• Geomechanics and Engineering
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• v.19 no.5
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• pp.421-434
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• 2019

It is of great significance to study the mechanical properties and failure mechanism of the defected rock for geological engineering. The defected sandstone modeling with power-law distribution of pre-cracks was built in this paper by Particle Flow Code software. Then the mechanical properties of sandstone and the corresponding failure process were meticulously analyzed by changing the power-law index (PLI) and the number of pre-cracks (NPC). The results show that (1) With the increase of the PLI, the proportion of prefabricated long cracks gradually decreases. (2) When the NPC is the same, the uniaxial compressive strength (UCS) of sandstone increases with the PLI; while when the PLI is the same, the UCS decreases with the NPC. (3) The damage model of rock strength is established based on the Mori-Tanaka method, which can be used to better describe the strength evolution of damaged rock. (4) The failure mode of the specimen is closely related to the total length of the pre-crack. As the total length of the pre-crack increases, the failure intensity of the specimen gradually becomes weaker. In addition, for the specimens with the total pre-crack length between 0.2-0.55 m, significant lateral expansion occurred during their failure process. (5) For the specimens with smaller PLI in the pre-peak loading process, the concentration of the force field inside is more serious than that of the specimens with larger PLI.

• Journal of the Korean Geophysical Society
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• v.5 no.3
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• pp.199-209
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• 2002

We have carried out a nondestructive close examination for the purpose of the structural safety diagnosis of the Dabo Pagoda of Bulkuk temple located in Kyungju, Kyungbuk Korea. For estimating the mechanical properties of each rock block of the pagoda, ultrasonic measurements were conducted at 641 points of 255 blocks. The P-wave velocity ranges from 584m/sec through 5,169m/sec, and averages 2,901m/sec Based on this result, the uniaxial compressive strength was estimated to be $93{\sim}1,943kg/cm^2\;with\;396kg/cm^2$ of average, and the index of weathering is $0.07{\sim}0.88$ with 0.43 of average, which means the moderate degree of weathering. The comparison of the rock strength of each block with the overburden acting on the block reveals that the rock blocks related to the structure of the pagoda are relatively sound for uniform stress, but it is highly possible for a concentrated stress to lead to a partial failure. We suggest a monitoring of cracks due to the concentrated stress. The parapets of 1st and 2nd floors composed of small rock pieces are severely weathered. However, this is not directly related to the structural safety of the pagoda.

• Applied Chemistry for Engineering
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• v.30 no.6
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• pp.698-706
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• 2019

Over the last few decades, there have been a lot of efforts to develop soft actuators, which can be external stimuli-responsive and applied to the human body. In order to fabricate medical soft actuators with a dynamic precision control, the 3D crosslinked poly(acrylic acid) (PAAc)/poly(vinyl alcohol) (PVA)/poly(ethylene glycol) (PEG) hydrogels were synthesized in this study by using a radiation technique without noxious chemical additives or initiators. After irradiation, all hydrogels showed high gel fraction over 75% and the ATR-FTIR spectra indicated that PAAc/PVA/PEG hydrogels were successfully synthesized. In addition, the gel fraction, equilibrium water content, and compressive strength were measured to determine the change in physical properties of PAAc/PVA/PEG hydrogels according to the irradiation dose and content ratio of constituents. As the irradiation dose and amount of poly(ethylene glycol) diacrylate (PEGDA) increased, the PAAc/PVA/PEG hydrogels showed a high crosslinking density and mechanical strength. It was also confirmed that PAAc/PVA/PEG hydrogels responded to electrical stimulation even at a low voltage of 3 V. The bending behavior of hydrogels under an electric field can be controlled by changing the crosslinking density, ionic group content, applied voltage, and ionic strength of swelling solution.

