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.
In the past, curing concrete was a process done using water from the outside in. But today, expanded shale and clay is used as a substitute for normal-weight aggregates to provide a source of moisture for internal curing that promotes more complete hydration of various cementitious materials.
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 or clay lightweight aggregate, which replaces some of the conventional aggregate in the mixture. IC is often referred to as “curing concrete from the inside out.” Due to the inherently low permeability of the surrounding environment, internal curing is particularly beneficial in concrete with a low-water-cementitious material ratio (w/cm) where external curing has little effect on hydration in the internal portion of the concrete. With an emphasis on durability, and with the trend to high performance concrete, internal curing is needed to reduce both autogenous shrinkage and cracking.
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.
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.
Charged with maintaining critical infrastructure while conserving taxpayer dollars, state Departments of Transportation are on the lookout for ways to improve the performance of roads and bridges. Deteriorated pavements irritate drivers and damage vehicles, while the need for frequent repairs wastes both time and money.
Internal curing has clearly demonstrated its beneficial effects on the performance of concrete bridge decks. The addition of pre-wetted lightweight aggregates to concrete mixtures can help reduce shrinkage stresses by reducing moisture differentials through the depth of a slab.
Denver Water Department’s three successful water tank projects drive an ongoing demand for internally cured concrete across multiple districts.
From an engineering perspective, Mitch Wyble really likes what he sees when he assesses the value of internally cured concrete (ICC), a concrete mixture where a portion of the fine aggregate is replaced with similar sized prewetted lightweight aggregate (LWA). Wyble first encountered ICC a few years ago while serving on the Technical Committee of the Concrete and Aggregates Association of Louisiana (CAAL).
When the Louisiana Department of Transportation & Development (LADOTD) needed to construct a bridge on U.S. 80 over the Kansas City Southern railroad tracks to accommodate two lanes of northbound and southbound traffic, it decided to incorporate internally cured concrete (ICC) into its completion plan.
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