## 1 Introduction  
Transport infrastructures need continuous maintenance due to deteriorations that may harm their serviceability and safety. These effects are the result of the combination of several factors, e.g., construction materials, quality of construction, traffic loads, and environmental factors. For this reason, it is necessary to carry out persistent monitoring of infrastructure health conditions in order to guarantee maximum safety at all times and support effective and economically sustainable maintenance. In the next few years, the problem of the resilience of the built environment will become increasingly important as the buildings and infrastructures built in reinforced concrete in the 20 th century are going to reach the end of their life. Accordingly, the monitoring activities will acquire increasing importance for the early identification of critical situations and the prioritization and planning of the interventions. In recent years, new technologies have been developed (mainly in the field of electromagnetic sensing and ICT) able to complement the analysis methods of civil engineering. This has led to the development of integrated systems for early warning, monitoring and quick damage assessment of the built environment and critical infrastructures [1; 2], able to achieve several aims, such as:  
* The implementation of a multi-disciplinary approach able to integrate different kinds of sensing/diagnostics technologies (including new concept of operation, such as the citizen as a sensor [3; 4] where personal smart devices collect and send information from all over the world in near real-time) with spatial data infrastructure and ICT architectures;
* The capability to assimilate monitoring data and indicators coming from the sensing in civil engineering analysis with the aim of assessing the loss of performance of structures. This knowledge is important to identify actions and strategies for an effective and economically sustainable management of the infrastructure;
* The possibility to couple current monitoring with early warning and quick damage assessment capabilities, in order to drive and plan remedial solutions in crisis situations. Several examples of the exploitation of this approach are in [5; 6], where the focus was mainly on its suitability for the long-term monitoring of the transport infrastructures. Anyway, in order to allow for the sustainability of such an integrated approach, it is crucial to design smart observational chains, which activate more sophisticated surveys only when necessary. An example of this \"smartness\" concept is reported in [2; 5], where different levels of observations of increasing complexity are identified. This means that the choice of the sensing technologies is made on the basis of the specific scenario (structure, territory, risks, etc.) and more sophisticated sensing techniques are activated only after criticalities are detected by cheaper and easier to deploy sensors. The first level of these observational chains is characterized by the integration of the technologies allowing to activate an almost entirely automated long-term monitoring and early warning strategy. In this way, it is possible to follow the life of an infrastructure continuously over time, by limiting the exploitation of personnel to a minimum, just for periodic inspections, and traffic interruptions. Therefore, this level has to be characterized by the use of technologies that require an extremely simplified logistics with no interruption of the operativity of the structure, by ensuring monitoring results at a large scale and in a fast way. The results deriving by the use of this level are exploited to decide whether to activate more complex analysis stages focused on the detailed diagnostics of the structure (or of the single elements of it), by means of in situ technologies, such as ground penetrating radar, infrared cameras, etc.