A. James Clark School of 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 Characterization of Delayed Ettringite Formation in Maryland Bridges(2007-04-25) Ceary, Micah; Amde, Amde M; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The research investigated the significance of Delayed Ettringite Formation (DEF) presences in the Maryland Bridge Inventory. The objective of the research included investigating possible presence of DEF, correlations between the presence of DEF in cast-in-place concrete and moist map cracking, and correlations between presence of DEF in cast-in-place concrete and air entrainment agents (AEA). The research required coring a target population, so that scanning electron microscope (SEM) with energy dispersive analysis x-ray (EDAX) could be utilized to identify DEF in the concrete. Combination of visual identification, SEM and elemental identification with EDAX are used to verify the presence of DEF in the concrete samples. The research was conducted in two phases with the first suggesting a possible link between moist map cracking and DEF. The research identified numerous ettringite morphologies in bridge concrete. Characterization of the ettringite morphologies suggested that lamellar ettringite could be linked to DEF-related damage. From this work, a second phase was developed to establish a link between degree of moist map cracking and DEF. Furthermore, other research showed that ettringite grew well in a solution of AEA. The second phase research developed a population to test both hypothesis and attempted to show positive or negative correlations between these two theories and DEF presence. Phase 2 research shows no correlation between concrete mixes with AEA and DEF. Therefore, the conclusion is that AEA does not have any measurable adverse effects regarding DEF. Whereas, the research shows a strong correlation between widespread moist map cracking and significant DEF quantities, but that a DEF presence is not necessarily indicative of DEF-related damages. Once a threshold is exceeded, DEF-related damages are observed and are found throughout the affected concrete. These results are independent of alkali-silica reaction (ASR), since ASR was not found in any of the Phase 2 samples selected. Overall, both phases of research shows that Maryland bridge concrete contains ettringite, but that not all ettringite formations appear to lead to damage of the concrete. Moist map cracking, a suggested DEF-related damage, appears to be correlated with lamellar ettringite formations and high quantities of DEF with in the concrete.