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5 Alkali-silica reactivity (ASR) is a chemical reaction that occurs in concrete between reactive silica present in the aggregate and the alkalis present in the pore solution of the hydrated cementitious paste. The reaction entails the dissolution of the silica, the formation of an alkali- silica gel, and interaction with calcium. As the gel takes up water, it expands significantly, destroying the integrity of the weakened aggregate particle and the surrounding cement paste. ASR distress has been confirmed at different levels and severities in the 48 contiguous states (Thomas et al. 2013c) as well as in many concrete pavement facilities worldwide, including at some commercial, military, and regional airports. Figure 1 shows several examples of ASR distress occurring on concrete airfield pavements. ASR is one of a family of materials-related distresses (MRDs) that can occur in concrete pavements as the result of the properties of the paving materials and their interaction with the environment (Van Dam et al. 2002a; ACI 2016). ASR in concrete pavements is commonly typified by a network of multiple, closely spaced cracks, often accentuated with staining or deposits. Over time it can also contribute to spalling, joint deterioration, and damage due to expansion, including blowups and shoving of fixed structures. Moreover, ASR commonly forms over the entire slab and not just at joints and cracks, although it can often first appear at joints and cracks before being observed in the interior portions of the slab. Common susceptible aggregate components include opal, chert, chalcedony, cristobalite, tridymite, strained and microcrystalline quartz, and volcanic glass, among others (ASTM C1778). ASR can take several years to develop and its appearance may look similar to other types of MRDs; therefore, visual inspections alone cannot confirm the presence or absence of ASR distress. Petrographic analysis of pavement core samples performed in accordance with ASTM C856 is required to definitively confirm the mechanisms that may be contributing to the distress. The development of ASR on concrete pavements can adversely affect its overall performance and significantly reduce its service life. As described above, some performance issues can result, including spalling, joint deterioration, and damage due to expansion (such as blowups and shoving of fixed structures), and these can lead to ongoing maintenance challenges and expen- ditures. Moreover, of great concern on airport facilities is the potential for ASR to produce FOD that could prove damaging to aircraft. A significant amount of research has been done to develop guidelines to prevent or mitigate ASR in new concrete pavement construction, with tangible benefits achieved through more rigid aggregate testing, reducing the total alkalis in the concrete, and using SCMs and lithium nitrate ASR-inhibitor as an additive. For example, for concrete airfield pavements, the FAA developed new concrete mix design requirements under Item P-501 to test aggregates and to mitigate ASR if the aggregates are determined to be potentially reactive (FAA 2014). Similarly, C H A P T E R 1 Introduction
6 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports for construction of military facilities, a new specification was issued in 2015 with updated ASR guidance (DOD 2015). As a result, it is generally accepted that mitigating the potential for ASR to develop in new concrete pavement construction can be achieved (although long-term monitoring of recent projects is needed to confirm this). Unfortunately, options to address ASR distress in existing pavement structures are limited. These pavementsâmost of which were constructed before the advent of the new material and construction specificationsâoften exhibit progressive map cracking and accompanying joint/crack spalling, and at times also experience significant slab movements that can produce blowups and shoving against light cans and other airport auxiliary features. The objectives of any mitigation measure then must be twofold: 1. Address the deterioration (spalling, cracking, blowups) that develops as the result of the ASR mechanism. 2. Minimize the potential for the continued ASR development. Although the first objective can be interpreted as relatively straightforward, it is only tempo- rarily effective unless the ASR mechanism can be halted or disrupted; otherwise the ASR distress continues to progress, resulting in a seemingly never-ending battle of repair and maintenance activities required to keep the pavement serviceable. With regard to the second objective, the methods to address continued ASR development are limited because the ASR mechanism feeds off the constituent materials in the concrete (alkalis in the pore solution most often provided by the cement and reactive silica components in the aggregate) in the presence of moisture. Efforts can be made to try to limit moisture ingress, but for pavement slabs on grade, sufficient moisture to support continued ASR development is nearly always available from subsurface sources. Furthermore, alteration or removal of the offending constituent materials (e.g., alkalis from the pore solution or reactive silica from the aggregate) is not possible from the existing concrete. Consequently, given the destructive nature of ASR and the already significant investment in the pavement structure, airport owner/agencies are interested in remediation measures or treatments that can be used to mitigate or retard the deleterious effects of ASR on existing airfield concrete pavements and thereby maintain serviceability and prolong life. Figure 1. ASR distress on concrete airfield pavements (courtesy of Applied Pavement Technology, Inc).
Introduction 7 Objectives The primary objectives of this project are to (1) identify the current state of the practice regarding the mitigation measures used on existing ASR-affected airport pavements that service aircraft and (2) summarize the experiences and practices of airports in dealing with the distress (including conventional treatments, but also any new or emerging technologies). The focus is on concrete airfield pavements in the United States that service aircraft (and thus are specified under the FAA AC 150/5370-10G Item P-501 or under UFGS Division 32 â Exterior Improvements; Section 32 13 11), and therefore is intended to reflect U.S. experience. This project does not consider the mitigation of ASR in the construction of new pavements nor does it examine all of the mechanisms, mix design issues, and laboratory test protocols associated with ASR because there is a large body of information available on those topics. Work Approach To meet the overall project objectives listed above, a three-pronged work approach was employed that consisted of the following activities: â¢ A literature review of national and international experiences in addressing ASR on pavement structures. This review focused on literature produced over the past 10 years on the topic of ASR, with an emphasis on mitigation methods and measures. Although the focus was on airports with ASR distresses, the experiences of highway agencies in addressing ASR on roadway facilities were also investigated. â¢ A survey of selected airport owner/agencies to determine their experiences in addressing ASR-affected pavements. Based on the input from the topic panel and from the literature review, a group of airports was identified with documented ASR distress on their concrete airfield pavements. An electronic survey was then sent to those airports to (1) document their use of and experience with various ASR mitigation methods and (2) determine if any new or innovative methods may be exhibiting promising results. â¢ Interviews held with selected airport owner/agencies to highlight specific mitigation treat- ments and approaches used (and featured in the case examples). Four airport owner/agencies with extensive experience in mitigating ASR on existing concrete airfield pavements were the subject of detailed interviews to more deeply investigate and document their experiences in dealing with ASR issues, particularly with regard to the performance of the different treatments and their short- and long-term plans in dealing with ASR. Report Organization This report is organized into five chapters (including this introductory chapter) that closely mirror the overall work approach described above. Chapter 2 presents a brief summary of the literature related to ASR, including its mechanisms, development, identification, prevention, and mitigation. Chapter 3 begins with a brief overview of the airport survey process, then pro- vides the compiled results from the surveyed airports as an indicator of the extent of the ASR problem and the use and effectiveness of various mitigation treatments. Chapter 4 presents a more detailed look at four airports with experience in managing concrete airfield pavements affected by ASR, focusing on their specific experiences with various mitigation treatments and other remediation measures. Finally, Chapter 5 provides a brief summary of the overall findings of the project and identifies several further research needs related to ASR on concrete airfield pavements.
8 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports Several appendices are included in support of the primary report chapters. Appendix A provides the blank survey questionnaire that was distributed to the selected airport owner/ agencies, while Appendix B presents a listing of the survey responses and the respondents who participated in the surveys. Finally, Appendix C includes a bibliographic listing of documents related to the treatment and mitigation of ASR on existing pavements that were identified and reviewed as part of the comprehensive literature search.