Numerical simulation of static and dynamic stability of air-bubbles in highly flowable cement-based materials

Abstract

A computational fluid dynamics (CFD) software and image analysis technique were used to simulate the flow performance and stability characteristics of air-void system (AVS) of fresh highly flowable cement-based materials as a three-phases gas-liquid-solid system using a coaxial cylinders tribometer set-up. The investigated characteristics consist of flow velocity, shear rate, and pressure magnitudes of the suspensions, as well as displacement, elongation, orientation angle, and rising velocity of the air bubbles in both parallel and perpendicular planes to flow directions. Static and dynamic shear modes as well as different shearing zones were evaluated. Different suspending fluids having different plastic viscosity and surface tension values varying between 5 and 30 Pa.s and 5 and 73 mN/m, respectively, were investigated. The suspensions consisted of spherical solid particles of 2.5, 4, and 5 mm diameter with 3.8 % volumetric content, as well as 18 and 150 air bubbles of 2- and 1-mm diameter, respectively, corresponding to 7 % volumetric content.According to the numerical results, the shearing mode and distance from the shearing source were shown to be the most dominant parameters affecting the flow performance and stability of air-void system in highly flowable cement-based materials, compared to the suspending fluid characteristics. Moreover, both the diameter and initial vertical positions of the air bubbles can significantly affect their stability characteristics.