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1 Alkali-silica reactivity (ASR) is an expansive distress occurring in portland cement concrete (PCC) as a result of a chemical reaction between certain siliceous minerals in susceptible aggregates and alkalis in the hydrated cement paste. ASR in concrete pave- ments is commonly typified by a network of multiple, closely spaced cracks and, over time, can also contribute to spalling, joint deterioration, foreign object debris (FOD) potential, and damage due to expansion, including blowups and shoving of fixed structures. The performance of some concrete pavements has been severely compromised by the presence of ASR. Given the destructive nature of ASR and the already significant investment made in the pavement, airport owners/agencies are interested in remediation measures or treat- ments that can be used to mitigate or retard the deleterious effects of ASR on existing airfield concrete pavements in order to maintain their functionality and to prolong life. Thus, the primary objectives of this project were to identify the current state-of-the-practice regarding the mitigation measures used on existing ASR-affected airport pavements that service air- craft and to summarize the experiences and practices of airports in dealing with the distress (including conventional treatments and any new or emerging technologies). To meet these project objectives, a review of the available literature was performed that focused on the treatment and mitigation of ASR on existing pavements; for complete- ness, the literature review also examined ASR mechanisms and distress manifestations, ASR identification measures, and ASR prevention. The literature review was supplemented by a web-based survey of airports known to have ASR issues, which yielded a total of 19 responses. Additionally, representatives of four airports were interviewed to gather more detailed information on their experiences in addressing ASR issues on existing concrete airfield pavements. The data collected under this project confirm that ASR is a problem at some airports and that corrective treatments are commonly used to address ASR and ASR-related issues: â¢ Partial-depth repairs (PDRs) are used to address spalling and other distresses associ- ated with ASR that are limited to the upper one-half to one-third of the slab thickness. The performance of these repairs may range from as little as a few years to up to 8 or more years, depending on the ASR severity and overall conditions, as well as the repair technique and material. Many airports use proprietary repair materials that set up quickly to allow rapid opening times. â¢ Full-depth repairs (FDRs) address more severe levels of ASR deterioration in which the structural capacity of a slab (or a series of slabs) may be compromised. These are more expensive than PDR but can provide longer performance, often 12 or more years. Materials used for FDR will depend on opening requirements. S U M M A R Y Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports
2 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports â¢ Pressure relief joints (PRJs), sometimes referred to as expansion joints, are installed after initial construction to relieve ASR-induced expansive forces in the pavement that could lead to joint spalling, blowups, or other pressure-related damage. These are used by some airports and are effective in providing relief and protecting adjacent pavements and fixed structures, but often close quickly and may require regular reestablishment. â¢ Structural overlay solutions, including both hot-mix asphalt (HMA) and PCC overlays, can be used to address ASR on existing pavements. Although such solutions do not directly address the ASR issue, they can effectively restore the serviceability of the facility and eliminate FOD potential. The underlying pavement may still experience ASR-related expansion that might require ongoing maintenance. â¢ Reconstruction of the ASR-affected pavement is the only completely effective means of addressing ASR, assuming the new pavement is constructed with a durable concrete mixture developed in accordance with prescribed specifications to prevent the recurrence of ASR. Several airports have selected the complete reconstruction alternative as they have been faced with few other options due to the severe and progressive nature of the ASR distress. PDR, FDR, and PRJ are commonly used by airports as interim treatments, not to address the ASR mechanism itself, but rather to address the symptoms of the distress and maintain the serviceability of the pavement. In this vein, these techniques can be effective short-term solutions until additional funding can be secured and more substantial rehabilitation can be programmed. There has been considerable interest in the use of surface treatments or surface sealers as a means of mitigating ASR on existing concrete pavements. These products include: â¢ Surface sealers, such as silanes and siloxanes. These are applied to the pavement surface to reduce or prevent the ingress of moisture (as well as other substances such as deicing chemicals) into the concrete. There is evidence from work done on highway pavements that suggests some short-term benefits (in terms of reduced expansion) may be achieved, but no airport reported direct experience with these products. â¢ Lithium compounds. These change the nature and behavior of the alkali-silica gel from expansive to non-expansive and have been applied to the surface of existing concrete pavements in hopes of reducing ASR expansion. Several airports have conducted small field trials of topical applications of lithium compounds on their ASR-affected concrete pavements, but results have generally shown no changes in performance or condition levels. A major concern documented in the field trials about the use of lithium com- pounds is achieving adequate depth of penetration. â¢ High-molecular weight methacrylate (HMWM). This product is intended to strengthen an existing concrete pavement affected by ASR by filling the cracks and bonding the cracked concrete pieces together. One Air Force base cited experience with the use of an HMWM product, but reported no difference in performance over the course of an 8-year monitoring period. From the review of available literature, airport surveys, and interviews, no new or innovative methods were identified that hold the promise of addressing ASR in existing concrete airfield pavements. Of positive note are the significant advancements that the concrete pavement community has made in the general understanding of the ASR mechanisms and in the prevention of ASR in new concrete construction. In the past 10 years or so, the Federal Aviation Administration (FAA) and the Department of Defense (DOD) have made great strides in developing standards and specifications that specifically address the prevention of
