## 6 Results and Discussion
### Evaluation on Asphalt Pavement Rutting Prediction Model
For STR5 with the smallest error, the MSE is 3.67 mm\({}^{2}\); the RMSE is 1.92 mm; the MAE is 1.69 mm; and the MAPE is 32.1%. The \(R^{2}\) of STR4, STR5, STR18, and STR19 structural rutting models, namely, \(-8.2491\), \(-1.2133\), \(-0.0792\), and \(-1.0787\), respectively, are all negative. According to the meaning of the determination coefficient \(R^{2}\), when \(R^{2}\) is negative, the effect of the simulated prediction is poor and even cannot reach the prediction accuracy of the average value curve of the measured value. Therefore, for Structures II and VII, the accuracy of the rutting model in China's Specifications is relatively poor. This model is not suitable for the rutting prediction. The reason for this phenomenon may be that the characteristics of the two structures are not fully considered when Equation (3) is established, thereby resulting in the poor prediction effect of the model. Figure 2 shows that Structure II belongs to the rigid base asphalt pavement. The lean concrete or cement concrete is adopted for the base, and the modulus and strength of the base are relatively high in value. Structure VII belongs to the full-depth asphalt pavement structure with the thickness of asphalt materials reaching 48-52 cm. According to [3], the rutting model in China's Specifications is established on the basis of the rutting data of 10 highways and 5 test sections. Semi-rigid materials are used as the base or subbase of more than 95% of asphalt pavement in China, and the thickness of the asphalt concrete structural layer generally does not exceed 30 cm. Therefore, to make the rutting model in the design specification more widely applicable, semi-rigid materials are used as the base in the construction of 10 highways and 5 test sections used in the model, and the thickness of the asphalt concrete is in the range of 16-28 cm. When establishing Equation (3), Structure II, which has lean concrete or cement concrete as the base, and Structure VII, which has 48-52 cm-thick asphalt material layer thickness, are not considered, which is also the main reason for the poor prediction effect of Equation (3) on the two kinds of structures. (3) For Structure III and STR10 in Structure IV, the predicted value is significantly larger than the measured value. The predicted curve is below the measured value. After the initial loading stage (\(N_{e}\) is about 5 million), the predicted curve is basically parallel to the measured curve, and the difference between the predicted value and the measured value is basically constant with the increase in cumulative ESALs. For STR7 with the smallest error, the MSE is 3.52 mm\({}^{2}\); the RMSE is 1.88 mm; the MAE is 1.64 mm; and the MAPE is 25.2%. The \(R^{2}\) of STR7, STR8, and STR10 structural rutting models are 0.3337, 0.2222, and 0.0469, respectively. \(R^{2}\) is in the range of 0-0.4. The prediction effect is not ideal. (4) For the remaining structures, namely, STR12 in Structures IV, V, and VI, an intersection between the model prediction curve and the measured curve of the rutting model exists with the increase in ESALs. Therefore, it can be obviously divided into two stages. The first stage is the first section of the whole rutting evolution curve. When the ESALs reach a certain critical value (\(N_{e}\)\({}^{\prime}\)) from 0 at the beginning, the predicted value of the rutting model is obviously less than the measured value. However, with the increase in ESALs, the error of the model prediction gradually becomes small. When it reaches the critical value (\(N_{e}\)\({}^{\prime}\)) of ESALs, the predicted value is almost the same as the measured value. The second stage is the latter section of the whole rutting evolution curve. When the number of ESALs is greater than the critical value (\(N_{e}\)\({}^{\prime}\)), the predicted value of the rutting model is higher than the measured value. With the increase in ESALs, the model prediction error gradually rises. For different structures, the critical values (\(N_{e}\)\({}^{\prime}\)) of ESALs vary greatly.