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排序方式： 共有654条查询结果，搜索用时 15 毫秒
41.
Although several research studies have been conducted on simply supported concrete elements reinforced with fibre reinforced polymer (FRP) bars, there is little reported work on the behaviour of continuous elements. This paper reports the testing of four continuously supported concrete slabs reinforced with carbon fibre reinforced polymer (CFRP) bars. Different arrangements of CFRP reinforcement at mid-span and over the middle support were considered. Two simply supported concrete slabs reinforced with under and over CFRP reinforcement and a continuous concrete slab reinforced with steel bars were also tested for comparison purposes. All continuous CFRP reinforced concrete slabs exhibited a combined shear–flexure failure mode. It was also shown that increasing the bottom mid-span CFRP reinforcement of continuous slabs is more effective than the top over middle support CFRP reinforcement in improving the load capacity and reducing mid-span deflections. The ACI 440.1R–06 formulas overestimated the experimental moment at failure but better predicted the load capacity of continuous CFRP reinforced concrete slabs tested. The ACI 440.1R–06, ISIS–M03–07 and CSA S806-06 design code equations reasonably predicted the deflections of the CFRP continuously supported slabs having under reinforcement at the bottom layer but underestimated deflections of continuous slabs with over-reinforcement at the bottom layer. 相似文献
42.
The Externally Bonded Reinforcement (EBR) technique using Carbon Fiber-Reinforced Polymers (CFRP) has been commonly used to strengthen concrete structures in flexure. The use of prestressed CFRP material offers several advantages well-reported in the literature. Regardless of such as benefits, several studies on different topics are missing. The present work intends to contribute to the knowledge of two commercially available systems that differ on the type of anchorage: (i) the Mechanical Anchorage (MA), and (ii) the Gradient Anchorage (GA). For that purpose, an experimental program was carried out with twelve slabs monotonically tested under displacement control up to failure by using a four-point bending test configuration. The effect of type of anchorage system (MA and GA), prestrain level (0 and 0.4%), width (50 mm and 80 mm) and thickness (1.2 mm and 1.4 mm) of the CFRP laminate, and the surface preparation (grinded and sandblasted) on the flexural response were the main studied parameters. Better performance was observed for the slabs: (i) with prestressed laminates, (ii) for the MA system, and (iii) with sandblasted surface preparation. 相似文献
43.
《Energy Policy》2013
Ancillary services are required to maintain the unity, stability, and quality of power systems. In Colombia these services are required to operate the national power system and the international connections with the neighboring countries. The system is influenced by factors ranging from system's topology to social and political aspects, such as the large number of terrorist attacks. In light of these particularities, we consider Colombia as a learning lab for ancillary services in the region. Colombia's power system relies on three ancillary services for its operation, namely frequency regulation, voltage control, and blackstart service. From 2010 Special Protection Systems were also added. In this paper we first analyze the technical aspects, operational restrictions, financial management, and the most relevant regulatory conditions of these ancillary services of the SIN. We also take into consideration the main regulatory characteristics and statistical data related to energy exchanges that have taken place between Colombia and Venezuela and Ecuador. Thereafter, we depict the main weaknesses and policy challenges that Colombia must address in order to increase the effectiveness and coverage of ancillary services in both the SIN and in the international interconnections. Finally, we propose new market oriented regulations to encourage investments and new tools for international connections. 相似文献
44.
《Annals of Nuclear Energy》1999,26(12):1083-1096
The growing interest of the scientific and public community in environmental problems has sparked the undertaking of several activities of protection from pollutants, of remediation of contaminated sites and of design of deposits for toxic or radioactive waste. The implementation and management of these activities require the use of mathematical models for predicting the behaviour of the pollutant in the geosphere; of particular interest is the study of the transport of contaminants in the groundwater system, which provides an important pathway of return of the hazard to the biosphere.In this paper a comparison is made between the traditional advection-dispersion model, which is the most widely used model of groundwater contaminant transport, and a stochastic model based on the Kolmogorov-Dmitriev theory of branching processes. The behaviour of the two models is analysed in the case of transport with and without retardation of contaminant in the solid matrix and, in the latter case, both equilibrium and non-equilibrium processes are considered.The results obtained show that for non-reactive contaminant and for reactive contaminant under equilibrium, the Kolmogorov-Dmitriev parameters are related by simple relations with the advection-dispersion parameters. Moreover, the Kolmogorov-Dmitriev model is capable of describing more realistic non-equilibrium phenomena, that the advection-dispersion cannot treat. This capability arises from the fact of treating explicitly the reactions of contaminant interchange between the liquid and solid phases. 相似文献
45.
《Combustion and Flame》1986,64(3):267-284
The dynamic parameters influencing soot formation and destruction in droplet burning are studied through time-resolved photography and sampling. Results show that, except for excessively sooty situations, the instantaneous amount of soot present follows the same trend as the instantaneous flame size, that near-complete oxidation of soot can be achieved by confining it within the regressing, closed flame, and that weak convection promotes soot oxidation while early extinction can lead to substantial soot emission. The effects of blending a sooty component with a nonsooty component of different relative volatility have also been investigated. 相似文献
46.
