Mechanical and Durability Performance of Recycled Coarse Aggregate Concrete Modified with Class-F Fly Ash for M50-Grade Pavement Applications

India generates roughly 150 million tonnes of construction & demolition (C&D) waste annually, of which less than 1 % is currently recycled into structural concrete. This study examines the suitability of recycled coarse aggregate (RCA) — sourced from demolished low-rise residential buildings in Buldhana district — as a partial replacement (0, 25, 50, 75, 100 %) for natural coarse aggregate in M50-grade pavement-quality concrete. Class-F fly ash from the Koradi thermal power station was incorporated at 0, 10, 20 and 30 % of the binder mass. Twenty mix proportions were cast and tested for compressive strength (3, 7, 28, 56, 90 d), water absorption, slump, hardened density and chloride permeability. The 50 % RCA + 20 % fly ash mix attained 36.2 MPa at 28 days (89 % of the control), reduced cement consumption by 20 %, embodied CO₂ by 17 %, and material cost by ₹680 per cubic metre. A four-parameter multiple-linear regression model with R² = 0.925 is proposed to assist site engineers in predicting strength.