Internal curing offers benefits of improved hydration, reduced chloride ingress, and reduced early age cracking, which helps concrete achieve its maximum potential as a sustainable building material by extending its service life.
Internally cured concrete is not a new concept; some might even say it is ancient since it can be considered to date back to concrete constructed during the Roman Empire. What is new, however, is a more complete understanding of how internal curing (IC) works and a way to design for IC. We also have a better understanding of why IC increases the durability and service life of concrete in an economical and practical way.
What is Internal Curing?
Internal Curing is a practical way of supplying additional curing water throughout the concrete mixture. This is done by using water absorbed in expanded shale, clay or slate (ESCS) lightweight aggregate, which replaces some of the conventional aggregate in the mixture. IC is often referred to as “curing concrete from the inside out.”
The American Concrete Institute defines internal curing as “supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation”
While internal curing occurs in conventional lightweight concrete, it is only recently that internal curing has been intentionally incorporated into normal weight concrete to improve its properties.
Why is Internal Curing Used?
Internal curing provides something that most concrete needs and conventional curing cannot provide: additional water that helps prevent early age shrinkage and increases hydration of cementitious materials throughout the concrete. Although IC has shown benefits at w/cm up to 0.55 (Espinoza-Hijazin and Lopez, 2010), the need for internal curing increases as the w/cm is lowered. Research shows that even in moderate w/cm (0.40 to 0.46) mixtures, the cement hydration is often not nearly complete, even after many months.
Once concrete sets, hydration creates partially-filled pores in the cement paste which causes stress that results in shrinkage. IC provides readily available additional water throughout the concrete, so hydration can continue while more of the pores in the cement paste remain saturated. This reduces shrinkage and early age curling/warping, increases strength, and lowers the permeability of the concrete, making it more resistant to chloride penetration.Internal curing has been shown to work well with supplementary cement materials (SCM), especially at higher dosage levels, because fly ash and slag have increased water demand during their reaction, compared to hydrating portland cement. Internal curing does not replace conventional surface curing, but works with it to make concrete better. Internal curing can also help compensate for less than ideal weather conditions and poor conventional curing that is often seen in the real world.
Internal Curing Q&A:
Jeff Speck, a past 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 General Sales Manager of Trinity Expanded Shale & Clay, a producer of high-quality expanded clay lightweight aggregates (LWA) based in Arlington, TX. He chaired the subcommittee tasked with drafting the standard for five years; and, although his term expired in December 2011, Speck remains active on the subcommittee. Here, he details the standards process and explains why the specification of using pre-moistened LWA for internal curing (IC) of concrete is important to the industry and for engineers and contractors.
Q: What led to the aggregates industry developing a standard to specify lightweight aggregates for internal curing?
Speck: 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.
The first known research dates back to Paul Klieger at PCA in 1957; however, it wasn’t until the 1990s that researchers began intensively quantifying the science so it could be confirmed. Through that extensive industry research, there is now a much greater understanding about the process and why IC using LWA not only increases durability, but also increases the service life of concrete for a better economical and practical value. Until now, there has never been a standard specifying the use of LWA for internal curing.
After years of intensive research with outstanding results, the aggregate industry began to look into the need for an ASTM specification. Dale Bentz of the National Institute of Standards and Technology (NIST), wrote the first draft of a standard specification in 2007. The aggregate industry reviewed and edited the Bentz draft, and debated whether to incorporate it into the existing ASTM Standard C330, Specification for Lightweight Aggregates for Structural Concrete, or to create a new standard from scratch. In December 2010, a task group was appointed to prepare a ballot item for voting in ASTM Subcommittee C09.21 – Lightweight Aggregates and Lightweight Concrete.
By July 18, 2011, the subcommittee was ready to submit version 9 of the draft standard to ASTM for the first ballot cycle. Less than a year later on June 15, after four more versions, one subcommittee ballot and two concurrent ballots, ASTM notified John Ries of the Expanded Shale, Clay and Slate Institute (ESCSI) that the document was officially approved, with the designation ASTM C1761-12, Standard Specification for Lightweight Aggregate for Internal Curing of Concrete.
Q: What is internal curing and what are its benefits?
Speck: 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. 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/cm 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.
Q: What is lightweight aggregate and how much is needed for IC?
Speck: 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 approximately 2000° F, 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. The amount of wetted lightweight aggregate needed is based on the absorption and desorption of the aggregate being used. For most practical concrete applications, 7 lbs. of IC water per 100 lbs. 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 cement materials.
Q: How is internal curing different from conventional curing?
Speck: 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. 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 contributes to increased strength.
Q: How will the new standard impact construction projects?
Speck: Although the concept of IC may be 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 benefitting 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 percent 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 percent reduction compared to a normal concrete deck!
This new standard will be useful in a variety of civil engineering projects, such as parking lots, roads, bridges, and water and sewage treatment tanks, and even residential driveways. Use of the standard will impact concrete projects contracted by the U.S. Federal Highway Administration, and state departments of transportation, as well as architects, and environmental, structural and civil engineers.