A wind-tunnel study has been carried out to assess wind pressures acting on parapets, including their top surfaces. Local and area-averaged pressure coefficients were measured on parapets of flat-roof models with a length to height ratio (L/H) of 1:1, 2:1, and 3:1. The results were obtained for full-scale equivalent parapet heights of 1 and 2 m and for wind directions ranging from zero to 315°. The local wind load on the parapet was found to be approximately 30% larger at the windward corner of the building than at the midspan location. Maximum parapet loads for the low building model were approximately 30% larger than those for the cubical model. Parapet height did not significantly affect the peak local load on the parapet except in the corner region, where the inward load (toward the roof) for the 1 m parapet was 25% higher than that for the 2 m parapet. 相似文献
47.
This study conducted an online experiment to examine how persuasion knowledge of online targeted advertising (OTA), coping self-efficacy, and product involvement influenced the perceptions of privacy concerns and ad intrusiveness. These findings indicated that persuasion knowledge increased coping self-efficacy, which, in turn, influenced privacy concerns and perceived ad intrusiveness. Furthermore, the relations between coping self-efficacy and the evaluations of the OTA varied, depending on levels of product involvement. Specifically, when participants had high product involvement, their perceptions of privacy concerns and ad intrusiveness increased as their coping self-efficacy enhanced. However, those relationships did not appear among participants with low levels of product involvement. Theoretical and managerial implications of these findings are discussed later. 相似文献
48.
《International Journal of Hydrogen Energy》2019,44(21):10744-10751
In this study, we report a superior dehydrogenation catalyst for dimethylamine borane, which exhibited one of the best catalytic activities. The newly formed catalyst system contains well dispersed ruthenium-copper nanomaterials on reduced graphene oxide (3.86 ± 0.47 nm), which was prepared by using the ultrasonic double reduction technique. The characterization of monodisperse ruthenium-copper alloy nanoparticles was performed using some advanced analytical methods such as TEM, HRTEM, XPS, Raman spectroscopic analysis. The experiments results revealed that the monodisperse ruthenium-copper alloy catalyst (RuCu@rGO) has one of the highest catalytic activity compared to previous studies, having a high turnover frequency value (256.70 h−1). The detailed kinetic parameters such as activation energy, enthalpy, and entropy values were also calculated for the dehydrogenation of dimethylamine borane at room temperature. Also, the results showed that the monodisperse RuCu@rGO catalyst has high durability and reusability as retained its 81% initial catalytic activity even after 4th runs for the dehydrogenation of dimethylamine borane. 相似文献
49.
《International Journal of Hydrogen Energy》2019,44(57):30014-30023
The conversion of bio-oxygenates into hydrogen (H2) via catalytic steam reforming is a green approach for H2 generation. In the present work, butanol was chosen as renewable feedstock for producing H2. Two catalysts supported on multiwalled carbon nanotubes, Ni/CNT and Co/CNT, were synthesized by the wetness impregnation method and used for butanol reforming. Trials were performed in a fixed-bed reactor in the 623–773 K range using S/C ratio equal to 33.3 mol/mol (here, S/C denotes steam to carbon ratio). The Ni/CNT catalyst exhibited higher reforming activity. The best catalytic performance for Ni/CNT was observed at T = 773 K. At this temperature, high values of butanol conversion (87.3%) and H2 yield (0.75 mol/mol) were observed at W/FA0 = 16.7 g h/mol (here, W is the catalyst mass and FA0 is the molar flow rate of butanol at the inlet). The performance of Ni/CNT catalyst for steam reforming of synthetic bio-butanol was also investigated at T = 773 K and H2 yield of 0.65 mol/mol was achieved. 相似文献
50.
Current service life models for predicting the time to chloride-induced corrosion initiation of steel reinforcement in concrete structures are based on a hypothesized surface chloride concentration, Cs, as a boundary condition. This is either assumed to be constant or varies with time, generally disregarding other factors that influence Cs. For example, Fickian models use a constant Cs and existing time-variant models assume Cs is only a function of time. In this paper, an improved time-variant Cs model is hypothesized using general physical concepts and is then validated by an empirical study. The proposed model, as opposed to the existing time-variant models, not only accounts for the variability of Cs with exposure time but also incorporates the effects of time to exposure and the effects of the concentration of chlorides in the exposure environment. The model assumes that Cs is sigmoidal in shape with an asymptote that is a function of the concentration of chlorides in the environment. The input variables for the proposed model were selected based on best subset sampling analysis on the results of the experimental work to determine the influence of water-to-cement ratio, time to exposure, the concentration of chlorides in the exposure environment, and exposure time on Cs. The accuracy of the proposed model is assessed versus existing time-variant Cs models and the results indicate that the proposed model better predicts the Cs in concrete exposed to chlorides. Moreover, results of service life predictions using the proposed model show that the proposed model yields more accurate results, better protecting owners from financial and engineering risks. 相似文献