Fibre reinforced polymers in civil engineering flexural strengthening of concrete structures with prestressed near surface mounted CFRP rods

Sammanfattning: Repair and/or upgrading of concrete structures with bonded steel or FRP (Fiber Reinforced Polymers) materials or by use of external tendons have been used for some time. In most cases the bonded on FRP materials are unstressed. However if prestressing could be applied better utilization of the strengthening material and a better strengthening result would most likely be achieved. More research is carried out in the area of repair and upgrading of concrete structures with prestressed FRP materials. Bonding prestressed FRP laminates or sheets to a concrete surface has proven to be efficient and also gives a better utilization of the strengthening material used. External prestressed cables of FRP materials have shown to be an alternative to steel cables in, for example, upgrading concrete structures. Good durability properties and a first-rate behaviour in creep and relaxation have given very good results so far. One weak part for both external prestressed cables as well as bonded laminates has shown to be anchorage. For cables this is due to lower lateral properties of the cables compared to the axial properties and for laminates due to the high peeling stresses at the cut off end of the laminate. Often the anchorage device has problem to handle the high stresses that would justify the use of FRP materials in prestressing, i.e. the stress that can be achieved due to the anchorage is not high enough. However this is changing with a number of research projects around the world focusing on the anchorage issue where a number of anchorage details have been developed The research presented in this thesis is focused on strengthening concrete structures with prestressed CFRP rods bonded in slots in the concrete cover. The prestressing force was transferred to the concrete via adhesive bond only; no mechanical anchorage was used during the tests. Three factors were varied during the tests; prestressing force, bond length (i.e. length of the rod) and stiffness of the rod. The results from the tests show a increased concrete cracking load as well as a increased steel yielding load. The thesis consists of a main body and three papers. The main body covers a literature survey of some of the work done in the area. Paper A covers tests made with Glass fibre beams combined with concrete, Paper B is about strengthening with Near Surface Mounted Reinforcement with CFRP rods and Paper C covers strengthening with Near Surface Reinforcement CFRP rods where the rods have been prestressed.