## 3 Results and Discussion
### Measured Temperature Profile and Temperature Gradient Frequency Distribution
By using the acquired temperature data from the three VWSGs in plate center/edge regions, the vertical temperature gradient can be calculated by using the following formula:  
\[T_{g}=\big{(}T_{top}-T_{bottom}\big{)}/\big{(}H_{top}-H_{bottom}\big{)} \tag{13}\]  
where \(T_{g}\) is the vertical temperature gradient (\({}^{\circ}\)C/m), -\(T_{top}\) and \(T_{bottom}\) are temperatures measured by two sensors at the top and bottom layers (\({}^{\circ}\)C), and \(H_{top}\) = 0.175 m and \(H_{bottom}\) = 0.025 m are the heights of the sensors. A frequency account analysis was carried out by using the hourly temperature gradient data from placement to the age of 150 days calculated by Equation (13). Figure 8 presents the comparison of temperature gradient frequency distributions in base plates under different curing conditions (Figure 8a for plate center and Figure 8b for plate edge). In general, the temperature gradient at the plate center was larger than that at the plate edge. This is attributed to only one direction of heat exchange with the air at the plate center, while two directions were attributed at the edge. Thus, the edge of the base plate had a narrower temperature gradient distribution compared to the center. As observed from Figure 8a, the temperature gradient at the plate center ranged from \(-\)30 to 45 \({}^{\circ}\)C/m, indicating that the absolute values of night time temperature gradients were not as extreme as the daytime gradients. For Series I, II, III, and IV, the most frequently occurring temperature gradient was around \(-\)9.0, \(-\)7.5, \(-\)3.5, and \(-\)3.0 \({}^{\circ}\)C/m, respectively. Negative temperature gradients were indicated to be more frequent in the tested days (in winter and spring). The absolute value of the most frequently temperature gradient is in a descending order from Series I, II, III to IV. Figure 7: Measured temperature in Plate I-a during a 24 h cycle (from 12 December 2020 to 13 December 2020, 3 days after concrete placement). The percentages of different levels of temperature gradient are shown in Table 7. The most frequent temperature gradient level was from \(-\)10 to 0 \({}^{\circ}\)C/m at 34.9% for Series I; 53.4% for Series II; 55.3% for Series III; and 66.5% for Series IV. The second frequently occurring temperature gradient level was from 0 to 10 \({}^{\circ}\)C/m, with the percentages of 17.2%, 18.4%, 25.5%, and 18.7% for Series I, II, III, and IV, respectively. For Series III and IV, more than 80% of the gradient was between \(-\)10 and 10 \({}^{\circ}\)C/m during the tested days. As for Series I and II, only 52.0% and 71.8% periods in the gradient level from \(-\)10 to 10 \({}^{\circ}\)C/m  
Figure 8: Comparison of temperature gradient frequency distributions in base plates under different curing conditions: (**a**) Position A (plate center); (**b**) Position B (plate edge). were observed due to the earlier exposure of these base plates to the environment. These results revealed the evident influence of curing conditions on temperature gradients. From Figure 8 and Table 7, we can remarkably observe that the maximum temperature gradient, either at the plate center or at the edge, was in an increasing order from Series IV, III, and II to I. The maximal positive gradient and the minimal negative gradient of Series I were clearly higher than those of the other test series during the same test seasons. Therefore, the use of geotextile or quilts as a thermal-protective cover can lower the temperature gradient of the base plate and is expected to decrease related stresses.