1952 Steel Seizure Revisited - Systematic Study In Presidential Decision-Making

Managemen

Development and Construction of Low-Cracking High-Performance Concrete (LC-HPC) Bridge Decks: Free Shrinkage, Mixture Optimization, and Concrete Production

The development and evaluation of low-cracking high-performance concrete (LC-HPC) for use in bridge decks is described based on laboratory test results and experience gained during the construction of 14 bridges. The study is divided into three parts covering (1) the development of an aggregate optimization and concrete mixture design program entitled KU Mix, (2) free-shrinkage tests to evaluate potential LC-HPC mixtures developed for use in bridge decks, and (3) the construction and preliminary evaluation of LC-HPC bridge decks constructed in Kansas. This report emphasizes the material aspects of the construction process; a companion report will provide a detailed discussion of the construction, design, and environmental factors affecting the performance of LC-HPC bridge decks. The KU Mix design methodology for determining an optimized combined gradation uses the percent retained chart and the Modified Coarseness Factor Chart. The process begins by developing an ideal gradation followed by the determination of an optimum blend of user-selected aggregates. A Microsoft® Excel workbook enhanced with Visual Basic for Applications is available to perform the optimization process at www.iri.ku.edu. Experiences with the KU Mix design methodology during the construction of several LC-HPC bridge decks indicate that the process is easily implemented and transferred to concrete suppliers and governing officials. The second portion of the study involves evaluating the effect of paste content, water-cement (w/c) ratio, coarse aggregate type, mineral admixture type (silica fume, slag cement, and Class F fly ash each at two levels of replacement), cement type and fineness, a shrinkage reducing admixture, and the duration of curing on the free-shrinkage characteristics of concrete mixtures in the laboratory tested in accordance with ASTM C 157. The evaluation of shrinkage properties includes a total of 56 individual concrete batches. Both a high-absorption (2.5 to 3.0%) coarse aggregate and a low-absorption (less than 0.7%) coarse aggregate are evaluated in many of the comparisons. The results indicate that a reduction in w/c ratio (achieved by reducing the water content), longer curing periods, and the addition of a shrinkage reducing admixture reduce concrete shrinkage. When cast with a high-absorption coarse aggregate, the addition of either silica fume or slag cement results in a reduction in shrinkage at all ages, while the addition of fly ash increases early-age shrinkage but has little or no effect on long-term shrinkage. For mixtures containing a low-absorption coarse aggregate, the addition of silica fume or slag cement results in increased early-age shrinkage if the specimens are cured for seven days. These mixtures exhibit reduced shrinkage at all ages when the curing period is increased to 14 days. The addition of fly ash increases shrinkage at all ages for either curing period. The high-absorption limestone used in the study provides internal curing water, which results in the shrinkage of mixtures containing slag cement or silica fume. The final portion of the study presents the specifications, construction experiences, and the preliminary evaluation of 14 LC-HPC bridge decks that have been built or are planned in Kansas. The techniques used to reduce cracking in these bridge decks are presented, and the field experiences for the 18 individual LC-HPC placements completed to date are presented. The results indicate that LC-HPC decks with an optimized aggregate gradation and design w/c ratios of 0.44 and 0.45 with cement contents of 317 and 320 kg/m3 (535 and 540 lb/yd3) have more than adequate workability, finishability, and pumpability, in addition to reduced cracking. A preliminary evaluation of these decks indicates that, on average, the LC-HPC decks are performing at a level approximately equal to or exceeding the best performing monolithic decks in Kansas surveyed over the past 15 years

Unsupervised machine learning for detection of phase transitions in off-lattice systems II. Applications

We outline how principal component analysis (PCA) can be applied to particle configuration data to detect a variety of phase transitions in off-lattice systems, both in and out of equilibrium. Specifically, we discuss its application to study 1) the nonequilibrium random organization (RandOrg) model that exhibits a phase transition from quiescent to steady-state behavior as a function of density, 2) orientationally and positionally driven equilibrium phase transitions for hard ellipses, and 3) compositionally driven demixing transitions in the non-additive binary Widom-Rowlinson mixture

