Effect of matrix Self-Healing on the Bond-Slip Behavior of Micro Steel Fibers in Ultra High-Performance Concrete

This study investigates the bond-slip behavior of micro steel fibers embedded into an Ultra High-Performance Concrete (UHPC) matrix as affected by the self-healing of the same matrix in different exposure conditions. The UHPC matrix contains a crystalline admixture as promoter of the autogenous self-healing specially added to enhance the durability in the cracked state. To the aforesaid purpose, some samples were partially pre-damaged with controlled preload (fiber pre-slip at different levels) and subjected to one-month exposure in 3.5% NaCl aqueous solution and in tap water to study the fiber corrosion, if any, and the effects of self-healing; after that, they were subjected to a pull-out test, to be compared with the behavior of analogous non pre-slipped samples undergoing the same curing history. Moreover, some samples were cured in the chloride solution, intended to simulate a marine environment, to study the effect of marine curing on the pull-out behavior of steel fiber. The steel fiber corrosion and self-healing products attached on the surface of steel fiber were analyzed via the Scanning Electron Microscopy (SEM), and Energy -Dispersive Spectroscopy (EDS). The results indicate that the new healed particles formed on the highly damaged fiber-matrix interface significantly enhance the friction phase of the bond-slip behavior and result into a significant residual capacity compared to non-pre-slipped specimens. On the other hand, the self-healing effect in specimens subjected to low damage pre-slip contributed more to the chemical adhesion region of the bond-slip behavior. Owning to the dense microstructure of the matrix, curing in 3.5% NaCl aqueous solution was not found to significantly affect the pull-out resistance as compared for the samples cured in tap water.