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26 questioned. Blank or "no" responses were common in the U.S. airports) responded to the ACRP survey. Two respon- questions concerning damage or deterioration of airfield dents specifically referred to the interim FAA recommenda- infrastructure. However, some responses provided more spe- tions concerning ASR and deicers. cific information regarding impacts on concrete and asphalt pavements as well as on other airfield infrastructure. ASR is a chemical reaction between alkalis present in the cement paste and siliceous minerals in the reactive IMPACT OF PAVEMENT DEICING PRODUCTS aggregates of PCC, which produces a hydrophilic gel that ON CONCRETE PAVEMENT expands when sufficient moisture is available. Such inter- nal expansive forces are deleterious to the concrete dura- This section synthesizes the information on the impact of bility and can cause cracks in both the cement paste and PDPs on concrete pavement. First, the potential role of PDPs aggregates. Failure of concrete structures later attributed to in the deterioration of concrete pavement is described, in ASR can be dated back to the late 1920s (Pan et al. 2006). terms of both chemical and physical effects associated with Typical ASR distress is manifested by cracking, popouts, the use of PDPs. The current understanding of the mecha- and expansion (as shown in Figure 13). Cracks allow more nisms of damage is then discussed, followed by the associ- water to enter the concrete, popouts create foreign object ated standards and test protocols, methods of prevention or damage hazard, and expansion can damage adjacent pave- mitigation, and finally knowledge gaps on this subject. ments and structures. The increased alkali content of mod- ern PCC, as well as the potential for additional alkali from fly As identified by a recent literature review (Pan et al. ash, admixtures, aggregates, mix water, etc., is the outcome 2006), the last decade has seen an increase in the premature of the competing forces of air emission standards and high deterioration of airfield PCC pavements with the use of energy costs. Thus, the low-alkali cement of today has more alkali-metal-salt-based PDPs (Maxwell 1999; Barett and alkali than cement manufactured before the 1970s, often Pigman 2001; Johnson 2001; Pisano 2004; Roosevelt 2004; around 0.6% sodium oxide equivalent (Na2Oeq). Accord- "New Anti-icing System . . ." 2005; Pinet and Griff 2005). ingly, aggregates that did not historically react to low-alkali Such PDPs have been used more extensively and for more cement may not have the same performance today (Pro-Act years in European countries for winter maintenance than in Fact Sheet . . . 2006). the United States. The degree of distress in the PCC pave- ments of European facilities ranged from mild to severe in In addition to ASR, physical distresses such as scaling and terms of surface cracking and repair and rehabilitation efforts spalling are common forms of deterioration of hardened con- needed (Pan et al. 2006). crete (Figure 14a and b, respectively), and both can occur in the absence of deicers. Scaling is physical damage of concrete Recent research conducted at Clemson University found surface often shown as local flaking or peeling, owing to the that the acetate/formate-based deicers could induce increased hydraulic pressures from freezingthawing cycles of concrete levels of expansion in concrete with aggregates susceptible to pore solution (ACI Committee 302 1996). Freezing of water the alkali-silica reaction (ASR), and could trigger ASR in con- in saturated concrete generates expansive forces that are detri- crete that previously did not show susceptibility to ASR (Ran- mental to the concrete surface, especially when it is not ade- garaju et al. 2005, 2006; Rangaraju and Desai 2006). The lab- quately protected with entrained air. Similar to chloride-based oratory results from a modified ASTM C1260 mortar bar test salts, alkali-metal-salt-based PDPs may exacerbate scaling and a modified ASTM C1293 concrete prism test indicated when used at a concentration high enough to induce osmotic that both KAc- and NaAc-based deicer solutions showed sig- pressure upon moisture (Pan et al. 2006). In addition, the nificant potential to promote ASR in mortar bar specimens that application of deicers to pavements increases the rate of cool- contained reactive aggregates. Such solutions were also found ing, which increases the number of freezethaw cycles over to cause more rapid and higher levels of expansion within ambient conditions and thus the risk for scaling (Mussato et al. 14 days of testing and to lead to lower dynamic modulus of 2005). The use of properly cured, air-entrained PCC can pre- elasticity, compared with 1N sodium hydroxide (NaOH) solu- vent scaling. Entrained air provides spaces within the concrete tion (Rangaraju et al. 2006). Increasing temperature or deicer matrix for expanding water to move into, thereby reducing the concentration was found to accelerate the deleterious effects potential stress and associated deterioration pertinent to freeze of deicers on the ASR in concrete. thaw cycling. It is believed that high quality concrete with 5%7% entrained air is more resistant to freezethaw cycles Based on the responses to the ACRP survey, concrete life and scaling (Williams 2003). spans at U.S. airports varied from 20 to 50 years, and changes in mix design and construction are consistent with FAA spec- The ingress of chloride into concrete and subsequent ifications. Only isolated cases of KAc accelerating ASR in reinforcement corrosion has been extensively studied, and some concrete pavements were reported by the survey respon- these eventually lead to concrete cracking or spalling. How- dents, with freezethaw cycles also contributing to the dam- ever, little research has been conducted to examine the ingress age. It should be cautioned, however, that among the more of alkali metal salts (e.g., KAc, KF, NaAc, and NaF) into than 50 U.S. airports contacted, only 12 (along with 3 non- concrete or their interaction with metallic reinforcement.

