This research examines the effect of fiber alignment on the performance of an exceptionally tough 3D-printable short carbon fiber reinforced cementitious composite material, the flexural strength of which can exceed 100 N/mm2. The material shows pseudoductility caused by strain-hardening and microcracking. An extrusion-based manufacturing process allows accurate control over the spatial alignment of the fibers’ orientation, since extrusion through a tight nozzle leads to nearly unidirectional alignment of the fibers with respect to the directional movement of the nozzle. Specimens were investigated using mechanical tests (flexural and tensile load), augmented by non-destructive methods such as X-ray 3D computed tomography and acoustic emission analysis to gain insight into the microstructure. Additionally, digital image correlation is used to visualize the microcracking process. X-ray CT confirms that about 70% of fibers show less than 10° deviation from the extrusion direction. Systematic variations of the fiber alignment with respect to the direction of tensile load show that carbon fibers enhance the flexural strength of the test specimens as long as their alignment angle does not deviate by more than 20° from the direction of the acting tensile stress. Acoustic emission analysis is capable of evaluating the spatiotemporal degradation behavior during loading and shows consistent results with the microstructural damage observed in CT scans. The strong connection of fiber alignment and flexural strength ties into a change from ductile to brittle failure caused by degradation on a microstructural level, as seen by complementary results acquired from the aforementioned methods of investigation.

Kurzfassung Die Verwendung von Hochleistungs‐ und Ultrahochleistungsbeton hat in den letzten Jahren vielversprechende Ergebnisse gezeigt und es wurden erste Brückenbauwerke und Gebäude aus diesen Werkstoffen hergestellt. Dabei kommen im Regelfall Stahl‐ und Mikrostahlfasern zur Verstärkung der Zugzone zum Einsatz. Es wird jedoch nur in wenigen Fällen ein überkritischer Fasergehalt angestrebt, da damit einerseits die Verarbeitbarkeit erschwert wird und andererseits auch die Kosten entsprechend ansteigen. Anders verhalten sich Carbonkurzfasern, die mittels einer Düsentechnik gezielt ausgerichtet werden und so die Zugfestigkeit des Verbundwerkstoffs um ein Vielfaches steigern können. Bei einem solchen „Carbonkurzfaserbeton“ (carbon short fibre reinforced concrete, CSFRC) liegt das Augenmerk ausdrücklich auf dem für überkritische Fasergehalte typischen Tragverhalten. Dabei lassen sich besonders leistungsfähige Bauteile herstellen (i. d. R. im 3‐D‐Druckverfahren), wenn die Ausrichtung der in die Zementmatrix eingebetteten Carbonfasern am Verlauf der Hauptzugspannungen orientiert wird.

By using a novel fiber-reinforced cement paste in a 3D-printing process, a material with vastly improved flexural and tensile strength can be created. The extrusion-based process leads to a high degree of fiber orientation. A fiber content of 3 vol. % results in static flexural strengths of above 100 MPa. High-strength fiber-reinforced materials have potential to be used in lightweight construction. Due to the susceptibility of lightweight structural members to oscillating dynamic loads, cyclic flexural and tensile tests with up to 10 million cycles are carried out. To characterize the complex (micro-)cracking behavior of the material, the tests are augmented by external deformation sensors (strain gauges, fiber-optic sensors, photogrammetry) and microstructural analysis (X-ray CT, acoustic emission sensors, coda-wave interferometry). The experimental data will serve as the basis for multiscale-modelling approach which uses representative volume elements created from high-resolution CT scans.

Carbon fiber reinforcement used in concrete has become a remarkable alternative to steel fibers. Admixing short fibers to fresh concrete and processing the material with a 3D printer leads to an orientation of fibers and a material with high uniaxial strength properties, which offers an economic use of fibers. To investigate its mechanical behavior, the material is subjected to flexural and tensional tests, combining several measuring techniques. Numerical analysis complements this research. Computed tomography is used with several post-processing algorithms for separating matrix and fibers. This helps to validate fiber alignment and serves as input data for numerical analysis with representative volume elements concatenating real fiber position and orientation with the three-dimensional stress tensor. Flexural and uniaxial tensional tests are performed combining multiple measuring techniques. Next to conventional displacement and strain measuring methods, sound emission analysis, in terms of quantitative event analysis and amplitude appraisal, and also high-resolution digital image correlation accompany the tests. Due to the electrical conductibility of carbon fibers, the material’s resistivity could be measured during testing. All sensors detect the material’s degradation behavior comparably, showing a strain-hardening effect, which results from multiple, yet locally restricted and distributed, microcracks arising in combination with plastic deformation.

As research interest in the additive manufacturing of cementitious materials for structural uses has been continuously increasing, the question of how to incorporate tensile reinforcement in an automated process has gained further importance. Our research describes a carbon fiber-reinforced cementitious composite produced by common extrusion techniques applied in 3D printing as a means to effectively control fiber alignment. Optimization of the mixture design and consistency allows for admixing up to 3 vol.-% chopped carbon fibers, leading to specimens that can reach a flexural strength exceeding 100 N/mm2 without the addition of further continuous reinforcement. Fiber integrity during the process was checked using optical microscopy. Analysis of the microstructure shows that approximately 70% of the fibers are aligned within ± 5° of a preferential direction. Micromechanical single-fiber push-out tests confirm an interfacial fracture toughness typical for strain-hardening systems. The first insights into a ‘lost formwork’ approach commonly employed in 3D printing show that the reinforcement remains effective even when combined with nonreinforced mortar.