A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Mitigation of Delayed Ettringite Formation in Laboratory Specimens(2007-04-25) Shaikh, Hafiz; Amde, Amde; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of this study was to determine if the growth of delayed ettringite formation (DEF) in existing concrete can be reduced or even prevented with commercial products. Additionally, the objectives were to determine if externally treating concrete specimens with water repellents and crystal growth inhibitors will decrease DEF-related expansion. The research required casting laboratory specimens for expansion and weight measurements, for strength testing, and for scanning electron microscope (SEM). Three of the four products, ChimneySaver, Dequest 2060S, and Good-Rite K752, reduced concrete expansion and weight change when compared to the control set. The study indicates that ettringite may have to be identified and mitigated early to prevent deleterious effects.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 Characterization of Pore Structure and Crack Propagation in Concrete Using X-Ray Computed Tomography(2004-12-13) Hunter, Opio Konata; Amde, Amde M; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis utilized the method of computed tomography to quantify the pore structure and the development of cracks in concrete specimens due to delayed ettringite formation. The pore structure is responsible for the passage of harmful substances through the concrete matrix, while delayed ettringite formation causes expansion and internal cracking in concrete structures; particularly in prestressed and precast concrete elements. Computed tomography is a relatively new technique that can be utilized to determine the pore structure and crack propagation in concrete specimens within the mm to micrometer range. The experiment was successful in quantifying the pore structure in the specimens. The pore sizes and total porosity of each specimen were determined along with the pore size distribution on diameter and volume. A comparison was then made with the mercury intrusion porosimetry technique. However, the second objective of analyzing internal cracking due to delayed ettringite formation was not met.