Investigation of Portland Cement Concrete Mix Consistency and Concrete Performance Using a Two-Stage Mixing Process

The objective of this study was to investigate the effects of different mixing processes, particularly a two-stage mixing process, on fresh and hardened characteristics of pastes and concrete. Characteristics studied for pastes included rheological effects (i.e., yield stress, thixotropy, viscosity, and peak stress) and compressive strength for mixes prepared in a Hobart mixer and a high-shear mixer. Parameters measured for the concrete study included fresh concrete air and slump, compressive strength, and hardened concrete air void characteristics. The paste study results show that for increased mixing time, the compressive strength is not significantly influenced. The rheology results show that longer mixing times with a high-shear mixer lead to lower viscosity, thixotropy, and peak stress; this indicates better-mixed slurry when compared with a shorter mixing time and the Hobart mixer. The fresh concrete made with the two-stage mixing process shows reduction in air content, and the hardened concrete shows little change in compressive strength when compared with the concrete produced with the one-step mixing process. Rapid air void analysis results indicate that the two-stage mixing method may be beneficial for mixes containing portland cement as a sole binder in freeze–thaw durability.