Air void clustering in concrete and its effect on concrete strength

Air voids in concrete tend to cluster around coarse aggregate particles and potentially weaken the concrete. This study explored key factors influencing air void clustering and evaluated the effect of air void clustering on concrete strength. The six considered variables were cement type (low alkali cement and limestone cement), fly ash (with the low and high loss on ignition), coarse aggregate type (limestone and gravel), admixture combination type (stable and unstable air-entraining agents and compatible and incompatible water reducers), mixing temperature (70 and 90 degrees F), and workability (with and without retempering). A total of 64 mixtures were prepared and tested. The fresh concrete's slump, unit weight and air content were studied along with its compressive strength after 7 and 28 days. Air-void spacing factor, clustering and the voids' specific surface were evaluated. The results showed that retempering was the most significant factor influencing air void clustering. It was found that retempering can increase air content, create finer air voids and aggravate air void clustering. To reduce the risk of air void clustering, retempering and the mixing water temperature of 90 degrees F should be avoided. When the clustering rating exceeded 0.6, some strength loss was observed.