Constructed in 1969, the Chillon viaducts constitute a vital component of the Swiss roadway network. The bridge deck was strengthened in 2015 by means of an Ultra High Performance Fiber Reinforced Cement-based Composite for alleviating the effects of deterioration that has been induced by Alkali-Silica Reaction. In order to assess the efficacy of this rehabilitation solution, and to further evaluate its long-term performance, a Structural Health Monitoring (SHM) campaign was implemented, relying on low-cost, yet easily deployable, sensors. The deployed monitoring solution includes a suite of accelerometers, strain gauges, temperature and humidity sensors. This work studies and reports on the influence of operational (traffic) and environmental (temperature and humidity) conditions onto structural performance, on the basis of monitoring data extracted form the first moths of the monitoring campaign. To this end, the monitoring data are initially processed by means of standard modal analysis methods, as well as non-stationary time-series analysis tools. In a second step, a finite element model of the system is built and updated, and subsequently exploited to deliver a comparison between the original and rehabilitated structure. Finally, by exploiting the updated system model, a reliability analysis is carried out, which confirms the efficiency of the strengthening solution.