Optimisation of mechanical properties of polyethylene terephthalate fibre/fly ash hybrid concrete composite

The study focuses on the prediction and optimisation using modelling with respect to the fresh and hardened properties of polyethylene terephthalate (PET) fibre reinforced concrete containing partial cement replacement with fly ash. Full factorial experimental design methodology was employed to fabricate the test specimens by simultaneously varying the independent factors to develop a model for overall response variation. Numerical optimisation was carried out concerning the fibre reinforced concrete's fresh and hardened mechanical properties. Predictive modified quadratic equations were developed for slump value, compressive, flexural, split tensile strength and total cost. Analysis of variance test was carried out for all the responses indicated and the developed model could predict the slump value and mechanical properties of the fibre reinforced concrete correctly and effectively with a coefficient of determination in the range of 0.5–0.9467. The optimum constituent combination for maximum mechanical strength at the lowest possible cost was found to be 15.76 % fly ash and 0.32 % PET fibre. These optimum values corresponded to responses of 25.65 MPa, 3.69 MPa, 2.36 MPa, 31.48 mm, R2744.17 for compressive strength, flexural strength, tensile strength, slump value and total cost, respectively. These predictions were validated experimentally, and a good correlation was observed between the actual and predicted values based on the observed standard deviations of 0.1335, 0.031, 0.005, 0.676, 0.02 for compressive strength, flexural strength, tensile strength, slump value and cost, respectively. Concrete slabs were optimised for various possible end uses, and the optimum PET fibre % and fly ash % were ascertained. The optimum combination for suspended slabs was 19.09 % for fly ash and 0.34 % PET fibre. These values corresponded to responses of R2742.03/kg/m3, 2.30 MPa, 3.68 MPa, 24.77 MPa, 30.60 mm for the total cost, split tensile, flexural, compressive strength and slump value. For foundation slabs, the optimum combination was 3.94 % fly ash and 0.65 % PET fibre, and the corresponding response values are R 2760.47 /kg/m3, 1.82 MPa, 3.26 MPa, 19.32 MPa and 10.78 mm for a total cost, split tensile, flexural, compressive strength and slump value, respectively. For paving slabs, the optimum combination was 0.65 % PET fibre giving a response of R 2775.13 kg/m3, 1.41 MPa, 3.29 MPa, 19.11 MPa and 9.92 mm for total cost, split tensile, flexural, compressive strength and slump value, respectively.