Rheology of concentrated suspensions of spherefiber fiber mixtures in view of applications to reinforced cementitious materials

Abstract

Discontinuous shear thickening (DST) in dense non-Brownian suspensions is a welldocumented phenomenon in scientific research, however its origins and effects are still under

discussion

nowadays. Not being able to recognize the direct cause of the phenomenon represents a point of weakness in many industrial fields, especially in the field of concrete and fiber reinforced concrete production. In this thesis, we will study experimentally as well as theoretically the DST in a suspension of calcium carbonate microparticles loaded with rigid polyamide and glass fibers. The rheology of this suspension simulates the behavior of fiber-reinforced concrete. We reproduce on a reduced scale different types of flows occurring in the concrete placement process, such as double-helix mixing, pumping through tubes, jet flow at the tube outlet, and we derive fundamental rheological behaviors applicable to any type of "sphere-fiber" mixtures. Using "mixing" type rheometry (double helix tools adapted to the rotational rheometer), we will first show that the addition of fibers shifts the DST transition to lower critical shear rates, which is explained by an increase in the viscosity of the suspension, so that the shear rate to reach the DST onset stress decreases. However, the mixture jams at a fiber volume fraction greater than or equal to 4% vol which is interpreted in terms of the percolation threshold of the fiber network in the shear thickening matrix of calcium carbonate. We will show in a second stage that the rheological behavior in a flow induced by a pressure gradient through a capillary remains quite similar to that in a simple shear but only at low fiber volume fractions ?? ? 1 % vol, and if the Mouney-Rabinowitch correction is correctly applied. Above this volume fraction, the flow ?? curves in the capillary rheometry become very different from those measured in simple shear likely because of microstructural difference in two different flow geometries. The theoretical model based on the homogenization approach allows to reproduce at least semi-quantitatively the flow curves in the " mixing" type and capillary rheometry at low fiber volume fractions but fails to capture microstructural changes at higher volume fractions. Finally, the instabilities of the calcium carbonate suspension jet under gravity with and without polyamide fibers will be studied. This instability is manifested by strong lateral oscillations of the axial symmetry axis of the jet accompanied by a slight undulation of the jet surface. We will perform for the first time a two-dimensional direct Fourier transform (2D DFT) analysis of the spatiotemporal variation of the jet diameter and the lateral deflection of the jet in the DST regime. We will show that the addition of polyamide fibers at different concentrations in the suspension allows for jet stability and promotes jet fractures. A theoretical explanation for the onset of jet lateral instability and the stabilizing effect of the fibers will then be developed based on the evaluations of tensile stresses and the lower and upper thresholds of the DST.