Experience has shown that when the models and methods that have been developed for the design of new structures are â blindlyâ used for the examination of existing ones, the management of civil infrastructure lacks substantially in financial efficiency and social sustainabilityâ one feels that in these cases the above models and methods are unduly conservative. This is not surprising since the design of new structures is performed under conditions of larger uncertainty and lower risk-reduction-cost comparing to the conditions under which the examination of an existing structure is usually carried out. Motivated by these observations this work develops around the specific problem of safety verification of existing reinforced concrete slabs (and more precisely of their bending steel reinforcement) subject to railway traffic action. The problem is tackled from both (a) a resistance standpoint (trough the development of advanced fatigue-resistance / damage-accumulation models) and (b) an action-effect standpoint (trough the development of suitable structural response monitoring schemes). The two approaches are combined in the development of safety verification methods for the ultimate limit states of: (a) excessive deformation (or fracture) and (b) fatigue failure. Accordingly, the work is organized in four chapters. Chapter 1 deals with the development of a mesoscopic model of resistance to fatigue flaw initiation in the vicinity of a stress concentration (as observed in the case of ribbed reinforcement bars, on the surface of which, fatigue flaws always initiate near the root of the ribs). The model is validated and calibrated by comparison to experimental results taken from the literature and derived from constant and variable amplitude fatigue tests on notched steel components. Chapter 2 presents a novel monitoring scheme oriented towards the structural examination of reinforced concrete slabs subject to railway traffic action. Monitoring is based on the measurement of the mean axial strain profile variation in the slab. The technical aspects of the implementation of the above scheme on a short span railway underpass for a period of seven months and the processing of the acquired signals are presented in detail. Some interesting monitoring records are provided over which the efficiency of the proposed scheme is discussed. Chapter 3 proposes a monitoring based safety verification format for the ultimate limit state of excessive deformation (or fracture). This format provides the examination value of the critical traffic action effect directly from monitoring data (without resorting to action models and structural analysis). The development of the verification format is based on the data obtained from the monitoring campaign described in Chapter 2. Chapter 4 focuses on the fatigue verification of the reinforcement bars which are practically always the critical component of a RC element with respect to fatigue failure. The following questions are addressed: (a) construction of case specific S-N curves for the steel reinforcement bars embedded in an existing structural element taking into account their surface hardness and the geometry of their ribs; (b) investigation of the load sequence effect using the fatigue resistance model developed in Chapter 1 and nominal stress histories obtained from structural response monitoring; (c) calculation of fatigue reliability taking into account the existence of a constant amplitude fatigue limit.