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February 14th, 2013

Internal curing standards and recent work on extending life of concrete structures

Published on February 14th, 2013 | By: Eileen De Guire

Credit: Purdue University and INDOT Joint Transportation Research Program.

Many people in the field of high-performance cements and materials have been working on the goal of improving the performance of structures such as roadways and bridge decks, and recently there have been interesting developments in regard to the use of internal curing (IC) techniques and the creation of a new standard specification by ASTM International.

For researchers involved in cements and concrete, a fundamental task has been to prevent deterioration caused, to a large extent by ions from salts and other materials that can lead to crack formation and corrosion of steel reinforcements. A basic consideration is that cement systems must “cure” or hydrate sufficiently to become useful. A nemesis is early-age cracking that can lead to accelerated deterioration of concrete, which can lead to catastrophic outcome in the case of concrete bridge components.

A couple of factors come into play. First, curing is not instantaneous and requires access to water. Curing to a serviceable extent (e.g., to 75 percent of full curing) is typically measured in days and weeks, but the curing process can go on for years if conditions are right.

Another factor is the composition of the concrete constituents. Engineers are employing both “high-performance” concrete and cementitious materials that can substitute for some of the cements, such as fly ash. Unfortunately, both can also lead to curing problems. In the case of the former, although the high-performance materials have the positive property of limiting the ingress of briny fluids and destructive ions, according to John Ries, technical director of the Expanded Shale, Clay and Slate Institute, “these properties also limit the ability of externally applied curing water to reach the interior of the concrete.”

In the case of the latter, cement alternatives can lead to extended curing times. In a recent NIST Tech Beat story, NIST engineer Dale Bentz explains, “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.”

In both cases, the solution is to achieve a way to encourage internal curing and, says Reis, “provide a source of additional water to maintain saturation of the cementitious paste and avoid its self-desiccation.”

As discussed in the above video, engineers from Purdue University and the Indiana Department of Transportation (INDOT) have been developing an IC approach that involves creating a longer-term internal water source instead of relying water in the mix or externally applied water. A Purdue news release reports that the IC approach is based on creating “water pockets” formed from small porous stones—or fine aggregate, as it is known in the industry—to replace some of the sand in the mixture. Purdue’s Jason Weiss says, “A key step in the process is to pre-wet the lightweight aggregate with water before mixing the concrete.”

Weiss, who is a professor of civil engineering and director of the Pankow Materials Laboratory, as well as a long-time collaborator on the annual meetings of ACerS’ Cements Division, reports that coming up with a suitable IC system did not happen overnight. “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,” Weiss says.

The video and the Purdue release say a real-world IC study is underway. In 2010, INDOT (with the support of NIST, Lafarge North America and the Expanded Shale Clay and Slate Institute) built two adjacent bridges—one based on IC specifications and one based on traditional specs—and, so far, the results are looking good. In Purdue release, Weiss reports, “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.”

Another recent development is that NIST and Purdue successfully gained the approval of the ASTM’s Standard Specification for Lightweight Aggregate for Internal Curing of Concrete (ASTM C1761-12).

Finally, this is a good place to mention that the “4th Advances in Cement-based Materials: Characterization, Processing, Modeling and Sensing” meeting co-organized by ACerS’s Cements Division and the Center for Advanced Cement-based Materials will be he held July 8-10, 2013, at the University of Illinois at Urbana-Champaign.

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