Short and long term performance of concrete structures repaired/strengthened with FRP
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Fibre Reinforced Polymer (FRP) composites have lately become a popular choice for strengthening and/or repairing of reinforced concrete (RC) structures due to their advantageous properties such as high strength- to-weight ratio, high corrosion resistance and easy application process. As the performance of FRP bonded RC structures depends on the effective stress transfer between FRP and concrete, extensive research has been conducted on the FRP-concrete bond system under short term loads. However, studies on the long term performance of FRP-concrete bond subjected to environmental conditions are very limited. Experimental studies on the long term performance of FRP strengthened structures to-date include the study of the effect of various environmental conditions using a variety of test set-ups such as pull-off, bending tests of beams and direct shear tests. However, the available studies based on various conditions and set-ups make it difficult to compare the findings. As the effectiveness of FRP-strengthening schemes, either used for flexural or shear strengthening, lies in the shear stress transfer between FRP and concrete, study of FRP-concrete bond subjected to different environmental conditions by direct shear tests were suggested by some of the researchers. Even sensitivity of this set-up to environmental conditions was also reported. Therefore, more research with similar test set-ups to create a large database of FRP-concrete bond behaviour under various environmental conditions can be of immense value. In addition, using very high temperature for accelerated ageing was found to be very common in available literatures. However, in reality structures may not be exposed to such high temperatures and using high temperature may lead to conservative prediction of long-term properties. Moreover, unavailability of test data for FRP-concrete bond subjected to natural ageing observed in the existing literatures necessitates the investigation on FRP-concrete behaviour under natural environment. In regards to the short term performance of reinforced concrete beams strengthened and/or repaired with FRP, extensive research have been conducted to-date in terms of experimental and analytical study. Some of these studies have also proposed design guidelines. However, the equations for prediction of load carrying capacity of severely damaged repaired beams, especially, considering the strain hardening after yielding are not recommended. Considering the identified gaps in the previous research on long term performance of FRP-concrete bond system and the short term performance of RC beams strengthened and/or repaired with FRP, the research study presented in this dissertation has mainly focused on the experimental investigation of the long-term performance of carbon FRP and glass FRP-concrete bond under three separate environmental conditions, namely, temperature cycles, wet-dry cycles and outdoor environment up to 18 months. The secondary objective is to investigate the effectiveness of typical FRP-strengthening schemes, used for strengthening of reinforced concrete beams, in the repair of severely damaged beams. The long term performance of two types of FRP (CFRP and GFRP)-concrete bond is studied by extensive experimental investigations using single shear tests (is referred to as pull-out tests). The maximum temperature of the temperature cycles is intentionally kept below the glass transition temperature of epoxy resin to avoid any over-degradation. In the wet-dry cycles, temperature close to ambient is maintained. Also, outdoor environmental exposure is applied to address the unavailability of test data of natural ageing of FRP-concrete bond system. Pull-out tests conducted after exposure durations are analysed based on the pull-out strength, failure modes and strain distributions along the bond length. In addition, material properties, namely, CFRP tensile strength and modulus of elasticity, and concrete compressive strength are determined to understand the effect of changing material properties on the pull-out strength by correlation of bond strength with failure modes. Curve fitting of shear stresses against slips of only CFRP-concrete bond is conducted to determine the fracture energy release rate and the effect of environmental conditions on it. In addition, interface laws are proposed for control and exposed conditions based on an existing model. Results obtained for long term performance of bond systems provide interesting findings due to imposed environmental conditions. Based on the observations, strength reduction factors for CFRP and GFRP-concrete bond are proposed. The short term performance of FRP-repaired beams is investigated both experimentally and analytically. Three severely damaged beams, fabricated from conventional concrete (normal concrete with water, cement and aggregate) and non-conventional concrete (concrete with additives such as fibres and rubbers) are repaired with CFRP for flexure. Anchorage provided by complete CFRP wrapping at two ends and mid-span is found to be effective for preventing the debonding of FRP at least partially. Analytical study is conducted to understand the effect of existing steel reinforcement on the response of repaired beams under flexure. Considering the strain hardening of steel after yielding, equations are also proposed for better prediction of load carrying capacity of the repaired beams and compared with experimental results. Finally, all the major findings of the two areas of research are summarised and recommendations for future research are made.
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