• Computers and Concrete
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• 제24권4호
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• pp.379-397
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• 2019

Steel-concrete composite beams are widely employed in constructions and their performance at the serviceability stage is of concern among practitioners and design regulations. In this context, an accurate evaluation of long-term deflections via various rheological concrete models is needed. In this work, the performance and predict capability of some concrete creep and shrinkage models ACI, CEB, B3, FIB and GL2000 are ascertained, and compared by using statistical bias indicators. Ten steel-concrete composite beams with existing experimental and numerical results are then modeled for this purpose. The proposed modeling technique uses the finite element method, where the concrete slab and steel beam are modeled with shell finite elements. Concrete is considered as an aging viscoelastic material and cracking is treated with the common smeared approach. The results show that when the experimental ultimate shrinkage strain is used for calibration, all studied rheological models predict nearly similar deflections, which agree with the experimental data. In contrast, significance differences are encountered for some models, when none calibration is made prior to. A value between twenty and thirty times the cracking strain is recommended for the ultimate tensile strain in the tension stiffening model. Also, increasing the relative humidity and decreasing the ambient temperature can lead to a substantial reduction of slab cracking for beams under negative flexure. Finally, there is not a unique rheological model that clearly excels in all scenarios.

• Nuclear Engineering and Technology
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• 제47권7호
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• pp.875-883
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• 2015

The use of subcritical aqueous homogenous reactors driven by accelerators presents an attractive alternative for producing $^{99}Mo$. In this method, the medical isotope production system itself is used to extract $^{99}Mo$ or other radioisotopes so that there is no need to irradiate common targets. In addition, it can operate at much lower power compared to a traditional reactor to produce the same amount of $^{99}Mo$ by irradiating targets. In this study, the neutronic performance and $^{99}Mo$, $^{89}Sr$, and $^{131}I$ production capacity of a subcritical aqueous homogenous reactor fueled with low-enriched uranyl nitrate was evaluated using the MCNPX code. A proton accelerator with a maximum 30-MeV accelerating power was used to run the subcritical core. The computational results indicate a good potential for the modeled system to produce the radioisotopes under completely safe conditions because of the high negative reactivity coefficients of the modeled core. The results show that application of an optimized beam window material can increase the fission power of the aqueous nitrate fuel up to 80%. This accelerator-based procedure using low enriched uranium nitrate fuel to produce radioisotopes presents a potentially competitive alternative in comparison with the reactor-based or other accelerator-based methods. This system produces ~1,500 Ci/wk (~325 6-day Ci) of $^{99}Mo$ at the end of a cycle.

• Nuclear Engineering and Technology
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• 제51권5호
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• pp.1444-1450
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• 2019

The fast-neutron shielding behaviour (FNSB) of two clay-materials (Ball clay and Kaolin)of Southwestern Nigeria ($7.49^{\circ}N$, $4.55^{\circ}E$) have been investigated using effective removal cross section, ${\Sigma}_R(cm^{-1})$, mass removal cross section, ${\Sigma}_{R/{\rho}}(cm^2g^{-1})$ and Mean free path, ${\lambda}$ (cm). These parameters decide neutron shielding behaviour of any material. A computer program - WinNC-Toolkit has been used for computation of these parameters. The toolkit evaluates these parameters by using elemental compositions and densities of samples. The proficiency of WinNC-Toolkit code was probe by using MCNPX and GEANT4 to model fast neutron transmission of the samples under narrow beam geometry, intending to represent the actual experimental setup. Direct calculation of effective removal cross section ($cm^{-1}$) of the samples was also carried out. The results from each of the methods for each types of the studied clay-materials (Ball clay and Kaolin) shows similar trend. The trend might be the fingerprint of water content retained in each of the samples being baked at different temperature. The compositions of each sample have been obtained by Particle-Induced X-ray Emission (PIXE) technique (Tandem Pelletron Accelerator: 1.7 MV, Model 5SDH). The FNSB of the selected clay-materials have been compared with standard concrete. The cognizance of various factors such as availability, thermo-chemical stability and water retaining ability by the clay-samples can be analyzed for efficacy of the material for their FNSB.

• Nuclear Engineering and Technology
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• 제51권3호
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• pp.755-760
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• 2019

