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34 Introduction As a means of determining the experiences in addressing ASR on concrete airfield pavement facilities, a web-based survey was prepared and distributed to selected airport owners/agencies. Some airports known to have ASR issues were earmarked, identified based on input from the technical panel, from contacts within the industry, and from the literature review. The contacts for the airports included airport managers/administrators, airport engineers, airport mainte- nance personnel, and airport engineering consultants. A total of 19 responses were received out of the 24 contacted, which came not only from airports, but also from several engineers who represent airports that have experienced ASR issues. (Note: survey responses reflect that some respondents had multiple answers to some questions.) The geographical distribution of respondents to the survey is shown in Figure 9. This chapter summarizes the findings from the airport surveys. A copy of the blank survey is provided in Appendix A and a summary of the survey responses is presented in Appendix B. Survey Content The web-based survey was developed in SurveyGizmo and designed to be completed in less than 15 minutes. It focused on the use of and experience with various ASR treatment and miti- gation methods and asked about the availability of any new or innovative methods exhibiting promising results. Recognizing that airports may have more than one facility affected by ASR, the survey allowed for the entry of up to three such facilities and the airportâs experience with each. A preliminary version of the questionnaire was reviewed and tested by the technical panel prior to distribution. Overall, the survey was organized into five primary sections: â¢ Section 1âGeneral. This section gathered contact information from the respondent, including the airport represented. â¢ Section 2âManifestations and Verification of ASR. This section solicited airport facilities (runway, taxiway, apron) affected with ASR (up to a maximum of three). For each facility entered, the year that the facility was built and the age that ASR was first identified were documented. It also gathered information on the type of distress manifestations that were exhibited, and how ASR was confirmed. â¢ Section 3âUse of Corrective Treatments to Address ASR. This section explored the types of corrective treatments (e.g., partial-depth repairs, full-depth repairs, overlays, reconstruction) used to address ASR, along with the performance of those treatments. C H A P T E R 3 Airport Surveys
Airport Surveys 35 â¢ Section 4âMitigation Strategies to Address ASR. This section investigated the use of mitigation strategies (e.g., waterproofing surface treatments, lithium compounds) by airports on their listed facilities to mitigate or reduce the progression of ASR. â¢ Section 5âOther. This section asked about experiences in preventing ASR on new construc- tion and any other information that might be pertinent to the study. The results of the survey are presented in the remainder of this chapter, corresponding with the sections of the web-based survey. Verification of ASR Facilities Affected By ASR Respondents could submit runways, taxiways, or apron areas as facilities affected by ASR. As shown in Figure 10, most of the airport facilities submitted were runway, but all three types were well represented. More than half of these total facilities had been built in the 1990s or later, although a significant number of them were older (dating to the 1950s and 1960s), with four having occurred since 2000 (see Figure 11). Most of these exhibited ASR after about 6 to 10 years of service, with a significant number occurring after 11 to 20 years of service (see Figure 12). This would seem to confirm the variability noted in the literature regarding the initiation of ASR, which is dependent on some key factors, including the degree of reactivity associated with the aggregate, the total alkalinity within the concrete mixture, and the field exposure conditions. Manifestation of ASR There are some indicators of ASR in concrete pavements, including staining, various forms of cracking, spalling, and signs of pavement expansion (e.g., blowups or shoving of fixed Figure 9. Geographical distribution of survey responses.
36 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports Figure 10. Types of airport facilities affected by ASR. Figure 12. Age of airfield facilities when ASR first observed. Figure 11. Construction decade of ASR-affected airfield facilities.
Airport Surveys 37 structures). These indicators were all commonly cited by the survey respondents as indicators of ASR on their concrete airfield pavements (see Figure 13). The definitive means of identifying ASR is through a petrographic analysis, and this was the primary means indicated by the survey respondents (see Figure 14). Two airports indicated the use of a field identification test. Corrective Treatments to Address ASR Corrective treatments are those that are used to address specific performance issues associ- ated with ASR. These can range from interim treatments (e.g., partial- and full-depth repairs) to structural overlays (HMA or PCC) to complete reconstruction. Figure 15 summarizes the types of corrective treatments used by the respondents, while Figure 16 indicates the range in ages when each treatment was applied. Not surprisingly, the interim treatments were more often applied early in the life of the concrete pavement (first 10 years), whereas overlays and reconstruction were applied later (more than 20 years). Figure 13. Indicators of ASR cited by survey respondents. Confirmation Method Re sp on se s Figure 14. Confirmation method for ASR identification.
