Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Investigation of Delayed Ettringite Formation Damage Process Using Simultaneous Neutron and X-ray Tomography(2019) Feuze Lekem, serge alain; Amde, Amde M; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Delayed ettringite formation (DEF) is a significant deterioration process in concrete which involves the growth of ettringite [Ca6Al2(SO4)3(OH)12 ·26H2O] crystals leading to cracking and reduction of compressive strength. Conditions leading to DEF are well known and include among others cement chemistry, presence of humidity, heat curing of concrete structures, and the presence of cracks. The mechanisms and kinetics by which deterioration occur is still not well understood despite numerous investigations. Understanding the mechanism and kinetics of concrete deterioration due to DEF is important in order to prevent such costly deterioration and to improve concrete durability. In this research, concrete specimens were prepared with type III Portland cement and under different conditions that were designed to either promote or inhibit DEF. These consisted of a control set, a set subjected to a heat cycle and a third set made with elevated potassium content of 1.72% and also thermally cycled. They were tested periodically up to 380 days by conventional methods such as expansion and weight change measurements and compressive strength testing. Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDAX) confirmed the presence and the morphology of ettringite in voids at different ages. Simultaneous neutron and X-ray tomography, a new nondestructive microscopic method was used to scan the specimens at regular intervals in order to assess the feasibility of the method in monitoring the progress and characterizing DEF induced damages. The linear regression analysis of the correlation of expansion with weight change data revealed that expansion and deterioration process occurred in three distinct successive stages. In the first stage, the ettringite fills the pores with little or no expansion; in the second, the expansion appears to be creep due to expansive stresses in the filled pores and in the third stage, crack propagation leads to significant expansion and loss of compressive strength. The results of the linear regression also revealed that the mechanism of DEF is the replacement of pre-existing calcium hydroxide crystals. Through non-linear curve fitting, the kinetic of deterioration was modeled using the Kolmogorov-Avrami-Johnson-Miehl model. The simultaneous neutron and X-ray tomography allowed visualization of the interior of the specimen due to enhance phase segmentation. MATLAB routines were developed to allow for correction for beam hardening and to enhance phase segmentation. The study showed that with improved resolution, proper sample sizing, the method can be effectively used to characterize concrete damage due to expansive phases.Item Mitigation of High Alkalinity in Leachates of Aged Steel Slag(2015) Ozkok, Enes; Davis, Allen P; Aydilek, Ahmet H; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Steel slag, an abundant by-product of the steel-making industry, after it is aged, has a huge potential for use as an aggregate in road construction. However, the high pH of steel slag seepage (pH≥12) is a major impediment in its beneficial use. Analyses on aged steel slag samples demonstrated that the alkalinity producing capacity of aged steel slag samples strongly correlated to Ca(OH)2 dissolution and that prolonged aging periods have marginal effects on overall alkalinity. Treatment methods that included bitumen-coating, bathing in Al(III) solutions and addition of an alum-based drinking water treatment residual (WTR) were evaluated based on reduction in pH levels and leachate alkalinity. 10% (wt./wt.) alum-based drinking water treatment residual (WTR) addition to slag was determined to be the most successful mitigation method, providing 65−70% reduction in alkalinity both in batch-type and column leach tests, but final leachate pH was only 0.5−1 units lower and leachates were contaminated by dissolved Al(+III) (≥3−4 mM). Based on the interpretation of calculated saturation indices and SEM and EDX analyses, formation of calcium sulfoaluminate phases (i.e., ettringite and monosulfate) was suggested as the mechanism behind alkalinity mitigation upon WTR-modification. The residual alkalinity in WTR-amended slag leachates was able to be completely eliminated utilizing a biosolids compost with high base neutralization capacity. In column leach tests, effluent pH levels below 7 were maintained for 58−74 pore volumes worth of WTR-amended slag leachate using 0.13 kg compost (dry wt.) per 1 kg WTR-amended slag on average; also, dissolved Al(+III) was strongly retained on the compost.Item LINEAR AND NON-LINEAR FREQUENCY DOMAIN TECHNIQUES FOR PROCESSING IMPACT ECHO SIGNALS TO EVALUATE DISTRIBUTED DAMAGE IN CONCRETE.(2009) McMorris, Nicolas A.; Amde, Amde M; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The condition evaluation of in-situ concrete with non-destructive testing is difficult at best. The concrete deterioration processes of alkali-silica reaction (ASR), delayed ettringite formation (DEF) and freeze-thaw cycles all produce distributed damage in the form of micro-cracking which results in loss of strength or stiffness. Presently, a suitable field applicable method for determining the degree of micro-cracking does not exist. The impact echo test is potentially the best candidate if improvements can be made in the signal processing techniques which are crucial for accurately interpreting the data retrieved from concrete with distributed damage. In this research, two batches of concrete specimens were prepared in accordance with standard procedures. A portion of each batch was subjected to either the Modified Duggan cycle or to Freeze Thaw cycles, both proven methods of inducing DEF and micro-cracking respectively. Curing techniques and materials were also chosen to accelerate distributed damage in the concrete specimens. In addition to the impact echo, a number of secondary tests were employed to monitor the progress of distributed damage in the concrete specimens. Previous research efforts utilizing the impact echo method have attempted to characterize damage in terms of P-wave attenuation or pulse velocity. This involves signal processing in the time domain. These are inherently linear dynamics methods whereas the development of micro-cracks in concrete, an inhomogeneous material, gives rise to non-linear dynamics. Non-linear approaches to signal processing in the frequency domain are proposed herein. One involves calculating the deviation of the peak of the response spectrum from the shape of an ideal Lorentzian function model. The other calculates the second order non-linear harmonic coefficient. The results showed that the potassium content, the curing methods and the Duggan and Freeze Thaw cycles had the desired effect of inducing distributed damage. The results of the signal processing of the impact echo data yielded more reasonable results for the specimens subjected to Freeze Thaw testing than for the specimens subjected to the Duggan Cycle. The results of the Freeze Thaw specimens suggest that the non-linear analysis of impact echo signals is capable of accurately quantifying distributed damage in concrete.Item APPLICATION OF LASER SHEAROGRAPHY FOR DETECTING MICROCRACKS IN CONCRETE(2007-02-08) Khong, Hung; Amde, Amde M; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Laser Shearography method (LSM) has shown a great potential for application in nondestructive testing (NDT) especially in the early detection of cracks in concrete. It is based on a refinement of electronic speckle pattern interferometry (ESPI) that records a sheared image of the speckle interferogram. The speckle interferograms are recorded before and after stressing and then compared to find the defects on the surface of tested object based on localized fringe irregularity. In this research project, LSM was employed on 3" x 3" x 11.25" concrete prisms subjected to Duggan heat or freeze-thaw treatment prior to storage in limewater. Lasershearography images were taken at distinct intervals to detect the onset of cracking and to measure its subsequent propagation. Data analysis of the results after treatment revealed fine cracks at early ages in the concrete, with resolution better than 10 microns. The freeze-thaw treated concrete specimens exhibited more cracks over time compared to those subjected to the Duggan heat cycle.