Characterization of Damage in Mortar and Concrete Specimens due to Delayed Ettringite Formation (DEF)

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2007-04-25

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Delayed ettringite formation (DEF) in concrete and mortar systems has been associated with deleterious expansion leading to its premature deterioration. DEF causes a characteristic form of damage which is highly influenced by the cement chemistry, curing methods and exposure conditions. While the cement paste expands, the aggregate does not and cracks around these aggregates that usually form during curing, grow over time and eventually become filled with DEF at a later age. Therefore, considerable research work in preventing such deterioration is required to improve the durability of concrete structures.

In this research, mortar and concrete specimens were prepared with different potassium contents and different curing methods before subsequently subjecting them to different exposure conditions. Duggan Heat and Freeze-Thaw cycles were employed to initiate microcracks. Length and weight change measurements of the specimens were periodically monitored for the research duration. Also, the pH value, Potassium ion [K+], Sodium ion [Na+] and Calcium ion [Ca2+] concentrations of the storage solutions were monitored. A scanning electron microscope (SEM) equipped with high-energy dispersive analysis X-ray (EDAX) was used to identify mineral deposits present in the cavities, transition zones and cracks to determine the failure mechanism. Laser shearography, a nondestructive technique, was used to detect the onset of cracking and its subsequent propagation due to DEF.

The results showed that potassium content, curing methods and exposure conditions are the most important factors responsible for expansion associated with DEF. Increasing the potassium content leads to deleterious expansion associated with DEF and is accompanied by an increase in the weight of the specimen. Steam-cured concretes showed relatively larger expansions than the room temperature-cured concretes. The exposure condition of the mortar and concrete specimens is very critical to the re-formation of ettringite. Leaching of alkalis from the specimens into the storage solution increased the rate of expansion, and hence enhancing the formation of ettringite. Laser shearography image analysis showed that an increase in crack growth over time is essential for expansion due to DEF. Extensive microstructural investigations of the specimens revealed presence of DEF without any sign of Alkali-Silica Reaction (ASR).

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