ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/297033
46 | ASPIRE , Fall 2012 F H WA F H WA A lkali-silica reactivity (ASR) is a durability problem that has resulted in premature deterioration of various types of concrete structures in the United States and throughout t h e w o r l d . S u p p l e m e n t a r y c e m e n t i t i o u s materials have been used for more than 50 years for preventing damage to concrete structures by controlling the expansion due to ASR. In recent years, lithium compounds have been used as an additive in concrete mixtures. ASR-induced damage is caused by the expansion resulting from the chemical reaction between the alkali and silica in the mixture in the presence of moisture. ASR damage in concrete structures is evidenced by the map-like cracking on the surfaces, surface discoloration and gel exudations, and the displacement of components. The Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) established funding for further development and deployment of techniques to prevent and mitigate ASR. In response to this act, the Federal Highway Administration (FHWA) initiated an ASR Development and Deployment Program to focus on preventing and mitigating ASR in concrete bridges, pavements, and other highway structures, such as median barriers and retaining walls. Elements Essential for ASR Three elements are essential for ASR to occur: reactive silica (from aggregates); alkalis (mainly from portland cement); and moisture (from drainage, leakage and/or high humidity). To effectively combat ASR, one or more of these elements must be controlled or eliminated. Reactive Silica The presence of reactive aggregates or another reactive silica source in concrete is necessary for ASR to occur. The term reactive refers to aggregates that tend to breakdown under exposure to the highly alkaline pore solution in concrete and subsequently react with the alkalis (sodium and potassium) to form an expansive ASR gel. Alkali The presence of sufficient alkalis is another required ingredient for ASR. Portland cement is considered the main contributor of alkalis. Other ingredients that may contribute to additional alkalis are fly ash, slag, silica fume, aggregates, chemical admixtures, seawater, and deicing chemicals. Moisture The presence of moisture is necessar y to cause the damaging effects of ASR in concrete structures. Concrete mixtures comprised of highly reactive aggregates and high-alkali cements have shown little or no expansion in certain very dry environments. Similarly, portions of the structure exposed to a constant or steady source of moisture have exhibited significant ASR-induced damage, while other portions of the structure that remain essentially dry have shown little or no damage. Preventing or Mitigating ASR Several viable methods exist to prevent ASR in new concrete structures, such as • u s e o f o n l y l o w - o r n o n - r e a c t i v e aggregates, • u s e o f l o w - a l k a l i c e m e n t , o r t h e addition of supplementary cementitious materials such as fly ash, slag, or silica fume, and • addition of lithium. Very few methods are available for mitigating further damage in structures already affected by ASR-induced expansion and cracking. Mitigating ASR in Existing Concrete Lithium has been shown in limited laboratory studies to have the potential of suppressing the expansion caused by ASR. Field studies have been conducted to introduce lithium into existing concrete: • Topical treatment—applying lithium to the surface and allowing the lithium to penetrate the concrete • E l e c t r o c h e m i c a l m i g r a t i o n w i t h lithium as electr olyte—using the electrochemical chloride extraction method with lithium as an electrolyte • Vacuum impregnation—similar to topical treatment, except a vacuum is used to enhance deeper penetration of the lithium into the concrete However, to-date the field application of lithium has proved to be challenging and in most cases has not proved to be effective in suppressing ASR. Other methods to mitigate the effects of ASR are being studied in the field, such as the application of sealers or coatings to limit ingress of moisture and reduce the internal humidity of the structure and restraining or confining expansion of the structure elements. Other methods that should be considered for mitigating the effects of ASR are: • treating existing cracks to minimize future expansion and avoid ingress of moisture, deicing salts, and the like, • avoiding the use of deicing salts high in alkali content, • providing proper drainage, and • sealing leaks. The FHWA ASR Development and Deployment Program The FHWA ASR Development and Deployment Program was initiated through SAFETEA-LU funding and addresses the needs of stakeholders. More information was needed on test methods and specifications to control reactive aggregates and ASR in new concrete structures and the methods and techniques to mitigate the effects dealing with Asr in Concrete structures by M. Myint Lwin and Gina Ahlstrom, Federal Highway Administration Signs of ASR-induced damage. Photo: Courtesy of Texas Department of Transportation. Copyright © 2002. All rights reserved. AspireBook_Fall12.indb 46 9/18/12 8:59 AM