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

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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.