## 2 Research Progress in In Situ Road Health Monitoring Technology
### Piezolectric Sensing Elements  
In recent years, the development of sensitive polymer composite materials has provided a new method for solving the problems of poor deformation coordination and insufficient working performance of strain sensors. Sensitive polymer composite materials are mainly composed of a polymer compound matrix and conductive material. They are  
Figure 9: Change of resistivity of conductive asphalt concrete with volume fraction of conductive filler and CT scan diagram (**a**) all the fibers (**b**) and only the connected fiber [70].  
a new type of functional material with self-perception of external factors, such as force, deformation, temperature, and humidity [67; 76; 77]. Due to their good adaptability to complex environments, material compatibility, piezoresistance performance, strain sensitivity, durability, and good repeatability, they have gained significant research attention in recent years and have shown good engineering application prospects.  
In addition, piezoelectric film sensors, piezoelectric quartz, and piezoelectric ceramics can also be used in situ to monitor civil engineering structures. A piezoelectric film sensor is a sensor based on polyvinylidene fluoride (PVDF) film, in which the core is the VDF-TrFE piezoelectric polymer located in the middle layer, composed of a repetitive monomer (-CH\({}_{2}\)-CF\({}_{2}\)-). By polarizing the piezoelectric material in a strong electric field, the noncrystalline polymer takes on a semi-crystalline form and has linear piezoelectric properties for stress or strain. [PERSON] [78] studied the dynamic strain sensing characteristics of PVDF piezoelectric films by encapsulating them with polytetrafluoroethylene and validated a device for health monitoring on offshore platforms. [PERSON] et al. [79] applied PVDF piezoelectric thin films onto copper sheets and designed a testing device to verify the strain sensing characteristics of thin piezoelectric films. [PERSON] et al. [80] studied the sensitivity and linearity of thin PVDF piezoelectric films by pasting thin piezoelectric films on civil engineering structures, where the high sensitivity and resolution in strain monitoring were verified. [PERSON] et al. [81] effectively monitored the initiation and propagation process of cracks using thin PVDF piezoelectric films. [PERSON] et al. [82] and [PERSON] et al. [83] used lead zirconate titanate (PZT) powder and PVDF to prepare sheet-like composite materials. They also used a 3D-printed cylindrical structure to encapsulate sheet-like composite materials and prepared piezoelectric sensing devices, which were embedded in asphalt mixtures. The effectiveness of the sensing signal output was tested by simulating vehicle loads. However, the application of PVDF and piezoelectric films in asphalt pavements is still in the exploratory stage at present. [PERSON] et al. [84] prepared \"smart aggregates\" by encapsulating PZT piezoelectric ceramic sheets after waterproofing with silicone gel for consistent monitoring of concrete structures. The authors of this paper have also carried out related explorations and research on the optimization, encapsulation, signal analysis, and monitoring system of PVDF piezoelectric films in recent years.