## 3 Results and Discussion
### Measured Strain Development and Strain Rate-Based Cracking Monitoring
A more precise and reliable determination of the time of cracking can be achieved based on the measured result of strain rate, which will be described later. Figure 9: _Cont._  
Figure 10a shows the variations of measured temperature and strain at the center (Position A) of Plate I-a during the 7 days after placement. It is observed that the measured temperature fluctuated due to the variations of air temperature and solar radiation. At early ages, the base plate experienced different combinations of temperature gradient, shrinkage, and swelling. The strain measured at the top layer was always larger than those measured at the middle and bottom layer during each temperature cycle (Figure 10b). As far as the author is concerned, two reasons contribute to this result. One is that the surface concrete swelled under wet-curing conditions. The other is that the plate top was subjected to lower external restraints compared with the middle and bottom portions. Figure 10c shows the measured strain rate at different depths (top, middle, and bottom) during 7 days after placement. The strain rate clearly showed positive and negative variations over time, and the varied amplitude of the strain rate decreased with the distance to the plate top surface. Figure 11 shows the measured strain rate at different plate depths at Position A (plate center, Figure 11a) and Position B (center of long edge, Figure 11b) of Plate I-a. The first  
Figure 9: Comparison of measured strains at different positions in four test series of base plates with different curing regimes: (**a**) plate I-a, (**b**) plate II-a, (**c**) plate III-a, and (**d**) plate IV-a. peak in the strain rate curve indicates the monitoring position started cracking, and the follow-up peaks reflect the sharp change of the measured strain produced by the existence of cracks. As inferred from Figure 11a, a crack occurred at A-top (plate center) on the 28 th day after placement (8 days after exposed to drying). This crack did not extend into the full depth of the base plate because the strain rate peaks measured at A-middle and A-bottom did not increase significantly over time. From Figure 11b, it is deduced that a crack occurred at B-top on the 25 th day after placement. As demonstrated in Figure 11, it is easy to track crack occurrence with the curves of strain rate. By this method, the initial time of cracking at each monitoring position was identified from the field strain rate data, and the results are summarized in Figure 12 and Table 8. As shown, the earlier the base plate was exposed to drying, the earlier cracks appeared. The initial cracking time is closely related to the duration of sealed-curing, although it is unlikely that early drying stress alone caused cracking. It is a remarkable fact that the time of cracking detected at the plate edge was obviously earlier than that at the center, or perhaps no crack was detected at the plate center (Figure 12). For the two base plates of Series I (covered by geotextile for 20 days), the average time of cracking at plate edge (Position B) was 30 days, while the average time of cracking at plate center (Position A) was 35 days. For Series II (covered by geotextile for 45 days), the time of cracking at plate edge was 65 days for Plate II-a, and no crack was detected in Plate II-b. For Series III (covered by geotextile for 120 days), the time of cracking at plate edge was 56 days (before being exposed to drying) for Plate II-a, and it was 129 days for Plate II-b. The results between Plate II-a and Plate II-b were scattered, possibly because the presence of the strain gauge and cable caused earlier cracking in Plate II-a. For Series IV (covered by geotextile and quilt for 120 days), no crack was detected except for the edge (Position B) of Plate IV-a. Figure 10: _Cont._  
Figure 10: (**a**) Measured temperature and strain at the center of Plate I-a. (**b**) Measured strain distribution along the depth during day 3 to 5. (**c**) Measured strain rate at the plate center.