## 2 Research Progress in In Situ Road Health Monitoring Technology
### Conductive Concrete Composite Materials  
Conductive concrete is mixed with conductive fillers to prepare conductive concrete so that for the conductive concrete with varying external loads (pressure and strain), temperatures, and other changes, its output electrical signal also changes accordingly, that is, it is a self-sensing material. The application of self-sensing materials in mechanical detection in the engineering field is mainly divided into two categories: composite materials with commonly used civil materials, such as matrix and polymer compounds, as the matrix [60; 61]. The commonly used composite materials in civil engineering include cement-based concrete composite materials and asphalt-based concrete composite materials, as shown in Figure 8. The conductive materials of these two types of conductive composite materials are usually graphite, CB, CNTs, carbon fiber, or metal powder. With respect to cement bases and asphalt concrete composite materials, research on their detection technology mainly focuses on the mix ratio of material designs; composite piezoelectric resistance tests (including stress, strain-resistance, temperature-resistance correlation, etc. ); and their conductive mechanism analysis and the establishment of a related model. At the same time, these materials have been employed in research on structural engineering, road engineering, and engineering applications [62; 63]. Cement-based conductive concrete composite materials have good piezoresistance performance and can be used for strain detection in cement concrete engineering [65; 66]. [PERSON] et al. [67] prepared CB nanoparticle cement conductive concrete with different specific surface areas, and the superior conductivity of the CB nanoparticles with specific structures was verified by conducted piezoresistive and mechanical performance tests. [PERSON] et al. [64] successfully prepared materials with piezoresistive effects by adding specific conductive materials to traditional cement mortar and conducted targeted research on the use of cement-based conductive concrete materials for stress-strain sensing and in  
Figure 8: Signal monitoring based on cement-based conductive concrete [64]. situ monitoring. [PERSON] et al. [68] used water-reducing agents, dispersants, and defoamers to improve the dispersibility of carbon fibers and prepared carbon fiber cement-based conductive concrete. The effects of the carbon fiber length and dosage on conductivity and changes in compressive resistance were investigated. However, the compatibility and deformation coordination of cement-based conductive concrete composite materials and asphalt materials were poor. It was difficult for the deformation of the two types of materials to be consistent for strain detection, so they cannot be applied for mechanical state detection of asphalt pavements. Asphalt-based conductive concrete composite materials and asphalt materials have good deformation consistency [69]. [PERSON] et al. [70] analyzed the changes in conductivity of carbon fiber conductive asphalt mixtures with the addition of conductive fillers and combined them with nano CT scanning images to analyze the situation of carbon fibers participating in the connection of conductive asphalt mixtures, as shown in Figure 9. [PERSON] et al. [71; 72; 73] prepared conductive asphalt concrete with good diagnostic abilities for strain caused by external stress by adding graphite to asphalt concrete. They verified pressure-sensitive effects and phased electrical signal changes for monitoring internal damage of asphalt concrete. [PERSON] et al. [74] investigated the water stability, crack resistance, strength performance, and force electric sensitivity characteristics of composite conductive asphalt concrete with graphite and short-cut carbon fiber. It was found that the addition of graphite reduces the road performance of asphalt concrete, while carbon fiber can improve the road performance of asphalt concrete.