This paper presents the results of 28-month-long monitoring of a slab portion of the Chillon viaducts in Switzerland using strain gauges and thermocouples. This post-tensioned reinforced concrete structure was strengthened with a layer of reinforced ultrahigh-performance fibre-reinforced cementitious composite (UHPFRC). The strain gauges are used to measure stresses in the bottom layer of reinforcement bars in the slab, while thermocouples explain the behaviour of the structure due to temperature variation. The response under both traffic and thermal actions is discussed. It is demonstrated that the stress variation due to the thermal action can be as large as the response due to traffic action even in such a massive structure. Furthermore, recommendations for analysing both traffic and thermal-induced stresses are given. The commonly used simplified method for calculation of fatigue stress is shown to be highly conservative, leading to overestimation of structural action effects by a factor of four.