Mechanism of the photovoltaic effects in 2-4 compounds Progress report, 1 Apr. - 30 Sep. 1968

Current gain mechanism in copper sulfide-cadmium sulfide diode upon photoexcitation in presence of reverse bia

PRESENCE AND PREVALENCE OF BD (BATRACHOCHYTRIUM DENDROBATIDIS) IN CENTRAL PENNSYLVANIAN WOODLAND VERNAL POOLS

Batrachochytrium dendrobatidis (Bd), a virulent chytrid fungus responsible for dramatic amphibian declines, has been detected in the northwestern and southeastern regions of Pennsylvania. However, little environmental Bd testing has been performed in central Pennsylvania, particularly in the unique and speciose habitats of woodland vernal pools. Our study included sampling in four vernal pools over a period of three months during amphibian breeding periods. Skin swabs were taken from three caudate and two anuran species, during the course of late winter and spring migrations (n = 143). Low Bd zoospore equivalent loads were detected in only a few individuals, in three of the five species but in all four vernal pools sampled. No significant trends were seen between zoospore loads and ambient temperature or migration timing across the species sampled

Lattice Universes in 2+1-dimensional gravity

Lattice universes are spatially closed space-times of spherical topology in the large, containing masses or black holes arranged in the symmetry of a regular polygon or polytope. Exact solutions for such spacetimes are found in 2+1 dimensions for Einstein gravity with a non-positive cosmological constant. By means of a mapping that preserves the essential nature of geodesics we establish analogies between the flat and the negative curvature cases. This map also allows treatment of point particles and black holes on a similar footing.Comment: 14 pages 7 figures, to appear in Festschrift for Vince Moncrief (CQG

On bare masses in time-symmetric initial-value solutions for two black holes

The Brill-Lindquist time-symmetric initial-value solution for two uncharged black holes is rederived using the Hamiltonian constraint equation with Dirac delta distributions as a source for the binary black-hole field. The bare masses of the Brill-Lindquist black holes are introduced in a way which is applied, after straightforward modification, to the Misner-Linquist binary black-hole solution.Comment: LaTeX, 4 page

Development and Construction of Low-Cracking High-Performance Concrete (LC-HPC) Bridge Decks: Free Shrinkage, Mixture Optimization, and Concrete Production

The development and evaluation of low-cracking high-performance concrete (LC-HPC) for use in bridge decks is described based on laboratory test results and experience gained during the construction of 14 bridges. The study is divided into three parts covering (1) the development of an aggregate optimization and concrete mixture design program entitled KU Mix, (2) free-shrinkage tests to evaluate potential LC-HPC mixtures developed for use in bridge decks, and (3) the construction and preliminary evaluation of LC-HPC bridge decks constructed in Kansas. This report emphasizes the material aspects of the construction process; a companion report will provide a detailed discussion of the construction, design, and environmental factors affecting the performance of LC-HPC bridge decks. The KU Mix design methodology for determining an optimized combined gradation uses the percent retained chart and the Modified Coarseness Factor Chart. The process begins by developing an ideal gradation followed by the determination of an optimum blend of user-selected aggregates. A Microsoft® Excel workbook enhanced with Visual Basic for Applications is available to perform the optimization process at www.iri.ku.edu. Experiences with the KU Mix design methodology during the construction of several LC-HPC bridge decks indicate that the process is easily implemented and transferred to concrete suppliers and governing officials. The second portion of the study involves evaluating the effect of paste content, water-cement (w/c) ratio, coarse aggregate type, mineral admixture type (silica fume, slag cement, and Class F fly ash each at two levels of replacement), cement type and fineness, a shrinkage reducing admixture, and the duration of curing on the free-shrinkage characteristics of concrete mixtures in the laboratory tested in accordance with ASTM C 157. The evaluation of shrinkage properties includes a total of 56 individual concrete batches. Both a high-absorption (2.5 to 3.0%) coarse iii aggregate and a low-absorption (less than 0.7%) coarse aggregate are evaluated in many of the comparisons. The results indicate that a reduction in w/c ratio (achieved by reducing the water content), longer curing periods, and the addition of a shrinkage reducing admixture reduce concrete shrinkage. When cast with a high-absorption coarse aggregate, the addition of either silica fume or slag cement results in a reduction in shrinkage at all ages, while the addition of fly ash increases early-age shrinkage but has little or no effect on long-term shrinkage. For mixtures containing a low-absorption coarse aggregate, the addition of silica fume or slag cement results in increased early-age shrinkage if the specimens are cured for seven days. These mixtures exhibit reduced shrinkage at all ages when the curing period is increased to 14 days. The addition of fly ash increases shrinkage at all ages for either curing period. The high-absorption limestone used in the study provides internal curing water, which results in the shrinkage of mixtures containing slag cement or silica fume. The final portion of the study presents the specifications, construction experiences, and the preliminary evaluation of 14 LC-HPC bridge decks that have been built or are planned in Kansas. The techniques used to reduce cracking in these bridge decks are presented, and the field experiences for the 18 individual LC-HPC placements completed to date are presented. The results indicate that LC-HPC decks with an optimized aggregate gradation and design w/c ratios of 0.44 and 0.45 with cement contents of 317 and 320 kg/m3 (535 and 540 lb/yd3) have more than adequate workability, finishability, and pumpability, in addition to reduced cracking. A preliminary evaluation of these decks indicates that, on average, the LC-HPC decks are performing at a level approximately equal to or exceeding the best performing monolithic decks in Kansas surveyed over the past 15 years

