Sustainable solution to recycled concrete: Improving structural applications

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Walker, La Sasha
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Construction and demolition (C&D) or debris concrete is an underutilized resource often landfilled or recycled as a non-structural fill material. This study’s main objective is to examine the potential to expand reuse options for recycling concrete, specifically for use in structural applications. In 2014, the United States landfilled approximately 375 million tons of concrete, some of which could have been diverted to offset the 1 billion tons of stone aggregate produced that same year. Thus, expanding productive reuse options for debris concrete can be an important contribution to sustainability. This study will address the knowledge gaps surrounding expanding utilization of recycled concrete in structural concrete by focusing on two main aspects: (1) use of recycled concrete aggregate as replacement of coarse aggregate in structural concrete, and (2) use of recycled concrete fines as partial replacement of cement. On the structural scale, locally sourced recycled concrete was used as 100% of the coarse aggregate in the recycled aggregate concrete, and the influence of maximum size aggregate was assessed in shear and flexure. The durability of recycled aggregate concrete was assessed through measures of permeability, resistance to alkali–silica reaction, and service life modeling of structural elements in chloride-rich environments, all in comparison to companion concrete using natural coarse aggregate. The potential use of recycled concrete fines, smaller than 74μm, as a supplementary cementitious material or as filler at 20% by replacement of cement was explored. To increase the reactivity of recycled concrete fines, several techniques were used to activate the fines, including calcining and ball milling. The recycled aggregate concretes produced exhibited comparable strength and durability comparable to natural aggregate concrete at 100% replacement of coarse aggregate. Recycled aggregate concrete in comparison to natural aggregate concrete at the same maximum size aggregate had -9% to 3%, -0.5 to 2.5%, -15% to 6.9% difference in compressive strength of 6 ksi at 28 days, flexural strength of 780 psi at 28 days, and shear strength of 800 psi at 28 days, respectively. Both natural aggregate concrete and recycled aggregate concrete had high to moderate permeability level. Maximum aggregate had negotiable effect on the performance of recycled concrete aggregate. Nevertheless, of the three maximum size aggregates tested in general 0.75-inch best structural performance. Thermogravimetric analysis show that recycled concrete fines do not exhibit pozzolanic reactivity and instead act as filler in cement systems. Activation techniques did enhance the reactivity of recycled concrete fines, as assessed through examination cement hydration kinetics and these improvements were linked to increases in surface area, reductions in impurities, and modified chemical composition. Of the methods explored calcining at 750°C produced the most reactive fines. This work demonstrates that, with proper mix design, recycled concrete can be used as coarse aggregate – at 100% rate of aggregate replacement – and as SCMs or fillers – at 20% rate of cement replacement – in structural concrete.
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