Professional engineers and contractors want to install concrete that can stand the test of time. So when it comes to using material that is proven to increase hydration, reduce shrinkage and cracking, improve durability, and ensure better quality, while ultimately reducing costs, a new standard sets the stage for projects using concrete and internal curing.
That was the conclusion ASTM Committee C09 on Concrete and Concrete Aggregates and Subcommittee C09.21 on Lightweight Aggregates and Lightweight Concrete came to when it wrote the ASTM Standard Specification for Lightweight Aggregate for Internal Curing of Concrete. The new standard was approved in June 2012, just 17 months after the first draft was circulated to the Subcommittee Task Group. Now, more than a year later, it’s worth reviewing how this specification is affecting and benefiting concrete producers and contractors.
Jeff Speck, a former chairman of the C09.21 subcommittee, credits the past decade of intensified research and industry collaboration for the expedient progress made on writing and passing the standard. Speck is vice president of sales and marketing for Big River Industries, a producer of expanded clay lightweight aggregates (LWA) based in Atlanta. He chaired the ASTM subcommittee tasked with drafting the standard for five years. Although his term expired in December 2011, Speck remains active on the subcommittee.
Although most people may not be aware of it, the use of internally cured concrete in construction is not new. “Those of us working in the lightweight aggregate industry have known anecdotally for decades that LWA will hold water and give it back to the cement paste as the hydration process occurs,” says Speck.
The first known research dates back to Paul Klieger at PCA in 1957. However, it was not until the 1990s that researchers began intensively quantifying the science so it could be confirmed. Through that extensive research, there is now a much greater understanding about the process and why internal curing (IC) using LWA not only increases durability, but also increases the service life of concrete for a better economical and practical value. Until 2012, there was never a standard specifying the use of LWA for internal curing.
The benefits of internal curing
Internal curing provides a supply of moisture from within the concrete for the development of cement hydration with age. Through the use of pre-moistened lightweight, porous aggregate, which replaces some of the conventional aggregate in the mixture, a high relative humidity (RH) can be maintained within the pore structure of the concrete, extending hydration and increasing strength and durability performance.
“The lightweight aggregate particles release moisture as necessary, increasing hydration of cementitious materials throughout the concrete over time, reducing shrinkage and warping of concrete, and lowering concrete permeability, making it more resistant to chloride penetration,” says Speck. “Because the IC water is absorbed water in the lightweight aggregate, it is not part of the mixing water and does not affect the w/c ratio.”
Even distribution of additional water sources within concrete leads to greater uniformity of moisture throughout the thickness of the concrete, and thus reduces internal stresses due to differential drying. While drying shrinkage may not be completely prevented long term, delayed drying will allow the mixture to gain strength and help resist associated stresses.
The fundamental role of lightweight aggregate
Expanded shale, clay and slate (ESCS) has a long track record of quality and performance. ESCS is composed of selectively mined materials that are fired in a rotary kiln at about 2000° F, and then processed to precise gradations for a variety of applications. The aggregate size used for IC is normally a fine (sand) grading, which provides a more even distribution of the IC water throughout the cementitious paste.
The same amount of water concentrated only in coarse aggregate can leave part of the cementitious paste unprotected by internal curing. This is because the water only travels a limited distance. In some mixtures, intermediate size aggregate may be used to optimize total aggregate grading and provide IC.
According to Speck, the amount of wetted lightweight aggregate needed is based on the absorption and desorption of the aggregate being used. For most practical concrete applications, seven pounds of IC water per 100 pounds of cementitious material provides an appropriate value for the amount of IC moisture needed. However, the amount of IC water may be increased to accommodate evaporation or to satisfy the higher water demand in mixtures with supplemental cementitious materials.
Internal vs. conventional curing
Concrete, especially high-performance concrete, is designed to limit permeability and reduce chloride ingress, but these properties also limit the ability of externally applied curing water, typically placed on top of the concrete after it has been mixed and poured, to reach the concrete’s interior.
“Conventional curing may only penetrate a few millimeters into the concrete,” says Speck. Once it sets, chemical shrinkage continues in the cementitious paste as hydration progresses, and creates pores within the concrete. These unfilled pores create stress, which causes shrinkage.”
IC provides additional water throughout the concrete, so more of the pores remain water-filled, minimizing stress and strain development. This reduces or eliminates early age cracking of the concrete and promotes maximum hydration, which can contribute to increased strength.
The new standard at work
Although the concept of IC may still be somewhat new to many in the concrete industry, since 2003, more than 2 million cubic yards of IC normal weight concrete, including 1.3 million cubic yards in low slump pavements, have been placed. Projects using high-performance concrete are benefiting from IC as well.
IC results in only slightly higher initial costs. However, when considering the extended service life, these costs are far outmatched by the value IC provides. Predictions based on 2010 research by Daniel Cusson of the National Research Council Canada maintain that service life of a normal concrete deck is 22 years with a present value life cycle cost of $783 per square meter, while a high-performance concrete deck has a projected service life of 40 years with a life cycle cost of $472 per square meter, a 40% reduction. A high-performance concrete deck with IC has a predicted service life of 63 years with a life cycle cost of $292 per square meter, a 63% reduction compared to a normal concrete deck.
“The ASTM standard that was approved in 2012 has been substantiated in a variety of civil engineering projects, such as parking lots, roads, bridges, and water and sewage treatment tanks, and even for residential driveways,” says Speck.
During the last year, the use of the standard has begun to impact concrete projects contracted by the U.S. Federal Highway Administration and state departments of transportation, as well as architects, contractors, producers, and environmental, structural, and civil engineers.
Don Eberly is president and CEO of Eberly & Collard Public Relations. E-mail email@example.com.
Case Study: I-90 Bridge in Hampshire, Ill.
Owner: Illinois State Tollway Highway Authority
Concrete Producer: Super Mix Inc., Hampshire, Ill., plant
Contractor: Acura, Inc., Bensenville, Ill.
LWA Supplier: Big River Industries, Atlanta
In January 2013, the Illinois State Tollway Highway Authority solicited bids for the I-90 bridge that crosses U.S. Route 20 in Hampshire, Ill. The specification included a provision for high-performance concrete mix designs for the concrete superstructure, with the primary goals of reducing or eliminating cracking in the bridge decks by drastically reducing shrinkage, and increasing the service life of the structures.
The performance-based specification allowed Super Mix to develop a mix design that met the various criteria set forth in the special provision. The producer selected Riverlite, an expanded lightweight clay aggregate (LWA) in conjunction with a high-range water reducer, shrinkage reducing admixture, hydration stabilizer to produce a mix design that met the various criteria specified by the highway authority.
Super Mix technicians conducted both laboratory and field test batches on the mix design with the assistance of Matthew D’Ambrosia of CTLGroup in Skokie, Ill. The team replaced a portion of sand with the Riverlite LWA to make available absorbed water for internal curing purposes.
By using Riverlite LWA, Super Mix was able to develop a mix that met the performance requirements of the Tollway Authority. The LWA was soaked with a sprinkler for several days and then allowed to drain for a day to reduce the free moisture on the surface of the material. After the stockpile had been thoroughly mixed, the concrete was batched with consistent slumps out of the plant.