# Evaluation of 3D Concrete Printing Extrusion Efficiency
## 3 Results
### Extrusion Efficiency Dependence on Reinforcing Fiber Addition  
Incorporating fibers into cement-based material offers several advantages in 3D printing. In addition to enhancing flexural strength, fibers reduce the risk of shrinkage-induced cracking. Therefore, it is crucial to assess their impact on the performance of the printing system. In this study, tests were conducted using a material containing polypropylene (PP) fibers in two lengths: 6 mm and 12 mm. The fibers were added during the mixing stage at a concentration of 0.04% by weight. The addition of fibers to the material occurred before the start of the test--the fibers were thoroughly mixed with the dry components of the mixture to ensure their uniform distribution within the tested material. Measurements were taken with two nozzle diameters, 20 mm and 12 mm, and the extruder rotational speed was set at 37.3 RPM, as shown in Figures 13 and 14, respectively. The material consistency corresponded to a flow table measurement of 150 mm. The presence of fibers noticeably reduces extrusion efficiency. For the 20 mm nozzle, adding fibers decreased the extrusion rate by 8.8 to 12.6% compared to fiber-free material. For the 12 mm nozzle, the efficiency loss was more pronounced, ranging from 18 to 28%. This reduction occurs due to the altered material structure; adding fibers increases material cohesion, thereby restricting pumpability. In both cases, material with longer fibers caused more significant changes in extrusion performance. Additionally, the 12 mm nozzle exhibited notably higher resistance, likely due to its geometry, which makes extruding fiber-laden material more challenging. Figure 15 presents the changes in extrusion performance associated with nozzle size and fiber addition. Notably, 12 mm fibers more severely restrict extrusion with the 12 mm nozzle, suggesting proximity to the maximum fiber length suitable for this configuration. Using longer fibers may risk clogging the system. Figure 12: Graphic depictions of printhead efficiency as a function of mass of material left in tank. Figure 16 confirms a significant reduction in extrusion resulting from both the use of a smaller nozzle and the incorporation of PP fibers. The graph also highlights considerable irregularities in extrusion, which in this case are more pronounced for tests conducted with the 12 mm nozzle, where greater fluctuations in printhead efficiency are observed. An interesting aspect is the rate of feedstock utilization within the print head, as well as the minimum material quantity required to initiate printing. The rate of feedstock used represents the efficiency of material usage in relation to the initial material input (Figure 17). Observations reveal a relatively low utilization rate, averaging approximately 68% across all tests. Based on this information and the graphs showing printhead efficiency relative to the amount of material inside the reservoir, it can be concluded that approximately 4 kg of material in the reservoir is necessary for the proper operation of the printhead. Figure 14: Graphic depictions of printhead efficiency depending on the reinforcement fibers used— nozzle diameter 12 mm. Figure 13: Graphic depictions of printhead efficiency depending on the reinforcement fibers used— nozzle diameter 20 mm. The highest feedstock utilization rates within the print head were observed in tests conducted with the base mix and a 20 mm nozzle. In the results from the first test on rotational speed, the lowest performance was recorded at the lowest speed. A similar trend was noted in the test with the smaller nozzle, where lower rotational speeds also resulted in reduced feedstock utilization rates. Across the entire set of results, it was evident that the lowest rates were associated with factors related to mixture consistency and the inclusion of PP fibers. In the case of fibers, their length did not have a definitive impact on the utilization rate.