Erdem, Savas
(2012)
Impact load-induced microstructural damage of concrete made with unconventional aggregates.
PhD thesis, University of Nottingham.
Abstract
Understanding the correlation between the mix proportions, micro structural characteristics, and macro-scale properties of concrete (i.e. the process-structure-properties relationship) is fundamental to achieving a more advanced understanding of how to apply and optimise this abundant engineering material. Although, concrete has been traditionally evaluated by its physico-mechanical and functional properties; development of advanced and effective inspection techniques during the last decade has demonstrated that fundamental macro-level properties of concrete depend, to a great extent, on its properties at the micro- and nano levels. This research was intended to make a quantitative assessment of impact load-induced micro-structural damage in concrete and, more particularly, to investigate the influence of ITZ micro-and nano local properties (as influenced by aggregate characteristics) on the impact load-induced cracking behaviour of concrete.
Five different types of concrete mixtures were designed with the same total water cement ratio either by using natural aggregates as reference or by totally replacing the natural coarse aggregate with unconventional aggregates (such as copper slag, blue brick, sintered fly ash and tyre rubber) having significant differences in strength, shape and surface texture, porosity and roughness, and elasticity. A range of advanced techniques including X-ray diffraction, mercury porosimetry, 3D X-ray computed tomography coupled with digital image analysis, laser surface profilometry, 3D nanotech vertical scanning interferometry and scanning electron microscopy fitted with energy-dispersive X-ray spectrometer were used to characterize the aggregates and the concrete micro-structures. Based on the results obtained a possible mechanism for the micro-structural damage in concrete was proposed.
Poorer aggregate characteristics alone could be responsible for a greater ITZ deterioration after loading but the results demonstrated that in fact, the aggregate causes a change in the ITZ conditions and it is these altered ITZ conditions that have a major effect on overall mix behaviour and govern the damaging process of concrete under impact loading. It was also concluded that the presence of a weak and porous ITZ has two opposite effects on the failure process. First, the chemical and porosity heterogeneities within the ITZ can cause fluctuations/disordering in the cracking (fracture) path, resulting in an increase in the tortuosity and corresponding fracture energy dissipation. Second, a weak and porous ITZ transfers less stress from the matrix to the aggregate particles. This leads to a lower compressive strength but increased toughness due to micro crack path lengthening and energy dissipation.
Finally, the effect of the aggregate on the surface area roughness of the ITZ was established for the first time in the concrete literature. The roughness number of the area near the ITZ was found to positively correlate with dissipated surface fracture energy. An increase in the roughness number is associated with an increase in the dissipated fracture energy. The significance of this correlation however, lies in the fact that the rougher near – ITZ fraction of the bulk paste is more resistant to cracking at the macro level.
Findings from this study will lead to a better understanding of the impact load-induced micro-structural damage phenomena. In addition, the micro-structural data from SEM and X-ray CT obtained during impact and mechanical testing of the concrete mixtures could be used to develop a multi-scale finite element model to simulate and predict the behaviour and fracture damage of concrete subjected to dynamic loading.
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