## 5 Conclusions and Challenges  
In this paper, the existing research on in situ sensing technology for the internal conditions of layered road structures in the past 20 years was extensively reviewed and compared. The monitoring approach of the internal conditions of the road structures mainly involves buried sensors, which can obtain the accurate mechanical state of a road structure. These sensors mainly include fiber-optic grating sensors, resistive strain gauges, electrically conductive concrete composites, piezoelectric sensing elements, etc. This paper discussed in detail the principles, advantages, and disadvantages of each monitoring sensor and its applicability in road monitoring. Sensing technology based on conductive polymer self-sensing materials has broad application prospects. Meanwhile, the details of conductive polymer compositions, preparation processes, and sensing performance factors were discussed. Lastly, the application of epoxy-based conductive sensitive materials for in situ road-monitoring techniques was emphasized. Current sensors used for internal condition monitoring of road structures include fiber-optic grating sensors, resistive strain gauges, sensors based on conductive concrete composites, piezoelectric sensors, and wireless sensing elements. These sensors can acquire mechanical and environmental parameters, such as strain, stress, temperature, humidity, etc., of layered road structures by burying them inside the road. The monitoring accuracy of fiber grating sensors is very good, and the monitoring accuracy can reach \(\pm 1\)\(\mu\)\(\epsilon\), but it is not suitable for large deformation monitoring. The available research focuses on encapsulation for fiber gratings in order to adapt to the complex service environment of roads. Resistive strain gauges have better monitoring accuracy, but water and humidity have a greater effect on resistive sensors. Sensors based on conductive concrete composites are mainly used in monitoring the condition of structural concrete and are not suitable for road monitoring. Piezoelectric sensors can obtain the road surface stress state and vehicle axle weight and other information, while wireless sensing elements are mainly made of the encapsulation of microelectronic devices, which can be used as wireless road monitoring of the \"smart rock\". However, most of the sensors currently used in road engineering have been transplanted directly from structural engineering and other fields. Current sensors are less compatible with flexible pavement structures, resulting in distorted sensor test data. Sensitive polymer composite materials have good adaptability to complex engineering environments, material compatibility, strain sensitivity, use of durability, and other advantages. Considering the harshness of pavement monitoring environments, conductive sensitive polymer composites provide a reliable approach for the development of pavement monitoring sensors, and sensitive polymer composite-based sensors show good durability and weather resistance. For conductive polymer sensing materials for road monitoring, it is recommended to use epoxy resin, nylon, and other polymer matrices with similar modulus properties to the road material. Nanoconductive fillers, such as CNTs, graphene, etc., are conducive to improving the response accuracy of composite materials. Meanwhile, the preparation process has a huge impact on the strain resistance effect of conductive composite materials, and it is recommended to use a combination of mechanical mixing and ultrasonic dispersion to prepare polymer conductive composite materials. Sensors based on epoxy conductive composites and in situ monitoring technology for roads show great application prospects. However, the current research on composite materials and sensors in the field of road engineering is relatively limited and mainly has the following urgent problems.