• Corrosion Science and Technology
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• v.4 no.4
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• pp.147-154
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• 2005

The creep resistance of geogrids is one of the most significant long-term safety characteristics used as the reinforcement in slopes and embankments. The failure of geogrids is defined as creep strain greater than 10%. In this study, the accelerated creep tests were applied to polyester geogrids at various loading levels of 30, 50% of the yield strengths and temperatures using newly designed test equipment. Also, the new test equipment permitted the creep testing at or above glass transition temperature($T_g$) of 75, 80, $85^{\circ}C$. The time-dependent creep behaviors were observed at various temperatures and loading levels. And then the creep curves were shifted and superposed in the time axis by applying time-temperature supposition principles. The shifting factors(AFs) were obtained using WLF equation. In predicting the lifetimes of geogrids, the underlying distribution for failure times were determined based on identification of the failure mechanism. The results confirmed that the failure distribution of geogrids followed Weibull distribution with increasing failure rate and the lifetimes of geogrids were close to 100 years which was required service life in the field with 1.75 of reduction factor of safety. Using the newly designed equipment, the creep test of geogrids was found to be highly accelerated. Furthermore, the time-temperature superposition with the newly designed test equipment was shown to be effective in predicting the lifetimes of geogrids with shorter test times and can be applied to the other geosynthetics.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2002.10b
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• pp.363-364
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• 2002

Although the factors that cause the failure of orthopedic implants were not clearly determined, it was reported that the shapes of wear debris affect the tribological behavior of artificial implant. Many researches were conducted to examine the wear mechanism by debris but the role of debris shape in inflammatory reaction remains unclear. To observe the debris shape by addition of reinforcement, carbon nanotubes ( CNTs ) were added to ultra high molecular weight polyethylene ( UHMWPE ) to investigate the reinforcement effect of CNTs. CNTs which have a diameter of about 10-50 nm, while their length is about 3-5 nm were produced by the catalytic decomposition of the acetylene gas using a tube furnace. Plate on disc type wear test were performed to evaluate the tribological performance of UHMWPE composites reinforced with CNTs in lubricating condition ( bovine serum ). The wear losses of CNT added UHMWPE in bovine serum were significantly reduced. Worn surface and wear debris of UHMWPE with CNTs and without CNTs were compared to investigate the reinforcement effect of CNT on tribological behavior.

• Journal of the Korean Applied Science and Technology
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• v.23 no.4
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• pp.290-299
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• 2006

This article investigated to polymer-clay nanocomposite, especially in interfacial respect clay structure, its dispersion into polymer matrix, and clay modification is studied. The cationic exchange of surfactants with clay gallery results in preparing organo-clay capable of compatiblizing to monomer or polymer and increasing interlayer adhesion energy due to expansion of interlayer spacing. The orientation of surfactant in clay gallery is affected by chemical structure and charge density of clay, and interlayer spacing and volume is increased with alkyl chain length of surfactant, or charge density of clay. Also, the interaction between clay and polymer in preparing polymer-clay nanocomposite is explained thermodynamically. In the future, the study and development of polymer-clay nanocomposite is paid attention to the interfacial adhesion, clay dispersion within polymer, mechanism of clay intercalation or exfoliation.

• Earthquakes and Structures
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• v.17 no.6
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• pp.567-575
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• 2019

In order to investigate the dynamic impact resistance of steel fiber reinforced full light-weight concretes, we implemented drop weight impact test on a total of 6 reinforced beams with 0, 1 and 2%, steel fiber volume fraction. The purpose of this test was to determine the failure modes of beams under different impact energies. Then, we compared and analyzed the time-history curves of impact force, midspan displacement and reinforcement strain. The obtained results indicated that the deformations of samples and their steel fibers were proportional to impact energy, impact force, and impact time. Within reasonable ranges of parameter values, the effects of impact size and impact time were similar for all volumetric contents of steel fibers, but they significantly affected the crack propagation mechanism and damage characteristics of samples. Increase of the volumetric contents of steel fibers not only effectively reduced the midspan displacement and reinforcement strain of concrete samples, but also inhibited crack initiation and propagation such that cracks were concentrated in the midspan areas of beams and the frequency of cracks at supports was reduced. As a result, the tensile strength and impact resistance of full light-weight concrete beams were significantly improved.

• Earthquakes and Structures
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• v.14 no.3
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• pp.261-271
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• 2018

Fiber reinforced cementitious composites (FRCC) materials that exhibit strain-hardening and multiple cracking properties under tension were recently developed as innovative building materials for construction. This study aims at exploring the use of FRCC on the seismic performance of coupling beams with conventional reinforcement. Experimental tests were conducted on seven FRCC precast coupling beams with small span-to-depth ratios and one ordinary concrete coupling beam for comparison. The crack and failure modes of the specimens under the low cycle reversed loading were observed, and the hysteretic characteristics, deformation capacity, energy dissipation capacity and stiffness degradation were also investigated. The results show that the FRCC coupling beams have good ductility and energy dissipation capacities compared with the ordinary concrete coupling beam. As the confinement stirrups and span-to-depth ratio increase, the deformation capacity and energy dissipation capacity of coupling beams can be improved significantly. Finally, based on the experimental analysis and shear mechanism, a formula for the shear capacity of the coupling beams with small span-to-depth ratios was also presented, and the calculated results agreed well with the experimental results.

