Dr Juan Manuel Gallego
During the last decades, traffic loads by heavy vehicles have increased due to requirements of the current society and bridges are getting older and this fact has to be considered when the state of existing bridges is evaluated. Many concrete structures, for instance lateral cantilevers of highway box-girder bridges, can be strengthened on the upper side with externally bonded (EB) Carbon Fiber Reinforced Polymer (CFRP) strips in order to increase their load-bearing capacity in flexure. These additional reinforcements can be applied as an unstressed or prestressed system. The main goal of this research project is to analyse the effect of the temperature on the bond between the concrete substrate, the epoxy resin layer, and the externally bonded (prestressed and unstressed) Carbon Fiber Reinforced Polymer strip. A focus is on the special anchorage method for the prestressed CFRP strips, the so-called gradient anchorage.
If CFRP strips and epoxy adhesives are applied on the upper side of bridges, they can reach elevated temperatures due to the sealing process and the subsequent application of the asphalt layer during the construction or afterwards due to the sun heating. In spite of the great number of experimental works carried out concerning the behavior of reinforced concrete elements strengthened with unstressed and prestressed CFRP strips, few experimental investigations have been done up to now regarding the behavior of such elements strengthened with prestressed CFRP strips anchored with the gradient anchorage under elevated temperatures. In the frame of the research project, an experimental campaign is being carried out at Empa in order to assess the short and long-term behavior of prestressed and unstressed CFRP strips glued on concrete under different elevated temperature environments.
Dr. Juan Manuel Gallego is doing a 2-year post-doc at Empa in the framework of this project. In Fig.1, he is preparing an experiment together with Dr. Julien Michels from the Structural Engineering Research Laboratory of Empa.
Fig.1 - Test preparation
Aims and Objectives
The research project focuses on the effect of elevated temperatures on CFRP strips anchored to the concrete surface in unstressed and prestressed state.
The simulation of an elevated temperature scenario in EB CFRP strips is related to the following consequences that have to be studied in detail:
- Development of thermal shear stresses in the joint due to the different coefficients of thermal expansion
- Decrease of the Young’s modulus in the epoxy adhesive
- Redistribution of the shear stresses along the bonded area
- Creep deformation of the epoxy adhesive
- Change of the failure mode from concrete failure to adhesion failure
- Deterioration of the material properties
In the project, the influence of these different effects shall be quantified.
Methodology and Results
Dr. Gallego is engaged with the following tasks:
- Introduction in a new research field (EB CFRP reinforcements)
- Learn new methods and laboratory technics as for example the image correlation measurement system (ICS)
- State-of-the-art review concerning the long-term behavior at elevated temperatures of EB CFRP reinforcements
- Assisting in an experiment campaign in large-scale RC elements strengthened with EB CFRP strips
- Performing, analysing and interpretation of the experimental results (ICS)
- Interpretation and modelling the experimental results
- Analyse the temperature stability of prestressed EB CFRP strips under elevated temperatures
- Planning an assisting in an experiment campaign in large-scale RC elements strengthened with EB CFRP strips subjected to fatigue loads at elevated temperatures
Several conclusions have been obtained so far from the investigation:
- The surrounding testing temperature influences the bond strength of a CFRP/epoxy/concrete joint. Different exposure temperatures generally lead to differences in anchorage resistances in lap-shear tests
- Elevated temperatures lead in several cases to a change in the failure type. Under room temperature, concrete is generally the weakest link, in which cracking eventually leads to failure. At higher temperatures, however, a shift from the concrete substrate to an interface failure epoxy/concrete has been noticed in several research activities
- The application of warm mastic asphalt can influence the behavior of an EB CFRP strip used for negative bending strengthening (i.e on the top side of the structure). In case of prestressed systems with the gradient anchorage, the loss in stiffness leads to a stress redistribution over the bond length
- A first simplified modeling for the behaviour of the prestressed CFRP strips under elevated temperature was developed
- According to the tests results the asphalt scenario has been verified as the critical elevate temperature scenario
- Through small-scale tests on prestress CFRP strips a stable prestress state with a slight decrease of the initial prestress force due to the asphalt scenario was observed. Besides, a slight reduction of the anchorage force resistance in lap-shear has been measured in those tests performed after the simulation of the asphalt scenario
- Large-scale tests on RC beams strengthened with unstressed EB CFRP strips have been performed to study the short- and long-term behavior of lateral cantilever of highway concrete box-girder bridges. A similar behavior of the beams tested at room temperature with and without a previous asphalt scenario was observed
- One large-scale test on a RC slab strengthened with two prestressed EB CFRP strips has been performed in order to study the temperature stability of such reinforcements subjected to elevated temperature. The application of a real mastic asphalt layer on the top surface was simulated. According to the experimental results this elevated temperature scenario was too high for prestressed strips with gradient anchorages. During the simulation of the asphalt scenario one of the prestressed strips suddenly collapsed while the other remained stable. For this reason, the gradient anchorage technique is not recommended for such applications under mastic asphalt layers. However, for other applications the long-term durability is investigated with a long-term test under environmental conditions.
The main dissemination activities carried out from Dr. Gallego so far are the following:
- Poster competition: Influence of temperature on the bond of prestressed CFRP strips. Technical University of Kaiserslautern 23.10.2014
- ETH Zurich Course “Fibre Composite Materials in Structural Engineering”. Help in the preparation of a laboratory exercise. Courses 2014/2015 and 2015/2016
- FRP training school: Reinforcing and strengthening of structures with FRP reinforcements. Location: Ghent, Belgium. Dates: 27-30.1.2015
- Endure meeting 05. Location: Empa, Dübendorf, Switzerland. Dates: 29-30.6.2015 and 1-2.7.2015. Activities: Meetings + presentations + lab competition. Dr. Gallego was heavily involved in preparation and organization of this event
- Conference publication: Influence of temperature on the bond of prestressed CFRP strips. SMAR 2015: 3rd conference on Smart Monitoring, assessment and rehabilitation of civil structures. Antalya, Turkey 7-9.9.2015
- Conference publication (Endure ESR presentation competition): Towards modelling the long-term behavior of prestressed CFRP strips under elevated temperature. SMAR 2015: 3rd conference on Smart Monitoring, assessment and rehabilitation of civil structures. Antalya, Turkey 7-9.9.2015
- Journal publication (published online): Prestress force-release tests at elevated temperatures-gradient anchorage stability for prestressed EB CFRP strips. 2015. Composites Structures. Juan Manuel Gallego, Christoph Czaderski and Julien Michels
- Journal publication: Gallego, J.M., Czaderski, C., Michels, J. Prestress force-release tests at elevated temperatures-Gradient anchorage stability for prestressed EB CFRP strips. 2016. Composites Structures, 137:159-169.
- Cost Action TU 1207: Next generation design guidelines for composites in construction. General meeting and industry seminar. Lodz University of Technology. Lodz, Poland 4-6.4.2016.