Clustering of air voids around aggregates in air entrained concrete
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Recent research reports have suggested that one of the potential causes of low compressive strengths in air-entrained concrete is the clustering of air voids around the aggregates. The phenomenon of clustering of air voids has been associated with the use of non-vinsol resin air-entraining admixtures and retempering (i.e. the late addition of water). The clustering is thought to reduce the bond strength at the aggregate-paste interface resulting in substantial reduction of compressive strength. The purpose of this research was to try and reproduce the clustering of air voids around aggregates in the laboratory by retempering, determine the severity of the clustering, and to evaluate its effects on concrete strength properties. Review of low strength field cases was also performed to determine if any correlations exist. This research evaluated three different types of air-entraining admixtures (wood and gum rosin, vinsol resin, and a synthetic formulation) and two types of coarse aggregates (siliceous river gravel and crushed limestone). A total of 43 concrete mixtures were made in the laboratory. The mixing procedure focused on retempering the concrete with water followed by an agitation period, and was successful at producing clustering of air voids around the aggregates. However, compressive, splitting tensile, and flexural strengths all showed no signs of strength loss that could be attributed to clustering of the air voids. Review of available field data and petrographic reports found that the phenomenon of air void clustering has a good correlation with the use of non-vinsol resin air-entraining admixtures and retempering of field concrete.