Reference: D.P. Bentz, to be published in the Proceedings of the 4th International Seminar on Self-Desiccation and Its Importance in Concrete Technology (2005)

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Capitalizing On Self-Desiccation for Autogenous Distribution of Chemical Admixtures in Concrete

Dale P. Bentz
Building and Fire Research Laboratory
NIST, Gaithersburg, MD USA

 

Abstract

For many concretes, self-desiccation results in the creation of empty capillary porosity within the cement paste microstructure, accompanied by a decrease in achieved hydration, an increase in internal stresses, and an increased propensity for early age cracking. One solution to this problem for low water-to-cement ratio concretes where external curing is ineffective has been the development of internal curing. In internal curing, water reservoirs, typically being either fine lightweight aggregates with a high moisture content or superabsorbent polymer particles, are distributed uniformly throughout the concrete microstructure and undergo desiccation themselves, while maintaining saturated conditions within the hydrating cement paste. Since these reservoirs typically contain pores that are much larger than those in the hydrating cement paste, the internal stresses are significantly reduced and early age cracking can be avoided. To date, these internal reservoirs have been filled with water only. Here, it is proposed to fill them with solutions of chemical admixtures such as shrinkage-reducing admixtures or corrosion inhibitors. Two possible advantages of the autogenous distribution of chemical admixtures over delivery by conventional addition to the mixing water are the mitigation or avoidance of possible detrimental interactions between chemical admixtures and a potentially more efficient delivery of admixtures that are partially absorbed by the cement during hydration and in the resulting hydration products. Preliminary results for the FLAIR (Fine Lightweight Aggregates as Internal Reservoirs) system are presented for the case of a shrinkage-reducing admixture. It is envisioned that the FLAIR technology will have the greatest applicability to chemical admixtures that effect the properties of the hardened concrete (as opposed to the fresh concrete), such as shrinkage-reducing admixtures, corrosion inhibitors, and admixtures employed to mitigate alkali-silica reactions.



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