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Air void clustering.
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  • Abstract:
    Air void clustering around coarse aggregate in concrete has been identified as a potential source of

    low strengths in concrete mixes by several Departments of Transportation around the country. Research was

    carried out to (1) develop a quantitative measure of air void clustering around aggregates; (2) investigate

    whether air void clustering can be reproduced in a laboratory environment; (3) determine if air void

    clustering contributes to lower compressive strengths in concrete mixes; and (4) identify potential factors

    that may cause clustering. Five types of coarse aggregate and five different air entraining agents were

    included in the laboratory study to determine if aggregate type or chemical composition of the air entraining

    agent directly relates to air void clustering. A total of 65 mixes were made, implementing the frequently used

    technique of retempering that has been previously associated with air void clustering around aggregates.

    Cylinders for compressive strength testing as well as samples for total air void analysis in the hardened

    concrete were made. Compressive strength at 7 and 28 days was determined, and automated testing for total

    air void analysis (including a new method of clustering evaluation) was performed on all mixes. This study

    found that it is possible to reproduce air void clustering in laboratory conditions. However, the results have

    shown that retempering does not always cause air void clustering. It was also observed that air void

    clustering is not responsible for a decrease in compressive strength of retempered concrete, as neither

    aggregate type nor chemical composition of the air entraining agent had a significant impact on severity of

    void clustering around coarse aggregate particles. It was found that the total air content and an

    inhomogeneous microstructure of the cement paste, not air void clustering, were responsible for lower


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