Special Section on Advances in Internally Cured Concrete

Although conventional water curing of concrete consists of providing water at the surface shortly after the concrete is placed, internal curing (IC) supplies water throughout the fresh concrete using porous inclusions that release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation (American Concrete Institute 2010). Saturated lightweight aggregates (LWA) are the most common type of water reservoir that has been used for IC, however, researchers throughout the world are also investigating other internal agents including superabsorbent polymers (Jensen and Hansen 2001) and natural fibers (Mohr et al. 2005). Although the design of internally cured concrete is a relatively new concept, the benefits of water provided by porous LWA have been observed since the 1950s (Weiss et al. 2012; Klieger 1957). Research on intentionally using LWA for IC began to take shape in the late 1990s when a variety of research groups, primarily in Europe, began using prewetted LWAs (Weber and Reinhardt 1997; Hammer 1992).

A great deal of the early research focused on using IC to reduce autogenous shrinkage in low water-to-cement ratio concrete (Bentur et al. 2001; Lura et al. 2006; Henkensiefken et al. 2011; Trtik et al. 2011). In addition to having inclusions with sufficient porosity to supply enough curing water, it quickly became obvious that the distance water reservoirs (Zhutovsky et al. 2002) and the ability to release water at high relative humidity levels (Jensen and Lura 2006) were also important. Design procedures were developed that enable both the spatial distribution and amount of water reservoirs to be computed (Jensen and Hansen 2001; Bentz and Snyder 1999; Bentz et al. 2005; Wyrzykowski et al. 2011). A review of these developments is available from a RILEM state-of-the-art report (Kovler and Jensen 2007). A second RILEM state-of-the-art report that focuses specifically on the use of superabsorbent polymers has recently been published (Mechtcherine and Reinhardt 2012). IC is also moving from the laboratory to the field with several full-scale field trials in pavements, bridge decks, and water tanks. IC is becoming a mature technology as it provides great opportunities for a robust concrete construction (Bentz and Weiss 2011).

This special section of the Journal dedicated to IC aims at giving a snapshot of the current worldwide trends of research and application—the papers indeed originate from research groups in North and South America, Europe, and Asia.

The first three papers deal with characterization approaches for IC agents, targeting LWAs, wood fibers, and superabsorbent polymers. The first paper by Pour-Ghaz et al. is dedicated to measurement of sorption isotherms of fine lightweight aggregates in the high relative humidity range, close to 100% RH. To characterize this part of the sorption isotherm that is most significant for IC, a pressure plate apparatus, derived from soil science, was used. The second paper by Mezencevova et al. investigates the efficiency of different types of thermomechanical pulp fibers, both untreated and after chemical treatment to produce holocellulose and $\alpha$-cellulose, on cement hydration and autogenous shrinkage mitigation. Although all types of fibers were able to entrain water and reduce autogenous shrinkage, the best performing holocellulose, also showed a retarding effect on cement hydration that might need to be counteracted with accelerators. The third paper by Siriwatwechakul et al. studied different types of polyacrylamide-based superabsorbent polymers in high pH solutions, revealing spontaneous swelling and deswelling behavior in saturated conditions, and strong ion filtration behavior.

The next papers discuss how research is overcoming challenges of extending IC to very low water-to-cement ratios and mixtures with supplementary cementitious materials. This includes both advances in testing methods and numerical modeling approaches. Espinoza-Hijazin et al. apply IC concepts to concrete containing natural pozzolans. As mixtures with natural pozzolans showed higher chemical shrinkage and lower permeability than mixtures with portland cement, an increase in the volume of IC reservoirs was needed. The paper by Fu et al. extends the standard chemical shrinkage test [ASTM C1698 (2009)] with the aim of determining the long-term chemical shrinkage value for systems with supplementary cementitious materials. Castro et al. use isothermal calorimetry to determine the amount of mixing water absorbed by partially saturated LWA during the mixing and placement processes. The study shows that a significant amount of water (between 56 and 71% of the 24-h water absorption) is absorbed by oven dry aggregates. Wyrzykowski et al. use a poromechanics-based numerical model to study the kinetics of water migration from superabsorbent polymers into hydrating cement pastes of low water-to-cement ratio. They show that a part of the water received by the paste in the proximity of the water reservoir can be later redistributed to a large volume of hardening paste, even after the permeability has become very low.

In the final papers, innovative approaches are explored to replace the water that has been used in internal curing applications with other fluids. Snyder et al. use fluids with higher viscosity to slow ionic transport. By using an innovative X-ray fluorescence technique, they demonstrate that the effective diffusivity can be doubled. This in turn could result in concrete structures with substantially reduced rates of deterioration and improved service life. Byard et al. demonstrate how internal curing can be used to reduce the potential for cracking in bridge deck applications. They also show how internal curing increases the degree of hydration experienced by the system. Sakulich and Bentz replace the water in the LWA with phase-change materials to potentially increase the freeze-thaw resistance of the concrete by reducing the number of freezing and thawing events in the concrete.

This section draws attention to the potential that IC offers. The benefits of IC concrete are promising; however, it is important that we keep in mind that successful field application will only be possible with attention to mixture design and quality control. It is not recommended that IC be viewed as a cure all or as an excuse to stop providing curing protection that minimizes the evaporation of water from fresh concrete. Rather, IC offers the potential to produce more robust concrete with a reduced risk of cracking and enhanced hydration and durability. The section shows that although the field of internally cured concrete is maturing and internally cured concrete is finding its way into field trials and applications, we may only be beginning to scratch the surface on its true potential as a material/design approach that can be used to improve the service life of the concrete infrastructure.