Corrosion of partially and fully debonded steel fibers from ultra-high-performance concrete and its influence on pullout resistance

Abstract The corrosion behaviors of partially and fully debonded steel fibers embedded in ultra-high-performance concrete (UHPC) and their impact on the pullout behavior were investigated. For this purpose, pre-loaded dog-bone samples were immersed in a 3.5% standard sodium chloride (NaCl) solution for 4, 10, and 20 weeks and then pull-out tested. The degree of corrosion on the fiber surface was determined by energy-dispersive X-ray spectroscopy. The test results indicated that the bond strength and pullout energy are improved by 54% and 90% maximally due to surface corrosion up to an immersion duration of 20 weeks. The effect of surface corrosion on the pullout properties was insignificantly influenced by the fiber pullout state (partially or fully debonded) up to 10 weeks, but the fully debonded fiber was prematurely ruptured at the longest duration of 20 weeks. The corrosion resistance of steel fiber in UHPC was earlier deteriorated, given the full debonding state (or the larger slip). The chemically debonded region was evidently corroded due to ferric oxide formation even after the shortest immersion duration of 4 weeks, whereas the bonded region with highly densified microstructures was insignificantly corroded. By surface corrosion, the pullout energy of the steel fiber in UHPC increased more significantly as compared with the average bond strength, owing to the twice or more frictional shear stress acting at the fiber–matrix interface.