The air void analyzer (AVA) was used to study the effects of concrete materials and mixing procedures on air void characteristics of fresh concrete. Twenty-seven batches of concrete were prepared with three mixes (with and without fly ash or water reducing agent), five mixing procedures (one-step mixing for one, two, or four minutes, two-step mixing for four minutes, and ASTM standard lab mixing procedures), and two sizes of pan mixers (0.5- and 1.5-cf capacity). The air content, size distribution, specific surface, and spacing factor of all the batch mixtures were examined. The results indicated that incorporating 15% fly ash replacement or the recommended dosage of water reducing agent into concrete generally reduced the spacing factor of air voids. The two-step mixing method (mixing mortar first, then adding coarse aggregate) produced a lower air void spacing factor than the one-step (four-minute) mixing method (mixing all concrete materials together at once) and the ASTM mixing method. For concrete mixed with the one-step mixing method, the air void spacing factor reduced with mixing time. For a given concrete mix and mixing procedure, use of different sizes of pan mixers provided the mixtures with different air contents and spacing factors.
Abstract