Recycled materials make 100-year 'long life' bridges possible

05/12/04

Penn State engineers have designed 10 concrete mixtures containing industrial by-products that make it possible for concrete bridge decks to last three times longer or 75 to 100 years.

Dr. Paul J. Tikalsky, associate professor of civil and environmental engineering who led the study, says, "The exact life expectancy of bridges constructed with these mixtures will not be known for many years. However, in full scale trials, each of the mixtures optimizes the ingredients to produce concrete with substantially lower permeability, higher electrical resistivity and lower cracking potential than the standard bridge deck concrete used in Pennsylvania for the past 30 years. "

He adds, "The cost of bridges constructed with these mixtures is nearly identical to the previous generation of bridges. With life expectancies at least three times as long, the life-cycle cost savings will be more than $35 million annually in Pennsylvania with the added benefit of using environmentally friendly materials to contribute to a more sustainable future for the highway infrastructure."

Over the next two years, 10 bridges on Interstate-99 between Bald Eagle and Port Matilda, Pa., will be constructed using the mixtures designed and tested by the Penn State team. Tikalsky and his team will fit the bridges with sensors that will relay information about strain, temperature, and corrosion to a data station at the University. The strains and temperature in the bridge deck will be monitored at 30-minute intervals during construction and hourly thereafter. Measurements related to corrosion will be taken 3 months after construction and every two years thereafter.

Tikalsky presented the Penn State team's results in a paper, High-Performance Concrete Bridge Deck Initiative - Performance Based Specifications in Pennsylvania, at the Concrete Bridge Council Conference, in Charlotte, North Carolina, May 18. His co-authors are David G. Tepke, doctoral candidate in civil engineering, Geoffrey Kurgan, graduate assistant, and Dr. Andrea Schokker, assistant professor of civil and environmental engineering.

In the first phase of the project, the team developed mixture designs with 154 combinations of materials. Total cement content and percentages of additives, including fly ash, silica fume, ground granulated blast furnace slag and an alkaline earth mineral admixture, were varied and the mixtures tested to see which met current structural and physical requirements.

"Fly ash, silica fume and slag are all industrial waste products that have been used previously in some types of construction," Tikalsky notes. "These additives reduce the permeability of concrete and deter salts from entering. The additives also increase electrical resistance. So, in 40 or 50 years when water and salt eventually reach the steel reinforcement rods in the bridge deck, corrosion won't progress as rapidly. "

Twenty-five of the original concrete mixture designs were selected for additional testing. Selection was based on the mixtures meeting basic technical properties and offering a diverse range of solutions to long-term durability. Each of the mixtures was produced in a full truck trial. The concrete was delivered to the lab where durability samples were fabricated and tested over a two-year period.

A second phase of testing subjected the 25 phase one finalists to permeability, porosity, shrinkage, freeze-thaw, hydration, strength, salt scaling and resistivity studies that identified 10 mixtures for testing in 10 bridges in the "100 Year Highway" project on I-99.

"Long-term monitoring of these bridges is the only true measure of the success of this philosophy of extending the life of the infrastructure," Tikalsky says. "Benchmarking the permeability, shrinkage, corrosion potential, chloride ingress, cracking and other properties of the structure over a long period of time will allow successive generations of bridges to further optimize the design of bridges for durability."

Source: Eurekalert & others

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