RSM-CCD Analysis of Sustainable Concrete with Complete Replacement of Natural Coarse Aggregate Using Pine Cone Waste: Towards Performance and Optimization

The increasing threat of forest fires in pine-dominated areas, driven by the extremely flammable nature of pine cone waste, calls for sustainable management strategies. The present research proposes a novel use of pine cone scale (PCS) waste as a complete substitute for natural coarse aggregate in structural lightweight concrete. With Response Surface Methodology with Central Composite Design (RSM-CCD), the impact of cement content (480–550 kg/m³), water–cement ratio (0.4–0.7), and PCS/sand ratio (0.4–0.75) (volume ratio of PCS as coarse aggregate to sand as fine aggregate) on compressive strength, slump, and hardened density was examined. Statistically verified polynomial equations (R² > 0.95) predicted performance with satisfactory accuracy, with deviations less than 5%. Optimization selected two PCSC mixes (PCSC-I and PCSC-II) that were observed to have experimental compressive strength values of 17.86 MPa and 20.08 MPa, corresponding slump values of 42–50 mm, and oven-dry densities less than 2000 kg/m³, meeting ASTM C330 and IS 456:2000 specifications for structural lightweight concrete. The results confirm PCS as an efficient bio-based coarse aggregate with the twin advantages of waste valorization in a sustainable manner and reduction of wildfire risk while producing structurally efficient lightweight concrete.