In recent years, requirements in terms of service-life of civil engineering structures have become more and more stringent, so that the focus of designers and owners is now set on structural durability. Foreseeing structural failures and repairing damaged structures at an early stage has become a major stake. This approach calls for an accurate knowledge of the state of the structure at any point in its lifetime. This is the incentive for the world-wide development of various in-situ monitoring techniques for structural materials. However, by measuring global structural quantities only, the existing monitoring techniques provide only indirect information on the structural health of the structure
