Internal Curing of Concrete Using Lightweight Aggregates

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Title: Internal Curing of Concrete Using Lightweight Aggregates

Author(s): G.C. Hoff

Publication: Special Publication

Volume: 234

Issue:

Appears on pages(s): 621-640

Keywords: autogenous shrinkage; curing; high-performance concrete; high-strength concrete; lightweight aggregates; self-desiccation; silica fume

Date: 3/22/2006

Abstract:
This paper describes the use of near-saturated lightweight aggregate (LWA) as a replacement for a portion of the normal weight aggregate (NWA) in high-strength/high-performance concrete in order to mitigate or eliminate the self-desiccation and autogenous shrinkage that can occur. These forms of shrinkage can lead to early age cracking and long-term durability problems. This is achieved by moisture from the aggregate continually contributing to the internal curing of the binder in the concrete. The amount of LWA used to achieve beneficial internal curing is a function of the type of LWA, its size and amount, the degree of moisture preconditioning the LWA receives, the amount and type of binder(s) in the mixture, the water-binder ratio at mixing, and the amount and duration of external moist curing provided to the concrete element. In almost every study, the use of adequate amounts of near saturated LWA in adequate amounts has completely eliminated self-desiccation and autogenous shrinkage in the concrete. While any size LWA appears to be beneficial, the use of smaller size fractions of the LWA is preferable because of a more uniform distribution of those particles throughout the concrete and the improved proximity of the binder to these particles. The need for internal curing is more critical when the binder contains finely divided silica such as fly ash or silica fume. The addition of saturated LWA to the concrete does not appear to have any adverse effects on the mechanical properties of the concrete and, in most instances, small improvements have been noted.