Why is a plastic consistency usually desirable for placing concrete?

Q. Why is a plastic consistency usually desirable for placing concrete?

A. In a concrete of plastic consistency, the cementitious material paste is sufficient in amount and of such consistency that the aggregate particles are virtually floated in the paste. This permits the complete incorporation of all aggregate particles and eliminates the likelihood of rock pockets and other large, unfilled voids. With proper mechanical consolidation, the aggregate can be floated and plasticity obtained with a stiffer paste than would be needed for plasticity with hand placing, but the amount of paste still needs to be sufficient. In some uses of concrete, a nonplastic mixture is used. These include concrete for consolidation by roller compaction where the mixture is typically of zero slump, and various precast operations where concrete masonry units or concrete pipe are made. Some mixtures appropriate for such uses are “earth moist.”

References: SP-1(02); ACI 304R-00; ACI PRC-309.5-22

Topics in Concrete: Concrete Fundamentals; Placing