Compound samples based on the mineral-like magnesium potassium phosphate matrix $MgKPO_4{\times}6H_2O$ were synthesized by solidification of high level waste surrogate. Phase composition and structure of synthesized samples were studied by XRD and SEM methods. Compressive strength of the compounds is $12{\pm}3MPa$. Coefficient of thermal expansion of the samples in the range $250-550^{\circ}C$ is $(11.6{\pm}0.3){\times}10^{-6}1/^{\circ}C$, and coefficient of thermal conductivity in the range $20-500^{\circ}C$ is $0.5W/(m{\times}K)$. Differential leaching rate of elements from the compound, $g/(cm^2{\times}day)$: $Mg-6.7{\times}10^{-6}$, $K-3.0{\times}10^{-4}$, $P-1.2{\times}10^{-4}$, $^{137}Cs-4.6{\times}10^{-7}$; $^{90}Sr-9.6{\times}10^{-7}$; $^{239}Pu-3.7{\times}10^{-9}$, $^{241}Am-9.6{\times}10^{-10}$. Leaching mechanism of radionuclides from the samples at the first 1-2 weeks of the leaching test is determined by dissolution ($^{137}Cs$), wash off ($^{90}Sr$) or diffusion ($^{239}Pu$ and $^{241}Am$) from the compound surface, and when the tests continue to 90-91 days - by surface layer depletion of compound. Since the composition and physico-chemical properties of the compound after irradiation with an electron beam (absorbed dose of 1 MGy) are constant the radiation resistance of compound was established.

• Structural Engineering and Mechanics
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• 제78권4호
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• pp.485-496
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• 2021

So far analytical methods on collapse assessment of three-dimensional (3-D) steel frames have mainly focused on a single-column-removal scenario. However, the collapse of the Federal Building in the US due to car bomb explosion indicated that the loss of multiple columns may occur in the real structures, wherein the structures are more vulnerable to collapse. Meanwhile, the General Services Administration (GSA) in the US suggested that the removal of side columns of the structure has a great possibility to cause collapse. Therefore, this paper analytically deals with the robustness of 3-D steel frames in a two-side-column-removal (TSCR) scenario. Analytical method is first proposed to determine the collapse resistance of the frame during this column-removal procedure. The reliability of the analytical method is verified by the finite element results. Moreover, a design-based methodology is proposed to quickly assess the robustness of the frame due to a TSCR scenario. It is found the analytical method can reasonably predict the resistance-displacement relationship of the frame in the TSCR scenario, with an error generally less than 10%. The parametric numerical analyses suggest that the slab thickness mainly affects the plastic bearing capacity of the frame. The rebar diameter mainly affects the capacity of the frame at large displacement. However, the steel beam section height affects both the plastic and ultimate bearing capacity of the frame. A case study on a six-storey steel frame shows that the design-based methodology provides a conservative prediction on the robustness of the frame.

• 대한금속재료학회지
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• 제48권2호
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• pp.133-140
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• 2010

10 nm thick Ni layers were deposited on 200 nm $SiO_2/Si$ substrates using an e-beam evaporator. Then, 60 nm or 20 nm thick ${\alpha}$-Si:H layers were grown at low temperature (<$200^{\circ}C$) by a Catalytic-CVD. NiSi layers were already formed instantaneously during Cat-CVD process regardless of the thickness of the $\alpha$-Si. The resulting changes in sheet resistance, microstructure, phase, chemical composition, and surface roughness with the additional rapid thermal annealing up to $500^{\circ}C$ were examined using a four point probe, HRXRD, FE-SEM, TEM, AES, and SPM, respectively. The sheet resistance of the NiSi layer was 12${\Omega}$/□ regardless of the thickness of the ${\alpha}$-Si and kept stable even after the additional annealing process. The thickness of the NiSi layer was 30 nm with excellent uniformity and the surface roughness was maintained under 2 nm after the annealing. Accordingly, our result implies that the low temperature Cat-CVD process with proposed films stack sequence may have more advantages than the conventional CVD process for nano scale NiSi applications.

• 대한금속재료학회지
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• 제49권4호
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• pp.321-328
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• 2011

The 30 nm-thick Ni layers was deposited on a flexible polyimide substrate with an e-beam evaporation. Subsequently, we deposited a Si layer using a catalytic CVD (Cat-CVD) in a hydride amorphous silicon (${\alpha}$-Si:H) process of $T_{s}=180^{\circ}C$ with varying thicknesses of 55, 75, 145, and 220 nm. The sheet resistance, phase, degree of the crystallization, microstructure, composition, and surface roughness were measured by a four-point probe, HRXRD, micro-Raman spectroscopy, FE-SEM, TEM, AES, and SPM. We confirmed that our newly proposed Cat-CVD process simultaneously formed both NiSi and crystallized Si without additional annealing. The NiSi showed low sheet resistance of < $13{\Omega}$□, while carbon (C) diffused from the substrate led the resistance fluctuation with silicon deposition thickness. HRXRD and micro-Raman analysis also supported the existence of NiSi and crystallized (>66%) Si layers. TEM analysis showed uniform NiSi and silicon layers, and the thickness of the NiSi increased as Si deposition time increased. Based on the AES depth profiling, we confirmed that the carbon from the polyimide substrate diffused into the NiSi and Si layers during the Cat-CVD, which caused a pile-up of C at the interface. This carbon diffusion might lessen NiSi formation and increase the resistance of the NiSi.

