Optimization of Functionally Graded Concretes Incorporating Steel Fibres and Recycled Aggregates

Functionally graded material (FGM) refers to a class of material produced with grading composition and structure to achieve enhanced performance compared to homogeneous materials. Several studies have explored the application of the concept of FGM to enhance the flexural behaviour of concrete, producing functionally graded concrete (FGC). Previous results indicated that FGC produced with fibre reinforced recycled aggregate concrete (FRRAC) exhibited higher residual flexural strength than homogeneous FRRAC for ratios of reinforced height to total beam height (h/H) equal or higher than 0.75, demonstrating the benefits of FGC with FRRAC. Hence, this study aims to verify the optimum value of h/H to obtain the highest residual flexural performance of the FGC. To achieve this goal, an experimental program was carried out, in which, FGC fabricated with FRRAC was assessed under bending considering a content of fibre of 0.50% in volume, and values of h/H ranging from 0.70 to 1.00. The effect of h/H in fibre orientation was also evaluated using the inductive method. The results indicated that the highest residual flexural strength is obtained with h/H = 0.90. However, a balance between pre-cracking and post-cracking behaviour should be defined for each application, resulting in optimized values of h/H in FGC with FRRAC. Furthermore, since the fibre orientation was not affected by h/H, the same orientation factor used in the design of the fibre reinforced concrete elements can be adopted for FGC elements, increasing the potential application of FGC.