Ultra-fast NMR imaging of aggregate motion in highly flowable fresh concrete

将核磁共振成像技术应用于新拌混凝土圆锥管流动中骨料行为的测量。采用振幅法和梯度回波法相结合的方法检测任意截面固液多相流中的颗粒运动。一系列实验证明了这项技术在这类研究中是有用的，尽管由于现有核磁共振设备的限制，不得不使用一根半径为20 mm的小管。另一个问题是雷诺数超过0.01的流动的时间分辨问题。用纤维素醚(添加少量硫酸铜)的水溶液和椭圆形塑料颗粒分别作为模型砂浆和集料来配制模型混凝土。分析了高粘流截面图像的变化，以研究固体颗粒的曲折运动以及复杂非定常液相流的精细结构。模型混凝土的平均流态与Hagen-Poiseulle和圆锥管流动理论的结果进行了比较，证明前者至少为这种渐变的管道提供了一个可以接受的估计。通过多次实验研究了集料拱形形成堵塞流动的可能性，得出集料体积比是造成堵塞的主要因素而不是液体粘度的结论，回波平面法的应用将使测量具有更高的时间分辨率。空间标记法在大规模实验中的推广可能使材料偏析指数的定量评估成为可能，其中必须使用球形模型集料来稳定磁场。

Technique of NMR imaging was applied to measurement of aggregate behavior in a fresh concrete flow in a conic pipe. The amplitude and gradient echo methods were combined to detect the particle motion in an arbitrary cross-section of the solid-liquid multiphase flow. A series of experiments proved usefulness of the technique in this sort of study, although a pipe with a small radius of 20mm had to be employed from constraint of the present NMR equipment. Another problem is in time resolution for a flow with Reynold's number exceeding 0.01.A water solution of cellulose ether (with small amount of copper-sulphate added) and elliptic particles of plastics were used as model mortar and aggregate, respectively, to compose model concrete. Variation in cross-sectional image of the highly viscid flow was analyzed to investigate meandering motions of solid particles as well as fine structures of complicated unsteady liquid-phase flow. All these data obtained will be summarized in the form of a final report.Mean flow patterns of model concrete observed were compared with those from the Hagen-Poiseulle and conic-pipe flow theories, and it proved that the former theory provides an acceptable estimation at least for such a gradually tapered pipe. Probability of flow blockage due to aggregate arch formation was studied through a number of experimental runs, and it was concluded that the volume rate of aggregate is a main factor to cause the blockage rather than the viscosity of liquid-phase.Application of the echo-planar method will permit measurement with higher time resolution. Incorportation of the spatial tagging method in large scale experiments may possible enables quantitative evaluation of material segregation indices, where sphere-shaped model aggregate has to be employed for stabilization of magnetic field.