Structural health of bridges monitor, assess and retrofit

Sammanfattning: The research presented in this Licentiate thesis is intended to cover minor areas of the wide research area Structural Health Monitoring (SHM) of Civil Engineering Structures. SHM is broadly the knowledge maintaining our infrastructure by assessing the condition, often by monitoring or measurements, diagnose damages, analyse risk levels, by using inputs from measurements and diagnoses, and in the end retrofit, if needed. In particular the methodology and the interaction between the different parts in SHM, ranging from assessment to upgrading, are explained. The thesis give also a brief introduction to sensor technology, mainly designed for monitoring, and to damage detection methods in Civil Engineering structures, such as bridges. The thesis is of the form "combination thesis" which is characterised by a relatively extensive literature study and a summary of the research presented at the beginning. Beyond this, it contains four papers, which are summarised below. Paper A: "Concrete beams exposed to LiveLoading during CFRP strengthening", presents how strengthening of concrete beams can be done without stopping the traffic during execution. Results show that no significant decrease of the strengthening effect exists when carbon fibre laminates and quadratic rods (NSMR), is bonded with an epoxy adhesive, which had cured during the cyclic loading. If a cementitious bonding agent is used, cyclic loading decrease the strengthening effect dramatically. Paper B: "Large scale hybrid composite FRP girders for use in bridge structures - theory, test and field application". This paper presents the possibility to build a bridge in full scale using fiber reinforced polymers. Numerical analysis, full scale tests and theoretical investigations shows full composite action between the constituents. Paper C: "CFRP Strengthening and monitoring of the Gröndals Bridge in Sweden" discuss the retrofit of a large concrete box bridge using CFRP laminates and how the strengthening effect can be monitored and verified using fibre optical sensors (FOS). Due to shear cracks in the webs the Gröndals Bridge is strengthened with CFRP-laminates and post stressed with steel stays. Monitoring with FOS and conventional LVDT gauges shows that most of the measured strain is caused by natural temperature changes. The results also show that the CFRP laminates work as intended and that the crack propagation has stopped. Paper D: "Damage Detection of a Large-Scale 38-Years-Old Prestressed RC Bridge Girder using Natural Frequencies" gives and example how modal analysis can be used as a diagnostic tool to indicate damages. The results show very small changes of natural frequencies because no cracks during the whole tests occurred and the Young's modulus of the concrete was increasing after unloading.

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