Predictive Modeling and Optimization of Compressive Strength for Cold In-Place Recycling Base Course Incorporating Ground Coal Bottom Ash and Calcium Chloride

Pavement rehabilitation faces increasing challenges from aging infrastructure, moisture damage, and traffic-induced fatigue. Cold In-Place Recycling (CIPR) provides a sustainable solution by reusing reclaimed asphalt pavement (RAP) and reducing reliance on virgin materials. This study evaluated the compressive strength of CIPR base mixtures composed of crushed aggregate (CA) and RAP stabilized with Ordinary Portland Cement (OPC), ground coal bottom ash (GCBA), and calcium chloride (CaCl 2 ). Three CA-RAP ratios (CA75RAP25, CA50RAP50, and CA25RAP75) were tested, with total stabilizer content fixed at 4% and CaCl 2 varied from 0% to 3% of the total dry weight of the CA-RAP mixture. Unconfined Compressive Strength (UCS) tests were conducted at 1, 3, 7, 14, and 28 days to assess strength development. Response Surface Methodology (RSM) was employed to model and optimize the effects of CaCl 2 and curing duration, producing statistically significant models ( p < 0.05, R 2 > 0.79). Results indicated that CaCl 2 enhanced early-age strength, GCBA was most effective at moderate OPC levels, and UCS declined when CaCl 2 exceeded 2%. The CA75RAP25 mix with 2% OPC and 2% GCBA achieved the highest UCS improvement. These findings highlight the interactive effects of stabilizer contents and curing duration, offering guidance for optimizing CIPR mixtures. Incorporating GCBA and CaCl 2 can enhance compressive strength, support sustainable reuse of industrial by-products, and improve the performance of pavement rehabilitation projects.