# An Investigation of the Temperature-Drift Effect on Strain Measurement of Concrete Beams
## 1 Introduction  
In recent years, structural health monitoring (SHM) has been a popular topic. Structural health monitoring is the installation of various types of sensors (such as accelerometers, tiltmeters, displacement meters, strain gauges, and thermometers) in structures for monitoring structural responses to external stimuli [1; 2; 3; 4]. The measured structural response can be used to detect the structural damage or deterioration through the analysis of structural system characteristics, and, thus, the purpose of maintaining the structural safety of the structure, such as a bridge, can be achieved. The SHM system generally includes sensors, data acquisition systems, monitoring centers, and system identification technology [5; 6]. In SHM, a large number of sensors are installed and distributed across large areas of a structure. In the past, limited by the large size and high cost of commercially available analog-to-digital converters, the analog-to-digital conversion was often integrated into a multi-channel data logger for execution, causing the need to install multiple independent cables for connecting sensors with the data logger; this formed the traditional and commonly seen centralized data acquisition system. The centralized system has many shortcomings, including the need for a large number of cables, signal attenuation and interference, and difficulty in expansion and adjustment of the system [7; 8]. In contrast, a distributed system completes the digitization work at the sensor's end so as to enable the development of a wireless sensor system. Wireless sensors, owing to their easy installation, low maintenance cost, and flexibledeployment, are highly suitable for use in the SHM of bridges. Thus, monitoring systems that are based on wireless sensor networks (WSNs) have become mainstream [9; 10]. Strain is the most sensitive physical quantity of the mechanical behavior of concrete beams. External environmental changes (external force, support conditions, and temperature changes) or beam deterioration and damage (cracks, fire, and prestress loss) tend to be clearly reflected on the strain of the concrete beam; therefore, strain measurement should be the first choice for SHM. In the use of electric resistance strain (ERS) gauges on bridge structures, the measured apparent strain includes strain induced by temperature changes and mechanical strain. Theoretically, if the temperature-change-induced strain can be eliminated from the apparent strain, then the mechanical strain required for structural analysis can be obtained. However, because the ERS gauge is affected by long-term temperature changes, a drift phenomenon is present in the measured strain, causing the signal analysis to become more complex. The temperature-induced drift phenomenon is attributed to a nonlinear relationship between the ambient temperature and the measured strain as illustrated in Figure 1 even in the absence of external force. In addition to ERS gauges, vibration wire (VW) and fiber Bragg grating (FBG) sensors also suffer from strain drift caused by temperature changes [11; 12; 13; 14; 15; 16; 17; 18]. A technique based on the use of a dummy gauge can eliminate the thermal effect provided that the dummy gauge is attached on an additional material component that undergoes the same thermal variations as the active gauge but is free from mechanical strain. However, this method is practically difficult for a heterogeneous material, such as concrete. To deal with a temperature effect on strain measurement, statistical analysis, especially linear regression, has been widely adopted to find the relationship between the ambient temperature and the measured strain [14; 15; 17; 18]. The statistical approaches cannot remove the temperature-induced strain drift, so only qualitative analysis or trend interpretation of the experimental results can be performed.