## 2 Research Progress in In Situ Road Health Monitoring Technology
### Resistance-Type Strain Gauges
[48] buried pressure boxes, asphalt strain gauges, thermocouples, humidity probes, and vibrating wire strain gauges in a road of 3.2 km in Virginia, USA. They simulated the impact of different loads and driving speeds on the dynamic response of the road by passing through the sensor burial area with different numbers of passengers and speeds on trucks with known axle loads. In France, in the A41N highway [46], 47 strain sensors, 12 temperature sensors, and 2 fiber sensors were buried in the wearing layer, base course, and sub-base course. Trucks with known axle weights were driven at three different travel speeds and three different trajectories, and the signals were collected at 600 Hz to study the effect of two variables on the structural strain of the road, as shown in Figure 7. [PERSON] et al. 's [49] study of debonding slips at the top of the HMA surface and the bottom of the HMA overlay of a runway using ASG strain gauges at Newark International Airport. [PERSON] et al. [50] studied the mechanical properties of flexible asphalt pavements reinforced by geogrids using ASG strain gauges. [PERSON] et al. [51] buried of TML strain gauges (Tokyo, Japan) in the asphalt layer to monitor the transverse and longitudinal strains, respectively, under different loading conditions. The corresponding mechanical responses were analyzed with the classical multi-layer linear elastic pavement model and the viscoelastic pavement model, respectively, and a comparison concluded that the viscoelastic model had better predictions. Figure 6: ASG asphalt strain gauge produced by CTL from the USA and TML asphalt strain gauge from Japan. In China, [PERSON] et al. [53] utilized ASG asphalt strain gauges for monitoring the dynamic strain response of asphalt pavements and investigated the setting of data acquisition frequencies and signal processing methods through experiments. [PERSON], [PERSON] [54] measured the effects of temperature, vehicle speed, and axle load on the dynamic response of the bottom of the asphalt layer by comparing and contrasting actual measurements and proposed a prediction model for the strain response of the bottom of the asphalt layer based on the three parameters of temperature, vehicle speed, and axle load, respectively, by adopting the method of multivariate regression analysis. However, this work only considered several factors affecting the strain response of the asphalt layer, and there are many control factors in actual working conditions, so further research is needed to better understand and analyze the measured strain response of the asphalt layer. [PERSON] et al. [55] monitored a construction road by burying a vibrating wire strain gauge and compared the measured data with calculated values based on the elastic layer theory. The comparison results show that there was a certain deviation between the theoretically calculated values and the measured data. [PERSON] et al. [56] buried KM-100 HAS of TML (Tokyo, Japan) into asphalt mixture beamlets and verified the synergistic deformation of the sensor and asphalt concrete using two modes of step-by-step loading and dynamic loading. [PERSON] et al. [57] and [PERSON] et al. [58] also used ASG asphalt strain gauges to detect the strain of asphalt pavements to analyze the mechanical state of durable pavements. Commonly used asphalt strain gauge parameters are shown in Table 1. However, this kind of mature resistance strain gauge detection system is generally more expensive, which is difficult to promote and apply at a large scale in road monitoring and intelligent road construction.