• Tunnel and Underground Space
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• v.17 no.5
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• pp.411-427
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• 2007

Reliability in tunnel analysis is necessary to accomplish technically sound design and economical construction. For this, a thorough understanding of the construction procedure including the ground-support interaction has to be obtained. This paper describes a proper modelling technique to simulate the behavior of the steel fiber reinforced shotcrete (SFRS) which maintain the supporting capability in post-failure regime. The additional supporting effect of the steel support was also verified by 3-D analyses and a new load distribution factor were proposed. The use of the plastic moment limit (PML) alone can eliminate the occurrence of the awkwardly high tensile stress in the shotcrete and can successfully model the post-peak ductile behavior of the SFRS. But with this method, moment is limited whenever the stress caused by moment reaches tensile strength of the shotcrete irrespective of the stress by axial force. Therefore, it was necessary to find a more comprehensive method which can reflect the influence of the moment and axial force. This can be accomplished by the proper use of "liner element" which is the built-in model in FLAC. In this model, the peak and residual strength as well as the uniaxial compressive strength of the SFRS can be specified. Analyses were conducted with these two models on the 2-lane road tunnels excavated in class IV and V rock mass and results were compared with the conventional elastic beam model. Results showed that both models can reflect the fracture toughness of the SFRS which could not be accomplished by the elastic beam model.

• Journal of Korean Society of Steel Construction
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• v.9 no.3 s.32
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• pp.421-430
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• 1997

In this study, the static and the fatigue tests were performed with high tension bolted joints, of which the surfaces were spread with inorganic zinc-primer after shot-blast, and milling surface, and steel-natural surface, difference of friction surface condition were examined by comparing the esults of tests. From the result of synthetical investigation of this study. it is proper that using the torque management method in order to introduce design axial force to blots, and the provision of specifications that initial axial forces must be 110% of design axial forces is proper. Decreasing ratio of axial forces to initial force is proportional to common lorgarithms of time progress, it converge constant value after 20 hours, and decreasing ratio is little related to the roughness of friction surface. Sliding coefficient of milling, spreading inorganic zinc-primer, just producting is great in order and sliding forces are dependent on the applied axial forces, but if the applied axial forces are great, sliding coefficient become small by a loss of roughness. So it is confirmed that relation between the applied axial forces and the sliding forces are not proportional linearly. From the result of estimation on fatigue strength, all specimens satisfy the specifications with B-grade and milling surface is lower than the others about 14% in fatigue strength because in milling surface lose the function of friction-types joints at lower number of cycles. From the result of eximination for the distribution area of compressive force, friction area near to inside bolt is wider in the direction of stress than near to outside. It is guessed that this situation occurs because outside bolts firstly change from the friction connection to the bearing connection.

• Journal of the Korean Wood Science and Technology
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• v.39 no.1
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• pp.53-59
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• 2011

A connection was made between a single stretcher and 2 headers with 2 threaded nails (Type-A), and another one between 2 stretchers and 2 headers with 4 threaded nails (Type-B) to use as specimens. Type-C was the stretchers that are connected with 2 threaded nails by half lap joint at end-distance 5D to reinforce Type-B, Type-C1 the stretchers that are connected by half lap joint at end-distance 10D, and Type-C2 with 3 threaded nails at end-distance 10D. Compressive shear strength of Type-C, the supplementation of Type-B, was decreased by 30%, compared with that of Type-B. Those of Type-B and Type-C1 that used longer end-distance than Type-C were about the same, and that of Type-C2 connected with 3 threaded nails was 1.28-times stronger than that of Type-C1. Connection of the retaining wall using existing square timber has a problem between long and short stretchers and 2 headers. So it was investigated in the experiment to replace it. Therefore, if Type-B is replaced with Type-C2 in constructing the retaining wall, the crack and the rupture of timber caused by threaded nail as well as construction period can be reduced, and also it can be expected to increase their own strength.

• Journal of the Korea Concrete Institute
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• v.15 no.2
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• pp.197-208
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• 2003

This paper is to determine the possibility of re-using waste concrete from Incheon city area. The strength test was conducted with five aggregate compounds which was replaced a natural aggregate with recycled aggregate. After checking the physical characteristics of recycled aggregate compounds, the mix design of recycled concrete was conducted. For the relatively comparison between natural and recycled compounds, while the unit aggregate weight was changed, other conditions were fixed. The freezing and thawing test which included fly-ash and super-plastezer were performed to check the durability and workability when recycling waste concrete. In the physical characteristics of recycled aggregate, it was found that the specific gravity of recycled coarse aggregate and recycled fine aggregate satisfied the first grade of recycle specification(KS), and all compounds of recycled aggregate also satisfied the second grade of absorption specification, Especially up to the 50% substitution of recycled aggregate is equal to or a bit lower than that of convention aggregate. In comparison with conventional concrete, the recycled concrete is lower than maximum by 7% in compressive strength decreasing rate after freezing-thawing test. From now, although most of recycled concrete was used to the building lot, subgrade, asphalt admixture, through the result. It was proved that possibility of re-using recycled aggregate as the substructure of bridge, retaining wall, tunnel lining and concrete structure which is not attacked the drying shrinkage severely.

