Newswise — Effects of Moisture, Temperature, and Freezing-and-Thawing on Alkali-Silica Reaction
by Richard A. Deschenes Jr., Eric Giannini, Thano Drimalas, Benoit Fournier, and W. Micah Hale
American Concrete Institute ACI Materials Journal July 2018
Although concrete pavements are designed with a service life of 20 to 50 years or more, various environmental factors can cause premature deterioration of concrete pavements. This research focuses on the combined effects of moisture, temperature, and freezing-and-thawing on alkali-silica reaction (ASR). The objectives of this research were to (1) better understand the intricate relationship between internal relative humidity (moisture state), temperature, and exposure conditions on the formation and development of ASR; and (2) understand the coupled mechanism of ASR and freezing-and-thawing in concrete.
Alkali-silica reaction is when concrete deteriorates due to an internal chemical reaction between certain reactive aggregates and the cement. This reaction causes a gel product to form, which then absorbs water from the concrete and expands, leading to deterioration of the concrete that can leave the concrete prone to other durability issues.
The symptoms of ASR typically manifest as cracking and spalling of the concrete surface, which decreases the serviceability and quality of the pavement and requires continuous maintenance (Fig. 1).
Concrete pavements are often exposed to cyclic ambient conditions that may include warm, humid weather during the summer and cold or freezing conditions during the winter. The moisture state of concrete fluctuates with the ambient conditions, as concrete is permeable and exchanges moisture with the atmosphere or subgrade. When water within concrete cools and begins to freeze, the water expands and is transported through the concrete. The viscous resistance to transport of water causes pressure on the cement phase, which can lead to expansion and cracking.
When the concrete warms and ice melts, water returns to its initial state causing further deterioration. This mechanism is known as freezing-and-thawing. The ASR and freezing-and-thawing deterioration mechanisms depend on the presence of sufficient moisture within the concrete. Reducing the availability of moisture within concrete below a critical threshold may prevent deterioration of the concrete.
The deterioration mechanisms of ASR and freezing-and-thawing are both well documented and methods have been developed to prevent either from occurring in well-designed concrete pavements. However, cases have been documented where adequately designed concrete pavements still deteriorate prematurely due to a combination of ASR and freezing-and-thawing conditions. In one documented case, a concrete pavement with adequate air entrainment and ASR mitigation measures (fly ash) deteriorated prematurely when freezing-and-thawing distress materialized near the joints of the pavement. This appears to have occurred due to the higher moisture content of the pavement near the joints. It was hypothesized that the formation of ASR gel in the pavement lead to increased moisture within the concrete, which exacerbated pressures during freezing and lead to cracking and spalling at the joints. A petrographic investigation confirmed the presence of ASR gel and freezing-and-thawing cracks, with freezing-and-thawing being the primary mechanism of deterioration.
To evaluate this combined mechanism, various combinations of temperature, humidity, and exposure cycles were tested to replicate the field observations. The results indicate that certain marginally reactive concrete mixtures may pass standardized test methods for ASR and freezing-and-thawing, but still deteriorate rapidly when exposed to cycles of warm, humid air followed by freezing-and-thawing.
The results confirm the dependence of this mechanism on moisture and indicate that controlling or limiting the internal relative humidity (moisture) within the concrete may prevent deterioration from occurring, or slow ongoing deterioration. Treating concrete with a vapor transmissive coating, such as silane, may lower the moisture state within the concrete and slow the deterioration mechanisms sufficiently to improve the service life of concrete. Future research will focus on further understanding the coupled interaction of ASR and freezing-and-thawing deterioration that can occur early in the life of otherwise durable concrete.
The research can be found in a paper titled “Effects of Moisture, Temperature, and Freezing-and-Thawing on Alkali-Silica Reaction,” published by ACI Materials Journal.
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