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35 CHAPTER FIVE CONCLUSIONS Responses representing approximately 100 airports were gath- KF on airfield pavements leads to premature oxidation of ered from the recent (2006) EPA questionnaire, which indi- C/C composite brake components. As the brake frictional char- cated that potassium acetate (KAc) and sand are most widely acteristics are changed by the use of alkali-metal-salt-based used at U.S. airports for snow and ice control of airfield pave- deicers on airfield pavements, airline operators are concerned ments, followed by airside urea, sodium acetate, sodium for- about the adverse effect of these PDPs on the braking perfor- mate, propylene glycol-based fluids, ethylene glycol-based flu- mance and safety of aircraft. ids, and other. Responses representing 12 U.S. airports and 3 non-U.S. airports were gathered from the ACRP synthesis sur- There are potential opportunities for all stakeholder groups vey distributed in this project to the 50 busiest U.S. airports to collaborate to address the catalytic oxidation issue of C/C among others. From the ACRP survey results, the selection of aircraft brakes, with respect to aircraft and component design, pavement deicing products (PDPs) by airport staff was based brake testing, aircraft operations, airfield maintenance, etc. In on many factors, including cost, effectiveness, environmental the domain of brake technologies, the combination of chem- impact, risk of corrosion, and electrical conductivity. "Effec- ical modification of C/C with structural changes or defect tiveness" was ranked as the most important criterion and "elec- elimination seems to offer promising solutions to mitigating trical conductivity" as the least. The effectiveness criterion also catalytic oxidation. Catalytic oxidation of C/C brakes may exhibited the lowest standard deviation, with "corrosion risk" also be mitigated by utilizing more carbon-friendly PDPs on being the highest. Interestingly, the challenges and dilemmas airfield pavements. faced by the airports pertinent to snow and ice control were highlighted because no airport selected "unimportant" or "not · Cadmium Corrosion very important" for any of the criteria options in the survey. Cadmium (Cd) plating is the most popular surface treatment Alkali-metal-salt-based PDPs such as KAc and potassium technology for corrosion protection of aircraft steel parts formate (KF) entered the European market to a significant (e.g., airframe components and fasteners). Field reports in- extent in the mid- to late-1990s. A few years later, these mod- creasingly suggest that the contact with modern PDPs pro- ern PDPs entered the U.S. market. In both cases, these salts motes damage to aircraft components, including Cd-plated were introduced as alternatives to urea and glycols used in components. Until recently, the principal evidence connect- traditional PDPs for freezing point depression, to mitigate the ing alkali-metal-salt-based PDPs with Cd-plating corrosion environmental concerns related to airfield deicing and anti- has been the increasing number of reports of the latter occur- icing operations. It became apparent soon after their intro- ring concurrently with to the introduction of the former. duction that these new deicers presented new challenges, to both the aircraft and airfield infrastructure. There are potential opportunities for all stakeholder groups to collaborate to prevent and mitigate the effects of PDPs on · Catalytic Oxidation of CarbonCarbon Composite aircraft components, from aspects of aircraft and component Brakes design, aircraft operations, and airfield maintenance. In the domain of corrosion-inhibiting compounds, there is still great Thermal oxidation is the primary design specification govern- potential for improvement when it comes to mitigating the ing durability of aircraft carboncarbon (C/C) composite effect of PDPs on aircraft frames and components. Little aca- brakes. Catalytic oxidation of C/C composite brakes resulting demic research on interactions between alkali metal salts and from airfield PDPs has become a growing concern that needs Cd-plating is available, and still less is available on inhibition to be monitored in the ever-changing operation environment. of these interactions. In lieu of a comprehensive prevention In recent years, as non-traditional chemical contaminants, solution to Cd-plating corrosion or a satisfactory Cd-plating modern PDPs may be responsible for the more rapid structural replacement, shop-level mitigation practices such as addi- failure of C/C composite brakes. To avoid potential safety tional and enhanced maintenance and inspection should help implications, this concern has to be mitigated through more reduce the effects of PDPs on corrosion-prone, Cd-plated frequent proactive maintenance and inspection activities incur- steel aircraft components. Such best practices would also min- ring high direct and indirect costs. A growing body of field evi- imize the impact of PDPs on other aircraft components. In addi- dence from airline operators suggests that the use of KAc and tion, the corrosion of aircraft components (e.g., Cd-plating and
