Comparative study of the deformation and sorptivity of recycled aggregate concrete made by two mixing approaches

Abstract

Due to the seriousness of waste pollution in Hong Kong, it is difficult to dispose of the tons of construction and demolition (C&D) waste that are generated daily from construction sites. The recycling of concrete debris into recycled aggregates (RA) and the production of recycled aggregate concrete (RAC) are possible ways to reduce the burden on landfills in the territory.

As RA come from the crushing of old concrete debris, some defects are likely to exist that affect the properties of RAC. As RAC will be used in future building structures, durability tests are therefore required. To improve the quality of RAC, the two-stage mixing approach (TSMA) has been introduced, which forms a layer of cement slurry that fills the cracks and voids in the RA, thus producing an improvement in the interfacial zone of the aggregates at the beginning of the mixing stage.

In real-life situations, deformation and water sorptivity are the major concerns in concrete building structures. In this research project, drying shrinkage, creep, and water sorptivity tests on RAC that was mixed using the normal mixing approach (NMA) and the TSMA were carried out to investigate the impact of the two methods on the behavior and performance of RAC with different proportions of RA (0%, 20%, and 100% RA substitution).

The results showed that the TSMA performed better than the NMA, which can be explained by the improved interfacial transition zone (ITZ) in the RA that is generated by the former approach. However, the concrete sample that contained 20% RA performed best because the new cement slurry filled the cracks and pores in the RA, which resulted in stronger and denser RAC. In contrast, the sample with 100% had the worst performance because of the presence of multiple cracks and pores on the surface.