Hybrid Fiber Reinforcement in HDPE–Concrete: Predictive Analysis of Fresh and Hardened Properties Using Response Surface Methodology

Plastic waste accumulation has driven research into recycling solutions, such as using plastics as partial aggregate substitutes in concrete to meet construction needs, conserve resources, and reduce environmental impact. However, studies reveal that plastic aggregates weaken concrete strength, creating the need for reinforcement methods in plastic-containing concrete. This study used experimental data from 225 tested specimens to develop prediction models for the properties of concrete containing macro-synthetic fibers (MSFs), steel fibers (SFs), and high-density polyethylene (HDPE) plastic as a partial substitute for natural coarse aggregate (NCA) by volume utilizing response surface methodology (RSM). HDPE plastics were used as a partial substitute for NCA by volume at levels of 10%, 30%, and 50%. MSFs were added at levels of 0, 0.25%, 0.5%, and 1% by volume of concrete, while SFs were added at levels of 0, 0.5%, 1%, 1.5%, and 2% by volume of concrete. The input parameters for the models are the ratio of HDPE, the dose of MSF, and the dose of SF. The responses are the slump value, the compressive strength (CS), the splitting tensile strength (TS), and the flexural strength (FS) of concrete. The significance and suitability of the developed models were assessed and validated, and the parameters’ contribution was investigated using analysis of variance (ANOVA) and other statistical tests. Numerical optimization was used to determine the best HDPE, MSF, and SF ratios for optimizing the mechanical properties of concrete. The results demonstrated that replacing NCA with HDPE plastics increased the workability and decreased the strength of concrete. The results demonstrated the applicability of the developed models for predicting the properties of HDPE–concrete containing MSFs and SFs, which agreed well with the data from experiments. The created models have R2 values more than 0.92, adequate precision more than 4, and p-values less than 0.05, showing high correlation levels for prediction. The RSM modeling results indicate that the inclusion of MSFs and SFs improved the mechanical properties of HDPE–concrete. The optimum doses of MSFs and SFs were 0.73% and 0.74%, respectively, of volume of concrete, leading to improvement in the mechanical properties of HDPE–concrete. This approach reduces plastic waste and its detrimental environmental impact. Further development of models is needed to simulate the combined effects of different fiber types, shapes, and dosages on the performance and durability of plastic-containing concrete.