The dam that fly ash built

Area of Science:

• Civil Engineering
• Materials Science
• Geology

Background:

• Hungry Horse Dam, built in 1949, was the first to use coal combustion fly ash in concrete.
• Pozzolan cements were common in North American dams before Hungry Horse, but fly ash was novel.
• The dam's location connects it to the rise of coal power and current climate change concerns.

Purpose of the Study:

• To investigate the long-term effectiveness of fly ash in suppressing alkali-aggregate reaction (AAR).
• To microscopically compare concrete cores with and without fly ash from Hungry Horse Dam.

Main Methods:

• Microscopic examination of two concrete cores from Hungry Horse Dam.
• One core contained coal combustion fly ash; the other did not.
• Analysis focused on sandstone coarse aggregate and signs of alkali-aggregate reaction.

Main Results:

• The concrete core without fly ash showed clear evidence of AAR in sandstone aggregates.
• Sandstone aggregates in the core with fly ash exhibited no signs of AAR.
• Microscopic analysis of AAR revealed alkali-silica gel filling gaps between sand grains.

Conclusions:

• Coal fly ash effectively suppresses alkali-silica reaction in concrete over the long term.
• The presence of fly ash prevents the formation of deleterious alkali-silica gel.
• This finding highlights the durability benefits of using fly ash in concrete construction.