• Structural Engineering and Mechanics
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• v.89 no.1
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• pp.47-59
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• 2024

In this study, a new sustainable material was proposed to prepare precast tunnel lining segments (TLS), which were produced using a fiber-reinforced slag-based geopolymer composite. Slag was used as the geopolymer binder. In addition, polypropylene and carbon fibers were added to reinforce TLSs. TLSs were examined in terms of flexural performance, load-deflection response, ductility, toughness, crack characteristics, and tunnel boring machine (TBM) thrust force. Simultaneously, numerical simulation was performed using finite element analysis. The mechanical characteristics of the geopolymer composite with a fiber content of 1% were used. The results demonstrated that the flexural performance and load-deflection response of the precast TLSs were satisfactory. Furthermore, the numerical results were capable of predicting and realistically capturing the structural behavior of precast TLSs. Therefore, fiber-reinforced slag-based geopolymer composites can be applied as precast TLSs.

• Journal of the Korean GEO-environmental Society
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• v.22 no.12
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• pp.15-24
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• 2021

In Korea, which is mostly mountainous, the proportion of tunnel and underground space development are increasing. Although the ground is reinforced by applying the ground improvement method during underground space development, accidents still occur frequently in Korea. In the grouting method, a representative ground reinforcement method, the effect was judged by comparing the total amount of injection material with the amount of injection material used during the actual grouting construction. However, it is difficult to determine whether the ground reinforcement is properly performed during construction or within the target ground. In order to solve this problem, it is necessary to study a new method for quality control during or after construction by measuring electrical resistivity after performing grouting by mixing carbon fiber, which is a conductive material, and microcement, which is a grout material. In this study, as a basic study, a cement specimen mix ed with 0%, 3%, 5%, 7% of carbon fiber was prepared to evaluate the performance of the grout material mixed with carbon fiber, which is a conductive material. The prepared specimens were wet curing for 3 days, 7 days, and 28 days under 99% humidity, and then compression wave velocity and shear wave velocity were measured. As a result of the compression wave velocity and shear wave velocity measurement, it showed a tendency to increase with the increase in the compounding ratio of carbon fibers and the number of days of age, and it was confirmed that the elastic modulus and shear modulus, which are the stiffness of the material, also increased.

• Journal of the Korea Concrete Institute
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• v.21 no.3
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• pp.275-282
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• 2009

The effects of surface treatment method of reinforcing fiber on the bonding strength between carbon fiber reinforced polymer rebar (CFRP rebar) and high strength concrete have been evaluated in this study. The structural PVA fiber is coated with a proprietary hydrophobicoiling agent and crimped type polyolefin based structural synthetic fiber is deformed with a geometrical modification were used for the reinforcing fiber. The compressive tests have been performed to evaluate the strength property of high strength concrete depending on the surface treatment method of fiber. The bonding property between the high strength concrete and the CFRP rebar was evaluated by means of direct bonding test. The test results indicated that the surface treatment method of fiber effect on the bonding behavior of high strength concrete and CFRP rebar. Also, as the development and propagation of splitting cracks were controled by adding fibers into the high strength concrete, the bonding behavior, bond strength and relative bonding strength of CFRP rebar and high strength concrete were significantly improved.

• Journal of the Korean Society of Safety
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• v.17 no.3
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• pp.7-12
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• 2002

The object of this paper is to investigate collapse characteristics of CF/Epoxy(Carbon Fiber/Epoxy resin) composite tubes on the change of interlaminar number and fiber orientation angle of outer and to evaluate reappearance of collapse characteristics on the change of tension strength of fibers under static and impact axial compression loads. When a CF/Epoxy composite tube is mushed, static/impact energy is consumed by friction between the loading plate and the splayed fiends of the tube, by fracture of the fibers, matrix and their interface. In general, CF/Epoxy tube with 6 interlaminar number(C-type) absorbed more energy than other tubes(A, B, D-types). The maximum collapse load seemed to increase as the interlaminar number of such tubes increases. The collapse mode depended upon orientation angle of outer of CF/Epoxy tubes and loading status(static/impact). Typical collapse modes of CF/Epoxy tubes are wedge collapse mode, splaying collapse mode and fragmentation collapse mode. The wedge collapse mode was shorn in case of CF/Epoxy tubes with 0$^{\circ}$ orientation angle of outer under static and impact loadings. The splaying collapse mode was shown in only case of CF/Epoxy tubes with 90$^{\circ}$ orientation angie or outer under static loadings, however in impact tests those were collapsed in fragmentation mode. So that CF/Epoxy tube with 6 interlaminar number and 90$^{\circ}$ outer orientation angle presented to the optimal collapse characteristics.

