| Popis: |
The correlation between concrete rheological parameters and its pumping behavior under consideration of the so-called lubricating layer has been investigated for decades. In this thesis, flow-induced particle migration (FIPM) was studied in-depth, as the main underlying mechanism for the formation of the lubricating layer. Conventionally vibrated and self-compacting concretes were chosen as the target mixtures. Furthermore, cementitious model concretes, containing colored glass beads, were proposed to obtain further insights into the FIPM and its impact on pumping. The mixtures were differentiating with regard to particle volume fractions, mortars composition, maximum particle size and particle size distributions. In the experiments, various established methods were used to characterize the rheological properties and the pumping behavior of the concretes. New methodologies for estimating the thickness of the lubricating layer and the particle distribution in pumped cross-sections were proposed as well. The rheological properties and pumping behavior of real and model concretes were in agreement with the state-of-the-art literature. Based on the radial particle distributions, it was illustrated that the particle migration intensifies by increasing the particle size and decreasing the total volume fraction of solid particles. Furthermore, in highly-concentrated poly-dispersed model concretes, the particle concentration curves consist of a sudden jump close to the pipe wall and a relatively uniform distribution in the inner zone of the pipe. In these cases, the influence of FIPM on overall flow behavior cannot be neglected. Moreover, it was shown that the lubricating layer thickness is not constant for all concrete mixtures. For poly-dispersed suspensions, the shear-induced particle migration (SIPM) has a stronger impact than the wall for forming the lubricating layer. Finally, it was concluded that a solid-liquid threshold of 1 to 2 mm is a reliable choice when investigating concrete pipe flow. The faster the concrete is pumped or the lower the yield stress of the constitutive mortar is, the smaller is the solid-liquid threshold. Based on the experimental findings, a first-order physical correlation between pumping parameters (pressure and discharge rate), particle properties (volume fraction and packing properties) and lubricating layer properties (thickness and viscosity) was proposed. Accordingly, for a simple flow topology with a shearing lubricating layer and an extruding yield stress concrete, the lubricating layer thickness was computed from the competition between shear-induced fluxes of particles. In agreement with the driving equations of SIPM, it was assumed that the first flux correlates with the internal stress gradient in the plug concrete whereas the counteracting particle flux correlates with the shear rate in the lubricating layer. The model was validated for two flow types, pipe flow in the Sliper and Couette flow in a co-axial tribometer. The proposed model can capture the observed main features and their evolutions despite the absence of any fitting parameters. |
