Design Methodology for Partial Volumes of Internal Curing Water Based on the Reduction of Autogenous Shrinkage

Historically, the use of internal curing began with a low water-to-cement ratio ($\mathrm{w}/\mathrm{c}$) and high-strength concrete. More recently, the benefits of reduced autogenous shrinkage and improved hydration have been recognized over a wider range of mixtures. In North America, the internally cured mixtures are typically made using prewetted fine lightweight aggregates (FLWA). The volume of FLWA used in an internally cured concrete is usually determined on the principle that the FLWA provides a volume of internal curing water that is equivalent to the volume of chemical shrinkage. This study examines whether it is possible to reduce the volume of the FLWA while still achieving the benefits of internal curing with respect to the reduced autogenous shrinkage and increased relative humidity. An alternative mixture methodology is presented that utilizes the pore-size distribution of the cementitious paste to calculate the amount of internal curing water needed to maintain a specific relative humidity (that is, to provide water sufficient to keep pores of a certain size filled) in the mixture. The proposed approach can be used to calculate the volume of water required to fill the pores of a specific size that empty, so that the effects of self-desiccation are minimized to an acceptable level. This approach maintains a high relative humidity to reduce the autogenous shrinkage and increase the early-age hydration. Although many applications such as bridge decks are still designed with the more conventional design approach, it is hypothesized that this assumption generally overestimates the amount of water needed to effectively reduce the autogenous shrinkage of a concrete mixture. Other applications where larger volumes of materials are involved, such as pavements, may benefit from a design approach that optimizes the volumes of FLWA, due to the reduction in the raw materials required, which has benefits from the perspectives of both the cost and the staging.