Multi-recycling of different concrete products: Effects on recycled aggregate's physical characteristics and compressive strength

The utilisation of recycled aggregate from construction and demolition waste (CDW) as a replacement for fine and coarse natural aggregate has been increasing in recent years. The purpose of this study is to examine the feasibility and limitations of multiple recycling of concrete aggregates, which represents a novel contribution in understanding the extent to which CDW can be repeatedly reused. This research aims to reduce the amount of construction waste sent to landfill and reduce carbon emissions. An experimental investigation was carried out on eleven randomly selected natural aggregate concrete products available on the market. The parent concrete was used to create the first-generation recycled concrete aggregates by crushing with a hammer. Within the concrete products happened to have two different fibre-reinforced composites and one manufactured aggregate were examined as well. The investigation assessed the aggregate morphology, density and particle size distribution through three recycling cycles. The investigation found that increasing the number of recycling cycles for all types of aggregates increased the angularity, the volume of coarse aggregates and water absorption while fine particles was reduced giving way to mortar paste and the compressive strength of each subsequent concrete was reduced. By the end of the third recycling cycle, all aggregates turned into 80 % cement paste. The rate of physical and mechanical performance change decreased with each cycle but did not settle by the third cycle, thus a conclusive conclusion could not be formed, although the trend was noticed. The decrease was asymptotic with the number of recycling cycles. It was also discovered that the multiple recycling procedure replaced 80 % of the parent aggregate volume by the third recycling cycle and, for mixes containing fibres, it damaged 98 % of the fibres resulting in a full loss of fibre performance. These findings demonstrate that it is only possible to recycle concrete by a finite number of times before significant deterioration in quality occurs, limiting its long-term reuse potential.