Optimization of equivalent modulus of RAP-geopolymer-soil mixtures using response surface methodology

This study focuses on assessing the resilient characteristics of a clayey soil modified with a fly ash (FA)-based geopolymer and reclaimed asphalt pavement (RAP) as an unpaved road material. RAP-geopolymer-soil mixtures were designed using the response surface methodology-central composite design with 0–40% RAP and 0–25% FA. The repeated-load California bearing ratio (CBR) testing method was used to determine the recoverable and permanent deformations and then obtain the equivalent (resilient) modulus (M_equ). The M_equ values were used to develop predictive models and determine the optimum soil–RAP–geopolymer mixture. The effects of the load level and soaking period on the stiffness of the optimum mixture were also investigated. The results revealed that the geopolymer binder played a significant role in enhancing the stiffness of the mixtures, with the maximum M_equ obtained at 25% FA and 0% RAP. However, it was determined that RAP has an adverse effect on the stiffness for almost all the studied cases and more significantly for the 40% RAP and 0% FA mixture. The optimal mixture was found to be 25% FA and 30% RAP. The developed model exhibited excellent predictive capability based on ANOVA results. The optimum mixture exhibited stress-softening behavior at an increased load level. No clear trend was observed in the effect of the soaking period on the resilient modulus within the examined soaking period range. Overall, this study agrees with several pavement design guidelines to limit the RAP content used in road applications owing to uncertain adequacy. Additionally, it suggests that the geopolymer binder is an effective stabilizer with excellent environmental and economic potential.