Performance of Functionally Graded Concrete incorporating Recycled Aggregate Concrete and Fiber Reinforced Concrete Exposed to Elevated Temperatures

Abstract

The study explores the mechanical properties of Functionally Graded Concrete (FGC), a sustainable and innovative construction material in comparison to normal concrete. Five types of mixes were prepared namely NC with natural coarse aggregates, RBAC with recycled brick aggregates replacing natural aggregates by 25%, and RAC with recycled concrete aggregates replacing natural aggregates by 25%, CFRC and PFRC with coir fibers and macro polypropylene fibers utilized in concrete by 0.5% of its volume. Six FGC combinations with NC, RBAC and RAC were used in top layer and CFRC and PFRC were utilized in bottom layer. The compressive, split tensile and flexural strength experiments were conducted at both normal temperatures and elevated temperatures of 200°C, 400°C, and 600°C to evaluate the material performance. Results revealed that PFRC exhibits the highest compressive strength (44.23 MPa), outperforming NC by 10.77%. At elevated temperatures, PFRC shows a notable strength increase of 25.21% at 600°C compared to NC. CFRC also shows improved performance at higher temperatures, with a 5.76% increase at 600°C. All FGC combinations show a decrease in compressive strength as compared to NC at normal temperatures, while certain FGC combinations like NC+PFRC and NC+CFRC, exhibit increased strength of about 11% and 4.6 % at 600 °C as compared to NC All concrete mixes, except PFRC, experience a decline in elastic modulus at elevated temperatures, with PFRC showing improved elastic modulus. PFRC also shows an increase in compressive toughness across all temperatures. All FGC samples show overall improvement in compressive toughness compared to NC at both normal and elevated temperatures. PFRC also shows significant improvements in flexural strength compared to normal concrete. Both FGC Samples like RAC+PFRC and NC+PFRC display improvements in flexural strength at normal temperature. PFRC shows a 20% increase in flexural toughness at normal temperature and a 27% increase at 200°C. FGC combinations generally improve toughness with decrease in compressive strength and increase in flexural strength at normal temperatures as compared to NC, with certain combinations like NC+PFRC and considering recycled aggregates, RAC + PFRC being the best combinations.