## 3 Maintenance Strategies and Monitoring Procedures for Transport Structures  
Recent dramatic events occurring on the European transport network have pointed out that a more detailed network-level assessment of the transport asset condition is urgently required [37; 38]. Similar consideration could be given to the high exposure and vulnerability of the transport system to major natural events, such as floods or earthquakes [39; 37; 38]. It is worth mentioning that most of the highway and railway assets were constructed between the 1960s and the 1980s, hence their nominal service life of typically 25-40 years has already elapsed. This highlights the importance of providing a successful asset maintenance strategy. Effective planning of maintenance protects the infrastructure serviceability over time by decreasing the risk of collapse during ordinary operations (e.g., the Morandi Bridge in Genoa, Italy [39; 40] or the Majerhat Bridge in Kolkata, India [41]) or critical natural hazard events (e.g., Troja footbridge in Prague [42], Czech Republic [43] or Pfeifer Canyon Bridge, California, US [44]). On the other hand, only limited attention is paid towards the management of the existing assets, with potentially dramatic macroeconomic consequences [45]. Maintenance activities are traditionally organised based on different types of approaches. The main aim of these activities is the definition of an ideal time, localisation and modality (i.e., defining \"When, Where and How\") of inspection surveys on the infrastructure network [46]. A classification of maintenance types and their mutual relationship is indicated in the standards \"EN 13306, 2001\" [47], as shown in Figure 3:  
Accordingly, maintenance activities can be traditionally sorted in two main approaches, namely \"corrective\" (or \"prescriptive\") and \"preventive\" maintenance. * **Corrective maintenance** is implemented when breakdowns or evident failures occur on the network [48]. Corrective maintenance can be also performed with the system continuing to operate. Nevertheless, corrective maintenance can be extremely expensive, as costs of repairing are not linearly proportional to the scale and extent of the failure. * **Preventive maintenance*
* must be put in place before a fault surfaces, and it is scheduled according to fixed time intervals. This prevents any possible break-down and failure. A preservation approach towards the infrastructure asset maintenance is strongly recommended by experts, and to date, largely used by most of the asset owners. Preventive maintenance can be sorted into two sub-classes [49]:
* Predetermined maintenance: interventions are scheduled on a time-based criterion (e.g., road resurfacing for a fixed amount of time), without the need to investigate the infrastructure condition. This maintenance model is scheduled a priori and performed according to convenient time schedules to contain budget costs. It does not rely on the actual condition of the asset, and the time intervals for the implementation of the maintenance are established based on prediction  
Figure 3: Maintenance strategies overview, SS-EN 13306 (2001) [47]. models. It is to be noted that if the infrastructure does not require maintenance and this approach is followed, time intervals scheduled for maintenance that are too close together can result in economic losses [50]. Condition-based maintenance (CBM): According to SS-EN 13306 (2001) [47], this strategy is implemented by forecasting the decay trend of relevant infrastructure performance parameters, taken as the reference. Interventions are planned based on the actual need for maintenance, which is ascertained on site. Condition-based maintenance is based on gathering information about the actual and predicted element condition retrieved from scheduled, continuous or on-demand inspections of the infrastructure. CBM is an appropriate strategy for structural elements where the formation of decay can affect the operations, structural health or material properties, or it can have an impact on the surrounding environment.