• Geomechanics and Engineering
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• v.14 no.2
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• pp.195-202
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• 2018

Many laboratory experiments on crack propagation under uniaxial loading and biaxial loading have been conducted in the past using transparent materials such as resin, polymethyl methacrylate (PMMA), etc. However, propagation behaviors of three-dimensional (3D) cracks in rock or rock-like materials under tri-axial loading are often considerably different. In this study, a series of true tri-axial loading tests on the rock-like material with two semi-ellipse pre-existing cracks were performed in laboratory to investigate the acoustic emission (AE) characteristics and propagation characteristics of 3D crack groups influenced by intermediate principal stress. Compared with previous experiments under uniaxial loading and biaxial loading, the tests under true tri-axial loading showed that shear cracks, anti-wing cracks and secondary cracks were the main failure mechanisms, and the initiation and propagation of tensile cracks were limited. Shear cracks propagated in the direction parallel to pre-existing crack plane. With the increase of intermediate principal stress, the critical stress of crack initiation increased gradually, and secondary shear cracks may no longer coalesce in the rock bridge. Crack aperture decreased with the increase of intermediate principal stress, and the failure is dominated by shear fracturing. There are two stages of fracture development: stable propagation stage and unstable failure stage. The AE events occurred in a zone parallel to pre-existing crack plane, and the AE zone increased gradually with the increase of intermediate principal stress, eventually forming obvious shear rupture planes. This shows that shear cracks initiated and propagated in the pre-existing crack direction, forming a shear rupture plane inside the specimens. The paths of fracturing inside the specimens were observed using the Computerized Tomography (CT) scanning and reconstruction.

• Journal of the Korea institute for structural maintenance and inspection
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• v.9 no.3
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• pp.87-92
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• 2005

The permeability of concrete affects largely on the durability of concrete, therefore it is required that the correct assessment and improvement of permeability. Therefore it is rational method that the permeability of concrete structures is estimated in the common use states under loading than in the early sound conditions. In this study, to improve the permeable efficiency of concrete, some kinds of fiber and resin are mixed in making of concrete specimens. And also, for the reasonable assessment of permeability, after 50% and 70% pre-loadings of its compressive strength were acted on the specimens, the tests were executed. From the results of this study, in the case of 50% pre-loading coefficients of permeability were increased about 1.4times against the nonpre-loading specimens and in the case of 70% pre-loading they were increased about 17.8times. And it turned out that hybrid steel fiber reinforcement is most effective for the improvement of permeable efficiency of concrete.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2004.10a
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• pp.487-490
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• 2004

In this paper, we investigated the characteristics of initiation and propagation behavior for fatigue crack observed by changing various shapes of initial crack and magnitudes of loading in modified compact tension shear(CTS) specimen subjected to shear loading. In the low-loading condition, the secondary fatigue crack was created in the notch root due to friction on the pre-crack face grew to a main crack. In the high-loading condition, fatigue crack under shear loading propagated branching from the pre-crack tip. Influenced by the shear loading condition, fatigue crack propagation retardation appeared in the initial propagation region due to the reduction of crack driving force and friction on crack face. In both cases, however, fatigue cracks grew in tensile mode type. The propagation path of fatigue crack under the Mode II loading was 70 degree angle from the initial crack regardless of its shape and load magnitude.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.302-307
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• 2004

This paper reviewed characteristics of fatigue crack behavior observed by changing various shapes of initial crack and magnitudes of loading in compact tension shear(CTS) specimen subjected to shear loading. In the high-loading condition, fatigue crack under shear loading propagated branching from the pre-crack tip. Meanwhile, the secondary fatigue crack in the low-loading condition which was created in the notch root due to friction on the pre-crack face grew to a main crack. Influenced by the mode II loading condition, fatigue crack propagation retardation appeared in the initial propagation region due to the reduction of crack driving force and friction on crack face. In both cases, however, fatigue cracks grew in tensile mode type. Propagation path of fatigue crack under the shear loading was 70 degree angle from the initial crack regardless of its shape and load magnitude.

• Journal of Korean Tunnelling and Underground Space Association
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• v.9 no.4
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• pp.331-341
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• 2007

Pre-loads could be imposed on the braced wall to prevent the horizontal displacements during the ground excavation adjacent to the existing tunnel. For this purpose, new pre-loading system through which large pre-loads could be applied to the braced wall was used in the model tests. Large scale model tests were performed in the real scale test pit which was 2.0 m in width and 6.0 m in hight and 4.0 m in length. Test ground was constructed by sand. Model tunnel in 1.2 m diameter was constructed before test ground excavation. Test ground was excavated adjacent to existing tunnel and was braced. To investigate the effect of pre-loading, tests without pre-load were also performed. During the ground excavation were the behavior of braced wall, test tunnel, and ground measured. Model tests were also numerically analysed and their results were compared to that of the real scale tests. As a result, it was found that the stability of the existing tunnel was greatly enhanced when the horizontal displacements of braced wall was reduced by applying pre-load larger than the design load.

