Diffusion-controlled and creep-mitigated ASR damage via microplane model: II. Material degra- dation, drying, and verification

The theory for the material and structural damage due to the alkali-silica reaction (ASR) in concrete is calibrated and validated by finite element fitting of the main test results from the literature. The fracture mechanics aspects, and particularly the localization limiter, are handled by the crack band model. It is shown that the theory can capture the following features quite well: (1) the effects of various loading conditions and stress states on the ASR-induced expansion and its direction; (2) degradation of the mechanical properties of concrete, par- ticularly its tensile and compressive strength, and elastic modulus; (3) the effect of temperature on ASR-induced expansion; and (4) the effect of drying on the ASR, with or without simultaneous temperature effect. The finite element simulations use microplane model M7. The aging creep, embedded in M7, is found to mitigate the predicted ASR damage significantly. The crack band model is used to handle quasi-brittle fracture mechanics and serve as the localization limiter. The moisture diffusivity, both the global one for external drying and the local one for mortar near the aggregate, decreases by one to two orders of magnitude as the pore humidity drops. The fits of each experimenter’s data use the same material parameters. Close fits are achieved and the model appears ready for predicting the ASR effects in large structures.Peer ReviewedPostprint (published version

Century-long expansion of hydrating cement counteracting concrete shrinkage due to humidity drop from selfdesiccation or external drying

A physically based model for auotgenous shrinkage and swelling of portland cement paste is necessary for computation of long-time hydgrothermal effects in concrete structures. The goal is to propose such a model. As known since 1887, the volume of cement hydration products is slightly smaller than the original volume of cement and water (chemical shrinkage). Nevertheless, this does not imply that the hydration reaction results in contraction of the concrete and cement paste. According to the authors’ recently proposed paradigm, the opposite is true for the entire lifetime of porous cement paste as a whole. The hydration process causes permanent volume expansion of the porous cement paste as a whole, due to the growth of C–S–H shells around anhydrous cement grains which pushes the neighbors apart, while the volume reduction of hydration products contributes to porosity. Additional expansion can happen due to the growth of ettringite and portlandite crystals. On the material scale, the expansion always dominates over the contraction, i.e., the hydration per se is, in the bulk, always and permanently expansive, while the source of all of the observed shrinkage, both autogenous and drying, is the compressive elastic or viscoelastic strain in the solid skeleton caused by a decrease of chemical potential of pore water, along with the associated decrease in pore relative humidity. As a result, the selfdesiccation, shrinkage and swelling can all be predicted from one and the same unified model, in which, furthermore, the low-density and high-density C–S–H are distinguished. A new thermodynamic formulation of unsaturated poromechanics with capillarity and adsorption is presented. The recently formulated local continuum model for calculating the evolution of hydration degree and a new formulation of nonlinear desorption isotherm are important for realistic and efficient finite element analysis of shrinkage and swelling. Comparisons with the existing relevant experimental evidence validate the proposed model

Turbulent drag reduction by rotating rings and wall-distributed actuation

Turbulent channel flows altered by the combination of flush-mounted spinning rings and vertical-velocity opposition control or hydrophobic surfaces are studied through direct numerical simulations. The two types of distributed control are applied over the surface area that is not occupied by the spinning rings. The turbulent mean skin friction is reduced by 20% through the steady rotation of the rings and the effect is enhanced by the distributed controls, reaching a drag reduction of 27%. The numerically computed combined drag reduction is well predicted by an upper bound obtained by a simple idealized model. The turbulence statistics are highly nonuniform along the spanwise direction, but show a weak dependence along the streamwise direction. The wall-shear stress is highly reduced over the central region of the rings. Narrow streamwise-elongated structures forming between adjacent disks offer a detrimental global contribution to the Reynolds stresses, although locally they reduce drag. A spatially dependent form of the Fukagata-Iwamoto-Kasagi identity helps explain the different influence of the two distributed controls, although the global drag-reduction levels are similar. The opposition control is effective in altering the elongated structures between rings, but it does not contribute to enhance the drag reduction in the central-ring region. Hydrophobicity creates a more nonuniform flow and even enhances the intensity of the streamwise structures between rings, but further reduces the wall-shear stress in the central-ring region compared to the ring-only case. The mean wall-slip velocity is the additional beneficial effect offered by the hydrophobic surface between rings

