422 research outputs found
Toward Development of Self-Compacting No-Slump Concrete Mixtures
No full textNo-slump concrete (NSLC) is one of the commercial types of concrete that is known as a type of concrete with almost zero flowability. No-slump concrete is normally used for typical applications, like pavements construction and massive dam structures. The specific feature of a no-slump concrete is its high shape holding ability. No fixed formwork is required for the construction. The main disadvantage of this type of concrete is that a great amount of energy is required for a proper compaction. Self-compacting concrete (SCC) is another commercial type of concrete that is known as a type of concrete that flows and fills the formwork under its own weight without applying any external energy. Although SCC is a relatively recent development, it has demonstrated substantial economic and environmental benefits in terms of faster construction, reduction of required manpower, better surface finishing, easier and vibration-free placing and reduced noise level. Therefore, SCC has recently found a wide use for different applications and structural configurations. However in comparison with NSLC, its demolding time for preserving its shape is much longer and fixed formworks are required for the construction. The first objective of this research project is to study the possibility of developing a self-compacting no-slump concrete (SCNSLC), which does not need compaction and also has a high shape holding ability shortly after placing in the formwork. The second objective is to develop a model, which can predict the rheological behaviour of a mixture based on the properties and proportion of the mix components. The study starts with a comprehensive study of mechanisms that govern the rheological behaviour of concrete in the dormant period, i.e. when the hydration effect is still ignorable. A fresh concrete mixture is considered as a two-phase system, paste and aggregates. The paste itself is divided into two components, the “void paste” and the “excess paste”. The void paste is the part of the paste which fills the void space between the aggregates in a compacted state. The excess paste is the rest of the paste used to form a nominal layer with an average constant thickness around every single aggregate particle. The void paste tries to keep the aggregate particles in their positions, while the excess paste tries to push the aggregate particles apart and promote their mobility. With respect to the dominant mechanism, mixtures are divided into three main classes: \u95 Class 1: High Excess Paste volume mixtures, where the excess paste effect is dominant. For these mixtures the capacity to deform is maximal. \u95 Class 2: Intermediate Excess Paste volume mixtures, where the effect of the void paste becomes significant and gradually increases with decreasing excess paste volume. The capacity of the intermediate excess paste volume mixtures to deform is lower than the high excess paste volume mixtures. \u95 Class 3: Low Excess Paste volume mixtures, where the void paste effect is dominant and the excess paste does not significantly affect the rheological behavior. For these mixtures the capacity to deform is minimal. Mix design The shape holding ability of mixtures is characterized by the shape preservation factor 0<SPF?1. The SPF shows the ability of a mixture to preserve its shape in the slump test after demolding. The SPF is the ratio of the cross sectional area of a 3D sample after and before demolding. A SPF is about 1 for a mixture which shows almost no deformation, i.e. a no-slump concrete mixture (NSLC). For a conventional self-compacting concrete mixture (SCC), with a spread diameter?~600 mm, the SPF is less than about 0.4. In this thesis it is found that the maximum SPF for mixtures that can compact under their own weight is about 0.7. A mixture with SPF?0.7 is denoted a Self-Compacting High Shape Preserving Concrete (SCHSPC). A mix design method is proposed for mixtures with a shape holding ability ranging from no-slump concrete mixtures (NSLC) with the shape preservation factor SPF?1 to conventional self-compacting concrete mixtures (SCC) with a shape preservation factor SPF?0.4. First the volume of the excess paste with a certain consistency is determined for a required shape preservation factor SPF. Then the volume of the total paste, i.e. the void paste plus the excess paste, and the volume of the aggregate are determined to obtain a required packing density of aggregate. In the next step, the quantities of the paste composition, i.e. amount of Portland cement CEM I 52.5, limestone powder, water and superplasticizer, are determined for the required paste consistency. Finally the deformability of the obtained mixture is checked by the slump test (experiment or numerical simulation). Numerical modeling For numerical flow analysis, the particle flow code 2D (PFC2D), ITASCA, is used. This program is based on the principles of a discrete element method (DEM). In this program a material is considered to be built of a finite number of individual elements. The behavior of such a system is described in terms of the movement of elements and the inter-element forces. In this study the correlation between the mix composition and the deformability of the mixtures was studied. The deformability of a mixture is characterized by the spread diameter (D) and the slump value (Hs). The focus is on simulation of rheological behavior of mixtures made with aggregates with a narrow particle size distribution. In the model a mixture is considered as an assembly of a finite number of individual two-phase elements. A two-phase element is defined as an aggregate particle surrounded with an excess paste layer. The force-displacement relation, which is adopted between the elements, is defined according to the interaction diagram, developed in this thesis. The interaction force is related to the inter-element distance, element size, consistency of the paste and the thickness of the excess paste layer. The approach showed to be promising for predicting the deformability of the mixtures made with aggregates with a narrow particle size distribution (Rmax/Rmin?2.0). The deformability of a mixture made with aggregate with a broad particle size distribution is considered to be the same as that of a mixture made with aggregate with a narrow particle size distribution with the same consistency of the paste and the same volume of the excess paste. This assumption is validated for the mixtures made with the aggregate particles with a shape deviation of about 3 % from the spherical shape, specific density between 2500-2600 kg/m3, maximum size of 8 mm, minimum size of 0.125 mm and the maximum fineness modulus of 5.0. The minimum fineness modulus of granular material is limited to 2.0 and 3.5 for aggregates with a narrow particle size distribution and with a broad particle size distribution, respectively. 0.125 mm is the boundary size between the aggregate particles and the powder particles.Structural EngineeringCivil Engineering and Geoscience
Democratization, economic liberalization, and transformational slump: a cross-sectional analysis of twenty-one postcommunist countries
No full textIn this paper the author provides some empirical evidence to show that those old, new, or restored countries of the former Soviet-dominated bloc which embarked on the early institutional transformation of their political and economic systems in a broad and intensive way experienced more quickly the resumption of economic stability and growth and thus left behind the severe downturn of a transformational slump (that is, economic contraction, hyperinflation, and a highly imbalanced governmental budget). The early postcommunist transformation is examined in terms of a general transformational equation. The author provides a cross-sectional explanation of early differences throughout the twenty-one countries both in the institutional transformation and in the transformational slump. Two complementary LISREL (linear structural equations) models show that a successful early postcommunist transformation means (1) a quick resumption of macroeconomic balance and economic growth resting on (2) genuine democratization and economic liberalization, and (3) higher levels of inherited modernization.
