Abstract: Segregation behaviors of cast-in-place concrete in ventilation shaft secondary lining of the Zimuyan tunnel during high-drop chute pouring are investigated and its mechanical properties are examined. Field sampling and laboratory testing are implemented to analyze the mechanical properties and microstructural characteristics of concrete. The research involves simultaneous collection of concrete samples from both the top and bottom of the shaft, followed by compressive strength tests and nuclear magnetic resonance T₂ spectrum analysis, revealing the evolution mechanism of the high-drop free-fall process on concrete homogeneity and pore structure. The experimental results indicate that: (1) Concrete from the initial and final pouring stages exhibits visible segregation between aggregate and paste, while the concrete from the middle pouring stage meet the design requirements for compressive strength and shows no distinct segregation. (2) Compared to the initial and final stages, the concrete from the middle pouring stage demonstrates more uniform aggregate distribution after pouring. (3) For the Group e specimens from the middle pouring stage, the proportion of medium and large pores significantly decreases after pouring, while the proportion of medium pores increases markedly (reaching approximately 50%), reflecting more uniform internal particle distribution and better paste encapsulation at this stage.

Key words: shaft, cast-in-place concrete, segregation, nuclear magnetic resonance technology, compressive strength