• Journal of Korean Tunnelling and Underground Space Association
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• v.7 no.1
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• pp.51-61
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• 2005

In general the strength parameter of the ground will be changed by reinforcing the ground around tunnel. In this case, the concept of tunnel design, such as supporting system, excavation, lining and so on, should be modified based on the failure criteria or the ground changed by the reinforcement. This paper presents the variation mechanism of strength parameters and new failure criteria of the reinforced ground. In order to perform this research, theoretical and experimental works were carried out. It was clearly founded that the cohesion of strength parameters is only increased by reinforcement of ground, especially by rock bolting.

• Geomechanics and Engineering
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• v.14 no.1
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• pp.71-81
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• 2018

Soils are mostly nonhomogeneous and anisotropic in nature. In this study, nonhomogeneity and anisotropy of soil are taken into consideration by assuming that the cohesion increases with depth linearly and also varies with respect to direction at a particular point. A three-dimensional rotational failure mechanism is adopted, and then a three-dimensional stability analysis of slope is carried out with the failure surface in the shape of a curvilinear cone in virtue of the limit analysis method. A quasistatic approach is used to develop stability charts in nonhomogeneous and anisotropic soils. One can easily read the safety factors from the charts without the need for iterative procedures for safety factors calculation. The charts are of practical importance to prevent a plane failure in excavation slope whether it is physically constrained or not. Then the most suitable location of piles within the reinforced slope in nonhomogeneous and anisotropic soils is explored, as well as the interactions of nonhomogeneous and anisotropic coefficients on pile reinforcement effects. The results indicate that piles are more effective when they are located between the middle and the crest of the slope, and the nonhomogeneous coefficient as well as the anisotropic coefficient will not only influence the most suitable location for piles but also affect the calculated safety factor of existing reinforced slope. In addition, the two coefficients will interact with each other on the effect on slope reinforcement.

• Composites Research
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• v.27 no.5
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• pp.201-206
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• 2014

Utilizing waste materials in making useful products is a globally increasing trend. This can reduce the cost and environmental issues. In this study, oyster shell powder (OSP) is reinforced as a filler in polypropylene (PP) matrix to make a new composite. The purpose is to improve fire retardant properties of PP as a result of OSP reinforcement. Mechanical, fire retardant and water absorption properties of the new composite have been studied in this research. Concentration of OSP reinforcement in PP has been varied by wt% and its effect on the above mentioned properties has been observed. SEM (scanning electron microscopy) images of tensile and bending fractured surfaces have been taken to observe the failure mechanism during mechanical tests. An increase in the fire retardancy has been observed as a result of the OSP reinforcement while tensile strength decreased. Stiffness also increased with the addition of OSP in PP.

• Structural Engineering and Mechanics
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• v.74 no.2
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• pp.243-254
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• 2020

Devastating RC structural failures in the past have identified that the behavior of beam-column joints is more critical and significantly governs the global structural response under seismic loading. The congestion of reinforcement at the beam-column joints with other constructional difficulties has escalated the attention required for strengthening RC beam-column joints. In this context, numerous studies have been carried out in the past, which mainly focused on jacketing the joints with different materials. However, there is no comparative study of different approaches used to strengthen RC beam-column joints, from efficiency and cost perspective. This paper presents a detailed investigation carried out to study the various strengthening schemes of exterior RC beam-column joints, viz., steel fiber reinforcement, carbon fiber reinforced polymer (CFRP) strengthening, steel haunch strengthening, and confining joint reinforcement. The effectiveness of each scheme was evaluated experimentally. These specimens were tested under horizontal loading that produced opening moments on the joints and their behavior was studied with emphasis on strength, displacement ductility, stiffness, and failure mechanism. Special attention was given to the study of crack-width.

• Earthquakes and Structures
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• v.16 no.3
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• pp.279-293
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• 2019

The overall seismic performance of existing pre 1960-70s reinforced concrete (RC) structures is significantly affected by the inadequate length of columns' lap-spliced reinforcement. Due to this crucial structural deficiency, the cyclic response is dominated by premature bond - slip failure, strength and stiffness degradation, poor energy dissipation capacity and low ductility. Recent earthquakes worldwide highlighted the importance of improving the load transfer mechanism between lap-spliced bars, while it was clearly demonstrated that the failure of lap splices may result in a devastating effect on structural integrity. Extensive experimental and analytical research was carried out herein, to evaluate the effectiveness and reliability of strengthening techniques applied to RC columns with lap-spliced reinforcement and also accurately predict the columns' response during an earthquake. Ten large scale cantilever column subassemblages, representative of columns found in existing pre 1970s RC structures, were constructed and strengthened by steel or RC jacketing. The enhanced specimens were imposed to earthquake-type loading and their lateral response was evaluated with respect to the hysteresis of two original and two control subassemblages. The main variables examined were the lap splice length, the steel jacket width and the amount of additional confinement offered by the jackets. Moreover, an analytical formulation proposed by Tsonos (2007a, 2019) was modified appropriately and applied to the lap splice region, to calculate shear stress developed in the concrete and predict if yielding of reinforcement is achieved. The accuracy of the analytical method was checked against experimental results from both the literature and the experimental work included herein.