# An Investigation of the Temperature-Drift Effect on Strain Measurement of Concrete Beams
## 4 Residual Strain Monitoring Test for Nonelastic Response  
The previous section proves that the proposed data-processing method could effectively eliminate temperature effects and obtain the accurate mechanical strain. This section examined the feasibility of using this data-processing method to monitor the residual strain of concrete beams when the beams had the nonelastic response caused by a considerable load. If feasible, this technique could be used to assess the presence of residual strain after a bridge structure had been subjected to seismic forces, and could be a technique for easy and effective assessment of damage to bridge structures. The dimensions of the beam and the load configuration used for the test were the same as in Figure 9 in Section 3.2. A steel plate with a thickness of 1 mm was glued to the top of the concrete beam to avoid brittle failure of the beam. The test period was 44 hours, and three time points were selected for short-term load application and removal. To enable the test specimen to generate nonelastic responses, a relatively large load (approximately 110 kgf) was given; residual strain occurred after load removal. Figure 13 shows the time-history response of the apparent strain at the beam bottom obtained by the test; a relatively large strain response (marked by the red dotted-line rectangle) generated by the three short-term load applications can be observed in the figure. Generally, in static monitoring of bridges, data are mostly retrieved at 10-minute or longer intervals. Therefore, a short-term seismic force response could not be recorded in a static apparent strain response. To simulate this type of scenario, the study eliminated the short-term large apparent strain response marked by the three red dotted-line rectangles in Figure 13.  
Figure 14a shows the apparent strain time-history curve obtained after deleting the strain data caused by the short-term loads, and Figure 14b shows the temperature change curve recorded during the test. Figure 13c shows the mechanical strain (\(\varepsilon_{m}\)) obtained by eliminating the temperature strain (\(\varepsilon_{T}\)) induced by the temperature change (\(\Delta T\)) in Figure 14b from the apparent strain (\(\varepsilon\)) of Figure 14a. The mechanical strain in Figure 14c was significantly affected by temperature drift, and was practically impossible to analyze. Using the same processing method as used in the previous section, the adjacent  
Figure 13: The apparent strain response of the nonelastic response test for the concrete beam. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Load Changes** & \begin{tabular}{c} **Add Load** \\ **5 kgf** \\ \end{tabular} & \begin{tabular}{c} **Add Load** \\ **5 kgf** \\ \end{tabular} & \begin{tabular}{c} **Remove** \\ **Load 5 kgf** \\ \end{tabular} & \
\begin{tabular}{c} **Remove Load** \\ **Load 5 kgf** \\ \end{tabular} \\ \hline \multirow{4}{*}{
\begin{tabular}{} \end{tabular} } & Strain obtained in short-term load & \multicolumn{4}{c}{13.90} \\  & \multicolumn{4}{c}{addition test (\(\mu\varepsilon\))} \\  & Mechanical strain with temperature & \multicolumn{4}{c}{\(-\)12.72} \\  & \multicolumn{4}{c}{effect elimination (\(\mu\varepsilon\))} \\  & \multicolumn{4}{c}{Difference (\(\mu\varepsilon\))} \\  & Strain obtained in short-term load & \multicolumn{4}{c}{\(-\)13.70} \\  & \multicolumn{4}{c}{addition test (\(\mu\varepsilon\))} \\  & Mechanical strain with temperature & \multicolumn{4}{c}{\(-\)13.90} \\  & \multicolumn{4}{c}{effect elimination (\(\mu\varepsilon\))} \\  & \multicolumn{4}{c}{Difference (\(\mu\varepsilon\))} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Test error for mechanical strain with temperature effects eliminated. data subtraction method was applied to the mechanical strain data in Figure 14c to obtain the change in mechanical strain per unit time, as illustrated in Figure 14d. Figure 14d clearly indicates that residual strains of 3.54, 5.78, and 4.91 \(\upmu\epsilon\) occurred at the three short-term load application time points.