Pumping of concrete

Abstract

We show here that pumping mortars over long distances can lead to microstructural heterogeneities. This work follows numerous works on particle migration in ideal (Newtonian) fluids, where particles migrate from areas with higher shear rates (near the wall) to areas with lower shear rate (center of the pipe). We investigated here if migration of particles (sand, aggregates) in cement slurries may also occur as the result of shear induced migration. Looking at the work of Nott and Brady (1994), we calculated that the distance needed to reach steady-state in our mortars would be on the orders of 100s of meters (their theoretical model estimates that the distance depends on the pipe diameter and the particle diameter). Avoiding this difficult experimental challenge, and following the work of Butler, Majors & Bonnecaze (1999) and Snook, Butler & Guazzelli (2015), we built an oscillatory setup. We performed seven experiments with three different mixtures, where the yield stress and cover distance vary. Because we could not measure particle concentration in-situ during flow, we let the mortar set and measured solid volume fractions in various cut and polished sections of the pipe. Each image was treated with ImageJ where we analyzed local particle concentration. For optimal image treatment, we used red particles and white cement. Quantification of the wall steric hindrance, as well as the stability (lack of settling of particles) of all three mixtures were controlled in reference samples (tube of same diameter of pumping pipe, but where no pumping was imposed). In almost all cases, particle migration is clearly evidenced, with a dependence on the cement yield stress.