| Popis: |
The growth in urbanized world population and higher rates of mobilization impose an increased demand on the performance of civil infrastructures such as bridges, that suffer not only from increased traffic loading, but also from age- and environment-related deterioration. Considerable attention was given to strengthening methods for reinforced concrete (RC) infrastructures using externally bonded reinforcements (EBR) such as carbon fiber reinforced polymers (CFRP). Their main advantages such as high strength-to-weight ratio, corrosion resistance and ease of handling have rendered them a popular choice. CFRP strips can be employed in unstressed and prestressed configurations, where recently the latter has gained considerable attention due to higher utilization of the strip's full strength capacity. Prestressing forces at both strip-ends need to be transferred into the concrete, which necessitates the use of an anchorage system. The majority of commercial products currently rely on a mechanical solution. A non-mechanical alternative is the so-called gradient anchorage, developed at Empa. It relies on the accelerated curing property of epoxy under higher temperatures and segment-wise force releasing at both strip-ends. This technique was tested extensively and proven to be suitable for strengthening applications in a short-term horizon. Long-term durability aspects of such a system is of paramount interest due to the fact that the mentioned anchorage technique purely relies on the bond behaviour between its constituents. Knowledge on the long-term behaviour is vital not only to ensure safe design but also for a broader industry acceptance. The aim of this work is to investigate the long-term performance of prestressed EBR, anchored with the gradient method, from an experimental and numerical point-of-view. For this purpose, a series of lap-shear tests were performed on concrete blocks using prestressed CFRP strips, anchored with the non-mechanical gradient technique via prestress force-releasing. Prior to lap-shear testing, specimens were exposed to various accelerated ageing conditions such as concrete carbonation, freeze-thaw-cycles (FTC), constant high humidity and temperature (below the glass transition temperature), as well as their combination. The effect of prestressing on the long-term performance is also investigated by means of exposing unstressed specimens to FTC and subsequently testing them in the identical lap-shear test setup. 3D-digital image correlation measurements were utilized during prestress force-releasing and lap-shear tests in order to obtain full-field displacements. In addition to strengthened blocks, individual material tests were performed in parallel to characterize the isolated deterioration process of concrete and epoxy. For concrete, compression tests on cubes, double punch tests on cylinders and single-edge notched bending tests on prisms were conducted. Direct tensile tests on dog-bone specimens and differential scanning calorimetry on small samples were performed for the epoxy resin. Experimental findings have demonstrated a beneficial effect for carbonation owing to modification of the surface mineral structure following CO2 uptake. No particular effect was observed following constant high humidity and temperature exposure, likely due to the reversal of potential bond damage during drying. On the other hand, specimens subjected to FTC exhibited a significant reduction in the residual anchorage resistance, maximum slips and elastic stiffness. Prestressed specimens subjected to FTC appeared to have suffered a higher degree of deterioration when compared to unstressed specimens subjected to identical conditions. A shift in the failure mode was observed, from concrete cohesive failure to an epoxy-concrete interface failure. The potential mechanism of deterioration was linked to the destruction of chemical bonds between epoxy and concrete by water desorption to the interface and thermomechanical fatigue loading caused by temperature cycles. Contrastingly, individual material tests did not reveal any adverse effects of accelerated ageing conditions on bulk material parameters of concrete and epoxy. A framework for numerical modelling was established in order to complement experimental findings from specimens subjected to FTC and further explore the effect of prestressing level on the residual anchorage resistance. Based on the cohesive zone modelling scheme, simulations were set to encompass both continuum and fracture models of constituent materials and their interfaces. These material and interface laws were determined with the help of design guidelines and a variety of inverse analysis techniques. By establishing a relationship between prestressing level and bond shear stress-slip parameters at the interface, the behaviour of specimens subjected to FTC were simulated and validated against experimental findings. The interaction between prestressing force and residual anchorage resistance was then employed to modify the currently used analytical expression to encompass the effects of freeze-thaw cycles and humidity exposure. Modified analytical equations are then implemented into Swiss Code SIA 166 to assess the strip-end debonding criteria for prestressed EBR subjected to humidity and freeze-thaw cycles. |
