United States Department of Transportation
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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
strengths.
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