Impact of meta-stable chloride hydrate phases on concrete durability at near-freezing temperatures

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Winter maintenance practices for concrete pavements have undergone major changes within the last ten years. De-icing/anti-icing strategies based on traditional rock salt (NaCl) have expanded to include alternative chemicals based on magnesium chloride, calcium chloride, and potassium acetate, to name a few. The impact of these new chemicals and new application methodologies on the durability of concrete infrastructure is not fully understood. The goal of the research program is to understand concrete degradation mechanisms attributed to the expansive precipitation of secondary chloride phases that occur at temperatures near 0°C. At temperatures near the freezing point of water the formation of complex calcium oxychloride (Ca-oxy) hydrate phases has been shown to cause expansion and damage in concrete, but the instability of these phases at room temperature and humidity has hampered further study. This research program incorporates both an experimental laboratory approach, and the exploratory real-world sampling of concrete exposed to chloride ion solutions and freeze-thaw (F-T) environments. In the laboratory portion, carefully controlled constant-immersion and evaporative-front exposure experiments will be conducted on a matrix of concrete mixtures, and coupled with periodic sampling to assess the development of secondary minerals and damage through a combination of mass and length change measurements, strength testing, combined scanning electron microscopy energy dispersive x-ray spectrometry (SEM/EDS), x-ray micro-fluorescence (µXRF), x-ray computed tomography (CT), and x-ray diffraction (XRD) techniques. In addition, Ca-oxy hydrate will be synthesized and characterized to determine its crystallographic and thermodynamic properties. To compliment the laboratory study, a series of field sites will be identified and the concrete sampled and recovered in such a manner as to maintain field temperature and humidity conditions for XRD analysis, particularly for the preservation of Ca-oxy hydrate phases. The information collected will be used to help integrate the formation of these secondary phases into existing models for the prediction of chloride penetration and concrete degradation.