Insight into the boundary flow resistance of high-workable concrete in different states: placing and pumping

The flow resistance of modern high-workable concrete during pumping is usually considered linearly related to the flow rate, but some studies have found a non-linear relationship. An accurate assessment model of concrete flow resistance at different flow rates remains unclear. This study presents a methodology for calculating the boundary resistance of high-workable concrete based on other flow velocity ranges during the pumping and placing of the concrete. A modified interfacial rheometer was developed, and a particular test procedure was applied to test the flow resistance of 18 groups of high-workable concretes. The flow resistance changes during the increasing and decreasing stages of the flow speed were comprehensively analyzed. Both hydrodynamic interactions and particle friction characteristics have been considered. The results show a two-state linear relationship between boundary resistance and flow speed, corresponding to low flow speed (LFS, for placing) and high flow speed (HFS, for pumping). Finer sand, higher mortar volume, and a specific aggregate ratio reduce the viscous resistance constant, while a higher roundness of CA reduces the slip yield stress. Boundary resistance increases linearly with the interfacial self-friction coefficient, highlighting particle properties' critical role in flow resistance. This study reveals the origin of flow resistance and the factors affecting the interface rheological properties at different flow states and provides valuable insights for optimizing the pumping.