Understanding and Mitigating the Effects of Chloride Deicer Exposure on Concrete - UTC
Started: August, 2011 Ended: September, 2012 Project ID #4W3730 Status: Completed
Results & Findings
As described in the final report, 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.
The objective of this project is to investigate the effects of chloride deicers on Oregon DOT and Alaska DOT&PF concrete bridge decks and to identify and evaluate best practices and products to mitigate such effects in the states of Oregon and Alaska.
To maintain safe and productive roadways during the winter season, ODOT applies a magnesium chloride (MgCl2) solution for snow and ice control in accordance with guidelines established by the Pacific Northwest Snowfighters Association and others. The corrosive effect of chloride on embedded steel reinforcement is well known; however, it is unclear whether deicers based on chloride salts deteriorate concrete. Laboratory studies using concentrated solutions to accelerate testing have demonstrated that magnesium chloride has the potential to damage concrete. However, field cores extracted from sites in Colorado, Idaho, Iowa, Montana, and South Dakota did not exhibit damage conclusively attributable to deicers. Consequently, ODOT does not know the damaging effects that chloride deicers may impart on Oregon’s concrete infrastructure over time. The exposure to deicing chemicals at a specific site may depend not only on application frequency but also on other factors such as environmental and traffic conditions and bridge configuration. Deicer may be applied directly to a bridge deck or roadway, but other nearby concrete elements such as rails, barriers, and columns may be vulnerable due to splash. Consequently, not all bridges or concrete components (e.g., decks, beams/crossbeams, diaphragms, abutments, piers and piles) are expected to have the same likelihood of damage. However, there is no known practical means to measure deicer exposure in order to focus on those bridges or components potentially at highest risk. There is an urgent need for DOTs to identify and evaluate best practices and products for enhancing the strength and durability of concrete infrastructure in the presence of chloride deicers. The objectives of this project are to investigate the effects of chloride deicers on Oregon DOT and Alaska DOT&PF concrete bridge decks and to identify and evaluate best practices and products to mitigate such effects in the states of Oregon and Alaska. To this end, the project will: (1) determine whether the accumulated application of MgCl2 and sodium chloride (NaCl) deicers have caused significant damage to the concrete typically seen in the bridge decks maintained by the Oregon DOT and Alaska DOT&PF; (2) quantify the chloride ingress from winter road operations and estimate the potential for corrosion damage to reinforced concrete; (3) Develop a practical, on-site measurement method to assess the exposure of concrete components to chloride deicers; (4) Create a tool to estimate current and future damage states due to applying magnesium chloride deicer; and (5) identify, test, and recommend methods of mitigating deicer-induced damage to existing concrete infrastructure in the states of Oregon and Alaska. This project is jointed funded by the Oregon Department of Transportation (ODOT), the Alaska University Transportation Center (AUTC), and the WTI University Transportation Center (UTC).
Xianming Shi - PI
Robin Kline - Main External Contact
Files & Documents
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- Federal Highway Administration (FHWA) Sponsor