Laminar and turbulent flows over hydrophobic surfaces with shear-dependent slip length

Motivated by extensive discussion in the literature, by experimental evidence and by recent direct numerical simulations, we study flows over hydrophobic surfaces with shear-dependent slip lengths and we report their drag-reduction properties. The laminar channel-flow and pipe-flow solutions are derived and the effects of hydrophobicity are quantified by the decrease of the streamwise pressure gradient for constant mass flow rate and by the increase of the mass flow rate for constant streamwise pressure gradient. The nonlinear Lyapunov stability analysis, first applied to a two-dimensional channel flow by Balogh et al. [“Stability enhancement by boundary control in 2-D channel flow,” IEEE Trans. Autom. Control 46, 1696-1711 (2001)], is employed on the three-dimensional channel flow with walls featuring shear-dependent slip lengths. The feedback law extracted through the stability analysis is recognized for the first time to coincide with the slip-length model used to represent the hydrophobic surfaces, thereby providing a precise physical interpretation for the feedback law advanced by Balogh et al. The theoretical framework by Fukagata et al. [“A theoretical prediction of friction drag reduction in turbulent flow by superhydrophobic surfaces,” Phys. Fluids 18, 051703 (2006)] is employed to model the drag-reduction effect engendered by the shear-dependent slip-length surfaces and the theoretical drag-reduction values are in very good agreement with our direct numerical simulation data. The turbulent drag reduction is measured as a function of the hydrophobic-surface parameters and is found to be a function of the time- and space-averaged slip length, irrespective of the local and instantaneous slip behaviour at the wall. For slip parameters and flow conditions that could be realized in the laboratory, the maximum computed turbulent drag reduction is 50% and the drag reduction effect degrades when slip along the spanwise direction is considered. The power spent by the turbulent flow on the hydrophobic walls is computed for the first time and is found to be a non-negligible portion of the power saved through drag reduction, thereby recognizing the hydrophobic surfaces as a passive-absorbing drag-reduction method. The turbulent flow is further investigated through flow visualizations and statistics of the relevant quantities, such as vorticity and strain rates. When rescaled in drag-reduction viscous units, the streamwise vortices over the hydrophobic surface are strongly altered, while the low-speed streaks maintain their characteristic spanwise spacing. We finally show that the reduction of vortex stretching and enstrophy production is primarily caused by the eigenvectors of the strain rate tensor orienting perpendicularly to the vorticity vector

Role of Nitric Oxide in the Altered Calcium Homeostasis of Platelets from Rats with Biliary Cirrhosis.

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The comparison between two methods of basic life support instruction: Video self-instruction versus traditional method

Introduction: Medical education is changing and evolving. Teachers need to re-evaluate their medical teaching practice to enhance student learning. The data about the ideal training method of Basic Life Support (BLS) is lacking. The goal of this study was to analyse the use and performance of video self-instruction (VSI) method in BLS, in order to develop an efficient BLS training method. Methods: Eighty-one undergraduate medical interns were enrolled in a prospective clinical study in 2011. They were divided into VSI group and traditional group. We provided the first group with a DVD containing a 20-minute training video while the second group took part in a 4-hour training class of BLS. Subjects participated in a pre-test and post-test based on 2010 American Heart Association Resuscitation guideline. Results: The average scores of VSI group and the traditional group before training were 8.85±2.42 and 8.57±2.22 respectively (p=0.592). After training, the average scores of the VSI and the traditional group were 20.24±0.83 and 18.05±1.86 respectively. VSI group achieved slightly better scores compared with the traditional group (p<0.001). Conclusions: Training through VSI achieves more satisfying results than the traditional lecture method. VSI method can be considered a useful technique in undergraduate educational programs. Developing VSI can increase significantly the access to the BLS training. © 2015, Medcom Limited. All rights reserved