It is known that subsidence cracking may cause premature deterioration of concrete slab structures in salt laden environments. Chlorides from either deicing salts or marine environments may cause chloride-induced corrosion of the reinforcing steel resulting in spalling of the cover concrete. Concrete specimens with 16 mm (# 5) diameter bars were cast with various cover depths, bar spacing and two concrete mixture types to determine the influence that epoxy coated reinforcement, cement type and bar spacing may have on the probability of subsidence cracking in bridge deck slabs. It was determined that there is not a significant difference in the probability of cracking of concrete between concrete cast with epoxy coated reinforcing steel and bare reinforcing steel. Concrete subsidence cracking was found to be dependent upon the clear cover depth and cement type. / Master of Science

The process of catalytic cracking of petroleum hydrocarbons invariably produces a carbonaceous deposit on the surface of the solid catalyst which serves to reduce the activity of the catalyst. This investigation was undertaken to determine the effect of the feed rate and the length of the cracking period on the carbon formation on the catalyst. A vertical, fixed-bed, externally heated reactor was constructed. The reactor contained a bed of synthetic silica-alumina Socony-Vacuum bead catalyst. The necessary auxiliary apparatus required to handle the feed and the products of reaction was provided. The amount of carbon deposited on the catalyst was determined by burning it and measuring the carbon dioxide thus formed. The feed material used was Esso Diesel Oil (208). It was exposed to the catalyst at a temperature of about 900 degrees Fahrenheit for a series of ten minute periods while the feed rate was varied from 0.42 to 5.55 volumes of feed per volume of catalyst per hour. Another series of tests was performed at the same temperature, the feed rate being held at approximately two volumes of feed per volume of catalyst per hour; while the length of the cracking period was varied between thirty-five seconds and thirty minutes. It was found that during the ten minute cracking periods the amount of carbon deposited on the catalyst was independent of the feed rate between the limits of 0.91 and 5.55 volumes of feed per volume of catalyst deposited on the catalyst was related to the length of the cracking period by a parabolic function. Further analysis of the data revealed that the amount of feed converted into carbon was related to the degree of conversion by a parabolic function. From the three relations mentioned above, an equation relating the conversion, the feed rate and the cracking period was derived. The form of this equation is as follows: V³⋅⁰ = (9.60 x 10⁵)/R_fv t⁰⋅⁷⁵ where V is the percent conversion, R_fv is the feed rate and t is the cracking time in minutes. It was further found that the activity of the catalyst was not materially decreased after a series of twenty-nine tests, but that the substitution of quartz chips for the catalyst in the reactor decreased the conversion obtained by 75 per cent. / M.S.

The problem examined is an infinite anisotropic layer with a through crack at arbitrary orientation, subjected to uniform in-plane remote loading. The purpose of this study is to gain a better understanding of several theoretical models for predicting the direction of crack propagation and the level of load causing crack extension, and to present a new model for predicting the critical load. The discussed models are particularly examined in detail with regard to the physical parameters affecting the results. Comparison is made with available experimental results. It is shown that the normal stress ratio theory provides good agreement with experimental crack growth direction, independent of physical parameters, and that the newly proposed traction ratio theory predicts well the critical load causing crack extension. / M.S.

This thesis presents the work performed to evaluate the susceptibility of AISI 304 to stress-corrosion cracking in terms of time to crack nucleation and rate of crack propagation. U-bend specimens were exposed to magnesium chloride solutions boiling at atmospheric pressure for some predetermined time. The concentrations of magnesium chloride employed were 40, 42, and 44 per cent by weight. After exposure, specimens were microscopically examined and crack depths were measured and recorded. It was found that a straight-line relationship existed between maximum crack depth and exposure time which may be expressed by the empirical equation Log t = D/M+ Log C. The constants M and C are characteristic of the conditions of exposure and increase with a decrease in chloride ion concentration. The rate of crack propagation was found to be inversely proportional to time. Microscopic examination revealed that cracking was both transgranular and intergranular. There are indications that intergranular cracking was more pronounced at the low concentrations of the chloride ion. The results obtained justify the continuation of the work to determine factors affecting susceptibility in terms of constants M and C. / Master of Science

