The use of MRI to observe the structure of concrete

摘要

A range of magnetic resonance imaging (MRI) measurements of water in concrete samples and their constituent materials is described and it is demonstrated that MRI can be used to discriminate the spatial distribution of aggregates within a fresh sample, measure changes in water content during the hardening process of cement, and detect fractures and voids within a water-saturated sample. The requirements of the magnetic resonance technique that the sample be free of iron impurities necessitated an investigation of a range of materials for manufacturing suitable concrete samples. This led to a mix design which is representative of normal concretes yet MRI compatible. MRI can achieve image acquisition using a number of different protocols which are defined and optimised according to the equipment available, the properties of the sample and the particular type of image required. Suitable protocols for detecting the water in concrete are described. Magnetic resonance (MR) images oj the internal structure of concrete are presented, which allow the aggregate, sand and matrix to be distinguished as well as any voids that are present. It is shown from our experiments that the measurable water present in wet concrete is in two forms, and the amount and proportions of these two components alters during hardening, which allows the progress of the associated chemical reaction to be studied. It is shown that fractures can be detected, but it is concluded that rather than attempting to determine both the structure and the fracture state from a single MRI image, it is far better to measure two separate images, one optimised to show the structure and the second for delineating the fractures.