# Carbon Fiber Traces in Cracked Surfaces of Mortar Prisms
## 4 Fiber Traces on the Cracked Surfaces
The reason for the roughness growth was explained in the description regarding Figure 2. This growth will lead to the increase in the surface area of the cracked surface. In the homogeneous samples, the lowest resistance place will be the cross section with the smallest cross-sectional area. Hence, as the surface area of the cracked surface increases, the resistance to the imposed force should also grow. However, in reality, the resistance is reduced by the fibers inducing the traces on the cracked surface. These fibers are not nullifying the resistance increase induced by the surface area growth but are diminishing it. The presence of any inhomogeneous materials in a homogeneous object can reduce the strength of the object as the plane mortar shows in Figure 2. The plane prism in Figure 2 shows the longest propagation distance to the side, i.e., the cracked surface has the widest surface area. This is probably because of the large amount of air pockets as shown in Figure 6. It is assumed that the total area occupied by the air pockets on the cracked surface will be greater than the area increase from 40 mm \(\times\) 40 mm, by having the longest propagation distance. Since the fibers in the mortar prisms are inhomogeneous materials, if their amount become more than a certain quantity, the strength of the prisms will go down again. As the fiber amounts appearing at the cracked surface grows as the percentage increases, the surface becomes more inhomogeneous with increasing fiber contents. Since the fibers appearing at the surface create specific traces, the number of traces can inform the relative amount of the fibers for prisms with different fiber contents. The results from Table 1 show that the fiber percentage 1.0 works as the turning point for changing the amount of decrement or increment in the number of each specific trace. This is because the highest increment in the number of the traces other than the uprootedness appears between CF 0.5 and CF 1.0. For the uprootedness case, the number stays almost the same as CF 0.5, but the numbers sharply reduce for CF 1.5 and CF 2.0. Moreover, the increasing number of the type II dividedness in the cases corresponding to the increasing fiber percentages shows that if the fiber content exceeds a certain amount, mixing more fibers in the prism samples will no longer increase their strengths because more portions of the cracked surface will be covered with fiber-laden mortars and cemented fiber clusters/chunks. The increasing amounts of the fiber-laden mortars and cementedfiber chunks/clusters will cause a further increase in the inhomogeneity of the mortar and reduction in the cohesive power of the cement. Hence, the strength growth achieved by mixing more fibers cannot match the strength reduction induced by the increased inhomogeneity and the reduced cohesive power. Hence, the strength of the prism samples will be reduced when the fiber content exceeds a certain value. As shown in Figures 10 and 11, the sampled areas of CF 1.5 and CF 2.0 are mostly covered with fiber-laden mortars and cemented fiber clusters/chunks. This means that the inhomogeneity increases more and the cohesive powers of CF 1.5 and CF 2.0 reduce more than those of CF 1.0. The flexural strength measurements of different fiber percentage mortar prism samples shown in Figure 3 indicate that CF 1.0 provides the highest flexural strength. This confirms that the increased inhomogeneity and the decreased cohesive power in CF 1.5 and CF 2.0 are the main reasons for the reduction in their flexural strengths to less than that of CF 1.0. Hence, 1.0% will be considered to be the certain amount mentioned above. The fiber content of 1.0%, as the turning point for the decrement or increment in the numbers of different traces, is the fiducial point for decreasing the strength of the sample prisms for fiber percentages greater or smaller than 1.0.