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Description: This research report on air-void clustering phenomenon outlines a methodology for reproducing air-void clustering in the laboratory, provides a visual rating scale for quantifying the extent of the problem through petrographic analysis, and examines several factors and their influence on clustering. Retempering and certain chemical classes of air-entraining admixtures, aggregate shape/mineralogy appear to be related to clustering and associated strength loss. This report is also found on DVD021.
Air-void clustering is a phenomenon in which entrained air-voids abnormally cluster around coarse aggregate particles. Numerous cases of low strength concrete have been attributed to this phenomenon and this study investigated the possible causes of the problem. There are many opinions as to why air-void clustering occurs, but to our knowledge, no study has ever been conducted and published to determine the specific cause(s) of the phenomenon. Therefore, various parameters were obtained through a literature search and interviews with industry experts to evaluate their potential influence on air-void clustering. A visual petrographic rating system was developed to establish a standardized method for determining the severity of clustering. A correlation was found between this rating system and compressive strength loss and was used to evaluate which parameters had the most influence on air-void clustering. Problematic air-void clustering was found to coincide with late additions of water to the concrete mixture. The type of air-entraining admixture used in the concrete was found to have a major influence on clustering. It was noted that longer mixing times exacerbated the condition and that air-void clustering tended to occur in relatively finer air-void systems.
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