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The Kentucky Transportation Center (KTC) conducted a study funded by the Kentucky Transportation Cabinet (KYTC) to assess the impact of chlorides on bridge coatings performance. The study included field measurements of chloride levels and associated coating performance of bridges. It also included laboratory accelerated weathering/corrosion performance of a variety of bridge coatings applied on steel panels charged with various concentrations of calcium chloride. Rusted steel panels were also subjected to abrasive blasting including some that were pressure washed. The cleaned panels were used for scanning electron microscopy (SEM) to analyze the disposition of chlorides not removed by the abrasive blasting process. The SEM tests revealed that abrasive blasting resulted in chloride-charged pits throughout the treated steel substrates. On other portions of the steel surface the chloride levels were low.
Two commercially available chloride extraction/measurement methods were used to assess both field chloride concentrations on thirteen bridges and specific concentrations of calcium chloride applied to laboratory test panels. Field tests indicated some variability in the two extraction/measurement methods. Laboratory tests indicated the two methods appear to accurately extract and measure chloride levels applied to the test panels. The KTC laboratory tests that were intended to simulate field performance of chlorides on the various paint systems did not consistently relate high chloride contamination levels to poor coatings performance
The conclusion drawn from the field measurements and laboratory testing is that the currently available methodologies for assessing chloride levels are insufficient in predicting coating performance. A key part of the problem may be pinpoint chloride concentrations or “hotspots”. Currently available chloride detection methods assess surface areas ranging from 10 to 100 cm2. A “hotspot” of chloride would be averaged across the test area and would yield a “low” chloride level but corrosion cells could remain to impact coating performance.