What is Internal Curing? (And What it Isn’t)

By Clint Chapman
Western Region Marketing & Technical Manager, Arcosa Lightweight

Internally cured concrete more readily resists early age shrinkage, curling, warping, and cracking by supplying water throughout the concrete mix over time. Internal curing can also increase a concrete’s strength and reduce its permeability. These qualities help prolong the concrete’s service life, even when it is placed in harsh environments.

While internal curing is not necessarily a new concept, there can be some confusion about what it is and how concrete producers can achieve mixes that provide water throughout their mixtures. By looking to definitions from the American Concrete Institute (ACI) and online tools, these professionals can easily adjust a mix to help their concretes reach maximum potential.

What is internal curing?

Generally, internal curing is the process of curing concrete from the inside out, which can include several methods. However, since 2013, ACI has defined internal curing as “a process by which the hydration of cement continues because of the availability of internal water that is not part of the mixing water” and has outlined specific standards in ACI 308.1.

What is important in this standard definition is that the concrete mix needs water in addition to the batch water used to make the mix. To do this, producers of internally cured concrete can replace some of the sand in conventional mixtures with an equal volume of prewetted expanded shale, clay or slate (ESCS) fine aggregate. This material stores extra water in its network of internal pores, releasing it slowly during the curing process to provide continuous hydration through the mix.

What internal curing is not

It can also be helpful to explain what internal curing is not. Although surface curing occurs alongside internal curing, on its own, it does not facilitate internal curing—even when the concrete is wet cured.

Likewise, there are a few different curing methods that mimic internal curing, such as introducing compounds that slow internal evaporation. That said, these methods do not necessarily meet the standards and definitions outlined by ACI. Additionally, they may alter the performance capabilities of the fully cured concrete.  

How does lightweight aggregate support internal curing?

Prewetted ESCS fine aggregate supplies additional water throughout the concrete mix. As the concrete cures, it draws the water out of the pores to keep the mix internally hydrated. The extra hydration minimizes cracks and curling, creating a less permeable and longer lasting concrete.

Likewise, concrete mixes that use ESCS lightweight aggregate for internal curing can adjust the ratio of aggregates to achieve dried weights up to 35 percent lighter than traditional concrete. Known as structural lightweight concrete, this internally cured concrete can provide several benefits to bridge decks, concrete pavements, high rises, stadiums, parking garages, and more.

Are there other benefits to using ESCS?

In addition to facilitating internal curing, ESCS also supports a stronger bond between the aggregate and the cementitious mix. It does this physically and chemically. First, because ESCS aggregate has an irregular surface, there is more area for the cement to bond to, increasing the mechanical adhesion through the contact zone.

The surface of ESCS is also considered pozzolanic. According to ACI, pozzolans within a concrete mix increase strength, impermeability, and sulfate resistance while also reducing expansion from the alkali-silica reaction that might otherwise take place. Because the additional water within the aggregates continues to hydrate the mix, it can improve the interfacial transition zone (ITZ) to further increase strength and reduce overall permeability.

Achieving internal curing with the right tools and knowledge

Using ESCS lightweight aggregate to facilitate internal curing requires planning, teamwork, and deviating slightly from traditional concrete mixtures. That said, there are tools available, like this online calculator, that make the process simpler.

Additionally, engineers, contractors and other building professionals interested in using internally cured concrete can reach out to any of the members of the Expanded Shale, Clay and Slate Institute to learn more about the material and its applications.

By ESCSI | February 1, 2024 | Articles

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