Investigating the effects of stabilization carried out using fly ash and polypropylene fiber on the properties of highway clay soils

Abstract

Improving highway subgrades is the most crucial step that increases the performance and longtime durability of pavements. Using fly ash to improve the properties of subgrades and synthetic fibers as additives to increase their strength in recent years is an effective method. It is essential to determine how fly ashes affect the strength properties of subgrade soils in the long term with a time-dependent curing effect and how synthetic fibers affect the strength properties of subgrade soils, especially in the short term before curing. Furthermore, another vital issue is determining the extent to which freeze–thaw cycles that occur during winter months due to regional climatic conditions reduce the strength of the subgrade soils. For this purpose, a regional high-plasticity soil representing the problematic subgrade was reinforced with fly ash at different percentages (10%, 20%, 30% and 40%), and a series of experiments were carried out to determine the plasticity, compressibility and strength properties of these soil mixtures. Besides the clayey soil and fly ash, the microstructures of these mixtures were also examined by XRF, XRD spectrometry, FT-IR spectroscopy and SEM analyses and the relationships between mechanical behavior and microstructure were tried to be explained. Furthermore, the impact of polypropylene fibers added to the soil at 0.5% of the dry mass and freeze–thaw cycles on the strength of the subgrade soils was determined. The results showed that the effective fly ash mixture ratio for stabilization with fly ash was 10%, and when the 28-day unconfined compressive strengths of the fly ash mixture and the natural soil were compared, the fly ash mixture increased the strength by 85.8%. Furthermore, the fiber additives were determined to increase the shear strength at ratios ranging from 3.0% to 21.0% and that approximately 80% strength loss occurred in the natural soil due to freeze–thaw cycles, but fly ash and synthetic fiber additives can increase the strength by compensating for the strength loss caused by freeze–thaw cycles.