## 3 Results and Discussion
### Measured Strain Development and Strain Rate-Based Cracking Monitoring  
Figure 9 shows the comparison of concrete strains measured at positions A-top (plate center), B-top (center of long edge), C-top (center of short edge), and D-top (corner) in the four series of field-instrumented concrete base plates cured with different regimes. Positive and negative strain values represent expansion and shrinkage, respectively. The initial expansion strain recorded during the first 14-day wet-curing period was attributed to swelling of surface concrete. It is observed from Figure 9a that after the 20 day long curing period, the measured concrete strains at positions C-top (center of short edge) and D-top (corner) of Plate I-a showed obvious decreasing trends over time because of concrete drying shrinkage and relatively weak restraints of the boundary conditions at these positions. In contrast, with respect to C-top (center of short edge) and D-top (corner), A-top (plate center) and B-top (center of long edge) exhibited much lower variations of strain with respect to the daily temperature cycles due to large structural constraints at the mid-length region. Consequently, large tensile stress can be induced in the mid-plate cross section of base plate. Generally, a suddenly increase in measured concrete strain can be used as a criterion for identifying crack occurrence, i.e., crack opening. However, this happens only if the crack is extended right across the strain gauge. On the other hand, an unusual decrease in measured strain, whether suddenly or gradually, may also indicate the occurrence of a crack near the measuring position. For the latter case, the explanation could be that the cracking phenomenon at the crack-formed position affects the surrounding area; stress is relieved at the crack, and the self-restraint becomes weak thereafter. Thus, theoretically, the time of cracking at each monitoring position can be deduced from the in-site strain measurement. For example, a sharp increase in measured strain could be observed at A-top (center) of Plate III-a (Figure 9c) and B-top (center of long edge) of Plate IV-a (Figure 9d) only several days after the 120 days long curing, which means that cracks appeared and passed through the strain gauges. There was another case where a progressive decrease in measured concrete strain was observed at Positions A-top and B-top of Plate II-a (Figure 9b), which indicated that cracks appeared near the strain gauge. From Figure 9a, it is observed that the measured strain at A-top and B-top of Plate I-a showed very large fluctuations with the daily temperature cycles after the 20 days long curing stage. The upward tendency of the measured strain was the result of continuous switching between crack opening and partial closure of the crack opening. In such cases, it is not easy to directly determine the exact time of cracking through the measured strain \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Test Series**} & \multirow{2}{*}{**Position**} & \multicolumn{5}{c}{**Level of Temperature Gradient (°C/m)**} \\ \cline{3-8}  & & \(-\)**45 to \(-\)20** & \(-\)**20 to \(-\)10** & \(-\)**10 to 0** & **0 to 10** & **10 to 20** & **20 to 45** \\ \hline \multirow{2}{*}{I} & A (center) & 3.9\% & 25.8\% & 34.9\% & 17.2\% & 8.5\% & 9.8\% \\  & B (edge) & 1.3\% & 20.4\% & 41.1\% & 17.3\% & 9.2\% & 10.8\% \\ \multirow{2}{*}{II} & A (center) & 3.4\% & 22.3\% & 53.4\% & 18.4\% & 2.5\% & 0.0\% \\  & B (edge) & 0.0\% & 2.6\% & 60.3\% & 26.1\% & 8.1\% & 3.3\% \\ \multirow{2}{*}{III} & A (center) & 0.3\% & 4.5\% & 55.3\% & 25.5\% & 14.5\% & 0.0\% \\  & B (edge) & 0.0\% & 0.3\% & 62.8\% & 35.9\% & 0.8\% & 0.1\% \\ \multirow{2}{*}{IV} & A (center) & 0.5\% & 13.0\% & 66.5\% & 18.7\% & 1.1\% & 0.3\% \\  & B (edge) & 0.0\% & 0.5\% & 72.6\% & 26.5\% & 0.5\% & 0.0\% \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Percentage of different levels of temperature gradient measured from filed base plates. curve.