The purpose of this paper is to investigate the diffusivity of the interfacial transition zone (ITZ) experimentally using cementitious composite materials made of rod-shaped aggregates and cement mortar. Two types of tests were conducted: the porosity distribution of the ITZ at different distances from aggregate surface was measured by backscattered electron images, and the water vapor diffusivity of the composites was measured by the penetration rate of water vapor through thin slides of specimens. The surface roughness of rod-shaped aggregates was considered as a testing parameter. The diffusivity of ITZ was calculated based on the measured diffusivities of the composite and it was also modeled according to the measured porosity distribution within the ITZ. The results showed that the porosity of ITZ decreases with increasing distance from the aggregate surface, a rougher aggregate surface decreases the porosity of ITZ in the first 10 μm from the aggregate surface, and the ratio of averaged wate...

Chloride-induced steel corrosion is the most concerning issue for the durability of concrete structures. Concrete and steel samples were obtained from a 30-year-old reinforced concrete bridge. The chloride content was measured by a potentiometric titration method and the microstructure of concrete was obtained by scanning electron microscopy and mercury intrusion porosimetry. The rust phases of the steel were detected by X-ray diffraction and Raman analysis. It was found that the convection depth for chloride transport in cracked concrete was significantly larger than that in uncracked concrete. The concrete in a pier column facing upstream had greater porosity due to the water impact and calcium leaching. The coefficients of variability of chloride diffusivity of concrete for the bridge deck and the pier column were significantly different. Rust phases including lepidocrocite, goethite, akaganeite, magnetite, and maghemite were detected using Raman spectroscopy and X-ray diffraction. The major phases of steel rust in the atmospheric zone were lepidocrocite and goethite, while they were lepidocrocite and maghemite in the tidal zone. The results of this study would provide information concerning the chloride-induced steel corrosion under a marine environment in order to predict long-term behaviors of a reinforced concrete structure.

The use of seawater as mixing water for concrete production is one of the most efficient strategies to solve freshwater shortage problems. The purpose of this paper is to investigate the effects of seawater mixing on hydration, microstructure, and strength development of cement pastes. Multiscale tests were used for cement pastes mixed with seawater, NaCl, and ${\mathrm{Na}}_2{\mathrm{SO}}_4$ solutions, respectively. The chloride ions in seawater increased the hydration degree during the process for nucleation and crystal growth and shortened the period for phase boundary reactions as a dominant process. At the age of 28 days, ettringite and Friedel’s salt coexisted for the seawater mixing paste, whereas the ettringite almost disappeared for the deionized water mixing. The seawater mixing led to a less porous cement matrix with much finer pores. Furthermore, seawater mixing enhanced the compressive strength and bending strength at early ages but later the enhancement decreased with curing time. ${\mathrm{Na}}_2{\mathrm{SO}}_4$ in the seawater enhanced the strength of the cement paste before 1 day but later inhibited the strength development.