Technical Bulletins > Cryotech NAAC®
5/24/07 - Relationship of Runway Deicers to ASR (Alkali-Silica Reaction) in Concrete
Airport officials have asked our opinion about ASR and what steps they might take with respect to their concrete surfaces. This bulletin summarizes information we have about the matter, including information posted on the recent FAA Engineering Brief No.70.
What is ASR?
ASR is the abbreviation for a naturally occurring phenomenon called alkali-silica reaction. Commonly defined, it is the expansive deterioration of concrete due to a chemical reaction involving concrete aggregate and cement paste.
ASR occurs when reactions between the aggregate and alkalis form a silica gel. The gel expands as it absorbs moisture, causing concrete to crack; generally, these cracks first appear at joints. This may happen anytime from a few months after installation to decades later, or not at all.
Three variables must be present for ASR to occur: silica-containing reactive rock and/or sand, moisture, and high-alkali conditions. Without careful concrete mix design, these conditions may be present. And even when present, ASR damage may or may not occur. What triggers ASR is not well understood.
Interestingly, ASR is found on airfield pavements in the United States but rarely in Canada or Europe, because their concrete specifications do not allow the use of alkali-silica reactive components. ASR is most common where moisture is abundant such as dams and retaining walls. But bridges, roads, parking structures and airport pavements are also susceptible.
ASR has been known to occur since the late 1920s. Yet ASR damage is not common today even though reactive aggregates are found throughout North America.
Why are airports concerned?
ASR has recently been found in airport runways and the causes are now under investigation. Some have conjectured that although deicers do not cause ASR they may accelerate it when the three aforementioned conditions exist.
What is being done?
Industry and federal government agencies are funding research into materials that cause ASR. Cryotech is a member of a privately funded research group, Innovative Pavement Research Foundation. We are actively working to assist in evaluating and understanding the relationship between ASR and deicers. In addition, research on new deicer formulations, sealers and surface treatments is being done to see if any of these can mitigate the effects of ASR.
What should airports do?
The FAA is not changing its policies on the use of deicing chemicals, as their benefit to safety is more significant than ASR related pavement concerns. Although airports should continue to use deicers, signs of ASR should be continuously monitored. Field history review by concrete experts is the best determinate for understanding if ASR conditions exist. Additionally, new tests are being developed that better identify the ASR potential in existing pavements. Cryotech will advise the industry as soon as these test protocols are available. In the meantime, before concrete materials are selected, care should be taken to understand their ASR potential for any new concrete applications.
Ref: Federal Aviation Administration; Portland Cement Assoc.; National Aggregates Assoc.; National Ready Mixed Assoc
