Extrusion of Lightweight Concrete: Rheological Investigations

Lightweight concrete enables the production of monolithic exterior wall components with sufficient thermal insulation even under Central European climatic conditions. If, in addition, additive manufacturing by extrusion is used for production, these components can be further improved by installing air chambers to increase thermal insulation of the component or varying wall thicknesses to reduce material demand. Furthermore, it is beneficial to use lightweight aggregate concrete (LAC) instead of e.g. aerated concrete due to its higher robustness with regard to the pumping process. However, LAC differs from normal concrete as the porous lightweight aggregates absorb water and thereby change the characteristics of the fresh mixture. This applies in particular if the concrete is put under pressure during production, e.g. during pumping. This paper focusses on the effect of rheological parameters (especially plastic viscosity) on the pumpability of LAC. We found for the tested mixtures, pumped with an eccentric screw mortar pump, that pumpability was independent of the yield stress. Plastic viscosity, on the other hand, had a decisive effect on the pumpability of the LAC. By varying the total water to binder ratio, we found a significant effect on the pumpability, which is in line with the effect of the plastic viscosity. Furthermore, alternative concrete additives can affect the flow in the pipe, as is known from normal concrete. Our experiments show that limestone powder influences the pumpability independently of plastic viscosity and yield stress. With our results, we contribute to the development of a lightweight, 3D printable concrete.