Effects of Materials and Mixing Procedures on Air Void Characteristics of Fresh Concrete

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 (0.5and 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.