Mechanical Properties of End-of-life Waste Tyres Steel Fibre Reinforced Concrete: RSM-Based Modelling and Optimisation

Abstract

The influence of fibres on the compressive strength of concrete is complex and is determined by the type, quantity, and characteristics of the fibres utilized in designing and forming the concrete. Designing fibre reinforced concrete (FRC) constituents is challenging and affects the concrete's performance and practicality. This study utilized response surface methodology (RSM) to optimize the properties of concrete containing steel fibre extracted from end-of-life tyres (ELTs). Face-cantered central composite design (FC-CCD) of RSM was used in the design of experiments (DOE) with aspect ratio (10-70) and volume fraction (0.5 % - 1.2 %) as the input variables. Three levels of each variable were used in forming the design matrix. The resulting concretes were then tested for slump and compressive strength at 7 and 28 days of curing. As the aspect ratio and volume fraction increased, slump values decreased, and 7- and 28-day compressive strength increased up to an aspect ratio of 45 and 1.0% volume fraction. Beyond an aspect ratio of 45 and 1.0% volume fraction, a declining trend in compressive strength at all curing ages was observed. The Analysis of Variance (ANOVA) revealed that the variables volume fraction and aspect ratio significantly affect the variability in the FRC models, with all models being statistically significant at the 95% level across all factor levels. A numerical optimization method was used to determine the optimal mix proportions for ELTFRC. The optimum response values were achieved by combining a 1.01% volume fraction and a 33.86 aspect ratio, resulting in a desirability of 0.73.