Long-term Resistance of Gradient Anchorage

Understanding the Durability of Prestressed Externally Bonded Reinforcements (PEBR) through Experiments and Simulations

The use of carbon fiber reinforced polymer (CFRP) strips as an externally bonded reinforcement (EBR) is currently a well-established technique for retrofitting existing deteriorated reinforced concrete (RC) structures. The majority of research and industry work has been focused on their use as a passive (unstressed) EBR. Better exploitation of CFRP as a prestressed EBR (PEBR) is attraction increased attention to such strengthening schemes. 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, whereas a non-mechanical alternative was proposed and developed by Empa. The so-called gradient anchorage relies on the accelerated curing property of epoxy under higher temperatures and segment-wise force releasing at both strip-ends. The working principles and failure mechanisms of this anchorage technique within lab-environments and short-time scales were explored in great depth, including a preceding PhD project by Czaderski (#200021_124401/1) that was completed in 2012. Studies within the literature hint towards a deteriorative effect of the outer environment (e.g. temperature, moisture) in the long-term behavior of an unstressed EBR. Since the gradient anchorage solely relies on its constituents and their interfaces, an assessment on its durability was deemed necessary for increased knowledge on its working principles and safer design practice. With this motivation, an experimental and numerical investigation was launched by Mr. Yunus Emre Harmanci (PhD candidate financed as part of this project).

The experimental and numerical work conducted within the context of this funded project resulted in a) an extensive experimental dataset to support durability research within the structural strengthening domain, b) a general-purpose numerical modelling scheme for such systems as well as c) proposal of modified design equations for PEBR considering hygrothermal exposure. The undertaken work unraveled the adverse effect of prestressing level on the residual anchorage resistance following ageing. This work has been conducted by Dr. Yunus Emre Harmanci (funded PhD candidate), as a collaboration between the Swiss Federal Laboratories for external pageMaterial Testing (Empa) (Supervisor: Dr. J. Michels) and the Chair of Structural Mechanics at ETH Zürich (Supervisor Prof. Dr. E. Chatzi).

Experimental Outcomes

Experimental findings have demonstrated a beneficial effect for carbonation owing to modification of the surface mineral structure following CO2 uptake. On the other hand, specimens subjected to freeze-thaw cycles (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. Contrastingly, individual material tests did not reveal any adverse effects of accelerated ageing conditions on bulk material parameters of concrete and epoxy.

Simulation Results

A relationship between prestressing level and identified interface parameters was constructed and 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.

Research Significance

The above-summarized results are considered to be the first lap-shear scale investigation on the durability of prestressed bonded connections, which can benefit to further the understanding of other strengthening systems as well as bonded connections within different engineering domains, and not only remain relevant to the gradient anchorage. The research findings are of high relevance regarding industry implementations as well, since current guidelines are well established for unstressed EBR, yet still lack aspects of durability and PEBR.

Output Data

Journals

Y. E. Harmanci, J. Michels, C. Czaderski, R. Loser, and E. Chatzi, “Long-Term Residual Anchorage Resistance of Gradient Anchorages for Prestressed CFRP Strips,” Compos. Part B Eng., 2018, external pagelink

Y. E. Harmanci, J. Michels, and E. Chatzi, “Behaviour of Prestressed CFRP Anchorages During and After Freeze-Thaw Cycle Exposure,” Polymers, vol. 10, no. 6, p. 565, 2018. Conference Papers, external pagelink

J. Michels, M. Staśkiewicz, C. Czaderski, R. Kotynia, Y. E. Harmanci, and M. Motavalli, “Prestressed {CFRP} Strips for Concrete Bridge Girder Retrofitting: Application and Static Loading Test,” J. Bridg. Eng., vol. 21, no. 5, 2016, external pagelink

Harmanci, Yunus E.; Michels, Julien; Czaderski, Christoph; Motavalli, Masoud (01.06.2016), Calculation Technique for Externally Unbonded CFRP Strips in Structural Concrete Retrofitting, in Journal of Engineering Mechanics, 142(6), 04016026 - 04016026, external pagelink

J. Sena-Cruz, J. Michels, Y. E. Harmanci, and L. Correia, “Flexural Strengthening of {RC} Slabs with Prestressed {CFRP} Strips using Different Anchorage Systems,” Polymers (Basel)., vol. 7, no. 10, pp. 2100–2118, 2015, external pagelink

Conference Presentations

Y. E. Harmanci, E. Zile, J. Michels, and E. Chatzi, “Cohesive Zone Modelling of a Prestressed Non-Mechanical CFRP Anchorage Subjected to Freeze-Thaw Cycles,” in 9th International Conference on Fibre-Reinforced Polymer (FRP) Composites in Civil Engineering (CICE 2018), 2018, p. 420, link

Harmanci, Yunus Emre; Michels, Julien; Chatzi, Eleni (2017), Behaviour of Prestressed CFRP Anchorages under Freeze-Thaw Cycle Exposure, in Fourth Conference on Smart Monitoring, Assessment and Rehabilitation of Civil Structures (SMAR 2017), external pagelink

Sena-Cruz, Jose; Michels, Julien; Correia, Luís ; Harmanci, Yunus Emre; Silva, Patricia; Manuel Gallego, Juan; Fernandes, Pedro; Czaderski, Christoph; Franca, Paulo M (2017), Recent contributions from UMinho and Empa on durability issues of flexural strengthening of RC slab with EB CFRP laminates, in Fourth Conference on Smart Monitoring, Assessment and Rehabilitation of Civil Structures (SMAR 2017), external pagelink

Y. E. Harmanci, J. Michels, and E. Chatzi, “Residual Resistance of Non-Mechanical Prestressed CFRP Anchorages Subjected to Environmental Conditions,” in 7th International Conference on Advanced Composite Materials in Bridges and Structures (ACMBS-VII), Vancouver, Canada, 2016, external pagelink