Untersuchungen zu den Korrosionsmechanismen von Stahlfasern in chloridhaltigem Beton

Steel in concrete is normally protected against corrosion due to a thin and dense passive layer formed in the alkaline medium of the pore solution of concrete (pH approx. 13.3). In the presence of chloride at the steel surface (due to de-icing or seawater salt) the passive layer breaks down when a critical chloride content is exceeded (c(Cl(crit)) and corrosion of the reinforcement can be initiated, causing deterioration of the reinforcement and thus of bridges or parking decks etc. This critical chloride content of black steel reinforcement is strongly dependent on the pH-value of the pore solution and on the structure of the interfacial concrete zone as well as on exposure- and steel-related parameters. Former investigations on the corrosion of steel fibres in concrete containing chlorides have led to the conclusion that the critical chloride content of steel fibres in concrete is much higher compared to the one of normal reinforcement, although identical steel and concrete compositions are used. In order to clarify the mechanisms of this increased corrosion resistance of steel fibres compared to rebars electrochemical tests have been carried out in artificial pore solution as well as in mortar and concrete containing chlorides. Besides established electrochemical methods (determination of polarisation resistance and current density-potential-curves) Electrochemical Impedance Spectroscopy (EIS) as well as Electrochemical Noise (EN) have been applied to investigate the passivity of steel fibres, cold drawn wires with different degrees of drawing and reinforcement. In particular potentiostep tests of steel electrodes in pore solution at different chloride concentrations have been conducted by simultaneously measuring the Electrochemical Current Noise to clarify the passivation and the depassivation mechanisms. Additional investigations using Scanning Electron (SEM) and Atomic Force (AFM) Microscopes as well as numerical simulations on the corrosion kinetics rounded out the experimental design. The results show a significant reduction of the decisive noise signals and the capacitive actions of the passive layer of passive steel fibres and cold drawn wires in pore solution. Considering additionally the depassivation of fibres and wires at clearly higher chloride contents, the formation of a more homogeneous and hence more stabile passive layer of steel fibres and wires can be concluded. The passivation and depassivation of steel fibres and wires in mortar and concrete is strongly dependent on the corrosion behaviour within the die scars on the fibre surface caused by the production process and filled with drawing grease, which at least decelerate the passivation. Thus localized secluded corrosion spots within the die scars can be formed with marginal corrosion rates due to the diffusion inhibition caused by the dense interfacial contact zone steel/concrete. These "sleeping corrosion spots" are unlikely to cause visual damages. Hence in practice harmful corrosion of steel fibres in uncracked concrete containing high content of chlorides is improbable inside the structure. But near the concrete surface corrosion can occur due to leaching processes of the concrete, which can be calculative estimated. The durability of steel fibres after the onset of corrosion is strongly dependent on the activity of the cathode. Though due to the small section of the fibres even low corrosion rates are sufficient to restrict the durability of corroding steel fibres

Untersuchungen zu den Korrosionsmechanismen von Stahlfasern in chloridhaltigem Beton

Steel in concrete is normally protected against corrosion due to a thin and dense passive layer formed in the alkaline medium of the pore solution of concrete (pH approx. 13.3). In the presence of chloride at the steel surface (due to de-icing or seawater salt) the passive layer breaks down when a critical chloride content is exceeded (c(Cl(crit)) and corrosion of the reinforcement can be initiated, causing deterioration of the reinforcement and thus of bridges or parking decks etc. This critical chloride content of black steel reinforcement is strongly dependent on the pH-value of the pore solution and on the structure of the interfacial concrete zone as well as on exposure- and steel-related parameters. Former investigations on the corrosion of steel fibres in concrete containing chlorides have led to the conclusion that the critical chloride content of steel fibres in concrete is much higher compared to the one of normal reinforcement, although identical steel and concrete compositions are used. In order to clarify the mechanisms of this increased corrosion resistance of steel fibres compared to rebars electrochemical tests have been carried out in artificial pore solution as well as in mortar and concrete containing chlorides. Besides established electrochemical methods (determination of polarisation resistance and current density-potential-curves) Electrochemical Impedance Spectroscopy (EIS) as well as Electrochemical Noise (EN) have been applied to investigate the passivity of steel fibres, cold drawn wires with different degrees of drawing and reinforcement. In particular potentiostep tests of steel electrodes in pore solution at different chloride concentrations have been conducted by simultaneously measuring the Electrochemical Current Noise to clarify the passivation and the depassivation mechanisms. Additional investigations using Scanning Electron (SEM) and Atomic Force (AFM) Microscopes as well as numerical simulations on the corrosion kinetics rounded out the experimental design. The results show a significant reduction of the decisive noise signals and the capacitive actions of the passive layer of passive steel fibres and cold drawn wires in pore solution. Considering additionally the depassivation of fibres and wires at clearly higher chloride contents, the formation of a more homogeneous and hence more stabile passive layer of steel fibres and wires can be concluded. The passivation and depassivation of steel fibres and wires in mortar and concrete is strongly dependent on the corrosion behaviour within the die scars on the fibre surface caused by the production process and filled with drawing grease, which at least decelerate the passivation. Thus localized secluded corrosion spots within the die scars can be formed with marginal corrosion rates due to the diffusion inhibition caused by the dense interfacial contact zone steel/concrete. These "sleeping corrosion spots" are unlikely to cause visual damages. Hence in practice harmful corrosion of steel fibres in uncracked concrete containing high content of chlorides is improbable inside the structure. But near the concrete surface corrosion can occur due to leaching processes of the concrete, which can be calculative estimated. The durability of steel fibres after the onset of corrosion is strongly dependent on the activity of the cathode. Though due to the small section of the fibres even low corrosion rates are sufficient to restrict the durability of corroding steel fibres