Insights into mechanical properties and sustainability of packing-enhanced recycled aggregate concrete

Abstract

The use of recycled concrete aggregates (RCAs) and recycled concrete powder (RCP), produced by processing waste concrete, is a promising strategy to mitigate the depletion of natural aggregates and the environmental burden associated with disposing of construction and demolition wastes. However, the use of recycled aggregates generally has negative impacts on the properties of concrete, particularly for those with a high RCA amount. To mitigate the adverse impacts of using recycled materials, one of the effective methods is to improve concrete mix design through particle packing optimisation.

This study first focuses on understanding the role of aggregate packing enhancement in the properties of natural aggregate concrete (NAC) and recycled aggregate concrete (RAC). The results indicate that increasing the aggregate packing density can help to densify the granular skeleton of aggregates in concrete by reducing the voids around the aggregates and the cement paste film thickness, which subsequently enhances the mechanical properties of the concrete, including the compressive strength, flexural strength, and Young's modulus. This enhancement is higher for RAC than NAC, particularly for Young's modulus. Optimising the aggregate packing status can decrease the number of macropores in RAC, contributing to the enhancement in the mechanical properties of RAC.

This study subsequently investigates the effects of cement paste volume (CPV) and sand-to-aggregate volume ratio (Bs) on the properties of packing-optimised RAC, followed by a microstructure study to reveal their influencing mechanisms. The results indicate that the slump of RAC increases with the CPV at a growing rate, which is mainly attributed to the increased lubrication effect of fresh paste on the aggregates. However, increasing the Bs decreases the slump of RAC due to the reduced cement paste film thickness. Increasing the CPV decreases the macroporosity of RAC, as it reduces the amount of RCAs, thereby enhancing its compressive strength and Young's modulus. However, excessive CPV increases the porosity and thickness of interfacial transition zones, which subsequently weakens the mechanical properties of RAC. The increase of Bs first enhances the granular skeleton by optimising the particle packing status, which is beneficial to RAC properties. However, excessive Bs can lead to an increase in macroporosity and mean pore size inside RAC, degrading its mechanical properties. Nonetheless, the flexural strength of the RAC is marginally influenced by the CPV or Bs.

This study further explores the use of RCP as a potential sand alternative by understanding its role in affecting the properties, particle packing density, hydration reaction, and microstructures of cement mortar. The results indicate that increasing the RCP content increases the water demand and decreases the dry bulk density of mortars due to the inferior characteristics of RCP. Replacing sand by 10-20% RCP has a negligible or slightly positive impact on the mechanical properties of mortars. This is mainly attributed to a decreased volume fraction of large capillary pores, air voids, and total porosity, which overcomes the negative effects induced by RCP. Specifically, incorporating RCP can reduce the fraction of large voids through its filling effect and slightly promote hydration in mortars. Besides, the drying shrinkage of mortars increases with the RCP replacement ratio due to the increased volume fraction of mesopores in the RCP mortars. However, an excessive amount of RCP increases the volume fraction of capillary pores and the total porosity of mortars due to the combined action of the decreased packing density and the high porousness of RCP particles, which subsequently decreases the mechanical properties of mortars and further increases the drying shrinkage of RCP mortars.

The environmental benefits of adopting packing-optimised RAC and using RCP as sand replacement are quantified via life cycle assessment (LCA). The environmental impacts of global warming, stratospheric ozone depletion, terrestrial acidification, freshwater eutrophication, land use, and non-renewable energy consumption are evaluated. The results indicate that the adoption of packing-optimised RAC can decrease most environmental impacts, except for land use. However, using RCP as sand replacement can further decrease all the considered environmental impacts of packing-optimised RAC, particularly for land use. Replacing 20% river sand with RCP can decrease the land use of packing-optimised RAC by 12%. Overall, the findings of the LCA study can offer valuable guidance for optimising the mix design of RAC with respect to environmental protection.