Resilient Characterization of RAP-Geopolymer- Soil Mixture in Roads

Abstract

There is an increasing demand to utilize secondary materials, such as construction wastes and industrial by-products, to meet sustainable development requirements. The use of reclaimed asphalt pavement (RAP) has become a widespread practice in road applications. RAP shows a fluctuated performance based on quality, age, gradation, and asphalt binder content, which limits the percentages of RAP used and requires enhancement of its properties. Geopolymer binder can be used as a stabilizer to improve RAP performance in place of Ordinary Portland Cement (OPC) to promote environmental protection. This study mainly focuses on the assessment of resilient characteristics of RAP-geopolymer-soil mixtures as unpaved road material. Fly ash (FA) is used as the main source of alumino-silicate material for the synthesis of geopolymer binder. A mixture of sodium silicate and sodium hydroxide is used as the alkali activator solution. The compaction characteristics of soil-RAP-FA at 0 – 40% RAP and 0 – 25% FA contents were first examined. Then, the effect of various alkali solution proportions (i.e., NaOH molarity and NaOH:Na2SiO3) on the unconfined compressive strength of soilgeopolymer mixtures, was investigated. 8 molarity of NaOH and 1 (50:50) NaOH:Na2SiO3 solution ratio was selected to activate FA for resilient characterization because it yielded the highest compressive strength of the soilgeopolymer mixture. RAP-geopolymer-soil mixtures were designed using the response surface methodology – central composite design by Stat-Ease Design- Expert® Software Version 7.1.6. Based on the experimental design matrix, eleven mixtures were prepared, each of a minimum of two replicates, at 0 – 40% and 0 – 25% RAP and FA content, respectively. The repeated load CBR (RL-CBR) testing method was employed to determine the recoverable (resilient) and permanent deformations and then obtain the equivalent (resilient) modulus (Mequ). Data of Mequ and cumulative permanent deformation (CPD) was used to develop predictive models and determine the optimum RAP-geopolymer-soil mixture based on Mequ model. The results of this study revealed that FA and RAP oppositely influence the compaction characteristics. FA increases the optimum moisture content (OMC) and decreases the maximum dry unit weight (MDU), whereas RAP decreases the OMC and increases the MDU. More importantly, it was shown that geopolymer binder ii plays a significant role in enhancing the stiffness of mixtures, with a maximum Mequ of 957.7 MPa and a minimum CPD of 0.01905 mm obtained at 25% FA and 0% RAP. On the other hand, it was found that RAP has an adverse effect on the stiffness for almost all cases studied, more significantly at 40% RAP 0%FA mixture, which showed the minimum Mequ and maximum CPD of 191.9 MPa and 0.8445 mm, respectively. The optimum mixture was found to be 25% FA and 30% RAP with the criteria of maximizing all of Mequ, FA content, and RAP content. Both of the developed models exhibited an excellent predictive capability based on Analysis of Variance (ANOVA) results. The stiffness of the optimum mixture decreased by about 11% and 31% when the level of load was increased to 10 kN and 15 kN, respectively. The decreasing trend was also observed when the specimen is soaked in water for 4 days. Contrarily, the Mequ was substantially increased by 43% compared to the unsoaked specimen after 7 days of soaking.