# An Investigation of the Temperature-Drift Effect on Strain Measurement of Concrete Beams
## 3 Strain Monitoring Tests for Beams
### Strain Monitoring Test for a Concrete Beam  
This subsection used a small concrete beam as the specimen for the strain monitoring test. The beam section measured 57 mm wide and 30 mm thick. The support length was 450 mm, and a welded wire mesh was placed at a distance of 5 mm from the bottom. Figure 9 is the schematic of the concrete beam test. During the test, a PFL-20 mm strain gauge was attached to the top and bottom of the midpoint of the beam, and two thermometers were positioned at the top and bottom of the beam midpoint, respectively. The test specimen was kept indoors for approximately 6 days. During the test, a load was applied or removed at different time points; the procedure was: add load 5 kgf, add load 5 kgf, remove load 5 kgf, and remove load 5 kgf. The strain and temperature data were recorded once per minute. Figure 10a,b illustrate the recorded apparent strain and temperature responses, respectively. In comparison with the steel beam test results of Figure 7a, the momentary mechanical strain response generated by load application or removal was difficult to identify from the apparent strain response recorded by the concrete beam test in Figure 10a; the entire apparent strain response was almost consistent with the temperature variation trend in Figure 10b. This phenomenon indicated that temperature changes dominated the entire apparent strain response and masked the mechanical strain response; therefore, the mechanical strain generated by load application or removal could not be determined. Figure 10b indicates that the temperature curves of the top and bottom of the beam are almost overlapping and  
Figure 9: Schematic of the concrete beam test. the largest difference was only 0.1 \({}^{\circ}\)C. Thus, this small concrete beam could be considered to have a uniform temperature distribution. Because it was not easy to observe the mechanical strain generated by load application or removal from the apparent strain data in Figure 10a, it was necessary to consider the effects of temperature changes. To obtain the mechanical strain (\(\varepsilon_{m}\)), the temperature strain (\(\varepsilon_{T}\)) induced by temperature changes (\(\Delta T\)) must be eliminated from the recorded apparent strain (\(\varepsilon\)), that is, \(\varepsilon_{m}=\varepsilon-\gamma\Delta T\). According to previous testing, the combined temperature effect coefficient (\(\gamma\)) of the PFL-20 mm strain gauge attached to the concrete was 20.17 \(\mu\)e/\({}^{\circ}\)C (refer to Figure 3). Hence, the apparent strain in Figure 10a and the temperature variation in Figure 10b could be combined to obtain the mechanical strain time-history response of the concrete beam, as shown in Figure 11a. After temperature correction, an abrupt mechanical strain change generated by load application or removal could already be slightly discerned from the mechanical strain time-history response in Figure 11a, as indicated by the arrow in the diagram. However, because the temperature drift was considerably significant, the mechanical strain response did not return to the 0 position even after the load had been completely removed. This phenomenon differed greatly from the steel beam test results in Figure 8a. Figure 11b is the mechanical strain change per unit time obtained by performing the adjacent data subtraction of Figure 11a. The adjacent data subtraction means that the current data were subtracted by their preceding data. In Figure 11b, only the four time points of load application and removal showed significant mechanical strain differences; the mechanical strain differences at all other time points were mostly within \(\pm\)3 \(\mu\)e. It is notable that, in Figure 11b, an unexpected noise occurred in the mechanical strain difference because of a minor temperature fluctuation near the 40 h time point of the x-axis (as shown in Figure 12). This type of noise can be ignored by observing the temperature curve; it would not affect the identification of the mechanical strain generated by load application or removal.