Understanding and Mitigating Effects of Chloride Deicer Exposure on Concrete

Field and laboratory investigations were conducted to examine the effects of chloride deicers on concrete bridge decks and to identify and evaluate best practices and products to mitigate such effects. The concrete bridge decks exposed to potassium acetate (KAc) or magnesium chloride (MgCl₂) deicer showed significant reductions in their compressive strength, splitting tensile strength and microhardness, whereas those exposed to sodium chloride (NaCl) deicer and without signs of surface distress did not. Visual inspection would be misleading for assessing the condition of concrete bridge decks exposed to MgCl₂ deicer, as the chemical attack by MgCl₂ generally does not exhibit apparent signs of distress. Chloride penetration as low as 0.1 in (2.5 mm) based on silver nitrate (AgNO₃) spray method does not guarantee the integrity of the concrete exposed to MgCl₂ deicer. At least half of cored Oregon Department of Transportation (ODOT) bridge decks exhibited air void spacing factor higher than 200 microns (0.008 inches) per the ASTM C457 test method, indicating that they no longer have a proper air-void system for freeze-thaw resistance. The role of MgCl₂ in the carbonation and alkali silica reaction (ASR) of field concrete, if any, is not significant, but KAc may play a significant role in contributing to ASR in concrete containing reactive aggregate. The microscopic evidence further suggests that the concrete in the field environment had been affected by both physical and chemical degradation by the joint action of freeze-thaw cycles and MgCl₂. A set of mortar samples can be deployed to assess the cumulative MgCl₂ exposure at a given site. A simplistic empirical-mechanistic model was developed to evaluate the conditions of the current bridge decks. Surface treatments, especially penetrating sealers and water repellents should be used to protect new concrete and existing concrete without too much chloride contamination. For any surface treatment to be used, it is important to select products with high resistance to both gas and water penetration to maximize the concrete’s resistance to “salt scaling”. When the concrete surface has deteriorated to a more severe degree, overlays should be used. For concrete decks exposed to freeze-thaw and wet-dry cycles and both NaCl and MgCl₂ deicers, silica fume modified cementitious overlays and micro-fiber modified cementitious overlays should be used. For decks mainly exposed to MgCl₂ deicer, Castek T48 polymer overlay is a good candidate. For areas that are also subjected to studded tires and high risk of abrasion, Castek T48 and KwikBond PPC-1121 polymer overlays should be used instead of cementitious overlays.