CFRP (Carbon Fibre Reinforced Polymer)-strengthening with plates and sheets of existing concrete structures is today a well-known method, proven with good results all over the world. However, limited research has been carried out regarding the behaviour of CFRP strengthen structures in cold climates. This paper addresses field testing of a CFRP strengthening pre-stressed concrete hollow box-girder bridge in Stockholm, Sweden, the Gröndals Bridge. Also how the strengthening behaves in cold climate is investigated. Two long term monitoring systems has been installed on a, one traditional system with LVDTs (Linear Vertical Displacement Transducers) mounted over cracks for continuous monitoring and one system with FOS (Fiber Optical Sensors) mounted over cracks and on CFRP-plates.Results show that difference in temperature causes most of the live load. Other results from summer and winter measurements shows that the CFRP plates are not tensioned during winter. A conclusion from this is that the strengthening would have been much more effective if the CFRP would be applied in the winter when the existing structure is contracted due to the temperature effect. Furthermore, the opening of cracks could be followed over time.