# Carbon Fiber Traces in Cracked Surfaces of Mortar Prisms
## 3 Properties of the Cracked Surfaces  
As mentioned before, in a prism, the cracked surface represents the lowest resistance places in the direction of the force from the UTM. The low resistance places in the prism can be: (1) air pockets; (2) fiber balls that represent the fiber clusters; (3) the boundaries between tips of the fiber strands/clusters and the mortar, between the cemented fiber clusters, i.e., fiber clusters covered by cement, and between the cluster and the mortar; (4) the mortar laden with fiber strands; (5) plane mortars where no fiber strands exist. As explained before, the dimension of the fiber balls can be much greater than the length of a single strand, and they can also be formed by overlapping between the cemented fiber clusters. The air pockets are observed in the plane prisms, but they almost disappear in the fiber-laden prisms. The air pockets work as a buffer zone for mitigating the internal stresses induced by the expanding and contracting of the prisms caused by the temperature changes. They are good for the durability but rather bad for the strength of the prisms. The fiber balls act as air pockets because of the air between the fibers working as the buffer zone, but the fibers themselves induce almost no internal stress because their temperature coefficient is almost zero. The fiber balls are essential for the durability of the prisms reinforced with the fibers. Nevertheless, they form low resistance places to the cracking forces when (1) their lengths are greater than that of the individual fiber strand and (2) their overlapped or folded parts are located along the force flowing direction. Since the fiber clusters forming the fiber balls are mostly bonded by loosely adhered cement, they can be easily divided or separated into two parts. The boundaries between the tips of the fiber strands/clusters and the mortar are the places where the fibers are shallowly buried in the mortar. They can be easily uprooted from the mortar. If the fiber clusters covered with the cement and/or fiber strands mixed with the mortar are in the force flowing direction, the fiber clusters can be easily: (1) avulsed from the mortar, and (2) separated/divided from each other because the adhering power of the cement on the  
Figure 1: Experimental set-up. Each camera is set to the single frame mode. surface of the clusters and the cohesiveness of the fiber-laden mortar are low. Hence, it is expected that the crack will pass through the air pockets, the fiber balls, the boundaries between the fiber tips and the mortar, as well as between the cemented fiber clusters and fiber-laden mortars/the plane mortar. However, since the frequencies of these traces appearing are different for different amounts of fibers, the crack surfaces for different fiber amounts will differ from each other. It is expected that more fiber clusters with bigger volumes will be formed as the fiber content increases. The cracked surface will become bumpier, i.e., the surface will have deeper hollows and higher humps/plateaus as the content increases. This is shown in Figure 2. Figure 2 presents a comparison of the crack lines on the bottom surfaces (1 st row), the surface showing the most changes in the crack line direction (2 nd row), the surface showing the longest crack line (3 rd row) and the surface morphologies of the cracked surface (4 th row) of the prisms with different fiber percentages. The force flowing direction is from right to left. The prism samples are identified by the description CF 0.0-2.0, where CF means carbon fiber, the 0.0 stands for the fiber percentages such as 0.0, 0.5, 1.0, 1.5 and 2.0, and the O the sample number such as 1 and 2. The 0.0 represents the plane prisms. In Figure 2, no sample numbers are given because the images have the maximum value in the same percentage samples in each row. The crack line of the CF 0.0's bottom surface shows the longest propagation distance and the most direction changes. The distance is even longer than that of the fiber-laden samples.