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Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure (2008)

Chapter: Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure

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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
×
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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
×
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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
×
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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
×
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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
×
Page 33
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Suggested Citation:"Chapter Four - Effects of Pavement Deicing Products on Airfield Infrastructure." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
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25 The use of traditional PDPs consisting of urea and/or glycols has become diminished on airfields as a result of their adverse environmental impacts. Modern PDPs often include KAc, NaAc, and NaF as the freezing point depressant, with other additives. This chapter synthesizes the effects of airport PDPs on pavement and other airfield infrastructure. The results of the ACRP survey distributed for this project are assimilated within the sections concerning effects on concrete pavement, asphalt pavement, and other infrastructure. In addition to the information obtained from the survey, the majority of this chapter synthesizes published data from a comprehensive lit- erature review. PAVEMENT DEICING PRODUCTS EFFECTS ON AIRFIELD INFRASTRUCTURE: FIELD EXPERIENCE This section reports on the field experience regarding the effects of PDPs on the durability of airfield infrastructure. It should be noted that any pattern derived from such field data should be treated with caution and needs validation from research conducted in a well-controlled laboratory setting because the durability of airfield infrastructure is affected by a wide variety of factors. The lack of documentation and/or control of other variables in the field environment presents a challenge for researchers to unravel the specific role played by the PDPs or to quantify their impact. For instance, the dif- ference in the performance of portland cement concrete (PCC) pavements at two airports could be potentially attrib- uted to the use of not only different PDPs, but also different types of aggregates (reactive vs. nonreactive), among many other variables (e.g., mix design, construction quality, cli- matic conditions, and traffic loading). Telephone interview results of 12 (primarily Canadian) airports/airport authorities conducted by George Comfort in 2000 indicated increased use of alkali-metal-salt-based PDPs (KAc, NaAc, and NaF) over urea. A majority of respondents to the interview indicated that pavement damage was not attributed to deicers, whereas four respondents suggested that no conclusive statements could be made. A few isolated responses to the interview indicated that crack and joint sealant might be affected, although no conclusive statements could be made to implicate aircraft or airfield deicers. Addi- tionally, two airports noted that KAc might provide an addi- tional benefit of removing rubber buildup on runways (Com- fort 2000). In a report produced by the Transportation Asso- ciation of Canada, it is stated that with good pavement design and construction, the effects of winter maintenance chemicals may be minimized (“Synthesis of Best Practices . . .” 2003). More recent field and laboratory experience, however, indicates probable impacts of alkali-metal-salt-based PDPs on both PCC and asphalt pavements (Nilsson 2003, 2006; Rangaraju et al. 2006; Pan et al. 2008). To reexamine the case, a portion of the survey for this synthesis was designed to gather input regarding the impacts of deicers on airfield pavement and infrastructure. The ACRP survey was distrib- uted to professionals representing the 50 busiest airports in the United States, among others. A total of 17 respon- dents were directed to this section of the survey based on an assessment of their initial responses. Among them, 14 were employed by airports, 12 of which are in the United States. Three additional respondents represented the Swedish Civil Aviation Administration (whose responses were specific to the Gothenburg–Landvetter Airport), the Innovative Pave- ment Research Foundation (IPRF), and the FAA. As such, the survey results provide information from a total of 15 air- ports. Ten of these 15 respondents indicated that their job title contained the word “Environmental”; other key words included Manager, Director, Coordinator, Administrator, Supervisor, Program, Deicing, Operations and Maintenance, Compliance, and Wastewater. Often the respondents con- sulted their pavement engineers before responding to some technical questions in the survey. Four airports responded with detailed information about the specific use of PDPs: the type, application rate, applica- tion frequency, and total amount applied for each of the pre- vious five seasons. In general, the results indicated increased use of KAc and less or no use of urea. For those that reported using both NaAc and NaF, the former has been used more frequently in the recent seasons. Even though most responses to the ACRP survey indi- cated little field observation or concern regarding the impacts of PDPs on the durability of airfield infrastructure, this may not necessarily represent the overall situation of U.S. airports considering the limited number of responses. Seven ques- tions solicited information regarding the role of PDPs in dete- riorating pavements, ground support equipment (GSE), light- ing fixtures, signage, and other infrastructure assets; the lifespan and design and material changes of these were also CHAPTER FOUR EFFECTS OF PAVEMENT DEICING PRODUCTS ON AIRFIELD INFRASTRUCTURE

