Investigation into the Role of Airfield Pavement Deicer in Concrete Pavement Deterioration

Premature deterioration of concrete due to materials-related distresses has been observed with increased frequency in airfield pavements across the United States in the past two decades. The use of a new breed of deicing chemicals based on alkali-acetates and alkali-formates, starting in the early 1990s, has been suspected to be one of the significant reasons for the increase in the observed deterioration. Previous laboratory-based studies have found that the early-age deterioration in concrete is associated with exposure to deicers causing acceleration of the Alkali Silica Reaction (ASR) deterioration mechanism and consequent failure. However, no systematic forensic studies on the impact of these deicers on field concrete were conducted to correlate the lab findings with the field performance. The principal objective of this study was to determine if the acetate-based deicer usage on airfield pavements does in fact cause the accelerated ASR distress in the field or if any other potential mechanisms are in play in the field concrete. While the fundamental objective was to establish the underlying mechanisms involved in the premature failure of concrete pavements and correlate to what is being observed in the lab conditions, it was evident that deicers are indeed capable of causing ASR distress in lab concrete, and therefore have a potential to cause distress in the field. Therefore, it was considered important to have a secondary objective of developing a quick and conservative test method that can be implemented to identify aggregate materials that are potentially susceptible to ASR phenomenon in the presence of deicing chemicals, so that potential problems associated with incompatible materials is minimized. Considering that the predominant deicer used in the field is based on potassium acetate, this deicer was exclusively used in this study. Following a comprehensive forensic investigation on concrete pavements from six different airports, the results of forensic study show that the airfield pavements do have some evidence of ASR, however other materials-related distresses were also observed, including alkali-carbonate reaction, D-cracking, shrinkage cracking, and a suite of defects related to poor construction practices. These defects were confirmed through petrographic analysis and macroscopic examinations. In general, the hardened air content, strength, and modulus of elasticity of concrete appeared to be adequate in the majority of the concrete investigated. Examination of concrete to determine the depth of penetration of deicer into concrete showed that in the majority of airfields, the penetration depth was limited to ½- ¾ inch from the pavement surface. However, where concrete exhibited some cracking, slightly deeper depths of deicer penetration was observed, preferentially along the cracks. The correlation between deicer usage and ASR occurrence was not strong due to this lack of adequate penetration. However, the results of the laboratory made samples show that there is strong potential for this acceleration if the deicer penetrated further into the pavement. Based on these results, it was considered important to explore and develop a test method to assess the sensitivity of aggregates to deicer chemicals. The objective of determining a potential develop a quick and conservative test method that can be implemented to reduce the chances of selecting alkali-reactive aggregates in the course of construction of new airfield pavements was achieved through revisions to an existing test method - deicer-modified mortar bar test method (EB-70 test method). A fundamental investigation conducted as part of this research study showed that pH-jump in deicer solution associated with interaction between deicers and alkali hydroxides was central to the development of the new test method. The new test method, Revised EB-70, was based on using a combination of 3 molar potassium acetate deicer and 1 normal sodium hydroxide captured the effects of pH jump while maintaining a high hydroxyl ion concentration. This revised test method produced conservative results when compared to the conventional test methods of ASTM C 1260 test and EB-70. The results produced higher or equivalent expansions in mortar bar samples when compared to the results of the standard ASTM C 1260 test method. It was also shown that the new test method can be used to select mitigation measures such as fly ash in helping to reduce deleterious ASR expansions. The overall results of this study show that it is important to select quality aggregates and construction methods along with deicer selection to ensure that the pavement system will meet its design life. Another distress mode in concrete pavements that deicers can actively play a role is the freeze-thaw damage. In this investigation, a deicer-modified ASTM C 666 testing regime was implemented to evaluate the impact of deicing chemicals on the premature deterioration of field concrete. The results from these tests showed that field concrete subjected to potassium acetate deicer in the modified ASTM C 666 protocol showed significant premature deterioration, compared to the control specimens that were subjected to plain water. Following these observations, fundamental studies conducted on cement paste and mortar specimens using cryogenic dilatometer exposed to deicers confirmed the potential role of deicers in causing the deterioration. Although additional testing and confirmation is needed, the principal freeze-thaw related mechanism that is promoted by deicer penetration into concrete is as follows: The penetration of deicer into a thin layer of concrete (approximately ½ in. to 1 inch) near pavement surface essentially renders this layer of concrete immune to freezing, depending on the concentration of the deicer. However, concrete that is significantly away from the pavement surface is devoid of deicer can exhibit freezing and the associated dilation. The differential expansion between the near-surface concrete and the deeper sections of the concrete pavement will subject the near-surface concrete to tension and cracking. The cracking observed in some of the field concrete is consistent with this proposed mechanism, in that the cracks are often shallow and not associated with any ASR distress. Additional testing, analysis and model development is required to gain a more comprehensive understanding of this phenomenon