Effects of rust layer and corrosion degree on the pullout behavior of steel fibers from ultra-high-performance concrete

This study investigates the effect of surface corrosion on the pullout behavior of straight steel fibers embedded in ultra-high-performance concrete (UHPC). To this aim, straight steel fibers, either with or without surface corrosion, were utilized, and various corrosion degrees from 2% to 15% by weight were considered. To evaluate the implication of rust layer on the pullout behavior of corroded steel fibers from UHPC, both washed and unwashed conditions were considered. The surface roughness of plain and corroded steel fibers was analyzed by means of scanning electron microscope and atomic force microscope (AFM) images. The test results indicated that surface corrosion is effective in enhancing the pullout resistance of straight steel fibers in UHPC when the fibers are completely pulled out from the matrix without breakage. The maximum average bond strength and pullout energy of moderately corroded fibers in UHPC were found to be 18.5 MPa and 715.7 N·mm, approximately 2.7 and 1.8 times higher than those of plain fibers in the same matrix at the aligned condition. The benefits of moderate surface corrosion on improving the pullout resistance were mitigated by inclining the fibers. A higher corrosion degree led to a better pullout resistance up to a certain value (2 or 5%); however, beyond such value, the resistance decreased significantly due to the rupture of fibers. A threshold value of 2% for the corrosion degree was thus suggested to achieve an excellent fiber bridging capability. The washed corroded fibers exhibited higher bond strength and pullout energy than the unwashed ones with the same degree of corrosion at aligned condition; however, the benefits of washing vanished when the fibers were inclined and ruptured prematurely. An obvious correlation between the bond strength and the surface roughness was observed from the AFM images.