• Journal of the Architectural Institute of Korea Structure & Construction
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• v.34 no.11
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• pp.19-26
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• 2018

This paper presents the shear strengthening effect of externally post-tensioning (EPT) method using high-strength steel rod in pre-cracked reinforced concrete (RC) beams. Three- and four-point bending tests were performed on a total of 8 specimens by adjusting the strengthening depths in the deviator position of EPT. The effective strengthening depths were 435, 535, and 610 mm. The pre-loading up to about 2/3 of ultimate load capacity measured in unstrengthened RC beam were applied in the beam to be post-tensioned. The EPT method was then applied to the pre-damaged RC beams and re-loading was added until the end of the test. EPT restored deflections of 3 mm or more, which account for about 40% of deflection when the pre-loading was applied. The shear strengthening increases more than 3 times and 36~107% in terms of the stiffness and load-carrying capacity compared to unstrengthening RC beams. The increased load-carrying capacities of the post-tensioned beam with strengthening depths of 435 and 535 mm are almost the same as 36~61%, and those of 610 mm are 84~107%, which shows the greatest shear strengthening effect.

• Journal of Korean Association for Spatial Structures
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• v.7 no.1 s.23
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• pp.79-85
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• 2007

Steel plate bonding method with epoxy is common applied to repair and strengthen RC structures, but Steel is apt to corrode quickly, hard to manufacture and heavy. To overcome these defects, it is carried out research on strengthening RC structures with FRP(Fibre Reinforced Polyrner/Plastic) FRP is generally used in the shape of Plate or Sheet, but it has weak point such as premature failure, difficult work. To cope with these problem, NSMR(Near Surface Mounted Reinforcement) which uses CFRP in the shape of Rod is proposed and carried out active research on strengthening effect of variables such as quantity, anchorage length and space of CFRP-Rod. Strengthening with CFRP-Rod is carried out under loading to some degree in fact, and so the amount of pre-loading is selected as variable in this research. The amount of pre-loading is chosen in proportion to nominal strength of non-strengthened specimen with CFRP-Rod.

• Structural Engineering and Mechanics
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• v.74 no.2
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• pp.283-296
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• 2020

The coupled hydro-mechanical loading conditions commonly occur in the geothermal and petroleum engineering projects, which is significantly important influence on the stability of rock masses. In this article, the influence of flaw inclination angle of fracture behaviors in rock-like materials subjected to both mechanical loads and internal hydraulic pressures is experimentally studied using the 3-D X-ray computed tomography combined with 3-D reconstruction techniques. Triaxial compression experiments under confining pressure of 8.0 MPa are first conducted for intact rock-like specimens using a rock mechanics testing system. Four pre-flawed rock-like specimens containing a single open flaw with different inclination angle under the coupled hydro-mechanical loading conditions are carried out. Then, the broken pre-flawed rock-like specimens are analyzed using a 3-D X-ray computed tomography (CT) scanning system. Subsequently, the internal damage behaviors of failed pre-flawed rock-like specimens are evaluated by the 3-D reconstruction techniques, according to the horizontal and vertical cross-sectional CT images. The present experimental does not only focus on the mechanical responses, but also pays attentions to the internal fracture characteristics of rock-like materials under the coupled hydro-mechanical loading conditions. The conclusion remarks are significant for predicting the rock instability in geothermal and unconventional petroleum engineering.

• Proceedings of the Korean Geotechical Society Conference
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• 2010.09a
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• pp.1110-1116
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• 2010

Void ratios are one of the key parameters for exact calculation of settlement of soft groundse. In the study, shear wave velocities of a soft ground were used to measure the field void ratio using bender elements sensors. The bender-element probes were installed in situ at the depths of 3, 5 and 8m on a pre-loading site near Incheon, Korea. During 90 days after installation, the changes of shear wave velocity and ground surface settlement were measured. The field void ratio was estimated from measured shear wave velocities. The void ratio estimated by the shear wave velocity measured by bender elements agrees well with the measured values in the field.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.05a
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• pp.92-95
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• 2004

The flexural strengthening effect of the RC beam strengthened with CFS under pre-loading condition was studied here. The beams were additionally strengthened at the each end with U type wrapping using the same CFS. Main variables considered were number of CFS plies(1,2) and pre-loading values(30,50,$70\%$ of the yield load of the control beam). The flexural strengthening effect was investigated through comparing the yield load, ultimate load, and ductility index of the specimens.