26 questioned. Blank or “no” responses were common in the questions concerning damage or deterioration of airfield infrastructure. However, some responses provided more spe- cific information regarding impacts on concrete and asphalt pavements as well as on other airfield infrastructure. IMPACT OF PAVEMENT DEICING PRODUCTS ON CONCRETE PAVEMENT This section synthesizes the information on the impact of PDPs on concrete pavement. First, the potential role of PDPs in the deterioration of concrete pavement is described, in terms of both chemical and physical effects associated with the use of PDPs. The current understanding of the mecha- nisms of damage is then discussed, followed by the associ- ated standards and test protocols, methods of prevention or mitigation, and finally knowledge gaps on this subject. As identified by a recent literature review (Pan et al. 2006), the last decade has seen an increase in the premature deterioration of airfield PCC pavements with the use of alkali-metal-salt-based PDPs (Maxwell 1999; Barett and Pigman 2001; Johnson 2001; Pisano 2004; Roosevelt 2004; “New Anti-icing System . . .” 2005; Pinet and Griff 2005). Such PDPs have been used more extensively and for more years in European countries for winter maintenance than in the United States. The degree of distress in the PCC pave- ments of European facilities ranged from mild to severe in terms of surface cracking and repair and rehabilitation efforts needed (Pan et al. 2006). Recent research conducted at Clemson University found that the acetate/formate-based deicers could induce increased levels of expansion in concrete with aggregates susceptible to the alkali-silica reaction (ASR), and could trigger ASR in con- crete that previously did not show susceptibility to ASR (Ran- garaju et al. 2005, 2006; Rangaraju and Desai 2006). The lab- oratory results from a modified ASTM C1260 mortar bar test and a modified ASTM C1293 concrete prism test indicated that both KAc- and NaAc-based deicer solutions showed sig- nificant potential to promote ASR in mortar bar specimens that contained reactive aggregates. Such solutions were also found to cause more rapid and higher levels of expansion within 14 days of testing and to lead to lower dynamic modulus of elasticity, compared with 1N sodium hydroxide (NaOH) solu- tion (Rangaraju et al. 2006). Increasing temperature or deicer concentration was found to accelerate the deleterious effects of deicers on the ASR in concrete. Based on the responses to the ACRP survey, concrete life spans at U.S. airports varied from 20 to 50 years, and changes in mix design and construction are consistent with FAA spec- ifications. Only isolated cases of KAc accelerating ASR in some concrete pavements were reported by the survey respon- dents, with freeze–thaw cycles also contributing to the dam- age. It should be cautioned, however, that among the more than 50 U.S. airports contacted, only 12 (along with 3 non- U.S. airports) responded to the ACRP survey. Two respon- dents specifically referred to the interim FAA recommenda- tions concerning ASR and deicers. ASR is a chemical reaction between alkalis present in the cement paste and siliceous minerals in the reactive aggregates of PCC, which produces a hydrophilic gel that expands when sufficient moisture is available. Such inter- nal expansive forces are deleterious to the concrete dura- bility and can cause cracks in both the cement paste and aggregates. Failure of concrete structures later attributed to ASR can be dated back to the late 1920s (Pan et al. 2006). Typical ASR distress is manifested by cracking, popouts, and expansion (as shown in Figure 13). Cracks allow more water to enter the concrete, popouts create foreign object damage hazard, and expansion can damage adjacent pave- ments and structures. The increased alkali content of mod- ern PCC, as well as the potential for additional alkali from fly ash, admixtures, aggregates, mix water, etc., is the outcome of the competing forces of air emission standards and high energy costs. Thus, the low-alkali cement of today has more alkali than cement manufactured before the 1970s, often around 0.6% sodium oxide equivalent (Na2Oeq). Accord- ingly, aggregates that did not historically react to low-alkali cement may not have the same performance today (Pro-Act Fact Sheet . . . 2006). In addition to ASR, physical distresses such as scaling and spalling are common forms of deterioration of hardened con- crete (Figure 14a and b, respectively), and both can occur in the absence of deicers. Scaling is physical damage of concrete surface often shown as local flaking or peeling, owing to the hydraulic pressures from freezing–thawing cycles of concrete pore solution (ACI Committee 302 1996). Freezing of water in saturated concrete generates expansive forces that are detri- mental to the concrete surface, especially when it is not ade- quately protected with entrained air. Similar to chloride-based salts, alkali-metal-salt-based PDPs may exacerbate scaling when used at a concentration high enough to induce osmotic pressure upon moisture (Pan et al. 2006). In addition, the application of deicers to pavements increases the rate of cool- ing, which increases the number of freeze–thaw cycles over ambient conditions and thus the risk for scaling (Mussato et al. 2005). The use of properly cured, air-entrained PCC can pre- vent scaling. Entrained air provides spaces within the concrete matrix for expanding water to move into, thereby reducing the potential stress and associated deterioration pertinent to freeze– thaw cycling. It is believed that high quality concrete with 5%–7% entrained air is more resistant to freeze–thaw cycles and scaling (Williams 2003). The ingress of chloride into concrete and subsequent reinforcement corrosion has been extensively studied, and these eventually lead to concrete cracking or spalling. How- ever, little research has been conducted to examine the ingress of alkali metal salts (e.g., KAc, KF, NaAc, and NaF) into concrete or their interaction with metallic reinforcement.

27 Interestingly, a very recent study reported the use of NaAc aqueous solution as a technology to reduce water perme- ability into PCC (Al-Otoom et al. 2007). The results indi- cated that the crystal growth of NaAc in concrete pores was relatively fast, which significantly reduced the water per- meability of the concrete after only a 7-day treatment. The PCC samples tested were porous with the following mix design: a water-to-cement ratio of 0.65:1, an aggregates-to- cement ratio of 4.5:1, and a sand-to-gravel ratio of 1:2 (all by weight). The treatment of PCC by the NaAc solution did not significantly affect its freeze–thaw resistance or com- pressive strength, and only slightly increased the pH of the concrete. Overall, the treatment was demonstrated to be ben- eficial to the concrete’s durability, especially at the optimum concentration of 20% NaAc (Al-Otoom et al. 2007). It is note- worthy that neither the details of aggregates used in the PCC samples were provided, nor was any ASR testing conducted in this specific research. Nature of the Effect of Modern Pavement Deicing Products on Portland Cement Concrete Pavement Deterioration Limited existing laboratory studies indicated that alkali-metal- salt-based deicers could cause or accelerate ASR distress in the surface of PCC pavement by increasing the pH of concrete pore solution. PCC pavements that were otherwise resistant to ASR might show rapid deterioration when exposed to these high alkali solutions. The nature of the reactions associated with increased expansions in mortar bar tests to date remains unclear owing to limited research conducted on this topic. It was proposed that such deicers react with one of the major (a) (b) (c) FIGURE 13 ASR-induced distresses at Air Force bases: (a) cracking, (b) popouts, and (c) asphalt shoulder heaving caused by ASR expansion in adjacent concrete [adapted from Pro-Act Fact Sheet . . . (2006)].