• Journal of the Korean Society for Nondestructive Testing
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• v.17 no.3
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• pp.151-155
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• 1997

In this research, nondestructive test using a radioisotope, $^{241}Am$ gamma-ray, was accomplished in order to evaluate the fiber volume fraction of the accumulated composite layers such as glass fiber/epoxy and carbon fiber/epoxy. Attenuation coefficients of the fiber and resin were measured respectively by NaI(T1) detector The fibers volume fraction was measured for various thickness of composite layers between 2 and 20mm. Fiber volume fraction of the composite layers were also measured for various amount of fibers. The experimental errors from nondestructive test using gamma-ray were in the range of ${\pm}1{\sim}2.5%$ in comparison with those from observation by optical microscopy. By selecting the optimum energy and activity of radioisotope, this method can provide a new means for the evaluation of the fiber volume fraction.

• Journal of Radiation Industry
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• v.6 no.1
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• pp.55-59
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• 2012

Polyacrylonitrile (PAN)-based carbon fibers have been widely used due to their unique chemical, electrical, and mechanical properties. Electron beam irradiation has been extensively employed as means of altering properties of polymeric materials. Electron beam irradiation can induce chemical reactions in materials without any catalyst. Electron beam irradiation may be useful in accelerating the thermal compression stabilization of PAN nanofibers. To investigate the irradiation effect on PAN fibers, PAN nanofibers were irradiated by electron beam at 1,000~5,000 kGy. Irradiated and non-irradiated PAN nanofibers were heated at 180 and $220^{\circ}C$ without applying pressure for 15 min. Then 1 metric ton has been applied for 5 min. SEM images have been found that the fiber kept its morphological behavior after the hot pressing up to electron beam irradiated 1,000 kGy. DSC thermograms showed that the peak temperatures of the exothermic reactions were found to decrease with increasing electron beam irradiation doses and temperature. FT-IR spectra have been found to decrease $C{\equiv}N$ stretch band with increasing the electron beam irradiation dose. These results indicate that the modification of PAN via reactions such as cyclization is significantly enhanced by electron beam irradiation and thermal compression technique.

• Steel and Composite Structures
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• v.38 no.5
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• pp.477-496
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• 2021

The main objective of this paper is to study vibration of sandwich open cylindrical panel with damaged core and FG face sheets based on three-dimensional theory of elasticity. The structures are made of a damaged isotropic core and two external face sheets. These skins are strengthened at the nanoscale level by randomly oriented Carbon nanotubes (CNTs) and are reinforced at the microscale stage by oriented straight fibers. These reinforcing phases are included in a polymer matrix and a three-phase approach based on the Eshelby-Mori-Tanaka scheme and on the Halpin-Tsai approach, which is developed to compute the overall mechanical properties of the composite material. Three complicated equations of motion for the panel under consideration are semi-analytically solved by using 2-D differential quadrature method. Several parametric analyses are carried out to investigate the mechanical behavior of these multi-layered structures depending on the damage features, through-the-thickness distribution and boundary conditions. It is seen that for the large amount of power-law index "P", increasing this parameter does not have significant effect on the non-dimensional natural frequency parameters of the FG sandwich curved panel. Results indicate that by increasing the value of isotropic damage parameter "D" up to the unity (fully damaged core) the frequency would tend to become zero. One can dictate the fiber variation profile through the radial direction of the sandwich panel via the amount of "P", "b" and "c" parameters. It should be noticed that with increase of volume fraction of fibers, the frequency parameter of the panels does not increase necessarily, so by considering suitable amounts of power-law index "P" and the parameters "b" and "c", one can get dynamic characteristics similar or better than the isotropic limit case for laminated FG curved panels.

