# The Recycling of Carbon Components and the Reuse of Carbon Fibers for Concrete Reinforcements
## 4 Results and Discussion
### Single-Fiber Tensile Tests  
The single-fiber tensile tests were conducted using a fiber tensile testing machine, following the ASTM D3822 standard [16]. Fibers were taken directly from the yarn for sample preparation and inserted into the testing machine. Subsequently, the individual 25 mm long fibers were loaded until failure. The loading speed was set to 1 mm/min. A total of 20 individual fibers were examined. In addition to the breaking force, other  
Figure 4: Schematic illustration of the laboratory profiling unit, reprinted from Ref. [13]. Copyright 2022, ITM.  
material properties such as diameter and Young's modulus of the fiber were determined. The average fiber diameter was 5.41 \(\upmu\)m, and the average Young's modulus was 229.54 GPa. The stress-strain curves are presented in Figure 5.  
The average fracture stress is 3.34 GPa of 20 samples. The stress-strain diagram also shows that the fiber's tensile stresses increase linearly. The failure of the fibers occurred suddenly and without prior notice. The strain of the fibers studied ranges from 0.87% to 2.02%. The average maximum strain of the fibers is 1.39%. All results are shown in Table 1. The exact material-specific properties (such as tensile strength) of the fibers, before the recycling process, could not be definitively determined. However, due to their similar diameter, it is assumed that they belong to the same type of fibers. In comparison, virgin carbon fibers with a diameter of 5 \(\upmu\)m have a tensile strength ranging from 5.9 GPa to 6.0 Gpa, and a Young's modulus between 280 Gpa and 290 Gpa. For a fiber diameter of 7.0 \(\upmu\)m, the tensile strength ranges from 4.1 Gpa to 5.1 Gpa [17]. Therefore, this would result in a strength loss of up to 10%, compared to a new fiber. According to [18], the recycled fibers achieved a tensile strength between 72% and 94% compared to the new fibers. In the experiments conducted by [19], the tensile strengths of the recycled fibers were found to be similar to those of the new fibers.  
From the diagram in Figure 5, it is evident that there is a large fluctuation in the individual breaking loads of the single-fiber test. The average fiber rupture stress is 3.34 GPa. However, the arithmetic mean is of limited use due to the variation of the 20 individual values. The correlation coefficient of Figure 5 is 0.99. The standard deviations of the conducted experiments are listed in the following Table 2.  
The large fluctuation in the breaking loads could be attributed to the manufacturing process, during which the yarns are subjected to different levels of stress during the dry-laid process, stretching and spinning. This caused defects in the individual fibers; therefore, they failed at different load levels. Since all the diameters of the fiber range between 5.14 \(\upmu\)m and 5.82 \(\upmu\)m, all individual values can be compared. The yarns used in these experiments were produced in a prototype manner. The individual fibers were extracted from the yarn and examined. Due to the manufacturing process using the dry-web method, the fibers experienced varying degrees of mechanical stress. Therefore, the fluctuations in the tensile strengths of the individual fibers can be attributed to this mechanical strain.  
Figure 5: Tension–strain diagram of single-fiber tensile tests.