hydrated products—calcium hydroxide—Ca(OH)2, and result in higher pH of the concrete pore solution. The high pH result- ing from these interactions is likely to have an accelerating effect on the expansions as a result of ASR. This mechanism was substantiated by the SEM-EDX investigation of mortar bars after deicer immersion, which was unable to detect Ca(OH)2 in the cement paste (Rangaraju and Olek 2007). There are other hypotheses that merit further investigation. A laboratory investigation using concrete samples obtained from existing Iowa highways suggested that magnesium and calcium deicers might accelerate highway concrete deterio- ration (Cody et al. 1996). Samples were experimentally dete- riorated using wet–dry, freeze–thaw, and continuous soak conditions in solutions of magnesium chloride, calcium chlo- ride, sodium chloride (NaCl), magnesium acetate (MgAc), magnesium nitrate, and distilled water. The magnesium and calcium salts were found to severely damage the concrete samples, whereas plain NaCl was the least harmful. This was possibly attributable to the reaction between magnesium and calcium cations (Mg2+ and Ca2+) and the cement hydration products, or to the accelerating effect of these cations on the alkali-carbonate reaction if the concrete contained reactive dolomite aggregates. Standards and Test Protocols The U.S. Air Force requires that aggregates for new con- crete pavements be tested according to ASTM C1260, Stan- dard Test Method for Potential Alkali Reactivity of Aggre- gates (Mortar-Bar Method). Another standard, ASTM C1293, Standard Test Method for Determination of Length Change in Concrete Due to Alkali-Silica Reaction, is pre- ferred but takes more than a year to complete. If it is not feasible to use only nonreactive aggregates, then mitigation methods are required. ASTM C1567, Standard Test Method for Determining the Potential for Alkali-Silica Reactivity of 28 Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method) or an equivalent test must be used. In ASTM C1260, the samples are soaked for 14 days in a solution of NaOH. For the Air Force, mixtures that experience expansion greater than 0.08% require mitigation (Pro-Act Fact Sheet . . . 2006). The FAA has recommended that ASTM C1260 testing for new concrete pavement mixtures be modi- fied by substituting a deicing agent for the NaOH solution, soaking for 28 days, and mitigating if expansion exceeds 0.10% (“Engineering Brief No. 70 . . .” 2005). Currently, these are interim recommendations until additional research is completed. The modifications to ASTM C1260, C1293, and C1567 are based on the research conducted at Clemson University. Additional research using these modified methods may be needed, especially for mitigation with lithium nitrate- based admixtures. The FAA will further refine the tests as part of the IPRF 05-7 project to make it a standard test method for evaluating the ASR susceptibility of PCC, which may be con- sidered for inclusion to the SAE AMS 1435 and AMS 1431, along with ASTM C672, Test Method to Assess Scaling Resistance of Concrete Exposed to Deicers. Prevention and Mitigation To prevent or mitigate the effects of PDPs on concrete pave- ment, the first and most important countermeasure is to fol- low best possible practices in concrete mix design and con- struction. For instance, the mix design should take into consideration supplementary cementitious material to allevi- ate excess bleed water, aggregate blends that do not lack mid- sized aggregate, and suitable air void systems. Proper mix designs will allow easier placement and consolation. In addi- tion, good curing practices should also be followed (Van Dam et al. 2006). When possible, polymer sealants can be used to minimize the contact between PDPs and concrete pavement (a) (b) FIGURE 14 Concrete scaling (a) and spalling (b) [adapted from Potomac Construction Industries (2007) and Cryotech Deicing Technology (2007), respectively].

