Effect of steel fibers with galvanized coatings on corrosion of steel bars embedded in UHPC

Abstract

Abstract Ultra-high-performance concrete (UHPC) is widely accepted for its super-high durability. However, the corrosion performance of steel reinforcing bars embedded in UHPC and the improved protection of UHPC cover to steel bars are seldom studied. Furthermore, whether using steel fibers in UHPC will accelerate or slow down the corrosion of steel bars is of special interest. In this study, the effects of two types of steel fibers with different galvanized coatings (brass and zinc coated fibers) on the electrical resistance of UHPC and corrosion resistance of embedded steel bars were investigated. Electrochemical tests were conducted for 1 year in 3.5\u202fwt % NaCl solution. The tests included open circuit potential (OCP), linear polarization resistance (LPR), Tafel polarization, and electrochemical impedance spectroscopy (EIS). Test results showed that ASTM C876 was not applicable to UHPC to determine corrosion state of embedded steel bars based on the OCP values. This is because the dense microstructure of UHPC can limit the penetration of oxygen and significantly lower OCP values. The corrosion rates in terms of corrosion current densities of steel bars from the LPR and Tafel polarization tests showed a decreasing trend with time, hence indicating that the steel bars were in passive state. Furthermore, chloride was not detected in the UHPC matrix after the electrochemical tests. This proved that corrosion did not occur on the steel bar surface in the testing period. The electrochemical resistance of UHPC matrix, the resistance of passive film, and the charge transfer resistance of the steel bars from the EIS test displayed a long-term increasing trend. However, when the conductive steel fibers increased from 0 to 2%, by volume, the UHPC matrix resistance decreased by more than 40%. It was also found that the fiber type and fiber content did not affect the resistance of passive film and charge transfer resistance of embedded steel bars. Galvanic corrosion did not happen due to uniform distribution of the fibers along the cast specimens. When proper casting method was employed, both brass and zinc-coated steel fibers can be safely used in UHPC for fiber volume up to 2%.