# Carbon Fiber Traces in Cracked Surfaces of Mortar Prisms
## 3 Properties of the Cracked Surfaces
However, the cracked surface looks very flat compared with the fiber added prism samples. As shown in Figure 2, the crack propagation distances and the crack lines' direction variations for the fiber-laden prism samples increase as the fiber percentage increases. The distance increase with increasing fiber percentages is specified in the 2 nd row images by two solid horizontal lines. The two solid lines and a solid vertical line in each image represent the distance, while the vertical broken lines in between the horizontally drawn broken line and the bottom straight line indicate the distance of the immediate left side image. The distances also increase as the fiber percentage increases. The increased direction variations are accompanied by the bumpier cracked surfaces. The crack lines and their corresponding edges of the cracked surfaces in the 4 th row are specified by solid lines starting from the 4 th row images. As the crack line shows more direction changes, the  
Figure 2: Crack lines and surface roughness of the cracked surfaces of prism sample of different fiber contents. cracked surface becomes bumpier. The surface shows the increased numbers of deeper hollows and higher humps/plateaus as the fiber percentage increases. Numerous dark holes of different sizes are observed on the surface of the plane prism, but it is hard to find the holes of the same kind in the fiber added prism samples. These holes are air pockets. Holes of this kind are not noticed in fiber-laden prisms as shown in Figure 2, probably because the numbers and sizes of the holes are too small to be recognized in the fiber-laden prisms. This informs us that the overlapped and folded fiber bundles absorb most of the air in the prism and act as air pockets as in the plane prism. The flexural strengths of the prism samples of different fiber contents are shown in Figure 3. The horizontal axis is the crack gap in mm and the vertical axis the applied forces in kN (Kilo-Newton). The value of 40 \(\upmu\)m corresponds to a pixel width of the Nikon D810. The force initiating the crack is almost the same for samples CF 0.0-, CF 0.5- and CF 1.0-, but CF 1.5- and CF 2.0- show 0.64 kN and 0.17 kN differences between samples, respectively. In fact, the flexural strength of CF 2.0- is within those of CF 1.5-1 and CF 1.5-2. However, the average strength of CF 1.5- is around 3 kN and that of CF 2.0 is 2.9 kN. CF 1.5- appears to be slightly higher than CF 2.0-. The average flexural strengths of CF 0.0-, CF 0.5-, CF 1.0-, CF 1.5- and 2.0- are 1.2 kN, 2.25 kN, 3.9 kN, 3.0 kN and 2.9 kN, respectively. The strength reduces as the gap increases, though the speed of reducing is different for different samples. This speed difference is caused by the presence of the fibers along the crack path. The CF 1.0 samples show the highest strength. As mentioned before, the fibers along the cracked surfaces reveal four distinctive fiber traces such as unrootedness, avulsion, separation and dividedness. There is also an overlay type trace. The uprootedness, avulsion, separation and dividedness pave the crack path by pulling out the tips of the fiber clusters from the mortar, detaching the fibers completely from the mortar, separating a cemented fiber cluster from the other cemented fiber clusters and breaking either the mortar or fiber strand mixed mortar into sets of a valley and a plateau/hill form, respectively. Figure 4 illustrates these five traces. The uprootedness and avulsion have two different types, the separation has three different types, the overlay has only one type, and the dividedness has two different types. The two types of the uprootedness are: (1) the pulled-out fiber bunches on one side and a grooved mortar area, where the fiber bunches are pulled-out, on the other side; (2) the grooved mortar area is also filled with fiber bunches in it. However, the second type is more frequent. The scale of the uprootedness is very small compared with other traces because of the involvement of the tip of a fiber cluster and a small groove.