Combining Wire and Arc Additive Manufacturing and Selective Paste-Intrusion for Additively Manufactured Structural Concrete (2021-11)¶

The Selective Paste Intrusion (SPI) is an additive manufacturing method in which thin layers of aggregates are bound selectively by cement paste only where the structure shall arise. In this way, concrete elements with complex geometries and structures can be produced. To meet the optimum between required layer bonding and sufficient shape accuracy, the rheological properties of the cement paste, i.e., its yield stress and dynamic viscosity, are crucial [1, 2]. The combination of the SPI process and the Wire and Arc Additive Manufacturing (WAAM) process enables the production of free-formed, high-strength reinforced concrete elements, which opens up a wide range of applications. However, the WAAM process generates high temperatures, which affect the rheological properties of the cement paste and thus the printing quality [3, 4]. Therefore, we analyzed the effect of external temperature loads on the rheological performance of cement paste over the entire SPI production period and derived a maximum acceptable temperature load for the combination of SPI and WAAM. The experiments showed decreasing viscosity and increasing yield stress values by stepwise increasing the paste temperature from 20 °C to 60 °C. Between 60 °C and 70 °C, the rheological behavior suddenly changed, and both viscosity and yield stress instantly increased to a multiple of their initial values. In a subsequent numerical simulation of the intrusion behavior of the paste in the particle bed, we could show that the high yield stress and viscosity lead to poor paste penetration and thus insufficient layer bonding, whereas paste temperatures up to 60 °C are not detrimental to the SPI process. Therefore, the results demonstrate that the combination of SPI and WAAM is possible if the WAAM process is adjusted by e.g. cooling strategies, increased distance of the welding point from the particle bed, or increased time intervals between the welding points to avoid paste temperatures exceeding 60 °C.