Browsing by Subject "Shrinkage"
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Item Machine vision system for simultaneous measurement of dimensional changes and soil release in printed fabric(2010-08) Hill, Matthew; Sari-Sarraf, Hamed; Hequet, Eric F.This thesis presents a protocol for creating digital images of printed fabric swatches and an algorithm that will automatically measure dimensional changes and segment stains so that soil release could be evaluated. The dimensional changes measured here are shrinkage and skew. Current methods for evaluating dimensional changes on printed fabrics are manual. There are no current methods for evaluation of soil release on printed fabrics and the segmentation that the proposed algorithm provides is a vital first step to such a system. This thesis proposes a system that could become a standard for making both measurements simultaneously. To make these measurements, printed fabric swatches are scanned before and after wash using an off-the-shelf scanner. Reference points (called shrinkage dots) are placed on the fabric swatches and then located in the scanned images. This is done using image registration and subtraction to remove the influence of the pattern followed by cross-correlation to then locate the shrinkage dots. The locations of the shrinkage dots are used both to calculate the dimensional changes and in locating the stains. Before a snake-based method segments the stain, the influence of the background pattern is removed using the same registration and subtraction method used for shrinkage dot detection. In an experiment involving 240 images and 10 different printed patterns, the algorithm was able to correctly identify 98.8% of the shrinkage dots and identify stains in 93.3% of stains that were determined to exist according to technicians. The segmentation accuracy is quantified by an average dice metric of .87 in a set of 50 potential stains when comparing to manual segmentations.Item Model of strain-related prestress losses in pretensioned simply supported bridge girders(2014-05) Gallardo Méndez, José Manuel; Bayrak, Oguzhan, 1969-Prestressed concrete construction relies on the application of compressive stresses to concrete elements. The prestressing force is typically applied through the tensioning of strands that react against the concrete and induce compression in the concrete. Loss of prestress is the decrease of this pre-applied stress. The conservative estimation of the prestress losses is imperative to prevent undesired cracking of the prestressed element under service loads. A large fraction of the prestress losses is a consequence of concrete deformations. This fraction of the losses can be identified as strain-related losses, and these occur due to instantaneous elastic shortening, and time-dependent creep and shrinkage. Creep and shrinkage of concrete depend on many factors that are extremely variable within concrete structures. The time-dependent behavior of concrete is not well-understood, but recent findings in the topics of concrete creep and shrinkage provide a better understanding of the underlying mechanisms affecting the nature of these two phenomena. However, current design practices and prestress loss estimation methods do not reflect the state-of-the-art knowledge regarding creep and shrinkage. The main objective of this dissertation was the study and estimation of strain-related prestress losses in simply supported pretensioned bridge girders. Simply supported pretensioned girders are widely designed, produced and frequently used in bridge construction. Due to this common use, pretensioned concrete bridge girders has become fairly standardized elements, which results in a reduced variability in the behavior of pretensioned bridge girders, as compare to that of less standardized concrete structures. Hence, a simplified method was calibrated to estimate prestress losses within pretensioned girders to an adequate level of accuracy. To achieve an acceptable accuracy experimental data from the monitoring of pretensioned simply supported girders was used for the calibration of the method. The accuracy of this simplified method is comparable to that achievable using more elaborate methods developed for generic concrete structures.Item Reflective cracking of shear keys in multi-beam bridges(2009-06-02) Sharpe, Graeme PeterMulti-beam bridges made from precast concrete box girders are one of the most common bridge types used in the United States. One problem that affects these bridges is the development of longitudinal or reflective cracks on the road surface because of failure of the shear keys. Some states have attempted to correct this problem by redesigning the shear key or adding post-tensioning, but the problem persists in many new bridges. The purpose of this study is to investigate why these shear key failures are occurring. This project studies two types of box girder designs, the common Precast/Prestressed Concrete Institute (PCI) box girder bridges and the Texas Department of Tranportation (TxDOT) box girder bridge. In the past, reflective cracking has occurred in bridges of both types. The analysis procedure involves finite element analyses of bridge models with realistic support and loading conditions, and comparing the PCI and TxDOT bridges. The results indicate that both PCI and TxDOT box girder have sufficient strength to resist cracking from vehicular loads, but uneven temperature changes and shrinkage strains cause high tensile stresses in the shear key regions and lead to reflective cracking. The analyses showed the highest stresses were often times near the supports, rather than at midspan. Past studies have proposed using larger composite deck slabs, transverse posttensioning, or full-depth shear keys to prevent shear key failure. Composite slabs were the most effective way to reduce high stresses in shear keys, and were effective for all loading cases considered. Post-tensioning and full-depth keys also showed a reduction in shear key stresses, but were less effective.