In order to predict the future needs of existing bridges, Bridge Management Systems use models to predict the time when damage will reach a level to cause repair, rehabilitation, or replacement of the structure. One such model is the deterioration model, which has three distinct phases. The second phase of the model, the corrosion phase, is the focus of this study. During the corrosion phase, chloride ion concentration reaches a threshold level at the depth of the reinforcing steel which initiates corrosion. The corrosion continues until sufficient pressure is exerted on the surrounding concrete to cause cracking. This study is a continuation of a study implemented in the Materials Division at Va Tech. The study includes the monitoring of the corrosion rate of steel reinforcing bars placed in simulated bridge decks. The corrosion rates were varied by placing between 0 - 9.6 Ibs/yd³ of chloride ions in the concrete to produce six different series. Also, the depth of concrete cover, bar spacing, bar size, and exposure conditions were varied. The specimens were monitored until the time that the cracking of the concrete was observed. At that time, samples of the steel reinforcing bars were removed and the actual amount of corrosion which had occurred was determined as the weight loss of the steel. The actual weight loss of the steel reinforcing bars was then compared to the predicted weight loss from the corrosion rate measurement devices. The time to cracking and the mode of cracking was compared to Bazant's equations for cracking which are the basis for the corrosion phase of the deterioration model. Although only one series cracked during the study, corrections in the use of Bazant's equations were proposed. / Master of Science

The work described herein examines the fracture behavior of steels used in bridge applications. As part of Transportation Pooled Fund (TPF) Project 5-238, Design and Fabrication Standards to Eliminate Fracture Critical Concerns in Steel Members Traditionally Classified as Fracture Critical, researchers aim to take advantage of advances made in both steel production technology and in the field of fracture mechanics. Testing and analysis of both conventional and High Performance Steel (HPS) grades of bridge steel was conducted as part of this study. This includes both Charpy V-Notch testing, as well as more rigorous elastic-plastic fracture toughness testing. Analysis includes the application of the master curve methodology to statistically characterize fracture behavior in the ductile to brittle transition region. In addition, a database of historic bridge fracture toughness data was compiled and re-analyzed using plasticity corrections to estimate elastic-plastic fracture toughness. Correlations between Charpy V-Notch impact energy and fracture toughness, which forms the basis for the current material specification, were also examined. Application of fracture toughness characterization of both new and historic data results in updated methodologies for addressing fracture in bridge design. / Ph. D.

An analytical study was conducted of the effect of cracks on creep of polycrystalline, brittle structural ceramics. Two independent mechanisms of contribution of cracks were defined. The mechanism of elastic creep by crack growth represents the rate of increase in strain, with time, resulting from the time-dependent decrease in elastic moduli of the material, due ·to crack growth. The mechanism of crack-enhanced creep provides a measure of the increase in creep rate over that in an identical but crack-free material, due to the local stress field associated with the cracks and the resultant transfer of stress to the adjacent, crack-free material. Creep rates due to these mechanisms were quantified for simple crack geometries. It was shown that the contribution of cracks can result in an idealized 4-stage creep curve for a brittle, polycrystalline ceramic, in contrast to the conventional 3-stage creep curve for metals. The four stages consist of a primary or crack incubation period, a secondary sigmoidal region resulting from growth of microcracks along grain boundary facets, a tertiary or crack-enhanced stage associated with arrested microcracks, and a quarternary stage comprising crack linkage and coalescence. It was demonstrated that the formation and growth of cracks during creep can result in apparent power-law creep, positive grain size dependence of the creep rate, and grain size-dependent creep activation energy. It can also account for observations of decreasing creep rate with increasing time in constant load creep tests, anomalous stress relaxation behavior in structural ceramics, significantly higher creep rates in tension tests than in compression tests, and discrepancy between diffusion coefficients inferred from creep studies and measured in diffusivity experiments. A simple model was presented for the effect of cracks on creep rate in bending, based on the time-rate of change of curvature of a bend specimen. Analysis of the effect of cracks on creep was extended to a general state of multiaxial stress, through matrix formulation of stress, creep rate, and creep compliance tensors. Derivation of components of the creep compliance tensor from analogs in elasticity was demonstrated for crack-enhanced creep, for uniaxial and uniform triaxial tension, for simple crack geometries. It was demonstrated that materials containing cracks can exhibit a finite rate of creep under hydrostatic tension, in contrast to a corresponding creep rate of zero in crack-free materials. Recommendations are made for analysis and interpretation of experimental creep data for structural ceramics. / Ph. D. / incomplete_metadata

