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concrete. "The control bridge has developed three cracks, but no cracks have developed in the internally cured bridge. Tests also show the internally cured concrete is approximately 30 percent more resistant to salt ingress," Weiss says. In addition to Indiana, Weiss has worked with several other states to accelerate the use of similar internally cured high-performance concretes, which can vary in composition from state to state depending on the materials that are available locally and the design of bridge deck components. In a review1 of the internal curing process, Bentz and Weiss comment that the technology has impacted modern infrastructure through its intentional use only within the last decade or so, although the process existed almost 1,900 years ago when the Romans built the Pantheon— the world's largest unreinforced solid concrete dome—and has inadvertently been employed by many lightweight concretes produced within the past 100 years. Increasingly, internally cured concrete is being used in the construction of bridge decks, pavements, parking structures, water tanks, and railway yards. They mention in the review that the path from research to practice has been a slow one, as with many new technologies; but as of 2010, hundreds of thousands of cubic meters of the lighter and more durable material have been successfully used in U.S. construction. According to one study cited in the review, bridge decks made with internally cured highperformance concrete were estimated to have a service life of 63 years as compared with 22 years for conventional concrete and 40 years for high-performance concrete without internal curing. Bentz comments that internal curing is particularly important for more environmentally friendly or "greener" concrete mixtures. The internal curing process allows engineers to reduce the amount of Portland cement used in concrete by replacing a portion of it with supplementary materials such as silica NACE International, Vol. 52, No. 5 fume, fy ash, and limestone. He notes that cement production is very energy intensive and has a signifcant carbon dioxide (CO2) footprint—producing a ton of cement creates almost a ton of CO2— and for sustainability, engineers are trying to take out more cement and replace it with other materials, such as fy ash. In these high-volume fy ash mixtures, internal curing is important because the fy ash takes longer to react with the cement (after 28 days, ~30% or less of the fy ash has reacted), so the concrete needs to be saturated with water for an extended period of time. "We just fnished a project for the Federal Highway Administration where we showed that we can take 60 percent of the cement out of a typical bridge deck concrete and obtain similar if not better performance for bridges by taking advantage of the benefts internal curing provides," Weiss says. A recent development stemming from the NIST and Purdue work on internal curing is the approval of a new ASTM International standard,2 which provides test methods and other information for evaluating and incorporating lightweight, absorbent aggregates for internal curing of concrete. Sources: Emil Venere, Science Writer, Purdue University, www.purdue.edu; and National Institute of Standards and Technology, www. nist.gov. References 1 D.P. Bentz and W.J. Weiss, "Internal Curing: A 2010 State-of-the-Art Review," National Institute of Standards and Technology, NISTIR 7765, February 2011. 2 ASTM C1761/C1761M-12, "Standard Specifcation for Lightweight Aggregate for Internal Curing of Concrete" (West Conshohocken, PA: ASTM International). —K.R. Larsen May 2013 MATERIALS PERFORMANCE 23