Freeze–thaw durability of cement-stabilized macadam subgrade and its compaction quality index

Cement-stabilized macadam (CSM) subgrade is widely used in high-speed railway (HSR) in cold regions owing to its good anti-frost heaving performance. However, the CSM subgrade still encounters some problems such as strength reduction, fractures, and soil-particle pulverization. Hence, this study focuses on the experimental study of compaction indexes and strength properties of CSM subgrade. Results show that admixture of cement significantly improves the compaction performance of graded macadam. The compaction quality indexes of CSM subgrade are 2–3 times higher than those of graded macadam subgrade. Under the effect of intense freezing and thawing, the unconfined compressive strength and flexural strength of CSM decrease rapidly; then, the decay rate declines and levels off gradually after 30 freeze–thaw cycles. The Morgan–Mercer–Flodin growth model and a hyperbolic model were employed to describe the effect of freeze–thaw cycles on measured strength. The flexural-compressive strength ratio was introduced to evaluate the flexibility of CSM. Under intensely freezing and thawing, the brittleness increases and the flexibility decreases. Finally, we proposed 5% cement content for HSR subgrade in cold regions taking multiple requirements into consideration.