Penetration of Cement Pastes into Particle-Beds: A Comparison of Penetration Models.

For the selective paste intrusion (SPI) method, thin layers of aggregate are locally bound by cement paste where the structure shall arise. After completion of the printing process, the structure is excavated from the particle-bed and the unbound particles are removed. However, for a sufficient layer bonding and shape accuracy, the rheology of the cement paste must be adapted to the flow resistance of the particle-bed. For practical application, that means mostly time and material consuming "trial and error" tests. To prevent that, analytical models can help to predict the penetration of the cement paste. This paper presents four analytical models to calculate the penetration depth of a cement paste into a particle packing. Based on Darcy's law, an already existing model is slightly modified (model A+) and a generalized (model C), an advanced generalized (model D) as well as a simplified model (model B/B+) are developed. Compared to conducted tests on the penetration depth, model B showed good accuracy (deviation <1.5 mm) for pastes with a yield stress ≥8.2 Pa, model A+/B+/C for ≥ 5.4 Pa and model D even for <5.4 Pa. Finally, an application guide for each model for practical use will be given.

Particle-Bed Binding by Selective Paste Intrusion-Strength and Durability of Printed Fine-Grain Concrete Members.

The selective paste intrusion (SPI) describes a selective binding, additive manufacturing method. SPI bonds thin layers of aggregate by cement paste locally. Currently, SPI can achieve higher compressive strength, durability, and easier unpacking behavior compared to other selective binding methods suitable for the production of concrete structures. Particle-bed based methods not only achieve much higher surface resolutions than depositing (extrusion)-based additive manufacturing methods but also have no restrictions in freedom of form. However, the mechanical performance of SPI components strongly depends on the void content between the individual layers and thus the penetration behavior of the cement paste. This paper presents direction-dependent measurements of the strength and durability of SPI-printed components compared to casted specimens with the same mixing composition. The results show compressive strength values between 70 and 78 MPa after 7 d, flexural strength of 1/10 without reinforcement, a high freeze-thaw resistance, no detectable carbonation after 182 days of exposure under ambient CO2-conditions, and after 28 days under increased CO2 content of 2 vol % as well as low chloride penetration resistances. All tests showed in almost all cases no dependency on the layer orientation.

Additive Manufacturing of Cementitious Materials by Selective Paste Intrusion: Numerical Modeling of the Flow Using a 2D Axisymmetric Phase Field Method.

The 3D printing of concrete has now entered a new era and a transformation of the construction sector is expected to reshape fabrication with concrete. This work focuses on the selective paste intrusion method, which consists of bonding dry particles of aggregate with a cement paste. This innovative technique could lead to the production of very precise component for specific applications. The main obstacle to tackle in order to reach a high shape accuracy of high mechanical performances of 3D printing elements by selectively activating the material is the control of the distribution of the cement paste through the particle bed. With the aim to better understand the path followed by the solution as it penetrates a cut-section of the granular packing, two-dimensional numerical modeling is carried out using Comsol software. A phase-field method combined with a continuous visco-plastic model has been used to study the influence of the average grain diameter, the contact angle, and the rheological properties of cement pastes on the penetration depth. We compare the numerical modeling results to existing experimental results from 3D experiments and a one-dimensional analytical model. We then highlight that the proposed numerical approach is reliable to predict the final penetration of the cement pastes.

Interlayer Reinforcement Combined with Fiber Reinforcement for Extruded Lightweight Mortar Elements.

Lightweight mortar extrusion enables the production of monolithic exterior wall components with improved thermal insulation by installing air chambers and reduced material demand compared to conventional construction techniques. However, without reinforcement, the systems are not capable of bearing high flexural forces and, thus, the application possibilities are limited. Furthermore, the layer bonding is a weak spot in the system. We investigate a reinforcement strategy combining fibers in the mortar matrix with vertically inserted elements to compensate the layer bonding. By implementing fibers in the extruded matrix, the flexural strength can be increased almost threefold parallel to the layers. However, there is still an anisotropy between the layers as fibers are oriented during deposition and the layer bond is still mainly depending on hydration processes. This can be compensated by the vertical insertion of reinforcement elements in the freshly deposited layers. Corrugated wire fibers as well as short steel reinforcement elements were suitable to increase the flexural strength between the layers. As shown, the potential increase in flexural strength could be of a factor six compared to the reference (12 N/mm2 instead of 1.9 N/mm2). Thus, the presented methods reduce anisotropy in flexural strength due to layered production.