# Carbon Fiber Traces in Cracked Surfaces of Mortar Prisms
## 2 Preparation of Experiment  
Carbon fiber-laden mortar prisms were manufactured to measure their flexural strengths. For this purpose, cement, sand, having grain sizes of not greater than 2 mm, and water were prepared to gain the weight ratio of 1.0:3.0:0.5. Carbons fibers having 0.5%, 1%, 1.5% and 2% volume of \"sand + cement\" along with polycarboxylic admixture having 1% weight of the cement were also prepared. These fibers were cut into 6 mm lengths. For each of the fiber percentages, the mixture of all these materials was prepared following the mixing procedures specified in KSL ISO 679 [12]: (1) cement and sand were mixed together for 60 s, (2) carbon fibers were added to the mixture by mixing for 90 s, and (3) water and admixture were poured into the carbon fiber added mixture, and then they were mixed together for 150 s. This mixture was turned to a prism shape by the test piece with the inner dimension of 40 mm \(\times\) 40 mm \(\times\) 160 mm by keeping for 24 h. After removing the test piece, the prism shape mortar was cured for 28 days in a water tank at a constant temperature of 20 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C. Three prism samples for each carbon fiber percentage including the one with no fiber added, i.e., plane, were prepared in this way. Thus, 15 prism samples were manufactured.  
The flexural strength measurement was performed with a universal testing machine (UTM). Three-point bending set-up was used by applying 50 N/sec force. The crack appearance on the prisms was observed with Nikon D810 placed 1 m away from the prism's front surface so that the horizontal size of the prism surface (160 mm \(\times\) 40 mm) was covered by all pixels in horizontal direction of the image sensor.  
The camera has the resolution of 7360 \(\times\) 4912 and a pixel size of 4.88 \(\times\) 4.88 \(\upmu\)m\({}^{2}\). For the cracked surface observation, three different cameras were used: the D810, a mobile phone (Samsung Galaxy s20 Ultra) camera, which has the resolution of 12,000 \(\times\) 9000 with a pixel size of 0.8 \(\times\) 0.8 \(\upmu\)m\({}^{2}\), and a USB microscope with 10\(\times\) magnification and 640 \(\times\) 480 camera resolution. They were set to single frame mode. Since the D810 has a stable focus distance compared with the mobile phone, it was used to observe the depth of the cracked surface at a close distance. The mobile phone camera was used to examine the details of the surface due to its high resolution, and the microscope camera was used to take the magnified image of a small area of the surface. The microscope and mobile phone images have almost the same image quality. Thus, the best images were selected from these two cameras.  
Each camera was set at a distance from the surface and its focus adjusted to compensate the height difference between the two pieces of the prism separated by the crack, as far as this height difference induces the size difference between the cracked surface image of left and right pieces. Hence, the surface images were digitally adjusted to have the same size. After adjusting the size, the images were overlapped with a wire grid to identify the matching fiber bundles in the left and right pieces. However, since the crack surface is not flat, the fiber clusters in slanted places often give the problem of identifying their matching clusters with the images. In this case, it was directly found from the left and right pieces. In the testing, because of a few errors in synchronizing the D810 to the UTM initiation and of the UTM operating mistakes, only two prism sample images were used to find the fiber traces in the cracked surface. In Figure 1, the experimental set-up for the flexural strength test and the cracked surfaces' photographing are presented. The set-up consisted of three cameras, UTM and the prism samples.