• Journal of the Korea Concrete Institute
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• v.24 no.5
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• pp.585-596
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• 2012

In this study, the flexural characteristics of reinforced polymer concrete T-beams strengthened with GFRP, typically used for bridges and parking structures, are investigated. A method to determine the flexural failure mode of reinforced polymer concrete T-beams comprised of compression failure (CF), tension failure (TF), and fiber sheet failure (FF) for different levels of GFRP strengthening is proposed. Moreover, the present study provides a formula to calculate the design flexural strength for each failure mode. In reinforced polymer concrete T-beams strengthened with GFRP, an ideal failure mode can be achieved when the failure occurs in the following order: 1) yield of steel reinforcement, 2) failure of GFRP, and 3) compression failure of concrete. In the case of FF mode, due to GFRP failure before the polymer concrete crushing in compression region, a concept of equivalent rectangular block based on the ultimate limit state of concrete should not be used. Thus, this study suggests an idealized stress-strain curve for polymer concrete and finds parameters for stress block, ${\alpha}$ and ${\beta}$ based on the strain distribution in polymer concrete. Furthermore, the present study suggests an aspect ratio of 2.5 by examining the compressive stress distribution and design flexural strength characteristics for different aspect ratio of T-beams. This study also provides a design flexural strength formula, and validates its acceptability based on experiment and theoretical analysis.

• Journal of the Korea Concrete Institute
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• v.21 no.5
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• pp.565-572
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• 2009

In this study, corrosion resistance of palm oil fuel ash (POFA) concrete as a blended concrete is evaluated by using electrochemical technique. The POFA is an industrial byproduct obtained from fuel ash after extracting palm oil from palm-tree. In order to obtain basic material characteristics of the POFA concrete, tests on compressive strength, slump, weight loss, bleeding and expansion ratio were carried out the early-aged POFA concrete. On the other hand, durability characteristics, both chloride penetration and carbonation depth test, were also conducted. Finally, corrosion resistance were evaluated by applying electro-chemical artificial crack healing technique, and the tests on the impressed voltage characteristic, galvanic current and linear polarization resistance. From the experimental results, it was found that long-term strength, bleeding, lower slump ratio, expansion ratio, chloride penetration, carbonation and corrosion resistance were improved by using the POFA due to activated pozzolanic reaction. It can be also mentioned that POFA concrete has a potential to be used as a cementitious binder for green-recycling resources.

• Tunnel and Underground Space
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• v.29 no.6
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• pp.532-541
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• 2019

Even though shotcrete is a main support for securing the stability of tunnel, the performance of shotcrete is not properly checked due to various difficulties arisen from the characteristics of materials themselves which constitute shotcrete, such as steel fibers and accelerators, and the on-site quality control. In this study, the actual conditions of shotcrete applied to expressway tunnel construction sites were tried to find out, and then some improvement was tried to derive. For this purpose, the characteristics of steel fibers and accelerators supplied to the expressway construction sites were investigated. Also, shotcrete specimens were prepared at the tunnel sites and performance tests were carried out. For steel fiber, domestic production states were investigated, and carbon content and tensile strength were measured using the steel fibers collected in the construction sites. For three types of accelerators such as aluminate, cement mineral and alkali-free, basic properties and total amount of alkali contents were analyzed. Shotcrete specimens were prepared using on-site shotcrete machine with regard to mix designs and types of accelerators. Using these specimens, uniaxial compression tests and flexural tests were performed. As the results, compressive strength, flexural strength, flexural toughness, and etc. were compared with types of acclerators and mix designs.