The influence of aggregate content and aggregate grading on the air bubble rise in fresh concrete

The durability and mechanical properties of hardened concrete are significantly affected by its porosity. Air voids can be trapped in fresh concrete during placement, particularly when insufficient workability is combined with inadequate processing. The mechanisms by which air bubbles rise in fresh concrete and interact with surrounding aggregates remain largely unknown. This study investigates how the rheological and granulometrical characteristics of fresh concrete affect de-airing behavior. Due to the opacity of concrete, X-ray techniques coupled to digital image analysis were used to study the bubble dynamics and the bubble-aggregate interactions. Concrete is idealised as a mix of glass beads as model aggregates (d_a ≤ 8 mm) suspended in a cement-water suspension, termed cement paste. The investigation focused on the relationship between the cement paste rheology, the aggregate properties and the speed, shape and trajectory of rising air bubbles. Further the effect of shear history and shear-induced particle migration on bubble rise was also investigated. The results show that the addition of aggregates to pure cement paste significantly alters the de-airing behavior and affects the bubble dynamics. As the bubble volume decreases, the bubble speed decreases. Increasing the aggregate content from 0 vol% to either 30 vol% or 60 vol% also results in a decrease in bubble speed. Small bubbles show minimal variation in rise trajectories across different aggregate volumes and gradings. In contrast, larger bubbles exhibit consistent paths with 30 vol% aggregates, but demonstrate dispersed trajectories at 60 vol%. Shear-induced particle migration depends on the bubble-to-aggregate size ratio. Finally, a dimensionless parameter is introduced that can be used to determine phase separation as a function of the ratio of bubble buoyancy to aggregate inertia. This research improves the understanding of concrete de-airing and is also applicable to other coarse granular suspensions in various industries.