Civil & Environmental Engineering
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Item Characterization of Damage in Mortar and Concrete Specimens due to Delayed Ettringite Formation (DEF)(2007-04-25) Ceesay, Jorgomai; Amde, Amde M.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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).Item THE INFLUENCE OF EXPOSURE CONDITIONS ON DELAYED ETTRINGITE FORMATION IN MORTAR SPECIMENS(2004-05-05) Ceesay, Jorgomai; Amde, Amde M; Civil EngineeringThe objectives of this research were to experimentally study the influence of exposure conditions on delayed ettringite formation (DEF) in mortar specimens, and to explore the complex processes of expansive cracking associated with DEF. Different exposure conditions were investigated while other parameters such as water-to-cement ratio, fine and coarse aggregates, cement and curing conditions were kept constant in order to study the influence of storage conditions on mortar damages associated with DEF. Mortar bars and cubes were prepared using Portland cement ASTM Type III, Frederick sand and ordinary tap water. The specimens were steam-cured and then subjected to the Duggan heat cycle to introduce microcracks. Control batches of mortar mixes with no additional potassium (K2O) content and sets with additional potassium increasing the overall potassium level to 1.5% K2O by weight of cement were prepared. Increasing the potassium level has a deleterious effect on the specimen expansion and causes deterioration in compressive strength. Length change measurements according to ASTM C490 and weight changes of the mortar bars were constantly monitored during the study. The pH value, Potassium ion [K+], Sodium ion [Na+] and Calcium ion [Ca2+] concentrations of the storage solutions were also monitored to analyze the amount of alkali being leached into the water. Scanning electron microscope (SEM) equipped with high-energy dispersive analysis X-ray was used to identify materials present in the cavities, transition zones and cracks to determine failure mechanism of the specimens. X-ray computed tomography (X-ray CT), a non-destructive method was employed to reveal the internal crack patterns. Ettringite crystals are mainly found in the cavities, in the interface between the aggregate and cement paste. Mortar bars subsequently stored in exposure conditions 1 (water pH maintained at 12.5) and 2 (plain water) after the Duggan heat cycle, clearly showed expansion against weight change results to be positive linearly correlated over the storage period. This was again observed in the mortar specimens with the higher potassium (K2O) content. Results from the petrography analyses of the expanded mortar specimens revealed delayed ettringite formation (DEF) without detecting alkali-silica reaction (ASR). The results also show that deleterious expansions caused by DEF prevails in exposure conditions with high potential leaching of alkali hydroxide to the storage solution.