The Chillon viaducts, in service since 1969, are two parallel structures with a total length of 2.1km each and spans varying from 92 to 104 meters. This post-tensioned precast concrete structure was strengthened with UHPFRC (Ultra High Performance Fiber Reinforced Cementitious composite) in 2014/2015 as, among other aspects, fatigue behavior was of concern. A monitoring campaign was commenced in May 2016 in order to verify the effectiveness of the UHPFRC-strengthening. The representative span of one of the viaducts was instrumented with strain gauges and thermocouples. The strain gauges were installed directly on the reinforcement bars in the longitudinal and transversal directions. This paper presents the main outcomes after more than 2 years of monitoring. The considerations on traffic-induced stress variation and thermal strain effects are presented. The results from the monitoring confirm the expected effectiveness of the UHPFRC strengthening.