Summary 3 ASR in new pavement construction. Furthermore, although limited, the web-based survey results and airport interviews confirm that ASR does not appear to be a problem in new construction. Four key elements in preventing ASR in new concrete construction include: 1. Avoiding the use of susceptible aggregates. Several laboratory test methods are available for evaluating the potential for ASR in aggregates. In FAA Advisory Circular AC 150/ 5370-10G, Item P-501 (FAA 2014), aggregate reactivity is assessed using expansion testing of mortar bars (ASTM C1260/C1567). Aggregates are tested separately in accor- dance with ASTM C1260 and combined in the job mixture proportions of aggregate and cement in accordance with ASTM C1567. Only the results of ASTM C1567 conducted on aggregates combined in the job mixture proportions are considered in the accep- tance of the aggregate, and if the ASTM C1567 test shows an expansion greater than 0.10% at 28 daysâ immersion, then the aggregate is considered to be potentially reactive and cannot be used without mitigation. UFGS 32 13 11 (DOD 2015) requires that the fine and coarse aggregates be evaluated separately using conditions as dictated in ASTM C1260. If the test results indicate an expansion of greater than 0.08% after 28 daysâ immersion, the aggregate is either rejected or additional testing is performed utilizing a mitigation measure. It is recognized that in many cases non-reactive aggregates simply may not be available in certain locations, or perhaps local reactive aggregates with otherwise suitable properties for use in concrete are available at low cost. In these cases, mitigation is needed. 2. Using supplementary cementitious materials (SCMs). SCMs are widely used and provide a very efficient approach to addressing ASR in new concrete construction. The addition of SCMs is effective because SCMs react with the calcium hydroxides in the concrete to produce additional calcium silicate hydrate, at the same time binding some of the alkalis in the newly created hydration products. In combination, this reduces the alkali hydroxides in the concrete pore solution, while also reducing concrete permeability, thereby reducing the availability of water to support the reaction. ASTM C618 Class F fly ash is generally considered very effective in addressing ASR because of its low calcium content, although slag cement (ASTM C989), silica fume (ASTM C1240), and natural pozzolans (ASTM C618 Class N) can also be effective, depending on the nature of the SCMs and the quantities used. FAA Item P-501 (FAA 2014) requires that ASTM C1567 be used to test the effectiveness of the SCM in mitigating ASR for aggregates combined in the job mixture proportions, specifying that the expansion be less than 0.10% at 28 daysâ immersion for acceptance. UFGS 32 13 11 (DOD 2015) requires the use of ASTM C1567 to evaluate SCMs, with the testing performed to determine the SCM content that lowers the expansion equal to or less than 0.08% at 28 daysâ immersion. 3. Minimizing the total alkalis in the concrete mixture. Although many specifications allow the use of potentially reactive aggregates, provided that the cement alkali content does not exceed 0.6% sodium oxide equivalent (Na2Oe, as defined in ASTM C150), cement equivalent alkali contents between 0.45 and 0.6% may still provide sufficient alkalis for ASR to develop. Furthermore, the 0.6% limit does not take into account the cement content of the concrete. Thus, it is now recognized that a more accurate index of the risk of expansion is to consider the total alkalis in the concrete mixture (considering both the cement alkalinity and the cement content). FAA Item P-501(FAA 2014) requires the use of low alkali cements when no other mitigating measures are added, while UFGS Section 32 13 11 (DOD 2015) requires the use of low alkali cement for all mixtures. 4. Using a lithium nitrate solution with or without SCMs. Lithium compounds added to a concrete mixture have been shown to be effective in mitigating ASR, but their effective- ness is highly dependent on the type of aggregate, the alkali content in the concrete, and the form of lithium.
4 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports FAA Item P-501 (FAA 2014) requires that the U.S. Army Corps of Engineers (USACE) Concrete Research Division (CRD) C662 be used to test the effectiveness of a lithium nitrate in mitigating ASR for the combined aggregate gradation. The expansion must be less than 0.10% at 28 daysâ immersion. FAA Item P-501 requires that the lithium nitrate admixture shall be nominal 30% Â± 0.5% weight lithium nitrate in water. UFGS Section 32 13 11 (DOD 2014) also requires the use of USACE CRD C662 for the evalu- ation of lithium nitrate in mitigating ASR. The expansion must be less than or equal to 0.08% after 28 daysâ immersion. Based on the findings from the literature review and the surveys conducted under this synthesis project, the following suggestions for further research are offered: â¢ Monitor the performance of new concrete pavements constructed under recent FAA and DOD specifications. Conduct detailed visual assessment using ASTM D5340 (which includes an ASR call-out) and perform coring and petrographic analysis to confirm the presence or absence of ASR. â¢ Document the regional performance of the effectiveness of corrective treatments in terms of life expectancy to develop realistic ideas of what airports can expect when treatments are tried under different conditions. â¢ Document the performance of structural overlays (both HMA and PCC) of ASR-affected pavements and their effectiveness in restoring the functional performance of airfield pave- ments. Investigate the effects of underlying movements of ASR on overlay performance. â¢ Determine the efficacy of using surface sealers as a means of reducing water intrusion into concrete pavements, and what role they may serve in an airfield environment to slow the development of ASR-related distress. â¢ Support continued work on the development of a more rapid and reliable laboratory test for aggregate ASR susceptibility. â¢ Investigate the use of alternative SCMs or other additives that could be used to mitigate ASR in new concrete mixtures.