Cracking and Chloride Contents in Reinforced Concrete Bridge Decks

The effects of material properties, design specifications, construction practices, and environmental site conditions on the performance of reinforced concrete bridge decks are evaluated. Field surveys were performed on 59 bridges to measure deck cracking, chloride ingress, and delaminated area. The surveys were limited to steel girder bridges – bridges that are generally agreed to exhibit the greatest amount of cracking in the concrete decks. The study includes two bridge deck types with silica fume overlays, one in which 5% of the cement is replaced by silica fume (19 bridges) and the other in which 7% of the cement is replaced by silica fume (11 bridges), plus decks with conventional overlays (16 bridges) and monolithic bridge decks (13 bridges). Information from the current study is combined with data from two earlier studies. In total, 27 variables are evaluated, covering bridge age, construction practices, material properties, site conditions, bridge design, and traffic volume. The performance of silica fume overlay decks relative to conventional overlay and monolithic decks is of particular interest due to the widespread use of silica fume overlays in the state of Kansas. The results of the study indicate that chloride contents increase with the age of the bridge deck, regardless of deck type. In addition, concrete for all bridge deck types sampled in the same age range exhibit similar chloride contents for samples taken both at and away from cracks, regardless of deck type. For bridges within the same age range, the average chloride concentration taken away from cracks at the level of the top transverse reinforcement rarely exceeds even the most conservative estimates of the corrosion threshold for conventional reinforcement. Chloride concentrations taken at crack locations, however, can exceed the corrosion threshold in as little as nine months. Based on these observations, it appears clear that attention iii should be focused on minimizing bridge deck cracking rather than concrete permeability. The study demonstrates that crack density increases with increases in the volume of cement paste and that neither higher compressive strengths nor higher concrete slumps are beneficial to bridge deck performance. In addition, crack density is higher in the end regions of decks that are integral with the abutments than decks with pin-ended girders. The results of the crack surveys indicate that cracking increases with age, although a large percentage of the cracking is established early in the life of the deck. Even with the increase in crack density over time, however, both monolithic and conventional overlay bridges cast in the 1980s exhibit less cracking than those cast in the 1990s. The differences are attributed to changes in material properties and construction procedures over the past 20 years. The trend in cracking for decks with silica fume overlays cast in the 1990s (containing 5% silica fume), however, is quite the opposite. A decrease in crack density is observed for 5% silica fume overlay decks, which appears to be the result of increased efforts to limit evaporation prior to the initiation of wet curing. Recently constructed 7% silica fume overlay decks, however, have not shown continued improvement. In light of the chloride and cracking observations, conventional high-density overlays are recommended in lieu of silica fume overlays, and full-depth monolithic decks are recommended for new deck construction