Optimization of UHPC Mix Design Using Polyacrylonitrile Fibers and Coarse Aggregates for Cost Reduction

To reduce the production cost of ultra-high performance concrete (UHPC), this study incorporated polyacrylonitrile (PAN) fibers and coarse aggregates (CA) to develop a novel UHPC with both excellent performance and reduced cost. A two-stage mortar-concrete design approach was employed to optimize the UHPC mix proportion. First, the mortar matrix was preliminarily optimized based on particle packing theory, and its strength development mechanism was analyzed. Subsequently, response surface methodology was applied to systematically investigate the effects of PAN fiber content, CA content, and superplasticizer (SP) dosage on the slump flow, compressive strength, flexural strength, indirect tensile strength, freeze–thaw resistance, and dynamic mechanical properties of UHPC. The entropy weight method was then adopted to determine the optimal mix proportion, followed by cost estimation. The results indicate that the optimal mortar matrix composition consists of 61.4% cement, 15% silica fume, and 23.6% fly ash, achieving a flow spread of 246 mm, a compressive strength of 117.2 MPa, and a flexural strength of 24.9 MPa. When the PAN fiber content, CA content, and SP dosage were 0.5%, 20%, and 3.8%, respectively, the prepared PAN-CA UHPC(PCUHPC) exhibited the best overall performance. Compared with conventional UHPC, the material cost was reduced by 81.7%, and the compressive strength-normalized cost decreased by 75.4%. The UHPC developed in this study, characterized by outstanding performance and significant cost advantages, provides a feasible solution and theoretical support for broader engineering applications.