## 2 Research Progress in In Situ Road Health Monitoring Technology
### Fiber-Optic Grating Sensors
[26] used a fiber-optic sensor that responds to deformation, and then, this information is further used for the estimation of the vehicle wheel's speed, contact width, and length and the axle's weight while in motion. Recorded signals from a truck passing over a group of fiber-optic sensors with various speeds and known weights are used as input data. [PERSON] et al. [27] and [PERSON] et al. [28] combined strain measurements and finite element simulations. A dynamic modulus test was therefore used to first obtain the viscoelastic characteristics of asphalt mixtures. The similarity theory with embedded fiber Bragg grating (FBG) sensors was used to develop a small-scale laboratory loading test. Studies were then carried out for measured strain validations in conjunction with a finite element method model. They also employed a fiber grating strain gauge that was based on polypropylene (PP) and fiber reinforced plastic (FRP) materials, and they embedded the calibrated fiber grating strain gauge in the asphalt concrete trabecular with conducted strain testing in static and dynamic loading modes. However, the FRP package sensor was separated from the matrix, and the range of the tests needs to be expanded to further verify the effectiveness of the measured strain of the fiber grating sensor [29; 30]. At the same time, by using the detection data of optical fiber grating strain gauges combined with  
Figure 2: Embedded FBG sensor installed in prefabricated base (**a**) and asphalt layer groove (**b**) [25]. the Burgers viscoelastic model, the permanent deformation of asphalt pavements can be calculated [31; 32; 33]. [PERSON] [34] deployed fiber-optic sensors on the prestressed tendons of bridge box girders to monitor the mechanical properties of bridge structures during construction and service periods. Although the monitoring survival time is longer compared to surface-mounted strain gauges, further research is needed on the durability of fiber-optic sensors. [PERSON] et al. [35; 36; 37] buried Bridgman-Wigner Time-Domain Analysis (BOTDR) distributed fiber-optic sensing technology in asphalt pavement layers to monitor cracks in asphalt concrete pavements. The results of experimental simulations and on-site monitoring indicate that the application of BOTDR technology has good monitoring accuracy and stability in monitoring the temperature, residual deformation, and cracks of actual road surfaces. [PERSON] et al. [38] developed a flexible asphalt-encapsulated fiber-optic sensor for monitoring the distributed strain of asphalt pavements by curling the fiber-optic sensor into asphalt and fine aggregates and analyzed the strain transfer theory of the fiber-optic sensor, as shown in Figure 4. Through an experimental test of an asphalt concrete beam and a field performance test under urban temperature and traffic loads of asphalt pavements, the designed fiber sensor can effectively detect the strain distribution of asphalt pavements, but the effectiveness of the monitoring data needs to be further verified. Researchers have also used optical fiber Bragg grating sensors for monitoring asphalt concrete strain tests combined with the requirements of pavement construction, and the monitoring performance of the sensor is tested by weight static loading and dynamic rut loading experiments, respectively [39; 40]. The above research provides high-precision optical fiber grating sensing technology for pavement structure detection. However, FBG and BOTDA optical fiber sensors still have the following defects: (1) The maximum ultimate tensile strain is only about 1.5% for quartz optical fibers of ordinary communication, which results in an optic grating strain gauge with poor shear resistance that has the problems of low survival rates and poor durability, especially in the harsh construction and working environments of the asphalt  
Figure 4: Diagram of flexible asphalt-encapsulated fiber sensors and indoor tests [38]. Figure 3: Fiber grating strain gauge based on different material packages [29]. pavement structure layer.