*3.8. Continuous Monitoring by Using Geophones and ASGs (French motorway)*

During the reconstruction of the slow lane of a French motorway, instrumentation was set up for continuous monitoring based on the coupling of ASGs with geophones and temperature probes [46]. The pavement structure and the sensors layout are shown in Figure 16. The PEGASE Platform was used as the data acquisition system. *Infrastructures* **2019**, *4*, x FOR PEER REVIEW 16 of 20

**Figure 16** French motorway. (**left**) Pavement structure and (**right**) sensors layout. (figures are reproduced from Duong et al. [46]) **Figure 16.** French motorway. (**left**) Pavement structure and (**right**) sensors layout. (figures are reproduced from Duong et al. [46]).

Due to the impossibility to close the lane and perform measurements with a known truck, the acquisition was carried out under real traffic. Measurements show high sensitivity of strains to vehicle type and wandering. Thus, the authors developed a methodology to select a signal associated to heavy traffic passing right above the strain sensors by analyzing the signal recorded by the geophones placed at different lateral positions (Figure 17). This showed the feasibility of pavement monitoring under real traffic.

Pavement calculations were performed by using the software ALIZE and Viscoroute© [47]. In the latter, the pavement was described as a multilayer structure. In addition, Viscoroute© allowed to account for the viscoelastic behavior of the asphalt materials through the Huet-Sayegh model [48,49]. Results highlighted the importance to take into account the viscoelastic properties of bituminous materials and to consider the interface contribution for a better prediction of the pavement response under loading at high temperatures.

Pavement calculations were performed by using the software ALIZE and Viscoroute© [47]. In the latter, the pavement was described as a multilayer structure. In addition, Viscoroute© allowed to account for the viscoelastic behavior of the asphalt materials through the Huet-Sayegh model [48,49]. Results highlighted the importance to take into account the viscoelastic properties of bituminous materials and to consider the interface contribution for a better prediction of the pavement response

Pavement management systems provide decision-makers with adapted and cost-effective maintenance strategies according to the needs of the road network. Thus, the quality of data collected via the adopted monitoring technology is decisive in defining the right actions to take and,

Road monitoring via embedded sensing technologies is considered a powerful tool for pavement auscultation. In fact, sensors embedded in the pavement allow remote and continuous control with a considerable saving of time and no traffic disruptions. At the same time, adapted data acquisition systems and well-defined strategies for the post-processing of data are required in order to enhance

Various sensing technologies for road instrumentation are available on the market. The most known and deployed are described in Section 2. They allow the assessment of pavement response under moving loads and due to climate changes in terms of stress, strain, deflection, temperature, and water content. Sensors are designed to be asphalt-compatible and provide very accurate

reproduced from Duong et al. [46]).

under loading at high temperatures.

consequently, for appropriate money investments [50].

**4. Discussion and Future Trends** 

an instrumentation project.

**Figure 16** French motorway. (**left**) Pavement structure and (**right**) sensors layout. (figures are

**Figure 17.** Lateral position of geophones G1, G3, and G4 installed on a French motorway (figures are reproduced from Duong et al. [46]). **Figure 17.** Lateral position of geophones G1, G3, and G4 installed on a French motorway (figures are reproduced from Duong et al. [46]).

### **4. Discussion and Future Trends**

Pavement calculations were performed by using the software ALIZE and Viscoroute© [47]. In the latter, the pavement was described as a multilayer structure. In addition, Viscoroute© allowed to account for the viscoelastic behavior of the asphalt materials through the Huet-Sayegh model [48,49]. Results highlighted the importance to take into account the viscoelastic properties of bituminous materials and to consider the interface contribution for a better prediction of the pavement response Pavement management systems provide decision-makers with adapted and cost-effective maintenance strategies according to the needs of the road network. Thus, the quality of data collected via the adopted monitoring technology is decisive in defining the right actions to take and, consequently, for appropriate money investments [50].

under loading at high temperatures. **4. Discussion and Future Trends**  Pavement management systems provide decision-makers with adapted and cost-effective Road monitoring via embedded sensing technologies is considered a powerful tool for pavement auscultation. In fact, sensors embedded in the pavement allow remote and continuous control with a considerable saving of time and no traffic disruptions. At the same time, adapted data acquisition systems and well-defined strategies for the post-processing of data are required in order to enhance an instrumentation project.

maintenance strategies according to the needs of the road network. Thus, the quality of data collected via the adopted monitoring technology is decisive in defining the right actions to take and, consequently, for appropriate money investments [50]. Road monitoring via embedded sensing technologies is considered a powerful tool for pavement auscultation. In fact, sensors embedded in the pavement allow remote and continuous control with a considerable saving of time and no traffic disruptions. At the same time, adapted data acquisition systems and well-defined strategies for the post-processing of data are required in order to enhance an instrumentation project. Various sensing technologies for road instrumentation are available on the market. The most known and deployed are described in Section 2. They allow the assessment of pavement response under moving loads and due to climate changes in terms of stress, strain, deflection, temperature, and water content. Sensors are designed to be asphalt-compatible and provide very accurate Various sensing technologies for road instrumentation are available on the market. The most known and deployed are described in Section 2. They allow the assessment of pavement response under moving loads and due to climate changes in terms of stress, strain, deflection, temperature, and water content. Sensors are designed to be asphalt-compatible and provide very accurate measurements. However, feedbacks from real applications have shown some drawbacks, and the need pursue further research in this field. ASGs still present a great loss of sensors in the installation phase, with vertical ASGs having a survival rate lower than the horizontal ASGs. This is primarily due to high temperatures, as well as rolling and vibrations generated during compaction. These conditions represent a restriction for the use FOSs as well. Still, the latter seem to be less intrusive than ASGs, due to their smaller dimensions. In terms of sensing configurations, similarly to ASGs, FP and FBG technologies can provide local strain measurements in the road pavement. Thus, the three of them seem to be more appropriate for monitoring of specific road sections. DFO sensors are sensitive at each point of their length; thus, they can monitor strain across a big portion of the structure under investigation, with higher spatial resolution. Some studies also show the potentiality of geophones and accelerometers to measure pavement deflections, which can be used for inverse calculation of pavement moduli. These devices are less intrusive than LVTDs, but require a more complex data treatment process, as the signal has to be integrated.

Concerning the inverse calculation of pavement health conditions, the studies described in Section 3 show that the general strategy is based on a purely mechanical approach. Due to the complexity of asphalt materials, measurements within the pavement are affected by temperature, vehicle speed, and vehicle transverse position with respect to sensors' locations. In addition, some studies have highlighted the fact that a viscoelastic model that takes into account the interface-effective bonding conditions can better predict pavement response under moving loads, especially at high temperatures.

Some innovative solutions that can overcome the aforementioned drawbacks are emerging and making their way in the field of pavement instrumentation. Alavi et al. [51] propose a smart-sensing technology based on the use of self-powered piezoelectric sensors that could be distributed in the pavement for continuous monitoring. Their methodology is based on relative damage; thus, there is no need to directly measure absolute strain in the pavement. In addition, they show an interest in the use of statistical techniques for data analysis. Lebental et al. [52] propose and approach the monitoring of construction materials that rely on the use of sensors based on nanomaterials. They demonstrate the embeddability of nanosensors, both in concrete and asphalt materials, and highlight the higher sensitivity of these devices with respect to traditional technologies.
