Variation of air in concrete at different stages of mixing and placement

In cold climates, the freeze-thaw durability of concrete depends heavily on a stable air-void system. This study investigates the impact of hydraulic pumping and the use of supplementary cementitious materials (SCMs)—specifically Class C Fly Ash (CFA) and Rice Husk Ash (RHA)—on the air-void characteristics and rheological properties of concrete. Laboratory mixes with varying dosages of air-entraining admixtures were tested at two stages: immediately after mixing and after discharge from a pump. A Super Air Meter (SAM) was used to precisely measure air content and bubble spacing factors. Results showed that pumping caused a reduction in air content in 58% of the tests, with a maximum loss of 24%. The incorporation of RHA significantly affected workability, reducing air content by up to 67%, slump by 45%, and unit weight by 1.5%, while other rheological properties remained stable. These fluctuations were primarily driven by boom configuration and the consolidation of concrete resulting from the pressurization and depressurization of air bubbles during the pumping process.