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36 aluminum parts) can be mitigated by using less corrosive PDPs both aggregates and asphalt mixes. Concurrent to the use of on airfield pavements. acetate and formate-based deicers in the 1990s, asphalt pave- ment in Europe saw an increase in pavement durability prob- · Interaction with Aircraft Deicing and Anti-Icing lems. At some Nordic airports, these problems emerged as Fluids degradation and disintegration of asphalt pavement, softening of asphalt binders, and stripping of asphalt mixes occurring Thickeners used in modern aircraft deicing and anti-icing flu- together with loose aggregates on the runways. Such problems ids increase viscosity through chargecharge interaction; were not identified before the airports changed from urea to organic salts such as KAc and KF are known to disrupt this KAc- and KF-based deicers. According to laboratory and field interaction and cause a measurable reduction in their viscos- investigations conducted under a joint research program--the ity. Not only do alkali-metal-salt-based PDPs accelerate the JÄPÄ Finnish De-icing Project--the damaging mechanism precipitation and buildup of thickener residues, but under the of asphalt pavement by modern PDPs appeared to be a combi- right conditions, they may also encourage greater moisture nation of chemical reactions, emulsification, and distillation, as uptake by the thickeners. well as the generation of additional stress inside the asphalt mix. Although interaction between runway and aircraft deicers is inevitable, there are opportunities to control the effects of To prevent or mitigate the effects of PDPs on asphalt pave- the interaction by means of enhanced operational practices. Nonetheless, challenges such as financial and environmental ment, the first and most important countermeasure is to fol- constraints remain for such operational practices in commer- low best possible practices in asphalt mix design and con- cial aviation. In addition, spray from PDP pools is unpredict- struction. Responses to the ACRP survey for this project able during aircraft take-off and landing. Interaction with pointed toward adoption of some of these preventive mea- Type IV aircraft deicing and anti-icing fluids has been seen sures: one European airport reduced asphalt pavement air to rapidly promote rough, persistent residue on wing leading void to 3.0%; another European airport indicated using edges with unfavorable aerodynamic properties. polymer-modified binder; and one U.S. airport changed the asphalt binder to PG 76-32, citing current FAA specifications. · Impact of Pavement Deicing Products on Concrete Nonetheless, the JÄPÄ Project research showed that the resis- Pavement tance of asphalt pavement to deicers can be improved only partially by mix design. According to the laboratory results, The last decade has seen an increase in the premature deteri- binders with high viscosity or polymer-modified binders were oration of airfield portland cement concrete (PCC) pavements recommended when formate/acetate-based deicers were to be with the use of alkali-metal-salt-based PDPs. Such PDPs have used. High-quality (sound) aggregates could also improve been used more extensively and for more years in European the durability of asphalt pavements in the presence of such countries for winter maintenance than in the United States. deicers, and so did the aggregates with higher pH. The void The degree of distress in the PCC pavements of European contents of the asphalt mixes were recommended to be kept facilities ranged from mild to severe in terms of surface crack- low enough to limit deicer solution in pores. ing, repair, and rehabilitation efforts needed. Limited exist- ing laboratory studies indicated that alkali-metal-salt-based · Impact of Pavement Deicing Products on Other deicers could cause or accelerate alkali-silica reaction (ASR) Airfield Infrastructure distress in the surface of PCC pavement by increasing the pH of concrete pore solution. Other airfield infrastructure that comes into contact with PDPs includes ground support equipment, signage, and lighting and To prevent or mitigate the effects of PDPs on concrete pavement, the first and most important countermeasure is to other electrical systems. Empirical evidence exists indicating follow best possible practices in concrete mix design and con- that PDPs are responsible for damaging such infrastructure. struction. ASR has been conventionally controlled by limit- However, no academic-peer-reviewed scientific information ing alkali content in cement and selecting aggregates of good could be found to corroborate these empirical observations. quality. Furthermore, efforts have been made to mitigate ASR by adding various supplementary cementitious materials or · Looking to the Future chemical admixtures such as lithium compounds. When it comes to airfield pavement deicing and anti-icing · Impact of Pavement Deicing Products on Asphalt there are no simple solutions to the competing, and sometimes Pavement conflicting, objectives of aircraft safety, environmental reg- ulatory compliance, materials compatibility, and operational In addition to the effects of PDPs on PCC pavement, their implementation viability. effects on asphalt pavement are also of increasing concern. A laboratory study found that the use of PDPs (sodium chloride, The ACRP survey distributed for this project provided a KAc, and sodium formate, as well as urea) was damaging to forum to describe knowledge gaps and research needs, as well
