Microstructural Characterization of 3D Printed Cementitious Materials

Abstract

Three-dimensional concrete printing (3DCP) has progressed rapidly in recent years. With the aim to realize both buildings and civil works without using any molding, not only has the need for reliable mechanical properties of printed concrete grown, but also the need for more durable and environmentally friendly materials. As a consequence of super positioning cementitious layers, voids are created which can negatively affect durability. This paper presents the results of an experimental study on the relationship between 3DCP process parameters and the formed microstructure. The effect of two different process parameters (printing speed and inter-layer time) on the microstructure was established for fresh and hardened states, and the results were correlated with mechanical performance. In the case of a higher printing speed, a lower surface roughness was created due to the higher kinetic energy of the sand particles and the higher force applied. Microstructural investigations revealed that the amount of unhydrated cement particles was higher in the case of a lower inter-layer interval (i.e., 10 min). This phenomenon could be related to the higher water demand of the printed layer in order to rebuild the early Calcium-Silicate-Hydrate (CSH) bridges and the lower amount of water available for further hydration. The number of pores and the pore distribution were also more pronounced in the case of lower time intervals. Increasing the inter-layer time interval or the printing speed both lowered the mechanical performance of the printed specimens. This study emphasizes that individual process parameters will affect not only the structural behavior of the material, but they will also affect the durability and consequently the resistance against aggressive chemical substances.

Extrusion-based concrete 3D printing from a material perspective: A state-of-the-art review

“…Roussel [13] showed that the bond strength could differ by even up to 50% when drying was occurring in the printed layers. Following this, studies have shown that the bond strength is dependent on the moisture content at the layer interface [120,121]. In summary, the bond strength is influenced not only by the mixture properties but also by the print parameters such as the time gap between the deposition of layers and the ambient atmospheric conditions such as temperature, relative humidity, wind velocity during printing.…”

Nailing of Layers: A Promising Way to Reinforce Concrete 3D Printing Structures

“…It can be noted that the bending resistance was significantly lower for this direction on loading due to the anisotropic behavior of the printed structure. This anisotropic behavior was due to the very high yield stress value of the printed cementitious materials, which led to an imperfect interface between layers [31].…”

“…Bos et al also studied the effect of a steel wire embedded in the deposited layer as a potential efficient reinforcement strategy [29]. However, those two previous methods did not provide reinforcement that crossed the interface between layers that were expected to be a potential weakness zone of the printed structures [11,[30][31][32].…”

Interlayer Reinforcement Combined with Fiber Reinforcement for Extruded Lightweight Mortar Elements

“…It was shown that the transmission of flexural loads can be enhanced and the effect of cold joints, as described e.g., in [5,6], reduced. As described in Section 1.2, the layer bond often represents a weakness zone in extrusion [22,23]. This is especially true for the production of larger building elements or the manufacturing of houses at side, where the time gap between layers increases and additionally, a need to pause the printing process might occur.…”

“…Thus, the strength of the layer bonding may either be not affected or even be deteriorated due to increased anisotropy. The layer bond was therefore identified as the weakest zone of the elements [22,23].…”