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 (1) discriminate the spatial distribution of aggregates within a fresh sample, (2) measure changes in water content during the hardening process of cement, and (3) 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 of 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. Concrete behaviourConcrete is a conglomerate made by mixing aggregates, sand, cement and water in suitable proportions. The resulting composite material is often described by its strength and stiffness, but these are the overall and averaged properties of a notional heterogeneous material and are not sufficient to describe local variations and

This paper describes apparatus that enables loading of a concrete beam reinforced with aramid fibre-reinforced plastic (AFRP) inside the magnet of a magnetic resonance imaging (MRI) scanner, and the subsequent measurement in three dimensions of the propagation of fractures within the concrete. These are then correlated with the internal structure of the concrete which had been previously determined by scanning the beam soon after casting. The particular requirements of the test frame for use within the scanner and the methods used to satisfy them are described in detail, as is the test procedure. It is shown that the resultant images can be visualised either as twodimensional (2-D) slices, or as 3-D data sets; the images can also be post-processed to highlight the particular feature of interest.

This study extends previous work and demonstrates that magnetic resonance imaging (MRI) of water allows the internal crack pattern of hardened concrete to be observed along with the internal aggregate structure. The ability to reconstruct three-dimensional images is demonstrated and the importance of image registration is discussed. By using aramid fibre-reinforced plastics (AFRP), it is possible to make reinforced concrete specimens that can be measured by MRI. Plain concrete cylinders and others externally confined by AFRP spirals have been tested in compression and scanned after various amounts of load have been applied to show the progress of fracture development. Samples have also been made with straight AFRP rods cast into the concrete which, when pulled out, give bond failures and internal cracking. It is possible not only to observe the fractures, but also to relate them to the internal concrete structure determined by previous scans of the freshly prepared concrete samples.

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