Optimum mix design of recycled concrete based on the fresh and hardened properties of concrete

Abstract

Due to the finite sources of natural aggregates, recycled concrete aggregates can be used as a suitable and economic material for the production of recycled concrete. Use of the recycled aggregates in concrete may undermine some mechanical properties of concrete. The aim of this study is to determine an optimum mix design made with the recycled aggregates, so that the environmental and economic aspects and also the fresh and hardened properties of concrete are taken into account. In the first phase of this research, different recycled concrete mixes with different levels of replacement of the recycled fine and recycled coarse aggregates are tested respectively instead of the natural fine and natural coarse aggregates. The results show the replacement of fine aggregates at the level of 30% has no significant negative effect on the expected common fresh and hardened properties of concrete such as fresh density, workability and compressive strength, and so this mix design can be selected as the optimum recycled mix design. In order to extend the application of the recycled concrete for the structures that tensile and flexural strengths may be demanded too, in the second phase of the research, the effect of the adding of polypropylene and steel fibers on the tensile and flexural strengths of the optimum mix design of the recycled concrete is investigated. The results show that the use of the fibers as the partial replacement of cement has no significant effect on the fresh properties of concrete, while effectively increases the tensile and flexural strengths of the recycled concrete and improve crack resistance and brittle behavior of the concrete when failure occurs.