29 and to reduce the ingress of water, PDPs, and other poten- tially deleterious contaminants into concrete. ASR has been conventionally controlled by limiting alkali content in cement and selecting aggregates of good quality. Mortar bars prepared with nonreactive aggregates did not exhibit ASR distress when exposed to the standard NaOH or deicer solutions, even when high alkali cement (0.82% Na2Oeq) was used in the mix (Rangaraju and Olek 2007). Based on these results, it appears that new concrete pavements should be pre- pared with nonreactive aggregates, if feasible. Nonreactive aggregates can likely be identified by a modified concrete prism test (ASTM C1293, with deicer soaking solution) in which expansion after one year is limited to 0.04% (Rangaraju and Olek 2007). Furthermore, efforts have been made to mitigate ASR by adding various supplementary cementitious materials or chemical admixtures. Research sponsored by the FHWA used lithium compounds to successfully reduce ASR induced by deicers. However, for existing concrete pavement, it is unlikely that the solution will penetrate significantly, and the most benefit may only be seen on the surface with reduced debris generation (Folliard et al. 2003). The potential for mit- igation of new concrete with lithium nitrate is more promis- ing, although additional research is needed to determine the appropriate dosage (Rangaraju 2007). The toxicity and envi- ronmental effects of lithium nitrate have not been evaluated (Materials Safety Data Sheet 2006). The effectiveness of min- eral admixtures was evaluated in reducing the ASR potential in the presence of KAc (Rangaraju and Desai 2006). The effectiveness of fly ash in mitigating ASR in the presence of KAc was found dependent on the lime content (Rangaraju and Desai 2006). Fly ash with lower lime content was more effec- tive in reducing the expansions, and greater amounts of fly ash (up to 35% by weight) were needed to replace cement when more reactive aggregate was used. Ground granulated blast fur- nace slag with a replacement level of 50% was needed to miti- gate the expansions; 40% replacement was found ineffective (Rangaraju 2007). Knowledge Gaps There is a need for research data from controlled field investi- gations regarding the effects of alkali-metal-salt-based PDPs on concrete pavement to help differentiate the contribution of such PDPs to concrete deterioration from other possible factors in the field environment. One challenge is that the durability of PCC pavement is often significantly affected by the mix design (water-to-cement ratio, type and amount of aggregates, air content, etc.), construction, curing and maintenance practices, exposure to various climatic conditions (e.g., wet–dry and freeze–thaw cycles), as well exposure to traffic loading. Furthermore, there is a need to unravel the specific mech- anism by which alkali metal salts cause or promote ASR. Knowing the mechanism(s) of damage will provide neces- sary guidance for preventing or mitigating such distress and for developing the next generation of PDPs and airfield con- crete pavement. Advances in technologies related to PDPs and concrete pavement will make both understanding their interaction and developing an appropriate compatibility test protocol a continued effort. The IPRF closed a request for proposals in October 2006 to address these knowledge gaps. IPRF Project 01-G-002-05-7, Performance of Concrete in the Presence of Airfield Pavement Deicers and Identification of Induced Distress Mechanisms, continues the investigation by Dr. Rangaraju that has previ- ously implicated acetate/formate-based deicers in the ASR increase in airfield pavements. Another IPRF project that recently underwent a request for proposals is Project 01-G-002- 06-5, Role of Dirty Aggregates in the Performance of Concrete Exposed to Airfield Pavement Deicer, which will examine the possibility of increased alkali content on dirty aggregates and its role in concrete durability in the presence of PDPs. IMPACT OF PAVEMENT DEICING PRODUCTS ON ASPHALT PAVEMENT This section will synthesize the information on the impact of PDPs on asphalt pavement. First, the potential role of PDPs in the deterioration of asphalt pavement is described. The current understanding of the mechanisms of damage is then discussed, followed by the associated standards and test protocols, methods of prevention or mitigation, and finally knowledge gaps. In addition to the effects of PDPs on PCC pavement, their effects on asphalt pavement are also of increasing concern. Canada’s contribution to this research subject began in the 1990s with a laboratory study comparing cores of asphalt mix immersed in distilled water and a 2.5% urea solution. After one freeze–thaw cycle, there was significantly more loss of indirect tensile strength in the sample immersed in urea compared with the one in distilled water. Field experi- ence did not coincide with these findings, but later studies continued to include urea among the other deicers evaluated (Hassan et al. 2000, 2002b). A laboratory study found that the use of PDPs (NaCl, KAc, NaF, as well as urea) was damaging to both aggregates and asphalt mixes (Hassan et al. 2002a). The PDPs were tested at a concentration of 2% of full saturation, previously determined to be a critical concentration capable of the greatest damage. Limestone and quartzite aggregate samples subject to freeze– thaw testing showed more serious damage in all deicers than in distilled water, as measured by accumulated weight loss. Aggregates immersed in urea exhibited the most weight loss for both types, whereas the least damaging deicer for limestone was NaCl and for quartzite was KAc. Asphalt pavement samples taken from the Ottawa Macdonald–Cartier Inter- national Airport were subjected to freeze–thaw cycles in closed

