# Concrete Crack Identification and Image Mosaic Based on Image Processing
## 3 Experimental Validation
### Results  
#### 3.2.1 Crack Width  
In this paper, six cracks of different brightness, shape and width were selected for characterization. Six measuring points were selected for each crack, so a total of 36 sets of crack width data were obtained. The crack width was calculated by the scale method and the object distance method, and the accuracy of the two methods was compared. The crack width estimation results are shown in Table 1:  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Region**} & \multicolumn{3}{c}{**Crack Width Calculation (mm)**} \\ \cline{2-4}  & **Microscope** & **Scale Method (Error (\%))** & **Object Distance Method (Error (\%))** \\ \hline \multirow{4}{*}{I} & 0.25 & 0.280 (12) & 0.282 (13) \\  & 0.37 & 0.330 (11) & 0.332 (10) \\  & 0.24 & 0.262 (9.2) & 0.263 (9.6) \\  & 0.44 & 0.431 (2.1) & 0.434 (1.4) \\  & 0.36 & 0.388 (7.8) & 0.390 (8.3) \\  & 0.44 & 0.422 (4.1) & 0.425 (3.4) \\ \hline \multirow{4}{*}{II} & 0.39 & 0.383 (1.8) & 0.386 (1.0) \\  & 0.39 & 0.396 (1.5) & 0.399 (2.3) \\  & 0.51 & 0.525 (2.9) & 0.529 (3.7) \\  & 0.43 & 0.413 (4.0) & 0.416 (3.3) \\  & 0.53 & 0.521 (1.7) & 0.525 (0.94) \\  & 0.56 & 0.544 (2.9) & 0.548 (2.2) \\ \hline \multirow{4}{*}{III} & 0.47 & 0.438 (6.8) & 0.446 (5.1) \\  & 0.51 & 0.523 (2.6) & 0.532 (4.3) \\  & 0.73 & 0.674 (7.7) & 0.686 (6.0) \\  & 0.73 & 0.697 (4.5) & 0.709 (2.9) \\  & 0.39 & 0.386 (1.0) & 0.393 (0.77) \\  & 0.71 & 0.737 (3.8) & 0.750 (5.6) \\ \hline \multirow{4}{*}{IV} & 0.17 & 0.200 (18) & 0.142 (16) \\  & 0.16 & 0.199 (24) & 0.142 (11) \\  & 0.17 & 0.214 (26) & 0.152 (11) \\  & 0.24 & 0.261 (8.8) & 0.251 (4.6) \\  & 0.23 & 0.220 (4.4) & 0.212 (7.8) \\  & 0.23 & 0.224 (2.6) & 0.216 (6.1) \\ \hline \multirow{4}{*}{VII} & 0.12 & 0.182 (52) & 0.134 (12) \\  & 0.13 & 0.156 (20) & 0.115 (12) \\  & 0.15 & 0.197 (31) & 0.145 (3.3) \\  & 0.16 & 0.206 (29) & 0.152 (5.0) \\  & 0.13 & 0.171 (32) & 0.126 (3.1) \\  & 0.12 & 0.161 (34) & 0.119 (0.83) \\ \hline \multirow{4}{*}{VIII} & 0.11 & 0.122 (11) & 0.094 (15) \\  & 0.08 & 0.123 (54) & 0.096 (20) \\  & 0.10 & 0.174 (74) & 0.134 (34) \\  & 0.07 & 0.122 (74) & 0.094 (34) \\  & 0.09 & 0.133 (48) & 0.103 (14) \\  & 0.11 & 0.171 (55) & 0.132 (20) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of the obtained crack widths.  
Figure 17: The image acquisition system.  
According to the data in Table 1, the error comparison curves of the object distance method and the scale method were obtained (for several cracks with the same width but different errors, the error was averaged), as shown in Figure 18. The identification accuracy of the two methods increased with increasing crack width. When the crack width was less than 0.2 mm, the accuracy of the object distance method was obviously higher than that of the scale method. A detailed comparison of the errors is shown in Table 2. The comparison results showed that when the crack width was greater than 0.20 mm, the average error of both methods was within 8.2%. When the width was less than 0.20 mm, the error of the scale method was larger and the average value was more than 30%. When the crack width was between 0.10 and 0.20 mm, the average accuracy of the object distance method could reach 90%; when the crack width was 0.10 mm or less, the accuracy of the object distance method could not meet the measurement requirements. In summary, the object distance method had a high accuracy and could accurately identify cracks with a width above 0.10 mm.