• Steel and Composite Structures
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• 제43권2호
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• pp.165-183
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• 2022

Most transportation departments have recognized and developed procedures to address the ever-increasing weights of trucks traveling on bridges in a service today. Transportation agencies also recognize the issues with overheight vehicles' collisions with bridges, but few stakeholders have definitive countermeasures. Bridges are becoming more vulnerable to collisions from overheight vehicles. The exact response under lateral impact force is difficult to predict. In this paper, nonlinear impact analysis shows that the degree of deformation recorded through the modeling of the unprotected vehicle-girder model provides realistic results compared to the observation from the US-61 bridge overheight vehicle impact. The predicted displacements are 0.229 m, 0.161 m, and 0.271 m in the girder bottom flange (lateral), bottom flange (vertical), and web (lateral) deformations, respectively, due to a truck traveling at 112.65 km/h. With such large deformations, the integrity of an impacted bridge becomes jeopardized, which in most cases requires closing the bridge for safety reasons and a need for rehabilitation. We proposed different sacrificial cushion systems to dissipate the energy of an overheight vehicle impact. The goal was to design and tune a suitable energy absorbing system that can protect the bridge and possibly reduce stresses in the overheight vehicle, minimizing the consequences of an impact. A material representing a Sorbothane high impact rubber was chosen and modeled in ANSYS. Out of three sacrificial schemes, a sandwich system is the best in protecting both the bridge and the overheight vehicle. The mitigation system reduced the lateral deflection in the bottom flange by 89%. The system decreased the stresses in the bridge girder and the top portion of the vehicle by 82% and 25%, respectively. The results reveal the capability of the proposed sacrificial system as an effective mitigation system.

• 치위생과학회지
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• 제22권3호
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• pp.191-198
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• 2022

Background: The purpose of this study was to evaluate the composition of the crystal phases of various calcium silicate-based materials (CSMs): ProRoot white MTA^Ⓡ (mineral trioxide aggregate) (WMTA), Ortho MTA^Ⓡ (OM), Endocem MTA^Ⓡ (EM), Retro MTA^Ⓡ (RM), Endocem Zr^Ⓡ (EN-Z), Biodentine^TM (BD), EZ-seal^TM (EZ), and OrthoMTA III (OM3). Methods: In a sample holder, 5 g of the powder sample was placed and the top surface of the material was packed flat using a sterilized glass slide. The prepared slides were mounted on an X-ray diffraction (XRD) instrument (D8 Advance; Bruker AXS GmbH, Germany). The X-ray beam 2θ angle range was set at 10~90° and scanned at 1.2° per minute. The Cu X-ray source set to operate at 40 kV and 40 mA in the continuous mode. The peaks in the diffraction pattern of each sample were analyzed using the software Diffrac (version 2.1). Then, the peaks were compared and matched with those of standard materials in the corresponding Powder Diffraction File (PDF-2, JCPDS International Center for Diffraction Data). A powder samples of the materials were analyzed using XRD and the peaks in diffraction pattern were compared to the Powder Diffraction File data. Results: Eight CSMs showed a similar diffraction pattern because their main component was calcium silicate. Eight CSMs showed similar diffraction peaks because calcium silicate was their main component. Two components were observed to have been added as radiopacifiers: bismuth oxide was detected in WMTA, OM, and EM while zirconium oxide was detected in RM, EN-Z, BD, EZ, and OM3. Unusual patterns were detected for the new material, OM3, which had strong peaks at low angles. Conclusion: It was caused by the presence of Brushite, which is believed to have resulted in crystal growth in a particular direction for a specific purpose.

• Structural Engineering and Mechanics
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• 제83권4호
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• pp.415-433
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• 2022

Eighteen (18) (120×300×2200 mm) beams were prepared and tested to evaluate the shear strength of Glass Fiber Reinforced Concrete (GFRC) beams with no shear reinforcement, and evaluate the effectiveness of various innovative strengthening systems to increase the shear capacity of the GFRC beams. The test variables are the amount of discrete glass fiber (0.0, 0.6, and 1.2% by volume of concrete) and the type of longitudinal reinforcement bars (steel or GFRP), the strengthening systems (externally bonded (EB) sheet, side near-surface mounted (SNSM) bars, or the two together), strengthening material (GFRP or steel) links, different configurations of NSM GFRP bars (side bonded links, full wrapped stirrups, side C-shaped stirrups, and side bent bars), link spacing, link inclination angle, and the number of bent bars. The experimental results showed that adding the discrete glass fiber to the concrete by 0.6%, and 1.2% enhanced the shear strength by 18.5% and 28%, respectively in addition to enhancing the ductility. The results testified the efficiency of different strengthening systems, where it is enhanced the shear capacity by a ratio of 28.4% to 120%, and that is a significant improvement. Providing SNSM bent bars with strips as a new strengthening technique exhibited better shear performance in terms of crack propagation, and improved shear capacity and ductility compared to other strengthening techniques. Based on the experimental shear behavior, an analytical study, which allows the estimation of the shear capacity of the strengthened beams, was proposed, the results of the experimental and analytical study were comparable by a ratio of 0.91 to 1.15.