Performance evaluation of lime, cement and alkali-activated binder in fiber-reinforced expansive subgrade soil: A comparative study

Expansive subgrade soils show early distresses causing distortion and premature failure to pavement structures. In the present study, an attempt is made to enhance the subgrade strength characteristics of the expansive soil by treatment with an alkali-activated binder (AAB) and reinforcement with four types of fibers (polypropylene [PF], glass, hemp, and coir fiber [CF]). The research also compares the effectiveness of fiber reinforcement in geopolymerized alkaline soil with the traditional lime and cement binders. AAB is produced by the reaction in an alkaline activator of sodiumsilicate and sodiumhydroxide with aluminosilicate precursors (Class- F fly ash and slag) by maintaining a 0.4 water-to-solid ratio. The effect of varying dosages of fibers and slag/fly ash ratio in the alkaline, lime, and cemented soil mixture on unconfined compressive strength (UCS), split tensile strength (STS), and flexural and penetration resistance is evaluated through a series of geomechanical and microstructural characterization tests. California bearing ratio (CBR) and flexural strength (Sf) are chosen as a subgrade strength performance indicator for fiber-reinforced soil. It is observed that PF-AAB soil shows higher interfacial bonding with strong interlocking density and tensile cracking resistance. The combined inclusion of fiber reinforcement (> 0.5 %) with 0.66 slags/fly ash ratio (S40 F60) in the alkaline mixture increased the subgrade strength performance indicator (flexural and elasticity) by around 35 %-40 % over the lime-cement soil-fiber mixture. The corresponding CBR, UCS, and STS values were also found to have increased by 55 %-65 %. Furthermore, the uncertainties associated with subgrade strength failure are predicted in the form of the CBR reliability index using Monte Carlo simulation through a nonlinear regression model developed from experimental results. The study shows that the optimumdosages of fiber reinforcement and slag/fly ash are essential factors for regulating the volumetric instability and tensile cracks of expansive soil.