Effect of endogenous chloride ion content and mineral admixtures on the passivation behavior of reinforcement embedded in sea-sand ultra-high performance concrete matrix

Abstract

Ultra-high performance concrete (UHPC) is typically defined as an advanced cementitious material which exhibits excellent mechanical property and superior durability. The development of UHPC made with sea-sand addresses the challenges associated with the shortage of river sand in producing concrete for a marine construction. In this study, UHPC matrix was prepared by using simulated sea sand with different levels of chloride and mineral admixtures included fly ash and slag. The passivation behavior of steel bars embedded in sea-sand UHPC specimens were investigated by electrochemical workstation. In addition, the microstructure of sea-sand UHPC matrix was characterized by rapid chloride test, alkalinity test and SEM-EDS. The experimental results showed that in the early age, the alkalinity of sea-sand UHPC matrix increases with the increment of endogenous chloride ion, and the introduction of fly ash in the matrix has higher alkalinity than slag. The corrosion current density icorr of steel bar in sea-sand UHPC gradually decreases with the age, and essentially being lower than 0.1 µA/cm2. Due to the pozzolanic effect, the utilization of fly ash and slag prolongs the stabilization age of icorr evolution. The reinforcement embedded is able to be in the stable passive state regardless of the endogenous chloride ion content. The incorporation of fly ash and slag has little effect on the densification of passivation film. Compared to the endogenous chloride ion level, the alkalinity of matrix has a more significant effect on the growth of the charge transfer resistance of steel bars in sea-sand UHPC, which is associated with the denser passive film and built-up of Ca(OH)2 layer near the steel bars.