## 2 Research Progress in In Situ Road Health Monitoring Technology
### Resistance-Type Strain Gauges  
Resistance strain gauges have the advantages of a light weight, a small size, high measurement accuracy, high fatigue resistance, and good stability. Therefore, a resistance strain gauge with corresponding elastic elements is widely used in the aerospace, electric power, and civil construction engineering fields [42]. The detection technique involves measuring the strain of the strain gauge on the measured structure by changing the strain on the measured structure of the strain gauge on the structure of the measured structure, as shown in Figure 5. When applied in civil structures, the strain is generally measured by sticking to the surface of the bridge or other cement concrete with a smooth surface. The quality of the gauge paste directly affects the accuracy and reliability of the test, resulting in a high degree of empirical control and the lack of reuse [43; 44]. In terms of asphalt pavements, due to the concealment of the monitoring position of the asphalt pavement structure layer, it is difficult to directly paste the strain patch. Under the harsh construction conditions and complex service environments of asphalt pavements, it is difficult for directly buried unpackaged strain patch elements to survive. On the other hand, the stiffness of a resistance strain gauge sensor is quite different from the stiffness of asphalt concrete, and the interface compatibility between the strain gauge and asphalt pavement material is poor, which affects the detection accuracy and reliability. Therefore, it is necessary to take the resistance strain gauge as the core element and to carry out research on the packaging and protection process to match the complex mechanical state of asphalt concrete. In road engineering, the ASG (asphal strain gauge) asphalt pavement resistance strain gauge developed by CTL (Hico, TX, USA) and the TML (Tokyo, Japan) asphalt strain gauge are relatively mature, with many engineering applications, as shown in Figure 6. According to the monitoring direction, the ASG asphalt strain gauge is divided into a horizontal asphalt strain gauge (I-shape) and a vertical asphalt strain gauge. The strain gauge involves sticking the resistance strain gauge on a threaded nylon rod member. Both ends of the nylon rod are connected to an aluminum rod to measure the axial strain. In the monitoring element area, polysulfur polymer and silicon butyl rubber materials are used to package and protect the strain sheet to adapt to the construction conditions and working environments of asphalt pavements. By collecting the output voltage of the sensor, the  
Figure 5: Typical structure of a resistance strain gauge. corresponding strain can be converted, as shown in Equation (2). The embedded asphalt strain gauge KM-100 HAS of TML (Tokyo, Japan) also adopts an I-shaped structure with an aluminum rod at each end, which is used to fix the intermediate axial shaft. In order to make coordinate deformation of the strain gauge and the asphalt mixture, the middle axial rod of the strain gauge is selected, and the asphalt mixture modulus is equivalent to the material, with high temperature, moisture, and other properties of the material. \[\frac{U_{SC}}{U_{0}}=\frac{[GF\times\varepsilon\times(1+\mu)]\times 10^{-3}}{2+[ GF\times\varepsilon\times(1+\mu)]\times 10^{-6}} \tag{2}\]  
where \(U_{sc}\) and \(U_{0}\) represent the excitation voltage and output voltage in V and mV, respectively, and \(GF\) is the sensitivity coefficient of the strain gauge. It is generally considered that the sensitivity coefficient of the resistive strain gauge is considered to be about 2.0; \(\varepsilon\) is the strain in \(\mu\varepsilon\), and \(\mu\) is the Poisson ratio of the nylon rod. Due to the relatively stable performance of the above asphalt strain gauges and their high detection accuracy, they have been widely used in the study of the structural mechanics of asphalt concrete pavements or airport runways at home and abroad [45; 46; 47]. In 1998, as a pioneer case in the field of road monitoring, [PERSON] et al.