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52 ASR is an expansive distress occurring in concrete as a result of a chemical reaction between certain siliceous minerals in susceptible aggregates and alkalis in the hydrated cement paste. The presence of ASR on concrete pavements can have a devastating effect on pavement performance, not only in terms of reduced functionality, but also in terms of shortened service lives. Although significant progress has been made in understanding and preventing ASR in new construction, some existing concrete airfield pavements are affected by ASR. Given the already significant investment in the pavement structure, airport owners/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. This synthesis was produced to identify the current state of the practice regarding the mitigation measures used on existing ASR-affected airport pavements that ser- vice aircraft and summarize the experiences and practices of airports in dealing with the distress (including conventional treatments, but also any new or emerging technologies). Overall Findings A review of the available literature was performed that focused on the treatment and miti- gation of ASR on existing pavements, and this was supplemented by a web-based survey of airports known to have ASR issues as well as interviews with four selected airports to gather more detailed information on their experiences in addressing ASR issues. Overall, no new or innovative methods were identified that hold the promise of addressing ASR in existing concrete airfield pavements, but it was confirmed that an assortment of corrective treatments are commonly used by airports to manage their ASR-affected pavements: â¢ Partial-depth repairs to address spalling and other distresses associated with ASR. â¢ Full-depth repairs to address more severe ASR deterioration in which the structural capacity of a slab (or series of slabs) may be compromised. â¢ Pressure relief joints, sometimes referred to as expansion joints, to relieve ASR-induced expansive forces in the pavement that could lead to joint spalling, blowups, or other pressure- related damage. â¢ Structural overlay solutions, including both hot-mix asphalt and PCC overlays, to restore functionality of existing pavements suffering severe ASR-related distress. â¢ Reconstruction of the ASR-affected pavement, which is the only fully effective means of addressing ASR in the existing pavement (assuming the new pavement is constructed with a durable concrete mixture developed in accordance with prescribed specifications to prevent the recurrence of ASR). The use of 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 C H A P T E R 5 Conclusions and Further Research
Conclusions and Further Research 53 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 rehabilita- tion can be programmed. Outside of the corrective treatments listed above, there is also considerable interest in the use of surface treatments or surface sealers as a means of mitigating ASR on existing concrete pavements, such as: â¢ Surface sealers, such as silanes and siloxanes, which 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. â¢ Lithium compounds, which have been used topically on existing concrete pavements in hopes of reducing ASR expansion by changing the nature and behavior of the alkali-silica gel from expansive to non-expansive. â¢ High-molecular weight methacrylate, which is intended to strengthen an existing concrete pavement affected by ASR by filling the cracks and bonding the cracked concrete pieces together. These products have not seen widespread use on concrete airfield pavements. A few airports have indicated some limited experience with lithium compounds and HMWM, yet neither treatment approach demonstrated any reductions in ASR progression or expansion. The web-based surveys and airport interviews did confirm the tremendous progress that the pavement community has made in the general understanding of the ASR mechanisms and in the prevention of ASR in new concrete pavement construction. The FAA and DOD have made great strides in developing effective standards and specifications that specifically address ASR issues; key approaches in the prevention of ASR in new concrete construction include: â¢ Avoid the use of susceptible aggregates by adopting a thorough testing program featuring a combination of petrographic analysis of aggregates (ASTM C295) and expansion testing of mortar (ASTM C1260) or concrete (ASTM C1293). In FAA Item P-501 (FAA 2014), aggregate reactivity is assessed using expansion testing of mortar bars (ASTM C1260/C1567). Aggregates are tested separately in accordance 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 are considered in the acceptance of the aggregate. 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 with the aggregate. â¢ Use effective supplementary cementitious materials, such as ASTM C618 Class F fly ash (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 the aggregate and cementitious materials in the job mixture proportions, requiring 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 quantity that lowers the expansion equal to or less than 0.08% at 28 daysâ immersion. â¢ Minimize the total alkalis in the concrete mixture to meet the prevention level required for the prevailing exposure conditions. FAA Item P-501 (FAA 2014) requires low alkali cement when no other mitigating measures are added, and UFGS 32 13 11 (DOD 2015) requires the use of low alkali cement for all mixtures.
54 Practices to Mitigate Alkali-Silica Reaction (ASR) Affected Pavements at Airports â¢ Use lithium-based admixtures in the concrete mixture, the effectiveness of which is highly dependent on the type of aggregate, the alkali content in the concrete, and the form of lithium. FAA Item P-501 (FAA 2014) requires that the USACE 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 2015) also requires the use of USACE CRD C662 for the evaluation of lithium nitrate in mitigating ASR. The expansion must be less than or equal to 0.08% after 28 daysâ immersion. Suggestions for Further Research Based on the results of the literature review and the surveys conducted under this synthesis project, the following suggestions for further research are offered: â¢ Monitor 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 effectiveness of corrective treatments in terms of life expectancy to develop realistic expectations 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 the effectiveness of overlap in restoring functional performance of 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 such sealers 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.