containers. After 25 and 50 cycles, indirect tensile strength, elastic modulus, and penetration tests were performed. The indirect tensile strengths of the samples exposed to deicers were mostly higher than those exposed to distilled water. The lowest average elastic modulus was associated with the samples in urea and visual inspection indicated significant damage by urea. Based on weight measurements and density calculations, the asphalt mix sample immersed in NaF experienced the most disintegration after 25 cycles, whereas urea (followed by KAc) was the most detrimental deicer after 50 cycles. Exposure to freeze–thaw cycles and deicers was found to affect the viscos- ity of the recovered asphalt binder and the gradation of recov- ered aggregates. The freeze–thaw cycles seemed to result in soft asphalt binder, whereas the deicers caused asphalt harden- ing. However, the authors noted that these findings were incon- clusive owing to the difficulties involved in testing and the inaccuracies in measuring the viscosity of the recovered asphalt. Overall, this laboratory investigation found urea to be the most detrimental deicer, whereas the other deicers “induced relatively small damage, comparable to that caused by distilled water” (Hassan et al. 2000). However, it was noted that chem- ical reactions would occur slowly at the temperatures involved in this study and that damage in the field could occur as a result of reactions between PDP residues and asphalt during hot sum- mer temperatures (Hassan et al. 2000). A follow-up study was conducted at higher temperatures on asphalt pavement samples taken from the Dorval Interna- tional Airport (Montreal, Canada) to clarify the role played by the PDPs (NaCl, KAc, NaF, as well as urea) in asphalt deterioration, and to determine whether the damage was attributable to the physical freeze–thaw action. Only 15 freeze– thaw cycles were performed before subjecting some samples to 40 wet–dry cycles at 40°C. This research confirmed the previous finding that softening occurs during freeze–thaw and exposure to deicers causes hardening. After the freeze– thaw and wet–dry cycles, the samples in NaAc showed the lowest strength, followed by those in NaF. Interestingly, all samples showed increased strength after the warm wet–dry cycles and all except NaF and NaAc showed increased elas- ticity after the warm wet–dry cycles. However, the dry sam- ples not exposed to freeze–thaw or wet–dry cycles had the greatest elasticity and nearly highest strength. Overall, the Canadian studies did not indicate significant damaging effects of KAc and NaF on asphalt pavement (Farha et al. 2002; Hassan et al. 2002a). It should be cautioned, however, that these results were based on laboratory experiments on only two samples of asphalt pavement and the mix design for each pavement was undeterminable from the reports. Addi- tionally, the deicer concentration was low (2%), which may not be conducive to simulate years of field exposure by means of accelerated laboratory testing. Concurrent to the use of acetate/formate-based deicers in the 1990s, asphalt pavement in Europe saw an increase in pavement durability problems. At some Nordic airports, these 30 problems emerged as degradation and disintegration of asphalt pavement, softening of asphalt binders, and stripping of asphalt mixes occurring together with loose aggregates on the run- ways (Nilsson 2003; Pan et al. 2006; Seminar on the Effects of De-Icing Chemicals on Asphalt 2006). Such problems were not identified before the airports changed from urea to KAc- and KF-based deicers (Pan et al. 2006). In 2001, serious asphalt durability problems were identified at airports in Nordic countries that used acetate/formate-based PDPs (Pan et al. 2006). Heavy binder bleeding and serious stripping problems were observed occurring together with loss of asphalt stabil- ity. Soft, sticky, and staining binder came to the surface, often leaving strong stains on electrical devices and on the airplanes. The binder of the asphalt base layer was “washed” off, and the aggregates experienced severe loss of strength. In the labora- tory, the tests indicated chemical changes in the binder after exposure to the deicer, in the form of emulsification, distilla- tion, and an increased amount of polycyclic aromatic hydro- carbons (PAHs). A field investigation was conducted there- after confirming the deleterious effects of acetate-based deicer on asphalt pavement. The bitumen and the mastic squeezed to the surface of the core, and the concentration of the deicer had a clear influence on its solubility. Some bitumen was dissolved into the pore liquid, and pure stone particles were found inside the core. The limestone filler was found fully dissolved by the PDP liquid and the rest of the mastic became brittle and grey- colored. A large increase in the porosity of asphalt was also noticed. To address the concerns over acetate-based deicers affect- ing asphalt pavement, a joint research program—the JÄPÄ Finnish De-icing Project—was established to conduct exten- sive laboratory and field investigations on this subject. The goal of JÄPÄ was to provide answers to three fundamental concerns: how the damages are generated, how to determine the compatibility between asphalt and de-icing materials, and how to prevent damages by mix design (Pan et al. 2006). The research showed that formate/acetate-based deicers signifi- cantly damaged asphalt pavements. The damaging mecha- nism seemed to be a combination of chemical reactions, emulsification, and distillation, as well as generation of addi- tional stress inside the asphalt mix. Asphalt binders soaked in the deicer solution were found to have lower softening points and tended to dissolve at temperatures as low as 20°C. Asphalt mixes soaked in the deicer solution were found to have lower surface tensile strength and lower adhesion (Nils- son 2003; Seminar on the Effects of De-Icing Chemicals on Asphalt 2006). It seemed to be clear that deicer (formate or acetate), water or moisture, and heat were necessary for the damage to occur. In the field, such damages mainly occurred during the repaving process or on hot summer days with residual deicers from the winter season, as dynamic loading and unloading reduced the time it took for damages to occur. Recent laboratory studies by a research group at Montana State University were able to reproduce acetate-induced emul-

