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FEATURE BY TOM KUENNEN quently take place between those components, the authors write. To get a handle on these potential conflicts, they investigated combinations of materials which can result in problems in generating and stabilizing air-void systems in both plain and fly ash mixtures. A low (0.3 percent) alkali Type I cement, a class F fly ash and four chemical admixtures were used in this study, the authors state. Two out of these four admixtures included different types of air-entraining agents, a vinsol resin-based air entraining agent, and synthetic-type air entraining agent. The other two admixtures included the lignin-based Type A water reducing agent and polycarboxylate-type superplasticizer. In addition to plain cementitious mixtures prepared from various combinations of these components, fly ash mixtures (with 20 and 60 percent weight replacement of cement by Class F fly) also were studied. "The results showed that the amount of air-entrainment admixture required to obtain the target air percentage increased with the increase in the fly ash FHWA REVISES ASR DETECTION GUIDE The updated Alkali-Silica Reactivity Field Identification Handbook is available in a compact, spiral-bound version— optimized for field use—or as free PDF download. Prepared under Federal Highway Administration's ASR Development and Deployment Program, it serves as a guide for recognizing and evaluating concrete with apparent alkali-silica reactivity (ASR) distress. A moisture-aggravated chemical reaction between particular combinations of cement and aggregates, ASR left untreated can considerably shorten the service life of concrete structures. Reactivity-affected concrete expands and cracks; in cases of prolonged damage, structures tend to shift, typically resulting in joint failure. Structures harboring ASR tend to exhibit initial symptoms five to 10 years after construction, although concrete in warmer, humid climates can show symptoms in less than two years. The handbook is FHWA's most comprehensive visual guide on the subject, presenting nearly 70 images of reactivity symptoms in varied conditions to help agencies identify ASR as quickly as possible. Early detection improves mitigation success and extends an affected concrete structure's service life. The handbook was developed under a contract between FHWA and The Transtec Group, an Austin, Texas-based pavement engineering and research firm. Requests for hard copies of Alkali-Silica Reactivity Field Identification Handbook—referencing report number FHWA-HIF-12-022—can be directed to Report.Center@dot.gov. The 80-page PDF version can be obtained at www.fhwa.dot.gov/pavement. 36 | MARCH 2013 content in the mixture," Paleti, Olek and Nantung say. "The lignin-based water-reducing agent had, in general, a higher air-entraining effect than the superplasticizer when used in combination with air entrainers. In general, mixtures prepared with synthetic air entraining agent exhibited a more stable foam system [in tests] than mixtures prepared with vinsol resin-based air entraining agent." The authors conclude: • The majority of the mixtures prepared with low (0.29 percent) alkali and high (3.98 percent) loss on ignitioncontent Class F ash resulted in incompatibility problems related to generation and stability of air void system. Also, literature review indicated problems with attaining desirable strength in low alkali cement mixtures prepared with certain admixture combinations. Therefore, it's recommended that in situations when the properties of the materials intended for particular application are similar to those indicated as potentially incompatible, these should be tested with different set of admixtures to identify appropriate combinations to avoid the problem. • The amount of air-entrainment agent dosage required to attain desirable airvoid content increased with an increase in fly ash content in mixtures that did not contain any water reducers, confirming previous findings from the literature. Similar trends were found in mixtures prepared with lignin-based water reducer. However the fly ash content did not have significant effect on the dosage of air-entrainment agent required in mixtures prepared with polycarboxylate type superplasticizer. It can thus be inferred that required air-entrainment dosage depends not only the fly ash content, but also the type of water reducer used in the mixture. • Water reducers, in particular ligninbased agents, were found to exhibit air-entraining characteristics. Therefore, one should account for this additional air entraining effect to avoid excessive air entrainment, thus preventing concretes with lower strengths. In addition, the quality of air-void system generated by water reducers may be inferior to that generated by air-entrainment agents. WWW.CONCRETEPRODUCTS.COM