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27 (a) (b) (c) FIGURE 13 ASR-induced distresses at Air Force bases: (a) cracking, (b) popouts, and (c) asphalt shoulder heaving caused by ASR expansion in adjacent concrete [adapted from Pro-Act Fact Sheet . . . (2006)]. Interestingly, a very recent study reported the use of NaAc samples were provided, nor was any ASR testing conducted aqueous solution as a technology to reduce water perme- in this specific research. ability into PCC (Al-Otoom et al. 2007). The results indi- cated that the crystal growth of NaAc in concrete pores was relatively fast, which significantly reduced the water per- Nature of the Effect of Modern Pavement Deicing Products on Portland Cement Concrete meability of the concrete after only a 7-day treatment. The Pavement Deterioration PCC samples tested were porous with the following mix design: a water-to-cement ratio of 0.65:1, an aggregates-to- Limited existing laboratory studies indicated that alkali-metal- cement ratio of 4.5:1, and a sand-to-gravel ratio of 1:2 (all salt-based deicers could cause or accelerate ASR distress in the by weight). The treatment of PCC by the NaAc solution did surface of PCC pavement by increasing the pH of concrete not significantly affect its freezethaw resistance or com- pore solution. PCC pavements that were otherwise resistant to pressive strength, and only slightly increased the pH of the ASR might show rapid deterioration when exposed to these concrete. Overall, the treatment was demonstrated to be ben- high alkali solutions. The nature of the reactions associated eficial to the concrete's durability, especially at the optimum with increased expansions in mortar bar tests to date remains concentration of 20% NaAc (Al-Otoom et al. 2007). It is note- unclear owing to limited research conducted on this topic. It worthy that neither the details of aggregates used in the PCC was proposed that such deicers react with one of the major

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28 (a) (b) FIGURE 14 Concrete scaling (a) and spalling (b) [adapted from Potomac Construction Industries (2007) and Cryotech Deicing Technology (2007), respectively]. hydrated products--calcium hydroxide--Ca(OH)2, and result Combinations of Cementitious Materials and Aggregate in higher pH of the concrete pore solution. The high pH result- (Accelerated Mortar-Bar Method) or an equivalent test ing from these interactions is likely to have an accelerating must be used. effect on the expansions as a result of ASR. This mechanism was substantiated by the SEM-EDX investigation of mortar In ASTM C1260, the samples are soaked for 14 days in a bars after deicer immersion, which was unable to detect solution of NaOH. For the Air Force, mixtures that experience Ca(OH)2 in the cement paste (Rangaraju and Olek 2007). expansion greater than 0.08% require mitigation (Pro-Act Fact Sheet . . . 2006). The FAA has recommended that ASTM There are other hypotheses that merit further investigation. C1260 testing for new concrete pavement mixtures be modi- A laboratory investigation using concrete samples obtained fied by substituting a deicing agent for the NaOH solution, from existing Iowa highways suggested that magnesium and soaking for 28 days, and mitigating if expansion exceeds calcium deicers might accelerate highway concrete deterio- 0.10% ("Engineering Brief No. 70 . . ." 2005). Currently, ration (Cody et al. 1996). Samples were experimentally dete- these are interim recommendations until additional research riorated using wetdry, freezethaw, and continuous soak is completed. The modifications to ASTM C1260, C1293, conditions in solutions of magnesium chloride, calcium chlo- and C1567 are based on the research conducted at Clemson ride, sodium chloride (NaCl), magnesium acetate (MgAc), University. Additional research using these modified methods magnesium nitrate, and distilled water. The magnesium and may be needed, especially for mitigation with lithium nitrate- calcium salts were found to severely damage the concrete based admixtures. The FAA will further refine the tests as part samples, whereas plain NaCl was the least harmful. This was of the IPRF 05-7 project to make it a standard test method for possibly attributable to the reaction between magnesium and evaluating the ASR susceptibility of PCC, which may be con- calcium cations (Mg2+ and Ca2+) and the cement hydration sidered for inclusion to the SAE AMS 1435 and AMS 1431, products, or to the accelerating effect of these cations on the along with ASTM C672, Test Method to Assess Scaling alkali-carbonate reaction if the concrete contained reactive Resistance of Concrete Exposed to Deicers. dolomite aggregates. Prevention and Mitigation Standards and Test Protocols To prevent or mitigate the effects of PDPs on concrete pave- The U.S. Air Force requires that aggregates for new con- ment, the first and most important countermeasure is to fol- crete pavements be tested according to ASTM C1260, Stan- low best possible practices in concrete mix design and con- dard Test Method for Potential Alkali Reactivity of Aggre- struction. For instance, the mix design should take into gates (Mortar-Bar Method). Another standard, ASTM consideration supplementary cementitious material to allevi- C1293, Standard Test Method for Determination of Length ate excess bleed water, aggregate blends that do not lack mid- Change in Concrete Due to Alkali-Silica Reaction, is pre- sized aggregate, and suitable air void systems. Proper mix ferred but takes more than a year to complete. If it is not designs will allow easier placement and consolation. In addi- feasible to use only nonreactive aggregates, then mitigation tion, good curing practices should also be followed (Van Dam methods are required. ASTM C1567, Standard Test Method et al. 2006). When possible, polymer sealants can be used to for Determining the Potential for Alkali-Silica Reactivity of minimize the contact between PDPs and concrete pavement