31 sification of asphalt similar to the field observations at Nordic airports (Pan et al. 2008). Aqueous solution tests of asphalt binder in water and four NaAc solutions of different concen- trations (5%–40%) showed a bilinear trend of weight loss increasing with the NaAc concentration. Both visual inspec- tion and optical microscopy (as shown in Figure 15) indicated that a significant amount of asphalt emulsification occurred in NaAc, but not in water or aqueous solutions of NaCl or NaOH with a pH of 9 (equivalent to the measured pH of 40% NaAc solution). For the two tested asphalt binders, PG 58-22 exhib- ited slightly higher emulsification than PG 67-22. In the calcium magnesium acetate aqueous solution, asphalt emulsi- fication occurred similarly to that in NaAc. These results con- firmed that asphalt emulsification should be attributed to the acetate anion, CH3COOH– and excluded the possibility that high alkalinity was responsible for the asphalt emulsifica- tion in NaAc. Asphalt emulsification also occurred in a NaF aqueous solution and more detailed laboratory testing is being conducted. The effects of NaAc on asphalt mixes were examined by conducting a modified ASTM D 3625-96 Boiling Water Test, which was originally designed to test the susceptibility of asphalt mixes to moisture damage, by accelerating the effect of water on bituminous-coated aggregate with boiling water. Stripping occurred for both crushed gravel and lime- stone aggregate particles included in the asphalt mix exposed to NaAc, suggesting that aggregate properties play at most a secondary role in asphalt emulsification (Pan et al. 2008). As indicated in Figure 16a, significant amounts of aggregates were stripped after exposure to the NaAc solutions and the aggregate stripping followed a bilinear trend with weight loss increasing with the NaAc concentration. Phase I of Airfield Asphalt Pavement Technology Pro- gram Project 05-03: Effect of Deicing Chemicals on HMA Airfield Pavements includes a literature review, interviews with 36 airports that use deicers and have asphalt pavement, as well as laboratory testing. Seven airports indicated that pavement deterioration had occurred, but the cause was unknown except in one case that was most likely attributable to the type and source of asphalt binder and aggregate. Pre- liminary laboratory testing was conducted of asphalt pave- ment samples composed of either a chert gravel or diabase with two binders (PG 64-22 and PG 58-28) exposed to KAc and NaF. The presence of PAHs was inconclusive after vacuum-induced saturated samples were stored for 4 days at 60°C. However, significant generation of carboxylate salts had developed after the asphalt mixes were exposed to the deicers, although this may not be related directly to deicer- induced damage. Indirect tensile strength tests showed PG 64-22 to be “somewhat more resistant” (Advanced Asphalt Technologies 2007) and that chert gravel had significantly less strength when exposed to deicers compared with water. A long-term durability test developed by Advanced Asphalt Technologies also showed chert to be very susceptible to moisture damage, particularly when exposed to KAc or NaF. Soundness tests of both types of aggregate in magnesium sul- fate, KAc, and NaF were acceptable and also showed that direct attack on the aggregate by the deicers was not occur- ring (Advanced Asphalt Technologies 2007). (a) (b) (c) (d) FIGURE 15 Digital photos (left) and optical microscopic images (right) showing the suspension solution of asphalt subsequent to the 60°C aqueous solution test. (a) and (b) Twenty-four hour reservation of 60°C aqueous solution test with the 20% acetate concentration; (c) and (d) Twenty-four hour reservation of the 60°C aqueous solution test with the 40% acetate solution [adapted from Pan et al. (2007)]. Sodium Acetate Concentration (wt. %) 0 0 10 20 30 40 10 20 30 40 Pe rc en t S tr ip pe d A gg re ga te (% ) FIGURE 16 Percent stripped aggregates for an asphalt mix exposed to different NaAc solutions after the Modified Boiling Water Test [adapted from Pan et al. (2007)].

Nature of the Effect of Modern Pavement Deicing Products on Asphalt Pavement Deterioration Canadian research demonstrating the damaging effects of urea on two types of asphalt pavement did not propose any mechanism or scientific explanation for the observations (Hassan et al. 2000, 2002b). The JÄPÄ–Finnish De-icing Project studied the ingredi- ent materials in asphalt pavement individually and their roles played in the damaging mechanism were ranked accordingly (Alatyppö 2005). The detailed test results of each ingredient material are included as follows, as summarized by Pan et al. (2006). Effects of Formate/Acetate-Based Deicers on Aggregates: The main reason for pavement damages was not due to poor quality of aggregates. Mineral aggregate might be a reason secondary to asphalt binders in pavement damage. The decom- position level of acidic aggregates was higher than for caus- tic aggregates, but was still acceptable. However attention should be paid to the weathering resistance of aggregates used in airfields to extend the life span of asphalt pavements. Physical Effects of Formate/Acetate-Based Deicers on Bitumen/Asphalt: (1) High density of deicer solution such as 1.34 kg/dm3 for the 50 wt.% solution enabled the deicer solu- tion to penetrate into bitumen by gravity. (2) Very low sur- face tension between deicer chemicals and asphalt facilitated stripping and emulsification of asphalt mixes. (3) Formate/ acetate-based deicers had pH values usually between 9 and 11; and the higher the pH, the more aggressive the deicer would be. (4) Formate/acetate-based deicers were very hygroscopic, which kept the road surface constantly wet and retained water inside asphalt to overfill the air voids. Chemical Effects of Formate/Acetate-Based Deicers on Bitumen/Asphalt: When exposed to deicers, composition changes of bitumen/asphalt occurred in the hydrocarbon clas- sification C10-C40. When exposed to deicers, large organic molecules such as the PAHs grew in bitumen. Deicers in asphalt were found in both the liquid and gas phases. PAHs in the asphalt samples could migrate and become dissolved in the deicer. Failure Process of Asphalt Pavements: Deicers migrate into the asphalt after application onto pavements and saturate asphalt mixes during the winter. The deicer solution intrudes into asphalt due to gravity and for other unknown reasons, espe- cially when asphalt temperature rises significantly (a result of a hot asphalt layer laid or summer weather). Due to the low sur- face tension between deicers and bitumen, the deicers are absorbed in the bitumen that in turn starts to emulsify. It is pos- sible that the chemical composition of the bitumen changes during emulsification. Due to emulsification the bitumen comes loose and the aggregate particles get cleaned, followed by bleeding and stripping. Ongoing research by Advanced Asphalt Technologies currently suggests that the damaging mechanism is mainly a disruption of the asphalt-aggregate bond as a result of ASR. Expansive pressures typical of ASR-damaged concrete are not perceived to be the problem, but rather the bond disrup- tion and increased susceptibility to moisture damage. Well- drained pavements may provide some protection because deicers are not applied during warm weather. Advanced Asphalt Technologies is currently working on Phase II that 32 includes more significant laboratory testing and field investi- gations (Advanced Asphalt Technologies 2007). However, the research by