The inverted T-beam system provides an accelerated bridge construction alternative. The system consists of adjacent precast inverted T-beams finished with a cast-in-place concrete topping. The system offers enhanced performance against reflective cracking and reduces the likelihood of cracking due to time dependent effects. Differential shrinkage is believed to be one of the causes of deck cracking in inverted T-beam systems. The objective of this study was to develop mix designs that exhibit lower shrinkage and higher creep compared to typical deck mixtures, recommend a prescriptive mix design and a performance criterion to VDOT that can be further investigated and used in the inverted T-beam system to combat effects of differential shrinkage. Ten different mix designs using different strategies to reduce shrinkage were tested for their compressive strength, splitting tensile strength, modulus of elasticity and unrestrained shrinkage. The four best performing mixes were selected for further study of their time dependent properties. The test data was compared against the data from various prediction models to determine the model that closely predicts the measured data. It was observed that ACI 209.2R-08 model best predicted the time dependent properties for the four mixes tested in this project. Tensile stresses in the composite cross-section of deck and girder, created due to difference in shrinkage and creep are quantified using an age adjusted effective modulus method. In this analysis, it was observed that mixes with normal weight coarse aggregate (NWCA) developed smaller stresses compared to those of mixes with lightweight coarse aggregate (LWCA). Mixes with fly ash as supplementary cementitious material (SCM) developed smaller stresses at the bottom of deck when compared to mixes with slag as the SCM. / Master of Science / The inverted T-beam system provides an accelerated bridge construction alternative. The system consists of adjacent precast inverted T-beams finished with a cast-in-place concrete deck. The system reduces the likelihood of cracking due to time dependent deformations of concrete – Shrinkage and Creep. The difference in rate of shrinkage of deck and the girder, also called as differential shrinkage, is believed to be one of the causes of deck cracking in inverted T-beam systems. The objective of this study was to develop concrete mix designs that exhibit lower shrinkage and higher creep that can be further investigated and used in the inverted T-beam system to combat effects of differential shrinkage. Studies resulted in the observation that ACI 209.2R-08 – model used to predict concrete behavior, best predicts the time dependent properties of the concrete tested in this project. Also, mixes with normal weight coarse aggregate (NWCA) developed smaller stresses compared to those of mixes with lightweight coarse aggregate (LWCA). Mixes with fly ash as supplementary cementitious material (SCM) developed smaller stresses when compared to mixes with slag as the SCM.

yes / Heavy petroleum industries, including the fluid catalytic cracking (FCC) unit, are useful for producing fuels but they are among some of the biggest contributors to global greenhouse gas (GHG) emissions. The recent global push for mitigation efforts against climate change has resulted in increased legislation that affects the operations and future of these industries. In terms of the FCC unit, on the riser side, more legislation is pushing towards them switching from petroleum-driven energy sources to more renewable sources such as solar and wind, which threatens the profitability of the unit. On the regenerator side, there is more legislation aimed at reducing emissions of GHGs from such units. As a result, it is more important than ever to develop models that are accurate and reliable, that will help optimise the unit for maximisation of profits under new regulations and changing trends, and that predict emissions of various GHGs to keep up with new reporting guide-lines. This article, split over two parts, reviews traditional modelling methodologies used in modelling and simulation of the FCC unit. In Part I, hydrodynamics and kinetics of the riser are dis-cussed in terms of experimental data and modelling approaches. A brief review of the FCC feed is undertaken in terms of characterisations and cracking reaction chemistry, and how these factors have affected modelling approaches. A brief overview of how vaporisation and catalyst deactiva-tion are addressed in the FCC modelling literature is also undertaken. Modelling of constitutive parts that are important to the FCC riser unit such as gas-solid cyclones, disengaging and stripping vessels, is also considered. This review then identifies areas where current models for the riser can be improved for the future. In Part II, a similar review is presented for the FCC regenerator system.

The ever-growing interest of the modern-day rotor dynamicist in the early detection of rotor cracks in turbomachinery has been the direct result of multiple catastrophic experiences that industry has had to face in recent times due to cracked rotors. The complete failure of the rotor due to crack propagation is easily recognized as one of the most serious modes of plant failure. Even so, this aspect has without question not received the attention it warrants. The last decade has, however, witnessed some laudable attempts that have been moderately successful in detecting cracked rotors. This work presents the design and set-up of a complete test rig that can be used for experimental research on response characteristics of cracked rotors. The results of this research will permit increased confidence in detecting the presence of rotor cracks in turbomachinery. The designed rig is capable of testing cracked shafts under the effect of lateral and coupled lateral-torsional vibrations. The conventional vibration signature analysis approach has been employed for the purpose of test rig evaluation and condition monitoring. The test rig has been shown to be essentially functional and the experimental data generated with the test rig are compared to appropriate analyses and published results. / Master of Science