Determination of aggregate shape properties using X-ray tomographic methods and the effect of shape on concrete rheology

The shape of aggregate particles can significantly influence certain properties of concrete, both in its fresh and hardened states. Therefore, there is a need to be able to completely characterize the shape of aggregate particles, in three-dimensions, in order to develop computational models which accurately predict properties. In the past, numerous methods have been suggested for this task; however these methods are often only applicable to two-dimensional images of particles, they output a single or a few values, and fail to characterize the true shape of the particle. X-ray tomographic techniques allow the capturing of the true shape of particles and have been applied to concrete aggregates. Computed tomography has been used to characterize coarse and fine aggregate particles, while X-ray microtomography has been used to characterize particles passing the 75µm sieve. Sample preparation methods and scanning parameters applicable to concrete aggregates have been developed. The spherical harmonic method was used to efficiently store shape information, and to calculate useful parameters for individual particles, such as volume and surface area. Comparisons of the results to properties determined using other techniques were made and it was determined that the results of indirect or two-dimensional shape and size characterization methods can be misleading. The shapes of aggregate particles particularly influence the rheological properties of concrete mixtures. However, it is not clear to what degree different-scale shape properties (the overall shape, angularity and texture) influence flow separately. Artificial aggregates were prepared in the laboratory and simplified test cases were chosen to independently investigate the effect of overall shape and surface texture on the yield stress and plastic viscosity of mixtures and to obtain a set of results which could be used to calibrate computational models. These tests revealed that the overall shape of coarse aggregate particles significantly influences the plastic viscosity of a mixture, but does not affect the yield stress visibly. Particle surface texture does not seem to noticeably influence either viscosity or yield stress, for the cases tested. The results were also used to verify the “Dissipative Particle Dynamics” model and showed good correlation with the predictions.