Internal curing was originally defined by the American Concrete Institute (ACI) 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.”
In 2013, ACI changed the definition of IC to “a process by which the hydration of cement continues because of the availability of internal water that is not part of the mixing water.”
In general conversation, IC is often referred to as “curing concrete from the inside out.” IC using prewetted expanded shale, clay or slate (ESCS) lightweight aggregate is a simple and practical way of supplying additional curing water throughout the concrete mixture. This is done by replacing some of the conventional sand in the mixture with an equal volume of prewetted ESCS fine aggregate.
ESCS was first used in 1918 by the USA government in the construction of concrete ships and is now being used extensive throughout Europe, Japan, Russia, United States and other parts of North and South America. ESCS is primarily used in lightweight concrete, however, it is also used in many other application like green roofs, horticulture, asphalt chip seal, lightweight geotechnical fills, etc.
Both slabs installed within minutes of each other. No conventional curing on either slab. Photograph taken 24 hours later.
Internal curing helps concrete realize its maximum potential in a simple, economical and sustainable way. IC improves hydration, reduces early cracking, reduces chloride ingress, reduces curling and improves durability, all of which extend the concrete’s service life.
Over one-hundred articles and papers have been written about IC. The following article gives a great overview of IC and a portion of it is quoted here.
Internal Curing: Constructing More Robust Concrete, by Jason Weiss, Dale Bentz, Anton Schindler and Pietro Lura, published in the January 2012 issue of STRUCTURE Magazine:
“…While lightweight aggregate is discussed in this article and is the most common method used as a water reservoir, researchers throughout the world are also investigating the use of superabsorbent polymers and natural fibers. Differences between conventional (external) curing and internal curing are shown in Figure 1. While external curing water is applied at the surface and its depth of penetration is influenced by the quality of the concrete, internal curing enables the water to be distributed more equally throughout the cross section.
While designing concrete specifically to provide internal curing is relatively new, the concept of lightweight aggregate improving the hydration of the cement paste was observed in the 1950s by Paul Klieger. Research on intentionally using lightweight aggregate for internal curing began to take shape in the late 1990s when a variety of research groups, primarily in Europe, began actively investigating whether mixtures could be designed with internal curing by using pre-wetted lightweight aggregates. Design procedures were then developed that enabled both the spatial distribution and amount of pre-wetted lightweight aggregate to be computed (Bentz et al. 2005). A review of the details on many of these developments can be found in the RILEM 2007, and Bentz and Weiss 2011. Internal curing is becoming a mature technology, and its use is increasing since it provides great opportunities for a robust concrete construction. Some benefits of internal curing as it relates to reducing the potential for cracking are described in the following section.”
Figure 1: Comparision of external (conventional) and internal curing using pre-wetted lightweight aggregate (LWA). (*Note that in practice the prewetted lightweight aggregates are placed sufficiently close to enable the cured zones to overlap allowing the entire paste to be cured.)
From a broader perspective, poor field curing along with concrete’s inability to fully cure internally has robbed the industry and society of the maximum potential that concrete has to offer. Being the most used building material known to man, concrete still falls short of delivering its true value on many occasions. This poor performance can be minimized by the age-old internal curing technology that dates back to concrete constructed during the Roman Empire using volcanic LWA materials.
Based on research conducted by several (see references below) we now have a more complete understanding of how internal curing (IC) works and a way to design it.
Internal curing provides something that most concrete needs and conventional curing cannot provide: additional internal water that helps prevent early age shrinkage (reducing early age cracking), and increases hydration of cementitious materials throughout the concrete.
Once concrete sets, hydration creates partially-filled pores in the cement paste which causes stresses that result 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, cracking, 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 cementitious materials (SCM), especially at higher dosage levels, because fly ash, slag and silica fume 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 more robust. Internal curing can also help compensate for less than ideal weather conditions and poor conventional curing that is often seen in the real world.
IC is gaining momentum in all areas of concrete construction including concrete paving, concrete flatwork, bridges, structural units, pavers and mass concrete applications.
Photo courtesy of P. Lura, O.M. Jensen, S.I. Igarashi. Experimental Observation of Internal Water Curing of Concrete. Materials and Structures, 2007, 40, 211-220.
For additional information on Internal Curing, see ESCSI publication 4362.1, Internal Curing: Helping Concrete Realize its Maximum Potential.
For a schematic diagram titled “Internal Curing at the Contact Zone” which shows the difference in the interfacial transition zone between lightweight aggregate and normal weight aggregate, see ESCSI publication 4362, Internal Curing Using Expanded Shale, Clay and Slate Lightweight Aggregate.