基于核磁共振成像的混凝土冻融损伤特征

为研究多次冻融循环过程中材料内部孔隙分布的变化特点,采用低强度磁场核磁共振成像扫描仪对经过0,50,100和200次冻融循环的混凝土试件进行扫描,获得了弛豫时间T₂分布谱和扫描图像。研究结果表明:随着冻融循环次数的增加,表征材料孔隙率大小的T₂谱面积呈现初期增速较快,后期增速减小的规律;小孔隙含量随循环次数增加而减小,大孔隙含量随循环次数增加而上升;不同损伤程度的混凝土试件的核磁共振(NMR)扫描图像与T₂分布谱的变化特点具有较好的同步性,体现了扫描结果的准确性。研究成果有助于加深对混凝土在冻融循环过程中结构损伤演化规律的认识。

Characteristics of Freeze-thaw Damage of Pore Structure in Concrete Material Using Nuclear Magnetic Resonance Technology

In the purpose of obtaining variation characteristics of pore distribution for material under freeze-thaw cycles, Nuclear magnetic resonance (NMR) technique was employed in this research to scan concrete specimens undergone 0, 50, 100, and 200 times of freeze-thaw cycles. The spectrum of relaxation time T₂ and NMR scanning images were acquired. The variation of T₂ distribution curve reflected the evolution and expansions of pores in the specimen undergone freeze-thaw cycles. With the proceeding of cyclic freezing and thawing, the T₂ spectral area which characterizes the porosity of the materials surged in the initial stage; but such increment attenuated later. The proportion of small pores reduced whereas the ratio of macropores increased. The NMR scanning images of concrete specimen with various damage degrees synchronized well with the variations of T₂ distribution spectrum, which implied the accuracy of scanning results. In conclusion, the joint failure due to the combined action of osmotic force and frost heave force in freeze-thaw cycles is the fundamental cause for the formation of connecting fracture.