Jan. 29, 1999 University Park, Pa. -- Penn State researchers have found that a commercially available additive can potentially double the life and lower the long term costs of concrete bridge decks by enhancing resistance to water, corrosion and deicing salt.
John J. Garvey, a master's degree candidate in civil and environmental engineering; Dr. Paul J. Tikalsky, associate professor of civil and environmental engineering; Dr. Barry Scheetz, professor of materials and senior scientist at Penn State's Materials Research Laboratory; and Bruce Grant, president of IPA Systems, Philadelphia, are the authors of a paper detailing the study.
The paper, "Influence of Alkaline Earth Silicate Admixture on Durability of Pennsylvania Turnpike Bridges," was presented Jan. 11 at the annual meeting of the Transportation Research Board (TRB) in Washington, D. C. The TRB is also distributing the paper nationally to all state highway departments.
The additive is a proprietary mixture of alkali earth silicates, calcium and nitrates tradenamed, Ipanex. The additive has been manufactured by IPA Systems, Phila., since 1972. IPA and The Pennsylvania Turnpike Commission funded the study which was conducted in cooperation with Penn State's Pennsylvania Transportation Institute.
"There is the potential to double the life span of most highway bridge decks with an increase in cost of less than five percent," Tikalsky says. "The interstate highway bridge decks on the turnpike see some of the heaviest traffic and salt applications in the country. This leads to a life of approximately 25 years but with this admixture the data shows that these decks are virtually new after 25 years. The repair, maintenance and replacement costs are essentially avoided by building a better initial deck. Our ongoing research is working on quantifying the value and additional life cycle benefits of the admixture."
The researchers inspected concrete deck structures that had been in service on six bridges in similar locations on the Pennsylvania Turnpike for 25 years or more. Bridge decks constructed with concrete to which Ipanex had been added were compared with those constructed without the additive. The researchers report that there was less tendency for water or salts to intrude into the concrete decks constructed using Ipanex. In decks constructed without Ipanex, the concrete between the surface and the embedded steel was observed to be peeling off, exposing the steel to the air.
In addition to evaluating the performance of the bridges along the turnpike, the Penn State research team also performed laboratory testing and chemical analysis of core samples taken from the bridge decks. The samples were subjected to water under pressure (300 psi) for seven days. No appreciable water penetrated the samples containing the additive while water penetrated the other samples.
Study of the cores with an scanning electron microscope showed that after 25 years, the additive had reacted with the cement to produce much finer structure in the concrete. A refined structure, with fewer and smaller spaces between the cement grains, makes concrete less penetrable by water and chloride ions from deicing salt.
"Chlorides from deicing salts and the saturation of concrete with water are the primary causes of premature corrosion of reinforcement in concrete bridge decks," Tikalsky notes. "The additive reduces the ability of water and chlorides to penetrate the concrete, delaying the onset of corrosion, and increasing the design life of bridges."
In their paper, the researchers conclude that the additive "greatly extended the life of the precast bridge panels." They added, "This has a positive impact on the life cycle cost of the bridges made with this material."
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