Investigation of fracture mechanics test methods to assess composite-concrete bond

In recent years, fiber reinforced polymer (FRP) composite materials have been successfully used in new construction and in repair or rehabilitation of existing structures as required for maintaining and upgrading civil infrastructure in many parts of the world. The durability performance of concrete structures reinforced with FRP strips depends on reliable bonding between the FRP and concrete. Bond failure often occurs in practice.; The broad objective of this research is to assist in the increasing use of FRP composites to reinforce tensile surfaces of concrete structures through in-situ repair techniques. The specific objective is to help develop an appropriate methodology for evaluating the long-term durability of the composite-concrete bond.; In the first phase of this research, a fundamental understanding of the stresses and strains that occur at the interface between a fiber-reinforced composite overlay and a concrete substrate in bridge rehabilitation applications is sought. This is accomplished by the finite element modeling of a scale T-beam with a bonded FRP strip on the tensile surface. Two situations are considered: with interface cracks and without interface cracks. In total, four crack locations were considered and modeled separately to study how the position of the interface crack affects the crack tip stress state. Also the effects of the composite material properties on the stress state are studied.; The second stage of this research focuses on the development of a test specimen geometry that replicates in-service loading of a composite-concrete structure with an interface crack. The finite element analysis is used to evaluate a modified double cantilever beam (MDCB) hinged specimen that has been reported in the literature. Results show that the MDCB hinged specimen gives good results for Mode I loading but not for Mode II loading. An MDCB tabbed specimen is proposed and evaluated for this purpose. Equations for calculating the strain energy release rate of the new sample are proposed. The finite element results for the MDCB tabbed specimen suggest that it may be used to determine the strain energy release rate of the composite-to-concrete bond under Mode II loading. Additional research examined how various test specimen parameters affect the behavior of the FRP overlay-concrete MDCB specimen. The parameters investigated were adhesive thickness, FRP overlay thickness, initial crack length, and specimen width and length. This study demonstrates that it is possible to use the finite element model to simulate the test specimen to evaluate the bond quality between the composite and concrete. It is believed the details obtained by finite element simulations will increase knowledge of confidence in using composite materials to repair civil structures.