Flexural strengthening of concrete structures with prestressed FRP composite

P Ahlgren, J Edwijn (Chalmers University of Technology)

This master thesis presents the results from a parametric study, with Finite Element Modelling, using a new prestressing method where a Carbon Fiber Reinforced Polymer (CFRP) laminate is Externally Bonded (EB) to a reinforced concrete beam. The results from the parametric study are compared with the results from already executed experiments. 

By prestressing the CFRP laminate the capacity of the composite material can be utilized more and by externally bond it to a reinforced concrete structure the Serviceability Limit State (SLS) will improve due to cracks closing up and the Ultimate Limit State (ULS) will improve due to higher ultimate strength.

The objective of this thesis is to create a model using the Finite Element Method (FEM) and optimizing the prestressing force, using the new method, in order to improve the structure in both SLS and ULS. The main focus of the project is to check for and optimize against crack width, ductility, and ultimate strength.

There are methods for strengthening concrete structures using prestressed Fiber Reinforced Polymer (FRP), but most of them require metal anchorage at the end of the laminate. Anchorage is needed due to the high shear forces created by the prestressing force and to transfer stresses into the structure to avoid debonding of the FRP laminate. A new method has been developed at Chalmers University of Technology where the prestressing force is applied in segments, reducing the shear force at the end of the laminate, making the need for anchorage redundant.

Initial experimental tests have been done on three 4.5 m long reinforced concrete beams subjected to four point bending at Chalmers, using the new prestressing method. One reference beam, one beam with EB passive CFRP, and one beam with EB prestressed CFRP.

The results from the Finite Element Analysis (FEA) show that, by prestressing the CFRP laminate to the concrete members, crack width can be reduced if the prestressing level is within a certain range. Prestressing with too high force will cause the concrete to crack on the top side and reducing the ductility of the beam. According to the results from the FEA and the optimization strategy, an optimal prestressing force will be between 27.90-29.17% utilization of the CFRPs ultimate tensile strength. The optimal prestressing force according to this thesis is when the top side cracking is kept at a minimum, failure of the concrete occurs as late as possible, and that there is sufficient utilization of the CFRP laminate. 

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