# An Investigation of the Temperature-Drift Effect on Strain Measurement of Concrete Beams
## 3 Strain Monitoring Tests for Beams  
### Strain Monitoring Test for a Steel Beam  
As shown in Figure 6, the study first used a simply supported steel beam for the mechanical strain measurement test. The steel beam's cross-section dimensions were 16 mm wide and 2 mm thick; the distance between the two load points was 340 mm; and the distance between the two support points was 480 mm. Two FLA-10 mm strain gauges were attached to the top and bottom at the midpoint of the steel beam, respectively, and a thermometer was installed on the steel beam. During the test, the specimen was kept indoors for two days and the load was changed at different times. The test procedure was: add a load of 100 g, then add another load of 100 g, then remove a load of 100 g. The strain and temperature data were recorded once per minute. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **A** & **B** & **C = A + B** & **D** & \\ \hline Gauge model No. & \(\alpha\) & \(\beta\) & Expected \(\gamma\) & Experimental \(\gamma\) & Difference between \\ (10\({}^{-6}\)/\({}^{\circ}\)C) & (10\({}^{-6}\)/\({}^{\circ}\)C) & (10\({}^{-6}\)/\({}^{\circ}\)C) & (10\({}^{-6}\)/\({}^{\circ}\)C) & C and D \\ PFL-20 mm & 11 & 8.58 & 19.58 & 20.17 & 2.9\% \\ PFL-30 mm & 11 & 7.66 & 18.66 & 18.89 & 1.2\% \\ PL-90 mm & 11 & 7.81 & 18.81 & 19.65 & 4.5\% \\ \hline \end{tabular}
\end{table} 
Table 1: Test results for the temperature-resistance effect of the strain gauge attached to concrete. Figure 5: The temperature-resistance effect of the strain gauge (PFL-20 mm) attached to the Zerodur glass ceramic plate: (**a**) temperature changes; (**b**) strain change and temperature relationship. Figure 7a,b show the recorded apparent strain and temperature responses. The apparent strain response in Figure 7a clearly indicated significant changes in strain response at the times of loading (position indicated by solid arrow) as well as unloading (position indicated by dotted arrow). Under the loading condition, the bending moment caused negative strain (compression) and positive strain (tension) on the top and bottom strain gauges, respectively; the opposite was observed under the unloading condition. Theoretically, when a load is not augmented or reduced, no strain change should exist. However, Figure 7a showed that a nonlinear apparent strain response was present during the load application intervals, and the undulations were consistent with the temperature trend in Figure 7b. This phenomenon indicated that temperature changes affected the apparent strain recorded during the test. To obtain the mechanical strain (\(\varepsilon_{m}\)), the temperature strain (\(\varepsilon_{T}\)) induced by temperature changes (\(\Delta T\)) must be subtracted from the recorded apparent strain (\(\varepsilon\)), that is, \(\varepsilon_{m}=\varepsilon-\gamma\Delta T\). According to the previous test, the combined temperature effect coefficient (\(\gamma\)) of the FLA-10 mm strain gauge affixed to the steel beam was 5.45 \(\mu\varepsilon/^{\circ}\)C (refer to Figure 1), so the apparent strain (\(\varepsilon\)) in Figure 6a and the temperature changes (\(\Delta T\)) in Figure 6b can be combined to obtain the mechanical strain response of the steel beam, as shown in Figure 8a. After temperature correction, the mechanical strain in Figure 8a showed that, when the system had no load addition or removal, the strain response did not fluctuate with the temperature changes but resembled an almost flat straight line, and after removal of all load (after 30 h), the mechanical strain also returned to close to the 0 position. This shows that the temperature-induced drift was not significant in such a case. Figure 8: The steel beam test: (**a**) mechanical strain; (**b**) change in mechanical strain per unit time. Figure 6: Schematic of the steel beam test. Figure 7: The steel beam test: (**a**) strain gauge response; (**b**) temperature changes.