking at the use of ultra-high performance concrete (UHPC) for bridge deck overlay.
According to Sritharan, the problem isn’t just the aging infrastructure, but also the continuous increase in both traffic volume and heavy traffic vehicles, which requires a critical look at the nation’s bridge stock.
“Right now, there is an urgent need to develop technologies that are not only economical and durable, but can also be safely and rapidly implemented in practice.”
And his project, which ended September 2017, is doing just that. Most bridge deterioration starts with deck cracking that in turn causes more severe and extensive damage to the superstructure. That’s where UHPC comes in.
In a UHPC study by Sritharan in 2014, he tested a concept of using UHPC as a thin layer on top of a normal concrete bridge deck.
“Since UHPC has a higher tensile strength and low permeability, cracking, as well as water and chloride ingression, can be prevented or considerably minimalized, which in turn will elongate the lifespan of the bridge. Moreover, UHPC is also deemed to have a higher fatigue resistance as compared to normal concrete.”
In his newest study, he involved a material supplier to formulate a new UHPC mix to accommodate road crowning and sloping deck surfaces and tested its applicability in the laboratory.
Using this new mix, the thin UHPC overlay concept was successfully implemented on the Mud Creek Bridge in Buchanan County, Iowa.
“The bridge overlay is periodically monitored, and, so far, there has been no issues found with regards to the UHPC overlay or the interface between the UHPC and the old deck surface,” says Sritharan.
In addition to the field implementation, three concrete slabs, with and without the UHPC overlay, were also tested in the laboratory. The results showed that UHPC overlay can be designed to increase the deck’s stiffness and strength if desired.
So, how does this study impact future research using UHPC overlay and bridge construction?
According to Sritharan, the use of a thin UHPC layer as a deck overlay has been shown to be a viable solution to prolong the lifespan of bridge decks. While its long-term performance in the field needs to be verified, technologies have to be developed to place the UHPC swiftly and uniformly on the bridge deck. Development of such technologies and further refinement to the UHPC mix will enable widespread application of UHPC overlay on our nation’s bridges.
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