Internal curing with lightweight aggregate gaining traction

Pile of lightweight aggregate
Photo credit: Lightweight Distributing,

Lightweight fine aggregate for internal curing is not a new technology, but it has been slow to gain traction in everyday use. Based on the number of times I’ve heard about the topic in the last few years, I have a feeling it will be implemented more frequently in years to come.

In October 2017, Bill Palmer wrote a blog post for Concrete Construction magazine, “The Best Thing Since Water-Cement Ratio.” He was referring to internal concrete curing using lightweight aggregate.

At the 2015 Minnesota Concrete Conference, Dr. Jason Weiss of Oregon State University’s School of Civil and Construction Engineering, gave the talk Ready, Set, Implement: Lessons Learned from 10 years of Internal Curing. I still remember his talk, and it was the second to last talk of a long day. Palmer, Weiss, and many others advocate the wonders of internal curing with lightweight aggregate and how it’s gaining traction.

Dr. Weiss’ talk was so engaging because he showed that internal curing with lightweight fine aggregate works, isn’t difficult to implement, and there is no reason why we shouldn’t all jump on the bandwagon.  It isn’t a proprietary technology with secrets or complicated process mechanisms.

Concrete slabs, including bridge decks, pavements, pavement repairs, suspended slabs, and slabs on grade, suffer cracking due to drying and autonomous shrinkage, warping, and curling. Internal curing with fine lightweight aggregate mitigates the shrinkage, warping, and curling, which mitigates cracking. Not all cracking is eliminated, but the cracks that do form are either less frequent and/or less wide than observed in companion conventional concrete slabs.

Lightweight aggregate used for internal curing is mostly pre-wetted, expanded clay, shale, and slate fine aggregate. Other materials are used, but not as frequently. Lightweight fine aggregate is used over lightweight coarse aggregate for even distribution of water compared to coarse aggregate where water would be localized.

When needed, water moves from the aggregate pores to the smaller, microscopic pores inside the paste fraction of the concrete and helps the cementitious portion of the concrete continue to hydrate. Water in the aggregates does not count towards the total water in the water-to-cement ratio because it does not start assisting the concrete until after the concrete has initially set.

Use specifications on internal curing with lightweight aggregate

ASTM C1761 Standard Specification for Lightweight Aggregate for Internal Curing of Concrete provides guidance on how to purchase lightweight fine aggregate, test methods for characterizing its absorption and desorption, and a method of analysis for determining the percent replacement of standard fine aggregate with lightweight fine aggregate. The FHWA Tech Brief “Internal Curing for Concrete Pavements” July 2016 FHWA-HIF-16-006, suggests up to 10% of fine aggregate can be replaced by saturated lightweight fines.

Besides the substitution of lightweight fines for standard fine aggregate, the mixture, batching, testing, and placing processes do not change. Yes, the pressure meter can still be used to determine plastic air when fine lightweight aggregate is used.

The technology is starting to be recognized in more places. Locally, the Minnesota Department of Transportation has implemented the use of lightweight fine aggregate in some of its recently-placed bridge deck mixtures.