• Advances in nano research
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• v.12 no.5
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• pp.515-527
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• 2022

The application of fibers in concrete obviously enhances the properties of concrete, also the application of natural fibers in concrete is raising due to the availability, low cost and environmentally friendly. Besides, predicting the mechanical properties of concrete in general and shear strength in particular is highly significant in concrete mixture with fiber nanocomposite reinforced concrete (FRC) in construction projects. Despite numerous studies in shear strength, determining this strength still needs more investigations. In this research, Adaptive Neuro-Fuzzy Inference System (ANFIS) have been employed to determine the strength of reinforced concrete with fiber. 180 empirical data were gathered from reliable literature to develop the methods. Models were developed, validated and their statistical results were compared through the root mean squared error (RMSE), determination coefficient (R²), mean absolute error (MAE) and Pearson correlation coefficient (r). Comparing the RMSE of PSO (0.8859) and ANFIS (0.6047) have emphasized the significant role of structural parameters on the shear strength of concrete, also effective depth, web width, and a clear depth rate are essential parameters in modeling the shear capacity of FRC. Considering the accuracy of our models in determining the shear strength of FRC, the outcomes have shown that the R² values of PSO (0.7487) was better than ANFIS (2.4048). Thus, in this research, PSO has demonstrated better performance than ANFIS in predicting the shear strength of FRC in case of accuracy and the least error ratio. Thus, PSO could be applied as a proper tool to maximum accuracy predict the shear strength of FRC.

• Composites Research
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• v.12 no.3
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• pp.84-90
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• 1999

High alumina cement(HAC) and polyvinyl alcohol(PVA) based macro-defect-free(MDF) cement composites were reinforced using short carbon fibers, 3mm in length, 1-4% in weight fraction and insoluble polymers such as polyurethane, epoxy, phenol resin, in order to increase mechanical properties and water stability. The specimens were manufactured by the low heat-press(warmpress) method. In addition, the interface and the cross-linking reaction of cement and polymers was also studied by the SEM and TEM. Flexural strength of HAC/PVA based MDF cementitious composites was proportionally decreased with increasing fiber contents due to the undensified structure around fibers. The flexural strength of insoluble polymer added specimen was decreased with increasing fiber contents, while water stability was dramatically improved. Epoxy resin added specimen showed the highest strength with increasing fiber contents, compared with other specimens. The water stability of fiber content 4% added specimen immersed in water presented about 95%, 87% at 3 and 7 days immersed in water, respectively. The interfacial adhesive strength of fiber-matrix was very much improved due to cross linking reaction of polymer and metal ions of cement. Tensile strength of insoluble polymers added composites as linearly increased with increasing the fiber contents. The epoxy resin added specimen also showed highest tensile strength. The 4% fiber added specimen presented 30~80% higher strength than controlled specimen.

• Journal of Environmental Impact Assessment
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• v.29 no.3
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• pp.198-209
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• 2020

In this study, electrospun nanofibers made of PAN (polyacrylonitrile) were activated through a physical method to obtain an optimized pore structure. In particular, to enhance the surface alkalinity, the activated carbon fibers (ANFs) were impregnated with tetraethylenepentamine (TEPA) with the aid of HNO₃. Then, the low level (3,000 ppm) CO₂ adsorption capacity for each ANF sample was evaluated. The specific surface area of ANFs increased from 308.4 ㎡/g to 839.4 ㎡/g and the total pore volume increased from 7.882 ㎤/g to 27.50 ㎤/g. Although the TEPA impregnation reduced the specific surface area and pore volume of the ANFs due to blocking of micropores, the HNO₃ pre-oxidation enhanced the amino groups tethered, increasing the amine content from 6.42% to 17.19%, and finally, increased the adsorption capacity of CO₂. This study showed that the sample 60-ANF-HNO₃-TEPA, which was activated for 60 minutes and was impregnated with HNO₃ and TEPA, had the best adsorption capacity for low level (0.3%) CO₂ (in a binary mixture with N₂).