Performance of cement stabilized cold-in-place recycling with ground coal bottom ash and calcium chloride

Cold In-Place Recycling (CIPR) is a pavement rehabilitation technique where existing deteriorated asphalt pavement is pulverized, stabilized, and compacted in place to form a new base layer. In Malaysia, Ordinary Portland Cement (OPC) is the most commonly used stabilizing agent. However, cement-treated layers are highly dependent on proper curing to achieve adequate hydration and early strength development. Insufficient curing often leads to premature cracking and long-term performance issues. This research investigated the use of Ground Coal Bottom Ash (GCBA) and Calcium Chloride (CaCl₂) with OPC to accelerate early strength development and improve the mechanical performance of CIPR base layer. In addition, predictive models were developed using Response Surface Methodology (RSM) to analyze the interaction effects among the variables and to optimize the material design parameters. Three combinations of Crushed Aggregate (CA) and Reclaimed Asphalt Pavement (RAP) were examined: CA75RAP25, CA50RAP50, and CA25RAP75. Each mix was stabilized with varying contents of OPC (1-4%), GCBA (0-3%), and CaCl₂ (0-3%) by weight of material. The research was conducted in four phases, beginning with the characterization of material properties, followed by compaction testing to determine Optimum Moisture Content (OMC), Maximum Dry Density (MDD) and bulk density. Strength evaluation was then conducted using Unconfined Compressive Strength (UCS), Indirect Tensile Strength (ITS), and Resilient Modulus (MR) tests at different curing periods (1, 3, 7, 14 and 28 days). In the final phase, RSM was employed to develop a predictive model describing the relationship between stabilizer content and strength performance, validate predictions, and identify optimal mix designs. The results revealed that the proposed mix design had a significant influence on the material characteristics and compaction properties. Higher CA content led to increased OMC, while bulk density showed only minor variation, indicating that the stabilizing agents improved bonding rather than particle packing. The CA50RAP50 mix achieved the highest UCS and ITS, especially during early curing, while CA75RAP25 demonstrated better MR performance. Although strength performance decreased with 3% GCBA, a mix combination of 0-1% GCBA, 1% CaCl₂, and 3-4% OPC produced better performance compared to the control specimen containing 4% OPC alone. This indicated enhanced binder efficiency and early strength development while reducing OPC usage. The developed RSM models showed strong predictive accuracy, with high R² values confirming their reliability. Model validation revealed close agreement with laboratory results, and the optimization analysis identified CA50RAP50 with 4% OPC and 1-2% CaCl₂ as the best performer in UCS and ITS, whereas CA75RAP25 achieved the highest MR with the same stabilizer composition. Overall, the findings highlight that the stabilizing agents mix comprising 0-1% GCBA, 1% CaCl₂, and 3-4% OPC offers a sustainable and practical alternative to conventional cement-only mixes for enhancing CIPR base performance in Malaysian road rehabilitation projects.