38 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports The life expectation for each treatment, as provided by the survey respondents, is presented in Figure 17. As expected, more significant treatments such as structural overlays and recon- struction provide more than 13 years of service (the maximum age of those treatments at the time of the survey), but it is interesting to note that, in a few cases, interim treatments such as PDR and FDR provided more than 8 years of life. The respondents were largely satisfied with the application of these treatments, as indicated in Figure 18. In addition to the corrective procedures described above, one respondent indicated that cutting expansion joints in ASR-affected concrete pavement helped minimize damage caused by panels expanding and moving. Full-depth joints, 2.5-inches (65 mm) wide, were cut across aprons and around building foundations, then filled with asphalt material. On an ASR-affected taxiway pavement abutting a runway, a 6-ft (1.8 m) wide expansion joint was created and filled with asphalt to prevent shoving damage to the runway pavement. Figure 15. Corrective treatments used to address ASR performance issues. Figure 16. Facility age when corrective treatments applied.
Airport Surveys 39 Another respondent indicated that recycling of ASR-affected pavements is a sound engi- neering choice. The airport has successfully recycled ASR-affected concrete pavement for use as a base course in new pavement construction, in accordance with FAA Item P-219. Mitigation Strategies to Address ASR Section 4 of the survey inquired about the use of various strategies to mitigate or reduce the progression of ASR. This was primarily looking at the use of different types of surface sealers (e.g., silanes, siloxanes) or the topical application of lithium compounds. Only three airports indicated experience with these mitigation treatments: two with applications of lithium nitrate and one with an application of a slurry seal. The respondents indicated that these treatments had no effect on the progression of the ASR, which is in line with the findings from the litera- ture review. Figure 17. Life expectations for corrective treatments. Figure 18. Respondent satisfaction with corrective treatments.
40 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports Other Comments Survey respondents had the opportunity to weigh on changes that have been made to their mix design specifications to help address ASR in new concrete construction. Major changes that were identified include: â¢ Identifying and avoiding potentially ASR-susceptible aggregates. Respondents cited the stringent requirements of UFGS 32 13 11 and FAA Item P-501 specifications and the use of ASTM C1260 (mortar-bar method) as contributory factors in eliminating problem aggregates. â¢ Using fly ash to help mitigate ASR. The use of ASTM C618 Class F fly ash was specifically called out, with between 20% and 30% by weight of total cementitious material as determined by trial mixes being recommended. One respondent commented on how suitable fly ash is becoming more difficult to obtain due to increasing regional shortages, which may require them to seek alternative ASR mitigation measures. â¢ One respondent pointed out that concrete pavements constructed following the more restrictive approaches have been in service for more than 24 years and have not exhibited any visible signs of ASR. Several respondents also commented on the regular inspections that are performed on their concrete pavement airfield structures as a way of monitoring conditions and identifying early signs of ASR distress. Summary Survey respondents indicated familiarity or experience with ASR issues on their concrete airfield pavement facilities, including runways, taxiways, and aprons. These facilities ranged in age from more than 50 years to as few as 5 years, with most showing signs of ASR after about 6 to 10 years of service, and a significant number of instances occurring after 11 to 20 years of service. Distress manifestations consisted of the typical indicators, including staining, crack- ing at joints, joint spalling, map cracking, and evidence of pavement expansion. Petrographic examination was the primary means of confirming ASR. To address ASR performance issues, respondents indicated general widespread use of cor- rective treatments, such as partial- and full-depth repairs, structural overlays, and reconstruc- tion. PDR and FDR were commonly used in the first 10 years of the pavementâs service life, whereas the other treatments were often used after the pavement was 20 years old or more. The PDR performance ranged from less than 3 years and up to 8 years, whereas the FDR typically provided 4 to 12 years of performance. The structural overlays and reconstruction provided more than 13 years of service. The respondents were largely satisfied with the performance of these treatments. One respondent commented on the benefits provided by creating expansion joints in the ASR-affected pavement to prevent pressure-related damage. Little experience was cited regarding the use of strategies to mitigate ASR on existing pave- ments. Two respondents indicated experience with the topical application of lithium nitrate, which had no effect on the progression of the ASR. In fact, no new treatments or emerging technologies were identified from the survey that can be used to mitigate ASR in existing concrete airfield pavements. Respondents generally indicated that ASR was not being observed in their more recent concrete construction. This was the result of the stringent specifications now in effect that test aggregates for reactivity and include ASR mitigation measures (e.g., low alkali cement, use of fly ash).