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X-ray computed tomography (CT) offers a nondestructive technique for visualizing features in the interior of opaque solid objects to obtain digital information on their 3-D geometry and topology. In the simplest approach, directing planar X-rays that pass through the specimen along several sequences of paths, in several different directions, produces the set of CT images. The intensity of X-rays is measured after it passes through the specimens. The scanning of a slice is complete after collecting the intensity measurements for a full rotation of the specimen. The specimen is then shifted vertically by a fixed amount and the entire procedure is repeated to generate additional slices. The minimum thickness of the slice, on the order of one millimeter, is set by X-ray tube and detector slit geometry. Reference [4] gives a good introduction to the general subject of CT.
The 3-D shape of the aggregate particles in a real concrete sample was captured using an X-ray CT system located at the Turner Fairbank Highway Research Center. Concrete prisms with a 75 mm x 75 mm cross sectional area were cast. The mix designs for all the prisms had a water/cement mass ratio of 0.5, with quartz sand used as the fine aggregate, and limestone used as the coarse aggregate. An industrial CT system operating at 420 keV and a 512 channel digital detector [5] was utilized to acquire images of the prisms. Horizontal slices of 0.5 mm thickness were captured every 0.4 mm and were saved in raw data format. The 20 % overlap was to ensure that all internal structures were captured in the image. The raw data were then transformed using a back projection reconstruction algorithm to generate a TIF format image. The captured image consists of 256 levels of gray intensity that correspond to different densities within the specimen. The gray scale image was then thresholded to a black and white image by recovering the known volume of aggregates contained in the sample.
Figure 1 shows a 2703 pixel portion of the final result, cut out of the original image (this size was chosen only for convenience, as much larger samples can be handled). Aggregates (high density) in Fig. 1 appear white, while the matrix, consisting of cement paste and any unresolved fine aggregate particles, appears black. The large flat areas on the aggregates showing on the faces of the cube are from the cut through the sample, and are not real. The pixels have real dimensions of approximately 0.4 mm per pixel in all directions. The physical size of the concrete sample shown in Fig. 1 is then about 108 mm x 108 mm x 108 mm. The image shown in Fig. 1 represents preliminary work, and was taken for the purpose of developing the mathematical algorithms described in subsequent sections.
Figure 1: Reconstructed x-ray computed tomograph of a concrete specimen (270 x 270 x 270 pixels) (about 108 mm x 108 mm x 108 mm). This cube has been "cut" from a larger image, so that the flat faces of the aggregates at the surfaces are artificial.