United States Department of Transportation
i
Performance of steel girders repaired with advanced composite sheets in a corrosive environment.
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2017-01-01
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Abstract:This report presents a two-phase research program studying i) galvanic current influencing deterioration of carbon fiber reinforced polymer (CFRP) sheets bonded to a steel substrate and ii) electrochemical reaction for steel beams strengthened with CFRP.
The first phase of the research presents an experimental program investigating the effect of galvanic current on the physical and mechanical characteristics of CFRP composite sheets bonded to a steel substrate. Electrochemical reaction is induced by galvanic interaction between anodes (CFRP-steel interface specimens) and cathodes (metallic strips) linked with an electrolyte (a 3.5% sodium chloride solution). Thirty-five test specimens are exposed to various periods of galvanic current from 0 to 72 hours until their corrosion rate is converged. Hydrated ferric oxide forms along the CFRP-steel interface with some concentration in the vicinity of its edge, which accompanies a loss in surface area and mass. The electrochemical reaction imposed by the galvanic current exponentially decays with an increase in exposure time. The initiation of corrosion noticeably affects the load-carrying capacity of the CFRP-steel interface, whereas its propagation is not a critical attribute until substantial corrosion damage occurs. CFRP-debonding is the governing failure mode of the interface, irrespective of the degree of galvanic current exposure. The stress-slip behavior of the interface is influenced by the electrochemical reaction and a geometric discontinuity associated with stress singularity. Corrosion-dependent interfacial fracture energy is probabilistically inferred and used for quantifying the degree of interface deterioration subjected to an aggressive corrosion environment.
The second phase of the research discusses the effects of an electrochemical reaction on the physical and chemical responses of steel beams strengthened with carbon fiber reinforced polymer (CFRP) sheets. An accelerated corrosion protocol is used for deteriorating the strengthened beams. Emphasis is placed on the electric potential, mass loss, corrosion current density, corrosion rate, flexural capacity, interfacial strain development, failure mode, and infrared spectroscopy of the beams. Corrosion damage is dispersed with increasing electrochemical reaction time; however, premature CFRP-debonding is not observed. The presence of surface rust tends to impede the flow of electric current and the diffusion of iron ions is reduced, thereby decreasing the rate of corrosion. The consequence of corrosion damage results in a decrease in load-carrying capacity of the strengthened beams along with two phases, such as initiation-propagation and steady-state. The electrochemical effect imposed alters the pattern of CFRP-debonding from discrete-discontinuous to smooth progression. Infrared spectroscopy illustrates that the functional group of the CFRP system changes from a chemistry standpoint as the degree of corrosion augments. Design recommendations are proposed to facilitate the use of CFRP-strengthening for steel members subjected to corrosive service environments.
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