Effect of using recycled concrete of different age as fine aggregate on concrete properties

In this study, the physical and mechanical effects of fine aggregates obtained from recycled concretes of different ages on new concrete mixtures were experimentally investigated. In the research, fine aggregate fractions were recovered in the laboratory from concretes that had reached the end of their service life, approximately 20 years old (Type I) and 40 years old (Type II). These recycled aggregates were used to replace natural aggregates at weight-based replacement ratios of 10%, 30%, and 50% to produce new concrete mixtures. The specimen dimensions were set at 10 × 10 × 10 cm.To determine workability, a slump test was performed; and to assess mechanical properties, compressive strength measurements were conducted after curing periods of 7, 28, and 90 days. Additionally, non-destructive tests including Schmidt hammer and ultrasonic pulse velocity (UPV) tests were carried out; SEM, EDS, XRD, FT-IR, and TGA analyses were performed for microstructural and thermal characterization. The results indicated that increasing the recycled fine aggregate replacement ratio led to a certain decreasing trend in compressive strength. However, 10% and 30% recycled fine aggregate replacement ratios, particularly with Type I aggregates, maintained the strength values at acceptable levels. UPV and Schmidt hammer test results supported this trend. Microstructural analyses revealed that the porous texture and residual paste present in the recycled aggregates could cause changes in the compactness and bonding properties of the new concrete depending on the recycled fine aggregate replacement ratio.In conclusion, the use of fine aggregates recovered from end-of-life concretes at specific replacement ratios contributes to environmental sustainability and provides the potential to optimize concrete performance through proper proportioning and material selection. However, parameters such as the recycled fine aggregate replacement ratio and the age of the recycled material should be carefully evaluated.