Toward a Mixture Design Methodology for Internally Cured Concrete that Considers Filling Specific Pore Sizes Caused by Self-Desiccation Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0g354k871

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  • Internal curing is a term used to describe a process in which curing water is provided to a concrete mixture from inside the mixture. Internal curing water has typically been provided in North America mixtures by using prewetted porous materials, like fine lightweight aggregate (FLWA). Alternative materials, such as superabsorbent polymers or absorptive fibers, have been suggested as potential alternative methods to supply internal curing water. Water moves from the FLWA to the cement paste when capillary suction develops in concrete mixtures as a result of self-desiccation. This capillary suction draws water out of the FLWA and into the paste of the concrete mixture. Internal curing is primarily used in low water-to-cementitious ratio concrete material applications, as these mixtures are more susceptible to early age autogenous shrinkage and cracking. Internal curing has received considerable research attention over the past two decades. Applications for internal curing include: bridge decks, water tanks, repair materials and pavements. The most common mixture design methodology for internally cured mixtures is based on an approach proposed by Bentz and Snyder. This design methodology provides a volume of internal curing water that is equivalent to the volume of the ultimate chemical shrinkage of the mixture. While this approach can reduce the autogenous shrinkage and early age cracking potential, it is believed to be conservative. For applications like concrete pavements where large volumes of materials are involved, it may be possible to reduce the volume of internal curing water while still maintain the benefits of internal curing. This thesis explores the benefits of using only a portion of the typical volume of internal curing water for internally cured mixtures with lightweight aggregates and superabsorbent polymers. A design methodology is examined that uses the pore size distribution of the cementitious matrix to quantify the volume of internal curing water needed to reduce shrinkage to a specific level. Results from mixtures that use a portion of water associated with the volume of chemical shrinkage reveal significant benefits in terms of increased relative humidity and reduced autogenous shrinkage. A relationship between the pore size distribution of the cement paste and the reduction in autogenous shrinkage is established. A comparison is made between the model and the experimental results to reveal that only a fraction of the provided internal curing water fills the vapor filled pores that are responsible for the autogenous shrinkage. Other factors, such as the increased degree of hydration of cement, increased degree of reaction of SCM and the partial desorption of the internal curing agent also need to be considered.The model was modified to take into account these additional factors. While further studies are needed to better quantify the impact of various factors which consume or restrict the availability of the internal curing water, the proposed design method shows promise as, by advancing the comprehension of internal curing and its effects, provides a way to safely reduce the volumes of internal curing water while still maintain the majority of the benefits.
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