## 3 Maintenance Strategies and Monitoring Procedures for Transport Structures
In contrast to the predetermined maintenance, this approach is suitable for issues with a random probability of failure or damage occurrence. On one hand, the CBM maintenance strategy is the most popular strategy amongst researchers [51; 52]. On the other hand, it requires a comprehensive survey plan to facilitate up-to-date knowledge of the asset condition. An example of the corrective, preventive, predetermined and CBM approaches is reported in Figure 4. Research has demonstrated that the CBM approach is the most suitable for infrastructure monitoring, as it minimises budget economic losses and the risk of implementing ineffective interventions. Conventional inspection methods for road infrastructure maintenance rely on the use of destructive techniques, such as coring, drilling or otherwise removing part of the structure to allow inner visual inspections (e.g., bridge deck inspections), or material performance checks in the laboratory environment. Despite the high reliability of these methods, they are costly and time-consuming. In addition, the outcomes have limited significance since they only represent local conditions of the infrastructure and cannot be projected to represent longer distances. These factors have huge impacts on the effectiveness of maintenance as well as on the costs and time of the decision-making process. To overcome the above limitations, several NDT methods, and more recently, satellite-based remote sensing technologies, have been introduced [13; 53; 54]. As the budget for road infrastructure maintenance is carefully managed to limit wrong allocations of funds, road authorities must increase the efficiency of measures and reduce the overall costs. This can be achieved by utilising new procedures and technologies, and it can benefit the sustainability of new projects and the implementation of maintenance on existing roads [55; 56]. Since maintenance costs usually cover most of the annual budget on road infrastructure, asset owners are continuously striving to develop more efficient and cost-effective methodologies. Several strategies and contract forms have been implemented to  
Figure 4: Maintenance overview: an example of the corrective, preventive, predictive and CBM approaches, SS-EN 13306 (2001) [47]. outsoourcing in competitive markets, as well as the development of life-cycle cost models, new funding and subsidiary forms [57]. The cost of a road structure over its service life is a function of design, quality of the construction, the maintenance strategies and the monitoring operations. Accordingly, an optimal asset management system requires the definition of life-cycle cost estimations for all the above-mentioned components, as well as the assessment of early-stage decays. In the last two decades, the use of NDTs, such as acoustic [58; 59], electromagnetic [60; 61; 62; 63; 64] and thermographic methods [65], has gained momentum for health monitoring of civil engineering infrastructures. These methods, although very effective, can provide information with gaps dictated by their own physics and working principles. As an example, LiDAR systems can provide high-density level of images and cloud-points for the inspected objects [66], but they cannot collect any information that is not visible on the surface. Therefore, the integration of datasets collected with different NDTs stands as a viable approach to fill technology-specific gaps and assure a more comprehensive assessment of the infrastructure [67]. In the last decade, satellite-based remote sensing techniques have proven to be faster and more cost effective for this purpose, thereby paving the way towards the implementation of a quasi-continuous and updated infrastructure information system at the network level.