Experimental Studies on Mechanical Properties of Recycled Concrete


Student thesis: Doctoral Thesis

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Awarding Institution
Award date12 Aug 2019


It is well known that the construction industry contributes to economic growth and infrastructural development of nations through provision of infrastructures such as housing, bridges, roads and diverse institutional buildings that are critical to improving the quality of lives of citizens of every nation. Unfortunately, the construction industry is also resource and energy intensive and contributes to environmental degradation through resource depletion, waste generation and disposal in landfills with attendant environmental impacts. In order to decouple economic growth from environmental degradation and meet both present and future infrastructural needs, the construction industry is being challenged to embrace sustainable development via sustainable construction and this challenge has attracted global attention of various national governments, business organizations, policy makers and numerous researchers across different fields.

The effects of elevated temperature on recycled and conventional concrete was investigated. The study revealed mass loss, thermal cracks and colour changes with increasing elevated temperature and significant reduction in load-carrying capacity. The experimental investigation also revealed dominance of shearing mode of failure at ambient and low elevated temperature as well as increasing predominance of buckling mode of failure at high elevated temperature. Therefore, utilization of shrinkage-resistant cement, high temperature grade recycled coarse aggregates and improvement of interfacial transition zone of the concrete matrix is recommended to improve fracture resistance of concrete at elevated temperature.

The present study also investigated closed-loop recycling of recycled aggregates from construction and demolition wastes (CDW) as one of the pragmatic ways to achieve sustainable construction and development, which has been highlighted by several reseachers. The study revealed that closed-loop recycling of mixed recycled aggregates is technically and economically feasible and environmentally friendly with a payback of three (3) years. The identified barriers to closed-loop recycling include behavioral, technical, legal and market barriers and can be overcomed through proposed measures such as government regulation, policies and incentive programs, upgrading and provision of decentralized recycling centres and sale outlets. Appropriate combinations of well-sorted and pre-treated recycled coarse aggregates (RCA) and recycled fine aggregates (RFA) were recommended to meet different durability requirements for structural applications under stringent and non-stringent conditions were derived. Also, adequate characterization, sorting and pre-treatment is recommended for heterogeneous mixed recycled aggregates.

Furthermore, this study also investigated the suitability of epoxy-coated CFRP (carbon fibre reinforced polymer) laminates as alternative reinforcements in recycled concrete beams containing 30% recycled coarse aggregates. This study revealed the applicability of the epoxy-coated CFRP composite to significantly improve the load-carrying capacity, stiffness, fracture toughness and fracture energy of recycled concrete beams and the improvement depends on the fibre architecture. The enhanced mechanical properties of the CFRP-reinforced recycled concrete beam was attributed to the strain-path changes known as Bauschinger effect which is common in multiphase composite materials and was responsible for the elasto-plastic deformation properties of the beams. Therefore, proper combination of CFRP laminates with recycled concrete is recommended and has high potential for achieving sustainable, eco-friendly construction.

In addition, the next stage of this research investigated the feasibility of epoxy-coated CFRP laminates as potential reinforcements and confinement in reinforced concrete columns containing recycled concrete. The study investigates the confinement effects of external epoxy-coated CFRP thickness and type as well as effects of internal diameter and spacing of epoxy-coated CFRP laminates as potential reinforcement. The optimized result was strip confinement with two layers and without laminates and indicated the superiority of strip confinement compared to full confinement in terms of fracture toughness, fracture energy and ductility. Strip confinement is recommended for confining concrete columns.

Lastly, this study also investigated the potential utilization of CFRP-reinforced mixed recycled concrete for sustainable construction. The study optimized the mechanical properties of the CFRP-reinforced mixed recycled concrete beams with a view to reduce material consumption in line with objectives of sustainable construction. The optimized result was recycled concrete beam internally reinforced with 120 mm spaced CFRP composite, two layers of inclined CFRP anchor, without external CFRP laminates and one layer of tensile (soffit) laminate. The study also revealed that deflection and ductility is largely controlled by anchor type, confinement type and spacing of internal CFRP confinement while fracture energy and fracture toughness is largely controlled by spacing of internal reinforcement and confinement type. The study suggested that the most significant factors to improve the mechanical performance of recycled concrete beams are the spacing of the CFRP reinforcement and the CFRP anchor type. Therefore, this study recommends CFRP-reinforced mixed recycled concrete for sustainable construction.

In summary, government needs to play active leading role through high-impact policy making and implementation in order to transition the construction industry towards sustainable development path. This can be achieved through putting in place appropriate regulatory framework and standards, demonstration projects of advanced manufacturing methods in construction characterized by zero-waste manufacturing and zero-emission, and mandating their applications in government-funded projects.