## 3 Results and Discussion
### Depth of Deterioration Analysis  
In order to estimate the propagation depth of deterioration from the damaged part of the surface of the slab to the lower part, the core specimens of the spalling were collected, and the ultrasonic velocity test was repeated while cutting the surface with doubtful deterioration at intervals of 1 cm. The ultrasonic velocity measurement proceeds until the velocity reaches 4100 m/s in the \"good\" state of soundness, and the total cutting thickness when reaching the reference ultrasonic velocity is assumed to be the propagation depth to which the deterioration has progressed. For in-depth analysis, the test results were grouped according to spalling severity to compare changes in ultrasonic velocity due to the removal of the areas where deterioration had occurred. The spalling severity classification was divided into spalls \(<\) 150 mm width, spalls with widths from 150 mm to 200 mm, and spalls with widths \(>\) 200 mm, referring to the Federal Highway Administration (FHWA) spalling severity criteria [17]. If the deterioration of concrete affects the ultrasonic velocity, the results of the tests performed while removing the deteriorated areas are expected to show a tendency to increase the ultrasonic velocity irradiated by the change in core thickness. Assuming that there is a correlation with the acceleration of deterioration to the lower part depending on the severity of the spalling, the higher the spalling severity, the greater the range of deterioration to the lower part of the slab in ultrasonic velocity analysis. A result of the analysis of the deterioration propagation depth of the slab by the ultrasonic test of the core specimens is shown in Figure 8. The first noticeable results showed that the ultrasonic velocity of the core specimens collected from the soundness of slab section exceeded 4100 m/s, while the ultrasonic velocities of the core specimens collected from the damaged section also were distributed in a low range of less than 4100 m/s. Comparing the results measured by dividing the severity of the spalling into damage widths, in all cases, the ultrasonic velocity tends to increase gradually, exceeding a certain cutting depth. The results verified the fact that the deterioration of the slab on the surface affects the ultrasonic velocity and the assumption of a test method that evaluates the total thickness of the cutting to reach the \"good\" standard ultrasonic velocity. As can be seen in Figure 7, the ultrasonic velocity of the core collected at the site under various conditions is larger than the results of the indoor experiment under limited conditions. This shows that since the ultrasonic wave speed is affected by environmental variables such as relative humidity and temperature, a difference in the ultrasonic wave speed may occur, even in a paved section in a good soundness state. In terms of conservative design and road management, this study developed a methodology for estimating the degradation depth of the pavement by testing the ultrasonic velocity for different locations and environmental conditions and assuming the minimum measurement results for maintaining sound conditions. This method should be understood and regarded as meaningful as a methodological result verifying the degradation depth estimation, which may be lower than the minimum ultrasonic velocity of the good soundness state pavement in this study and the mixing ratio used in the pavement. Figure 9 shows the test results for the spalls with width sections < 150 mm, which includes some measurements within the \"doubftul\" soundness range. When the cutting thickness exceeds 3 cm, the ultrasonic velocity increases slightly, and when the cutting thickness is 6 cm or more, the ultrasonic velocity exceeds the standard ultrasonic velocity of 4100 m/s. Figure 10 shows that most of the ultrasonic velocities observed in the spalls 150 mm to 200 mm in the width section are distributed within the doubtful soundness range.