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Publication

Etude de l’effet de la couche limite sur les profils de vitesse du béton pompé

Book - Dissertation

Subtitle:Influence of the slip layer on the velocity profiles in pumped concrete
The rheological properties of concrete are significantly influencing the relation between pumping pressure and discharge rate. The concrete rheology is often characterized by a rheological law in stationary conditions (time independent), giving the evolution of shear stresses as a function of shear rate. In case of traditional concrete, this evolution is typically described by a Bingham model, while for a self-compacting concrete with low water/powder ratio, the evolution often becomes non-linear and can be described by a modified Bingham or Herschel-Bulkley model, considering shear-thickening. In these models, a critical shear stress is typically considered above which the concrete starts to flow. Furthermore, a consistency parameter is considered (and in case of non-linear behavior also an index) to describe the intensity of the evolution. As a consequence, the relation between pressure and discharge rate can be linear or non-linear, depending on the concrete pumped. As the rheological parameters of the concrete are directly relevant for the prediction of the pumping pressure, the accurate measurement of these parameters is a challenging task. As the rheological properties of concrete cannot be directly measured as a physical quantity, concrete rheometers can only be used to determine the rheological parameters in an indirect way, by measuring other physical values like speed, couple or pressure. Different methods can be applied in order to convert the measured physical values to obtain the rheological properties. The most direct method consist of calibrating the rheometers by testing materials with known rheological parameters. A higher precision in this approach can be obtained by testing a higher number of known materials. However, in order to cover the whole range of rheological properties of concrete, a very high number of known materials would have to be tested, which would thus become very cumbersome. Instead of performing this calibration in an experimental way, it can be done in a numerical way. This kind of numerical calibration is the topic of chapter 2 of the doctoral thesis. Besides the rheology of the concrete, tribology is also an important factor determining the pumping characteristics. Tribology enables to characterize the behavior of concrete in the interface with the surface of the pumping pipe. For traditional concrete with high yield stress, the flow of concrete in the pumping pipe is dominated by the slip layer or lubrication layer near the surface, while the bulk of the concrete is flowing as a plug. This slip layer can only be formed due to shear stresses in this area, and is considered to be the consequence of three phenomenons: geometrical wall effect, structural breakdown, and dynamic segregation. These phenomenons induce a reduction of the viscosity of the concrete within a layer of a few millimeter near the surface of the pumping pipe. As a result, a non-homogeneous flow is induced. Due to the occurrence of the slip layer, an additional speed component is added to the speed profile already induced by shear of the concrete. In order to characterize the concrete behavior near the surface, tribometers are being used, simulating the relative movement between concrete and the surface. In case of traditional concrete, with high yield stress, due to the relative movement only the slip layer is sheared, while the bulk concrete is not sheared. In this case, the use of tribometers results in a yield stress and a viscous constant of the slip layer. These two parameters enable to define a linear relation between shear stress and shear rate in the interface. Meanwhile, in case of self-compacting concrete, the concrete is also sheared, leading to very complicated tribology measurements. For this reason, it is very difficult to characterize the behavior of self-compacting concrete near the interface by means of a tribometer. This situations complicates the prediction of the relation between pumping pressure and discharge rate. In order to characterize and quantify the effect of the slip layer, an experimental method enabling the measurement of the velocity profile in the flowing concrete in an open pipe has been developed. By measuring the velocity profile, it is possible to determine the thickness of the slip layer and the velocities in the slip layer. An experimental research program studying the effect of the mix design on the slip layer thickness has been carried out. An experimental method to sample the material of the slip layer has been developed in order to determine its rheological properties. This study, performed on self-compacting concrete, enables to relate the rheology of the slip layer to the rheology of the bulk concrete. Finally, the experimental results are implemented in a numerical simulation method. The results of the numerical simulation enable to validate the experimental methods and the obtained experimental results. Based on the available rheological models like the Bingham model, modified Bingham model or Herschel-Bulkley model, it is possible to predict the relation between the pressure gradients and the discharge rate, considering the Poiseuille formula for a laminar flow in a circular pipe. Knowing the thickness of the slip layer and the relation between rheological properties of slip layer and bulk concrete, it is possible to predict the relation between pumping pressures and discharge rates. A numerical application has been performed in order to compare the numerical results with the experimental results obtained within a series of previously performed pumping tests available in literature.
Publication year:2014
Accessibility:Open