Pan et al. (2008) shows: (1) asphalt emulsifi- cation occurring to asphalt mixes with both reactive and non- reactive aggregates, and (2) asphalt emulsification not occur- ring in NaOH solutions of the same pH values as the NaAc solution. Thus, the research indicates that asphalt emulsifica- tion may be a more critical mechanism of asphalt mix deteri- oration than ASR unless very reactive aggregates are used in the asphalt mix. Pan et al. (2008) proposed a detailed and specific mecha- nism of acetate-induced asphalt emulsification based on con- tact between acetate anions (CH3COOH–) and asphalt, which can be greatly increased at high summer and/or repaving tem- peratures owing to the tendency of asphalt to swell. For NaAc, aqueous solution tests of asphalt binder were performed at sev- eral concentrations and temperatures and the resulting sus- pended substance was examined using the Fourier Transform Infrared Spectroscopy. No significant amounts of new chemi- cals were identified, and intermolecular binding between the acetate anion CH3COOH– and the alkane component of asphalt was inferred (Pan et al. 2008). Van der Waals forces anchor the lipophilic organic chain (CH3–) of the acetate anion to the molecular chain of asphalt (CH3–CH2–). At the same time, the hydrophilic polar end of the acetate anion (COO–) forms hydrogen bonds with water molecules and pulls on the asphalt, overcoming the intermolecular forces within the asphalt. Asphalt emulsion is maintained by Brownian motion and repulsive forces on the floccules. The emulsification of asphalt reduces the asphalt-aggregate bond and can lead to adhesion failure in the pavement. There is also a potential that the aggregate preferentially bonds with the acetate anion, which has a higher polarity than the asphalt molecules. Standards and Test Protocols There are two existing Swedish test methods related to asphalt and deicers: the LFV Method 1-98, Bituminous Binders, Stor- age in De-icing Fluid, and the LFV Method 2-98, Effect of De-icing Fluid on the Surface Tensile Strength of Asphalt Concrete for Airfields—Adhesion Test. In 2006, results of round-robin testing of LFV Method 2-98 by seven laborato- ries were presented; however, the information was only avail- able in the form of Microsoft PowerPoint Slides and thus dif- ficult to follow. The confusion lies in the measurement units reported. The LFV 2-98 method indicates the surface strength at failure should be reported to the nearest 0.1 N/mm2, but the standard deviation of the round-robin test for repeatability was reported to be 130 N and 220 N for reproducibility (Nils- son 2006). One Norwegian airport reported the use of LFV Method 2-98, according to the ACRP survey results. The Aqueous Solution Test as developed by Pan et al. (2008) showed high efficiency in examining the emulsifiability of asphalt, and can be potentially established as a standard accel- erating test. The other test—the Modified Boiling Water Test—also proposed by Pan et al. (2008) can be used as a rou-

33 tine laboratory test for evaluating suspicious asphalt mixes when exposed to alkali-metal-salt-based deicers. Prevention and Mitigation To prevent or mitigate the effects of PDPs on asphalt pavement, the first and most important countermeasure is to follow best possible practices in asphalt mix design and paving. Responses to the survey for this project point toward adoption of some of these preventive measures: one European airport reduced asphalt pavement air void to 3.0%; another European airport indicated that polymer-modified binder is used; and one U.S. airport changed the asphalt binder to PG 76-32, citing current FAA specifications. Nonetheless, the JÄPÄ Project research showed that the resistance of asphalt pavement to deicers can be improved only partially by mix design. According to the laboratory results, binders with high viscosity or polymer- modified binders were recommended when formate/acetate- based deicers were to be used. High-quality (sound) aggre- gates could also improve the durability of asphalt pavements in the presence of such deicers, and so did the aggregates with higher pH (Pan et al. 2006). It was recommended that the void contents of the asphalt mixes be kept low enough to limit deicer solution in pores. Other suggestions to prevent asphalt damages are summarized here (Valtonen 2006): • Prefer harder bitumen (penetration max 70/100) or mod- ified bitumen. • Use alkaline aggregates and avoid limestone filler. • Test the compatibility of the materials in advance. • For security, do not use acetates and formates on asphalt structures. • When repaving, mill away the wearing course containing residual deicers and do not use the recycled asphalt pave- ment unless confirming that it is not hazardous (Alatyppö and Valtonen 2007). Knowledge Gaps Although it was observed in some Nordic airfields that exacer- bated asphalt deterioration occurred with applications of alkali- metal-salt-based PDPs, thus far little observation has been reported in U.S. or Canadian airports. Significantly accelerated deterioration of asphalt pavements was found in laboratories when exposed to acetate/formate-based deicers. There is a need for research data from controlled field investigation regarding the effects of alkali-metal-salt-based PDPs on asphalt pave- ment, which would help differentiate the contribution of such PDPs to asphalt deterioration from other possible factors in the field environment. One challenge is that the durability of asphalt pavement is often significantly affected by the mix design, paving, and maintenance practices, the exposure to cli- matic conditions, as well as the exposure to traffic loading. Furthermore, there is a need to unravel the specific mech- anisms by which alkali metal salts and other PDPs (e.g., bio- based deicers) deteriorate asphalt pavement. Knowing the mechanism(s) of damage will provide necessary guidance for preventing or mitigating such damage and for developing the next generation of PDPs and airfield asphalt pavement. Advances in technologies related to PDPs and asphalt pave- ment will make both understanding their interaction and developing an appropriate compatibility test protocol a con- tinued effort. IMPACT OF PAVEMENT DEICING PRODUCTS ON OTHER AIRFIELD INFRASTRUCTURE Other airfield infrastructure that comes into contact with PDPs includes ground support equipment (GSE), signage, lighting, and other electrical systems. Empirical evidence exists indi- cating that PDPs are responsible for damaging such infra- structure. However, no academic peer-reviewed scientific information could