International Concrete Abstracts Portal

A high cement content is often found in concrete mixture designs to achieve the unique fresh-state behavior requirements of three dimensional (3-D) printable concrete (3DPC) to ensure rapid stiffening of an extruded layer without collapsing under the stress applied by the following layers. Some materials with high water absorption, such as recycled concrete aggregates, have been incorporated in concrete mixture designs to minimize environmental impact; nevertheless, the fine powder fraction that remains from the recycled aggregate processing still poses a challenge. In the case of 3DCP, few studies are available regarding mixture designs using recycled concrete powder (RCP) for 3-D printing. In this context, this study presents the use of RCP as a filler to produce a printable mixture with low cement content. An RCP with 50 μm average particle size was obtained as a by-product from recycled concrete aggregate production. Portland cement pastes were produced with 0, 10, 20, 30, 40, and 50% of cement mass replacement by RCP to evaluate its effects on the hydration reaction, rheology, and compressive strength. It was found that the studied RCP replacement was not detrimental for the hydration reaction of portland cement during the initial hours, and at the same time, it was capable of modifying the rheological parameters of the paste proportionally to the packing density of its solid fraction. The obtained results indicated the viability of 3DCP with up to 50% cement replacement by RCP. It was concluded that RCP presents good potential for decreasing the cement consumption of 3DPC, which in turn could decrease its associated environmental impact while providing a destination for a by-product from recycled concrete aggregate production.

Reinforcing strategies for three-dimensional printing (3DP) of cementitious materials (mostly mortars) have been extensively studied in recent years. Among various reinforcement strategies available for 3DP of cementitious materials, the use of fibers is frequently mentioned as a promising approach to enhance their mechanical performance. This work aims to evaluate the influence of four types of fibers (polyvinyl alcohol [PVA], nylon, rayon, and basalt) on the flowability and flexural strength of mortars used in 3DP. The flexural behavior of 3DP beams was compared with that of cast specimens, and the digital image correlation (DIC) technique was used to evaluate the development of the cracks. The fiber orientation in the reference (cast) and 3DP samples was examined using optical microscopy. The results revealed that, among four types of fibers used, the PVA fibers were most effective in increasing the flexural strength of both the cast and 3DP specimens. In addition, the results show that all fibers preferentially aligned parallel to the printing direction. 3DP specimens with filaments aligned in the direction perpendicular to the direction of the applied load showed superior flexural strength when compared to the cast specimens.

Despite all the recent advances in the development of threedimensional (3-D) concrete printing (3DCP), this technology still has many unresolved problems. In most of the completed projects with the application of 3DCP, the focus was mainly on mastering the printing of vertical walls, while horizontal structural elements were produced with conventional methods—that is, using formwork, which reduces the level of technology automation, or using prefabricated elements, which makes the construction dependent on their availability and supply. In this contribution, the authors propose new methods of manufacturing slabs and beams directly on site by extruding concrete onto a textile reinforcement mesh laid on a flat surface. Specimens obtained from a slab produced following this method were used for mechanical testing and investigation of the concrete-reinforcement interface zone. Finally, as proof of the feasibility of the proposed approach, a demonstrator representing a full-scale door lintel was manufactured.

Extrusion-based concrete printing technology allows the fabrication of permanent formwork with intricate shapes, into which fresh concrete is cast to build structural members with complex geometries. This significantly enhances the geometric freedom of concrete structures without the use of expensive temporary formwork. In addition, with proper material choice for the permanent formwork, the load-bearing capacity and durability of the resulting structure can be improved. This paper investigates the concrete printing of permanent formwork for reinforced concrete (RC) beam construction. A three-dimensional (3-D)-printable engineered geopolymer composite or strain-hardening geopolymer composite (3DP-EGC or 3DP-SHGC), recently developed by the authors, was used to fabricate the permanent formwork. The 3DP-EGC exhibits strainhardening behavior under direct tension. Two different printing patterns were used for the soffit of the permanent formwork to investigate the effect of this parameter on the flexural performance of RC beams. A conventionally mold-cast RC beam was also prepared as the control beam for comparison purposes. The results showed that the RC beams constructed using the 3DP-EGC permanent formwork exhibited superior flexural performance to the control beam. Such beams yielded significantly higher cracking load (up to 43%), deflection at ultimate load (up to 60%), ductility index (50%), and absorbed energy (up to 107%) than those of the control beam. The ultimate load was comparable with or slightly higher than that of the control beam. Furthermore, the printing pattern at the soffit of the permanent formwork was found to significantly influence the flexural performance of the RC beams.

Three-dimensional (3D) printing of cement-based materials is carried out using extrusion, which requires a fundamental understanding of the non-Newtonian flow of pastes through capillaries, which is the focus of this paper. 3D-printable cementitious pastes, qualified using steady-state extrusion pressure, are subjected to multiple-speed extrusion tests under apparent shear rates that correspond to typical printing speeds. The true, non-Newtonian flow curves are obtained by carrying out the relevant end corrections, deconvoluting the apparent shear rate (or velocity) into its true and wall slip components and applying the Weissenberg-Rabinowitsch correction. An exponential relationship is observed between the slip velocity and the wall shear stress, which is used to determine the slip layer thickness. The velocity profiles in the capillary demonstrated the shear-thinning nature of the pastes and the existence of a plug-flow zone with invariant velocity, while the viscosity profiles showed the near-Newtonian response of the superplasticized paste at higher shear rates. The influence of printing speed, particle concentration, and the presence of superplasticizer on the slip layer thickness is explored. A particle-depleted slip layer could be beneficial in reducing the energy needed for printing but could have implications in interlayer bonding and durability. The flow characterization approach presented herein can be adopted to optimize the paste material design and printing characteristics for extrusion-based 3D printing.