Concrete pavements: deterioration due to de-icing salts and repair

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Ghazy, Ahmed
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With intensive use of chloride-based ice control chemicals for winter maintenance operations (de-icing, anti-icing and pre-wetting), concrete pavements may undergo significant alterations resulting in adverse consequences on their engineering properties. Formation of complex salts (oxychlorides) has been suspected for causing chemical degradation of concrete pavements, but without direct microstructural evidence due to their instability. Thus, an effort was made to elucidate the damage mechanisms of field and laboratory concrete exposed to the most widely used chloride-based de-icing salts (individual and combined) under different exposure conditions. This thesis aimed at capturing/linking the formation of these complex phases, if any, with the physico-mechanical properties of real cementitious systems/concretes and proposing their roles in the damage process. Also, this thesis explored the potential benefits of supplementary cementitious materials (SCMs: fly ash and nanosilica) and portland limestone cement (PLC) to mitigate this type of damage. The results indicated that the deterioration of concrete in the field is explained by a combination of physical and chemical aspects, due to the interaction of salts, freezing/thawing (F/T) and wetting/drying (W/D) cycles with the hydrated paste. Formation of acicular flattened blades of 3- and 5-form magnesium oxychloride (MOX) and tiny fibrous crystals, as well as subhedral pseudo-hexagonal calcium oxychloride plates (COX), were found in the deteriorated concrete specimens, depending on the type of solution. The overall results indicated that the restrictive limits on fly ash in concrete serving in chloride-rich environments may produce less durable concrete than alternative noncompliant mixtures with higher fly ash dosages. Moreover, the performance of the concrete was much enhanced by an innovative SCM such as nanosilica. PLC mixtures also exhibited better resistance to de-icing salts due to dual physical and chemical actions of limestone in the matrix. This thesis also aimed at introducing adaptive neuro-fuzzy inference systems to model the behavior of the concrete mixtures under various exposure regimes combined with de-icing salts. Finally, the thesis aimed at developing durable repair materials (nano-modified fly ash concrete) and enhancing the overall process of rehabilitating concrete pavements to achieve optimum performance, longevity and life-cycle cost effectiveness.
Concrete Pavement, De-icing Salts, Freeze-thaw cycles, Wet-dry cycles