INFLUENCE OF AIR BUBBLES SIZE AND VOLUME FRACTION ON RHEOLOGICAL PROPERTIES OF FRESH SELF-COMPACTING CONCRETE

摘要

In literature, in order to study the rheological properties of fresh concrete, the air content is mostly varied by means of the addition of air-entraining agents. As a result, no separation between the influence of the air bubbles themselves and the surfactant, which keeps the bubble stable, can be executed. By means of pumping, the air content in fresh concrete is gradually increased. Furthermore, a decrease in air bubbles size of around 1 order of magnitude has been observed. In this case, the bubbles are naturally formed, without the addition of any air-entraining agents. Calculation of the Capillary-number, which is defined as the ratio of the applied shear stresses to the surface tension, provides the ability to make a distinction between the effect of air on yield stress and plastic viscosity. If the Ca-number is much larger than 1, which is the case when studying the viscosity, the bubbles deform and decrease the flow resistance. For low Ca-numbers, for example when studying the yield stress, the bubbles remain spherical and increase the flow resistance. As a result, for small air bubbles, the yield stress has been found to increase, while the viscosity decreases. For larger air bubbles, the effect of air on the yield stress becomes unclear, resulting in a possible increase or decrease.