FHWA INNOVATION: A certain reaction

Gina Ahlstrom / March 10, 2011

Find solutions for diagnosing and mitigating alkali-silica reaction (ASR) in concrete through the many resources available from the Federal Highway Administration’s (FHWA) ASR program. Launched in 2006, the program’s goals are to increase concrete pavement and structure durability and performance and reduce life-cycle costs through the prevention and mitigation of ASR. ASR occurs when silicas in some aggregates and alkalis in concrete combine with water to form a gel-like substance. As the gel absorbs water and expands, it causes the concrete to crack. Over time, these cracks can then enable other modes of distress to occur, such as freeze-thaw damage or corrosion.

Guidance documents developed by the ASR program include the “Report on Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction” (Pub. No. FHWA-HIF-09-001). The report includes information on rapid test methods and evaluation processes for selecting job-specific ASR preventive procedures for new pavements, bridges and other highway structures. Tests covered include petrographic assessment, the accelerated mortar bar test and the concrete prism test. The report also looks at a prescriptive approach for selecting preventive measures, including assessing the degree of aggregate activity, the level of ASR risk and the level of prevention needed. Options for preventive measures are then briefly discussed.

In the recently released “Report on the Diagnosis, Prognosis, and Mitigation of Alkali-Silica Reaction (ASR) in Transportation Structures” (Pub. No. FHWA-HIF-09-004), engineers and other transportation professionals can find more detailed guidance on the necessary steps for detecting and evaluating ASR in a highway structure. Diagnosis begins with a condition survey to evaluate the presence and severity of distress, followed by a second level of investigation to document information, measure the cracking index, conduct a petrographic examination and obtain samples. A more detailed investigation is then conducted to determine the current rate of concrete expansion and cracking, the potential for future expansion and the risks posed by the presence of ASR. This third level of investigation includes both in-situ evaluations, such as examining surface cracking and taking expansion and deformation measurements, and laboratory tests.

The new report also examines the success of proposed mitigation measures for ASR, which are divided into measures that treat the causes of ASR and those that treat the symptoms. Measures to treat the causes include using lithium compounds to halt expansion in the concrete and applying sealants to reduce moisture. To treat the symptoms, options include crack-filling techniques and the confinement of an ASR-affected member.

To download the two ASR reports, visit www.fhwa.dot.gov/pavement/concrete/?­asr-protocols.cfm.

Additional ASR resources can be found at FHWA’s ASR Reference Center. Available online at www.fhwa.dot.gov/pavement/concrete/asr/reference.cfm, the center’s offerings include research reports, guidance documents, specifications related to ASR, field trials reports, case studies and helpful links. Visitors to the site also are encouraged to contribute new resources and information.

To stay up to date on all of the work of the ASR program, subscribe to FHWA’s Reactive Solutions technical update. The quarterly e-news report highlights new program activities, research results and publications. Readers also can submit ASR-related questions to the regular Ask the Experts feature. To be placed on the e-mail distribution list for Reactive Solutions, send an e-mail to [email protected]. Discover all of these resources and more, as well as the guidance you need to address your ASR-related challenges, by visiting www.fhwa?.dot.gov/pavement/concrete/asr.cfm today.

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