United States Department of Transportation
i
Understanding and mitigating effects of chloride deicer exposure on concrete.
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2014-09-01
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[PDF-5.89 MB]
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Abstract: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 KAc or MgCl2 deicer showed significant reductions in their compressive strength, splitting tensile strength and
microhardness, whereas those exposed to 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 AgNO3 spray
method does not guarantee the integrity of the concrete exposed to MgCl2 deicer. At least half of cored 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 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.
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