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.