A recent news story from Purdue University describes how Indiana state transportation officials are making concrete use of concrete research to extend the service life of bridge decks in the state and to lower repair and maintenance costs. The Indiana engineers are using new "internally cured" high-performance concrete, a technology championed by researchers from Purdue and the National Institute of Standards and Technology (NIST).
Water is an essential part of the chemical reaction that gives concrete its strength, explains NIST engineer Dale Bentz, and it can add to the strength of the material for a long time, if it's available. "The chemical reaction is ongoing for years and years," Bentz explains. "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."
Bentz worked with Purdue's Jason Weiss on the definitive review* of internal curing, a technique to extend the life and durability of concrete by distributing an extra supply of water uniformly throughout the concrete in absorbent materials that are mixed in with the cement and aggregate that make up concrete. (See the March, 2011, NIST Tech Beat story, "High-Tech Concrete Technology Has a Famous Past.")
One of the most important effects of internal curing is to make the concrete more resistant to early-age cracking, Bentz says. This is especially important for concrete bridge decks, where such cracks allow winter deicing salts to more rapidly infiltrate the concrete and attack steel reinforcing bars.
In addition, says Bentz, internal curing is particularly important for "greener," more environmentally friendly concrete mixtures. "For sustainability, engineers are trying to take out more cement and replace it with other materials, such as fly ash. Cement production is very energy intensive and has a significant CO2 footprint—making a ton of cement produces almost a ton of carbon dioxide. But in these high-volume fly ash mixtures, internal curing is important because while the fly ash will react with the cement, it takes a lot longer. After 28 days, maybe 30 percent or less of the fly ash has reacted, so you really need to keep the concrete saturated for an extended period of time."**
Read the Purdue University story, "Indiana using new concrete to increase bridge life span" at www.purdue.edu/newsroom/releases/2013/Q1/indiana-using-new-concrete-to-increase-bridge-life-span.html.
Another recent development of the NIST and Purdue work on internal curing is the approval, this past summer, of a new standard specification by ASTM International. ASTM C1761-12, Standard Specification for Lightweight Aggregate for Internal Curing of Concrete, provides test methods and other information for evaluating and incorporating lightweight, absorbent aggregates for internal curing of concrete.