Effect of ITZ on chloride ion transport in recycled aggregate concrete: Analytical and numerical studies

Abstract

The weaker ITZ is one of the significant differences between recycled concrete and ordinary concrete. How the weaker ITZ between recycled aggregate and mortar in recycled concrete affects the transport mechanism of chloride ions has become the main obstacle to the large-scale promotion and recycling of demolished building waste. To tackle this issue, a novel equivalent single-phase analytical method was proposed and a series of numerical simulations with different ITZ distributions and characteristics were carried out. The research results indicated that the transport of chloride ions in recycled aggregate concrete was primarily controlled by the diffusion coefficient, volume, and distribution of the ITZ (mainly affected by recycled aggregates). The current analytical methods effectively described the aggregate blocking effect and ITZ effect. The research results revealed that the addition of recycled aggregate within a certain range had little effect on chloride ion transport (the difference in the effect of 30 % and 35 % recycled aggregates addition on chloride ion transport was within 5 %). However, once the volume fraction of aggregates exceeded the threshold, the transfer rate of chloride ions increased sharply. Furthermore, the uniform mixing of recycled aggregates was crucial. As the ITZ non-uniformity coefficient increased, the ITZ diffusion coefficient increased. The weaker ITZ of recycled concrete amplified the impact of concrete cracks, ITZ-crack channels of chloride ions were formed even if minor cracks developed. Current research provided technical support for the durability design of recycled concrete structures.