be found to corroborate these empirical observations. The survey and other less technical information were heavily relied on in examining the case. In July 2004, the United Kingdom Civil Aviation Author- ity issued a Notice to Aerodrome License Holders, the authority’s standardized procedure for disseminating infor- mation about licensing of aerodromes, about the corrosion effects on ground lighting. Premature failure of an aeronau- tical ground lighting centerline fixture was partially attrib- uted to a rubber removal cleaner; the cleaner destroyed the passivated corrosion protection layer and cracking formed. The cracking significantly reduced the strength of the fitting. However, it was thought that deicers could also produce a similar effect. The recommended action was to inspect fit- tings, repassivate if needed, and prevent fittings from con- tacting fluid with a pH outside of the range of 4 to 8.5 (Aero- drome Standards Department 2004). In 2005, when one European airport switched from urea and ethylene glycol to formate-based products, corrosion of zinc-coated steel occurred on light fixtures, as well as on maintenance and ground operation vehicles. The same air- port now uses stainless steel light fixtures instead of zinc- coated steel. Another European airport found that washing airport vehicles has decreased the corrosion effects, accord- ing to the responses to the ACRP synthesis survey. The ACRP survey results indicated that lighting cable was also reported to deteriorate during or shortly following deic- ing events at two U.S. airports. One of these airports suspects that the aging of cable insulation plays a role in the deterio- ration. The other airport has upgraded to lighting cable with more resistance to deicers and is in the process of installing a system to remotely monitor the lighting electrical system. The FAA has approved a test method for airport contain- ers designed to serve as airport light bases, transformer hous- ings, junction boxes, and accessories in the presence of

deicers containing KAc. A 50% KAc deicing solution is left in the base for 21 days at 194°F. To pass, the base must have no evidence of corrosion or leakage (“Specification for Air- port Light Bases . . .” 2006). Finally, the only other information found concerning deicers and airfield infrastructure is the possible reduction in safety from dirty light fixtures. In 2002, the airfield duty manager of the Manchester Airport in the United Kingdom mentioned “. . . that deicing fluid makes light fixtures sticky and more dirt sticks to them in the winter months” (Flight Safety Foundation Editorial Staff 2002). CONCLUDING REMARKS Both traditional and modern PDPs have been observed to react with major pavement materials and deteriorate the integrity of airport pavements. Deterioration of PCC pavements owing to 34 deicers is a complex process that involves chemical and phys- ical alterations in aggregates and cement paste. Deterioration of hot mix asphalt caused or accelerated by the PDPs is a less explored area. In addition, current understanding of the impact of modern PDPs on both PCC and asphalt pavements is mostly based on macro-level observations and testing of properties, whereas mechanisms underlying the critical physical and chemical interactions are less known. Therefore, in-depth research using advanced techniques is needed to advance the knowledge base for better design, construction, and mainte- nance of pavement materials and to extend their service life in a cost-effective manner. In spite of their environmental advantages over older for- mulae such as urea and glycols, alkali-metal-salt-based PDPs present potential problems for the airfield infrastructure. Table 12 summarizes the effects of modern PDPs on airfield infrastructure, including the key findings and knowledge gaps. PDP Impact Information Sources What Is Known What Is Unknown 2. There is a need to unravel the specific m echanism by which alk ali me tal salts cause or prom ote ASR. 2. Si gn ificantly accelerated deterioration of asphalt pave me nts was found in laboratories when exposed to acetate/f orm ate- based deicers. 2. There is a need to unravel the specific m echanism s by which alk ali me tal salts and other PDPs (e.g., bio- based deicers) deteriorate asphalt pavem ent. Im pact of PDPs on asphalt pavemen t 1. Acade mi c-peer- reviewed literature 2. In dustry -p eer- reviewed p ubl ic ati ons and reports 3. Survey of stak eholder gr oups 1. Although it was observ ed in so me Nordic airf ields that exacerbated asphal t deterioration occurred w ith applications of alk ali- me ta l- salt-based PDPs, there is thus far littl e observ ation reported in U.S. or Canadian airports. 1. No academ ic- peer- rev iewed scienti fic inform ation could be found to corroborate these em pirical observ ations. 1. The last decade has seen an increase in the premature deterioration of airfield PCC pav em ents with the use of alka li- me ta l- salt-based PDPs. 1. There is a need for research data from controlled field in ve stig ation reg arding the effects of alk ali- me tal- salt-based PDPs on concrete pavem ent. Im pact of PDPs on concrete pavement 1. Academi c-peer- reviewed literature 2. In dustry -p eer- reviewed p ubl ic ati 2. Lim ited existin g laboratory studies indicated that alk ali- me tal- salt-based deicers could cause or accelerate ASR distress in the surface of PCC pav em ent, by in creasin g th e pH of concrete pore solution. ons and reports 3. Survey of stak eholder gr oups 1. There is a need for research data from controlled field in ve stig ation reg arding the effects of alk ali- me tal- salt-based PDPs on asphalt pa ve me nt . Im pact of PDPs on other airfield infrastructur e 1. In dustry -p eer- reviewed p ubl ic ati ons and reports 2. Survey of stak eholder gr oups 1. Em pirical evidence exists indicating that PDPs are responsible for dam ag in g other airf ield infrastructure (G SE , si gn ag e, lig htin g and other electrical system s). GSE = ground support equipment. TABLE 12 SUMMARY OF EFFECTS OF MODERN PAVEMENT DEICING PRODUCTS ON AIRFIELD INFRASTRUCTURE

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TRB’s Airport Cooperative Research Program (ACRP) Synthesis 6: Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure explores how airports chemically treat their airport pavements to mitigate snow and ice, and the chemicals used. The report also examines the effects of pavement deicing products on aircraft and airfield infrastructure, and highlights knowledge gaps in the subject.

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