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37 TABLE 13 SUMMARY OF PDP EFFECTS, KNOWLEDGE GAPS, AND ONGOING RESEARCH PDP Impact What Is Known What Is Unknown Ongoing Research 1. A growing body of field evidence from The SAE G-12 Carbon Oxidation airline operators suggests that the use of 1. There is still a need to establish a Working Group is in the process of KAc and KF on airfield pavements leads comprehensive PDP catalytic Catalytic refining a carbon compatibility test to catalytic oxidation of C/C composite oxidation test protocol. oxidation of protocol. brake components. carboncarbon composite 2. More research is needed to better 2. Existing research in the laboratory has The SAE A-5A Brake Manufacturers brakes understand relationships between demonstrated the catalytic effects of Working Group is in the process of brake design, AO treatment, and potassium, sodium, and calcium on carbon developing an oxidation test method PDP contamination as factors in oxidation. for AO-treated coupons. catalytic oxidation. 1. There is still a need to establish a 1.Until recently, the principal evidence comprehensive metallic corrosion connecting alkali-metal-salt-based PDPs test protocol for PDPs. with Cd-plating corrosion has been a trend of increased reports of the latter 2. More research is needed to better Corrosion of understand the interactions among A Cd-corrosion test protocol has been occurring simultaneously with the aircraft alloys the aircraft component design, the in development in the SAE G-12 Cd introduction of the former. (with a focus CICs used, and the contamination Corrosion Working Group since 2003 2. Very little research has been conducted on cadmium of PDPs in the processes of and is currently being refined for to investigate the mechanism of Cd plating) metallic corrosion. inclusion to AMS 1431 and 1435. corrosion or Cd-steel corrosion in the presence of alkali metal salts (e.g., KF and 3. There is still a lack of academic KAc), partly owing to the high toxicity research data from controlled field associated with Cd and its compounds. investigation regarding the aircraft metallic corrosion by PDPs. 1. The contamination effects of ADAFs by runway deicing fluids have been well-observed but not Interaction 1. Recent laboratory data appear to yet thoroughly quantified. with aircraft corroborate anecdotal reports of increased The SAE G-12 Fluid Residues deicing and rates of thickener residues in environments Working Group is leading research 2. Further research is needed to anti-icing where alkali-metal-salt-based PDPs have efforts in this field. better understand the interactions products been used. between ADAFs and PDPs, as new ADAFs and PDPs are continually introduced to the market. 1. There is a need for research data 1. The last decade has seen an increase in from controlled field investigation IPRF Project 05-7: Performance of the premature deterioration of airfield regarding the effects of alkali-metal- Concrete in the Presence of Airfield PCC pavements with the use of alkali- Salt-based PDPs on concrete Pavement Deicers and Identification of Impact of metal-salt-based PDPs. pavement. Induced Distress Mechanisms and PDPs on IPRF Project 06-5: Role of Dirty concrete 2. Limited existing laboratory studies Aggregates in the Performance of pavement indicated that alkali-metal-salt-based 2. There is a need to unravel the Concrete Exposed to Airfield deicers could cause or accelerate ASR specific mechanism by which alkali Pavement Deicer, both conducted by distress in the surface of PCC pavement, metal salts cause or promote ASR. Clemson University. by increasing the pH of concrete pore solution. 1. Although it was observed in some 1. There is a need for research data Nordic airfields that exacerbated asphalt AAPTP Project 05-03: Effect of from controlled field investigation deterioration occurred with applications of Deicing Chemicals on HMA Airfield regarding the effects of alkali-metal- alkali-metal-salt-based PDPs, there is thus Pavements, conducted by the Impact of salt-based PDPs on asphalt far little observation reported in U.S. or Advanced Asphalt Technologies. PDPs on pavement. Canadian airports. asphalt 2. There is a need to unravel the RITA Project: Mitigation of Moisture pavement 2. Significantly accelerated deterioration specific mechanisms by which and Deicer Effects on Asphalt Thermal of asphalt pavements was found in alkali metal salts and other PDPs Cracking through Polymer laboratories when exposed to (e.g., bio-based deicers) deteriorate Modification, conducted by Montana acetate/formate-based deicers. asphalt pavement. State University. Impact of 1. Empirical evidence exists indicating 1. No academic-peer-reviewed PDPs on other that PDPs are responsible for damaging scientific information could be N/A airfield other airfield infrastructure (GSE, signage, found to corroborate these infrastructure lighting and other electrical systems). empirical observations. AO = anti-oxidant; CICs = corrosion-inhibiting compounds; ADAFs = aircraft deicing/anti-icing fluids; ASR = alkali-silica reaction; N/A = not available.
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38 as potential challenges for the future of airfield pavement deicers with airfield infrastructure and airframe materials, as deicing and anti-icing. Some of the key findings from the sur- well as standards for environmental effects of deicers. There vey and the literature review are summarized in Table 13. was a strong call for best practices and new test methods with pass/fail criteria, but also skepticism about the actual A dominant theme throughout all the responses provided impacts of PDPs and whether scientific data are truly avail- by airports was the challenge of needing environmentally able to confirm these impacts. One suggestion noted was to benign products that are simultaneously safe for aircraft, keep precise records of PDP use to develop the knowledge pavement, and electrical systems. Several respondents indi- base needed to determine if PDPs are damaging aircraft and cated the need for standards concerning the compatibility of airfields.