Microstructural study of steel-concrete interface and its influence on bond strength of reinforced concrete

Abstract

In this investigation, the variations in steel-concrete interface (SCI) properties, such as porous zone thickness and calcium hydroxide content around the reinforcing steel, were studied with respect to curing time. Three kinds of commercially used cements, ordinary portland cement (OPC), portland pozzolana cement (PPC), and portland slag cement (PSC), were used, and their significance regarding SCI properties was investigated. A reliable thresholding grayscale-based technique was used to determine the porous zone thickness at the SCI. The properties of SCI were found to be quite influenced by the curing period. The PSC concrete showed significant reduction in mean porous zone thickness at SCI compared with OPC and PPC concrete after 90 days of curing. The reduction in mean porous zone thickness can be considered one of the many influencing factors that resulted in increased ultimate bond strength at 90 days of curing. Also, the variation in calcium hydroxide content from the SCI toward the bulk concrete was examined with a scanning electron microscope empowered with energy-dispersive spectroscopy. The findings indicate a gradual decrease in calcium hydroxide content away from the steel surface toward the bulk concrete. The prolonged curing resulted in a slightly higher reduction of calcium hydroxide content around the SCI for PPC and PSC concrete because of the pozzolanic reactions. Higher reduction of calcium hydroxide content around the SCI for PPC and PSC concrete is predicted to be the reason for improved ultimate bond strength after prolonged curing. © 2019 by ASTM International.