Research Summary Report of A02

[10.02.2023]

Hamilton, Leigh Duncan; Researcher; Leigh-Duncan.Hamilton@tu-braunschweig.de

Zetzener, Harald; Leading researcher; H.Zetzener@tu-braunschweig.de

Kwade, Arno; Project leader; A.Kwade@tu-braunschweig.de

All: TU Braunschweig, Institute for Particle Technology

Our main goal within project A02 is to unite two additive manufacturing (AM) processes, thereby, creating a hybrid AM process for structural concrete. The foundation of A02 is formed around the concrete 3D printing process Selective Paste Intrusion (SPI). SPI creates components in layers by first spreading coarse aggregates (usually quartz) on a surface or previous layer. Subsequently, the cement slurry is applied onto designated areas, where it fills void volumes between aggregate particles.

The second AM process is Wire and Arc Additive Manufacturing (WAAM). WAAM is essentially implemented as means of steel reinforcement in order to create structural concrete components [1].

Summary

While our project partners from TU Munich are investigating process parameters for SPI and WAAM as well as active cooling strategies to reduce heat transfer into the particle bed, we at the Institute for Particle Technology are, inter alia, responsible for functionalised particles and adjustments of particle sizes (grain sizes).

Firstly, it is safe to say that particles within a collective never have an equal size, which is why a collective is described by statistical distributions and relevant characteristic values, e.g., median value. Particle sizes are thereby commonly characterised by particle size distributions (PSD). Furthermore, PSDs can be manipulated by different kinds of processes like grinding, sieving or mixing with other particles.

With respect to our aggregate particle system, the PSD can have a major effect on SPI, and therefore, the shape accuracy as well as strength of finished concrete components. For instance, altering the PSD of a given bulk material can significantly change the specific surface area, meaning finer aggregate particles in our case have more contact areas for the intruding cement slurry. However, reducing the particle size also reduces the pore size (size of void volumes) between particles (see Fig. 1). A full and precise intrusion of the cement slurry may consequently be impeded.

Another approach of manipulating a PSD is by mixing coarse particles with a significantly finer fraction of particles. By doing so, a bimodal PSD is created and the packing density (filled volume) of a bulk material can be increased. For SPI, the packing density of our aggregate particle bed has a direct impact on the cement slurry consumption: Less void volumes mean lower amounts of cement required for the process, which, among other things, has the highest environmental impact. Nevertheless, challenges mentioned beforehand regarding the pore size still exist. For that reason, the aggregate particle packing density is our hot topic of today.

Current state of research

In an attempt to increase the packing density, different proportions of a finer PSD were mixed with coarser aggregates that are currently used in SPI (1-2 mm). The packing density was increased in this case by 3.5% and, thus, the porous volumes within the bulk material decreased equally, which can have a significant impact in large-scale operations. However, segregation effects do occur in the given particle size range due to the insignificance of necessary attractive forces between particles. Segregation can, on the other hand, be partially avoided by inducing adhesive forces between coarser and finer particles. Within this study, attractive forces were induced by mixing small amounts of water into the particulate system which creates liquid capillary bridges between particles and prevents enhanced segregation. Although the coarser particle fraction dominates the bimodal mixture, changes in the flow behaviour were observed. The flow behaviour of the aggregate material also has a substantial effect on SPI. As a result, the appropriate amount of water or rather strength of attractive forces is an interaction between the grade of segregation effects and bulk material flow requirements for SPI. Fig. 2 displays the effects of water has on a bimodal mixture, i.e., increased homogeneity of the particle distribution, a steeper angle of repose due to induced cohesion.