How did a domestic housing slump turn into a global financial crisis
No full textdiscussion and critical comment. References to International Finance Discussion Papers (other than an acknowledgment that the writer has had access to unpublished material) should be cleared with the author or authors. Recent IFDPs are available on the Web at www.federalreserve.gov/pubs/ifdp/. This paper can be downloaded without charge from Social Science Research Network electronic library at www.ssrn.com. How Did a Domestic Housing Slump Turn into a Global Financial Crisis
Thaw slump activity measured using stationary cameras in time-lapse and Structure-from-Motion photogrammetry
No full textThaw slumps are one of the most dynamic features in permafrost terrain. Improved temporal and spatial resolution monitoring of slump activity is required to better characterize their dynamics over the thaw season. We assess how a ground-based stationary camera array in a time-lapse configuration can be integrated with UAV-based surveys and Structure from Motion processing to monitor the activity of thaw slumps at high temporal and spatial resolutions. We successfully constructed point-clouds and digital surface models of the headwall area of a thaw slump at 6-13 day intervals over the summer, significantly improving the decadal to annual temporal resolution of previous studies. The successfully modeled headwall portion of the slump revealed that headwall retreat rates were significantly correlated with mean daily air temperature, thawing degree-days and average net short-wave radiation and suggest a two-phased slump activity. The main challenges were related to strong JPEG image compression, drifting camera clocks and highly dynamic nature of the feature. Combined with annual UAV-based surveys, the proposed methodology can address temporal gaps in our understanding of factors driving thaw slump activity. Such insight could help predict how slumps could modify their behavior under changing climate.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
In control. Methodological and clinical aspects of cerebral autoregulation and haemodynamics
No full textCerebral autoregulation (CA) is a key mechanism to protect the brain against excessive fluctuations in blood pressure (BP) and maintain cerebral blood flow. Analysing the relationship between spontaneous BP and cerebral blood flow velocity (CBFV) using transfer function analysis is a widely used technique to quantify CA in a non-invasive way. The objective of this review was to provide an overview of transfer function techniques used in the assessment of CA. For this review, 113 publications were included. This literature showed that there is no gold standard for the execution and implementation of the transfer function. There is a high diversity in settings and criteria used for transfer function analysis. Notable is also the high number of studies which report little on the settings. This disparity makes it difficult to replicate or compare the results of the different studies and further hinders the opportunity to make a distinction between intact and impaired CA in different patient groups. More research on the effects of different implementation techniques on outcomes for CA and optimization of the transfer function analysis is urgently needed. Furthermore, the results of this review show that international guidelines should be created to inform the minimal description of the applied technique and the interpretation of transfer function outcomes in scientific research
On the Determination of the Number and Multiplicity of Zeros of a Function
No full textIt is shown that certain simple integrals determine the number of zeros with a certain multiplicity of a function of one variable in an arbitrary interval. Several typical numerical examples are given.
On the Calculation of the Exact Number of Zeroes of a Set of Equations
No full textThe number of simple zeroes common to a set of nonlinear equations is calculated exactly and analytically in terms of an integral taken over the boundary of the domain of interest. The integrand consists only of simple algebraic quantities containing the functions involved as well as their derivatives up to second order. The numerical feasibility is shown by some computed examples.
Extracting cyclostationary features from single carrier signals
No full textThis paper contributes to the discussion about the usefulness of cyclostationary feature detection for the purpose of cognitive radio. From a simple but realistic radio signal model and an ideal channel, the power spectral density of the random signal component is derived, and compared with the periodical component that can be retrieved from the signal with a nonlinear operation
On the theory of the reconstruction of a weak phase-amplitudeobject from its images, especially in electron microscopy: III. Low-dose imaging using oblique illumination
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