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M AT E R I A L M AT T E R S Internally cured concrete improves service life of bridges From left, Purdue University graduate students Paul Imbrock, Kambiz Raouf, and John Schlitter pour concrete for a test specimen used for research to improve Indiana bridges. (Raouf and Schlitter recently graduated.) Photo by Andy Hancock/ Purdue University. A new internally cured high-performance concrete, a technology supported by researchers with Purdue University (West Lafayette, Indiana) and the National Institute of Standards and Technology (NIST) (Gaithersburg, Maryland), is helping Indiana state transportation offcials to reduce concrete cracking and damage caused by deicing salt and extend the service life of concrete bridge decks in the state. Concrete is typically made by mixing Portland cement with water, sand, and stone. Water is an essential part of the chemical reaction that gives concrete its strength during the curing or hardening process and it can add to the strength of the material for a long time, if it's available, explains NIST engineer Dale Bentz. "The chemical reaction is ongoing for years and years. It essentially never stops, but it keeps going slower and slower. Usually, about 75 percent of the reaction has occurred by 28 days, but the other 25 percent might happen over many, many years, as long as there is still water available and those reactions can still take place," he adds. 22 MATERIALS PERFORMANCE May 2013 The new internally cured concrete technology distributes a supply of water uniformly throughout the concrete by substituting pre-wetted, lightweight absorbent materials for some of the sand and/or coarse aggregates (stones) that are mixed with cement to make conventional concrete. The water-flled, lightweight aggregates dispersed throughout the mixture serve as reservoirs that release water on an as-needed basis to nearby hydrating cement particles. This internal curing process, where lightweight concrete sets and hardens from the inside out, enhances the material's strength, durability, resistance to cracking, and other properties. "A key step in the process is to pre-wet the lightweight aggregate with water before mixing the concrete," says Jason Weiss, civil engineering professor and director of Purdue's Pankow Materials Laboratory. "Nearly five years of research has been performed to fully understand how to proportion these mixtures and the level of performance that can be expected." One of the most important effects of internal curing is to make the concrete more resistant to early-age cracking, Bentz notes. This is especially important for concrete bridge decks, where such cracks allow winter deicing salts to more rapidly infltrate the concrete and attack steel reinforcing bars. "This material will reduce maintenance costs and allow bridge decks to last longer," says Weiss. "Our testing indicates that internally cured high-performance concrete experiences substantially less cracking and concrete damage caused by deicing salt and, when properly designed, the service life of bridge decks can be greatly extended." Weiss is working with the Joint Transportation Research Program, a partnership between the Indiana Department of Transportation (INDOT) and Purdue, to create specifcations for implementing the internally cured high-performance concrete, which will be used on four Indiana bridges in 2013. The frst bridge is on State Road 933 in St. Joseph County. "We anticipate these relatively minor changes to our concrete specifcations to substantially extend the life of our bridges," says Troy Woodruff, INDOT's chief of staff. "That means fewer traffc delays due to bridge maintenance and repair, and much lower expense." Jay Wasson, INDOT deputy commissioner for engineering and asset management, commented that "this collaboration between Purdue and INDOT to implement the research fndings not only benefts Indiana taxpayers, but also provides valuable full-scale living laboratories for study by Purdue students and faculty. As further feld data are collected by professor Weiss, we anticipate even broader deployment of this concrete specifcation." Purdue researchers assisted county offcials in Indiana's Monroe County with specifcations for internally cured concrete used in a bridge built in 2010, and they are now studying how well it performs compared to an adjacent bridge built the same year using conventional NACE International, Vol. 52, No. 5