• Computers and Concrete
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• v.11 no.2
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• pp.123-148
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• 2013

The nonlinear finite element method with eight noded isoparametric quadrilateral element for concrete and two noded element for reinforcement is used for the prediction of the behavior of reinforcement concrete structures. The disturbed state concept (DSC) including the hierarchical single surface (HISS) plasticity model with associated flow rule with modifications is used to characterize the constitutive behavior of concrete both in compression and in tension which is named DSC/HISS-CT. The HISS model is applied to shows the plastic behavior of concrete, and DSC for microcracking, fracture and softening simulations of concrete. It should be noted that the DSC expresses the behavior of a material element as a mixture of two interacting components and can include both softening and stiffening, while the classical damage approach assumes that cracks (damage) induced in a material treated acts as a void, with no strength. The DSC/HISS-CT is a unified model with different mechanism, which expresses the observed behavior in terms of interacting behavior of components; thus the mechanism in the DSC is much different than that of the damage model, which is based on physical cracks which has no strength and interaction with the undamaged part. This is the first time the DSC/HISS-CT model, with the capacity to account for both compression and tension yields, is applied for concrete materials. The DSC model allows also for the characterization of non-associative behavior through the use of disturbance. Elastic perfectly plastic behavior is assumed for modeling of steel reinforcement. The DSC model is validated at two levels: (1) specimen and (2) practical boundary value problem. For the specimen level, the predictions are obtained by the integration of the incremental constitutive relations. The FE procedure with DSC/HISS-CT model is used to obtain predictions for practical boundary value problems. Based on the comparisons between DSC/HISS-CT predictions, test data and ANSYS software predictions, it is found that the model provides highly satisfactory predictions. The model allows computation of microcracking during deformation leading to the fracture and failure; in the model, the critical disturbance, Dc, identifies fracture and failure.

• Tunnel and Underground Space
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• v.17 no.5
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• pp.350-359
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• 2007

Most large cut slopes of open pit mines, roadways, and railways are steeply inclined and composed with rocks that do not contain soils. However, these rock slopes suffer both weathering and fragmentation. In the case of steep slopes, falling rock and collapse of a slope may often occur due to surface erosion. Cast-in place concrete and rubble work are the most widely used earth structure-based pressure supports that act as restraints against the collapse of the rock slope. In order to overcome the shortcomings of conventional retaining walls, a segmental grid retaining wall is being used with connects precasted segments to construct the wall. In this study, laboratory model test was conducted to estimate deformation behavior of segmental grid retaining wall with configuration of rear strecher, height and inclination of the wall. In order to examine the behavior characteristics of a segmental grid retaining wall, this research analyzes the aspects of spacial displacement through relative displacement according to change in the inclination of the wall. Also, the walls behavior according to the formation and status of the rear stretcher which serves the role of transferring the load from the header and the stretcher which make up the wall, the displacement of backfill materials in the wall, and the location of the maximum load were surveyed and the characteristics of displacement in the segmental grid retaining wall were observed. The test results of the segmental grid retaining wall showed that there was a sudden increase in failure load according to the decrease in the wall's height and the size of the in was greatly decreased. Furthermore, it revealed that with identical inclination and height, the structure of the rear stitcher did not greatly affect the starting point or size of maximum horizontal displacement, but rather had a stronger effect on the inclination of the wall.

• Structural Engineering and Mechanics
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• v.70 no.1
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• pp.67-80
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• 2019

Strengthening with near surface mounted carbon fibre reinforced polymers (NSM-CFRP) is a strengthening technique that have been used for several decades to increase the load carrying capacity of reinforced concrete members. In Iraq, many concrete buildings and bridges were subjected to a wide range of damage as a result of the last war and many other events. Accordingly, there is a progressive increase in the strengthening of concrete structures, bridges in particular, by using CFRP strengthening techniques. Near-surface mounted carbon fibre polymer has been recently proved as a powerful strengthening technique in which the CFRP strips are sufficiently protected against external environmental conditions especially the high-temperature rates in Iraq. However, this technique has not been examined yet under repeated loading conditions such as traffic loads on bridge girders. The main objective of this research was to investigate the effectiveness of NSM-CFRP strips in reinforced concrete beams under repeated loads. Different parameters such as the number of strips, groove size, and two types of bonding materials (epoxy resin and cement-based adhesive) were considered. Fifteen NSM-CFRP strengthened beams were tested under concentrated monotonic and repeated loadings. Three beams were non-strengthened as reference specimens while the remaining were strengthened with NSM-CFRP strips and divided into three groups. Each group comprises two beams tested under monotonic loads and used as control for those tested under repeated loads in the same group. The experimental results are discussed in terms of load-deflection behavior up to failure, ductility factor, cumulative energy absorption, number of cycles to failure, and the mode of failure. The test results proved that strengthening with NSM-CFRP strips increased both the flexural strength and stiffness of the tested beams. An increase in load carrying capacity was obtained in a range of (1.47 to 4.49) times that for the non-strengthened specimens. Also, the increase in total area of CFRPs showed a slight increase in flexural capacity of (1.02) times the value of the control strengthened one tested under repeated loading. Increasing the total area of CFRP strips resulted in a reduction in ductility factor reached to (0.71) while the cumulative energy absorption increased by (1.22) times the values of the strengthened reference specimens tested under repeated loading. Moreover, the replacement of epoxy resin with cement-based adhesive as a bonding material exhibited higher ductility than specimen with epoxy resin tested under monotonic and repeated loading.

• Structural Engineering and Mechanics
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• v.55 no.4
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• pp.857-869
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• 2015

The aluminum 7075-T6 is known as an alloy widely used in aircraft structural applications, which does not exhibit strain rate sensitivity during dynamic compressive tests. Despite mechanical importance of the material, there is not enough attention to determine appropriate sample dimensions such as a sample diameter relative to the device bar diameter and sample length to diameter (L/D) ratio for dynamic tests and how these two parameters can change mechanical behaviors of the sample under dynamic loading condition. In this study, various samples which have different diameters of 31.8, 25.4, 15.9, and 9.5 mm and sample L/D ratios of 2.0, 1.5, 1.0, 0.5, and 0.25 were tested using Split Hopkinson Pressure Bar (SHPB), as this testing device is proper to characterize mechanical behaviors of solid materials at high strain rates. The mechanical behavior of this alloy was examined under ${\sim}200-5,500s^{-1}$ dynamic strain rate. Aluminum samples of 2.0, 1.5 and 1.0 of L/D ratios were well fitted into the stress-strain curve, Madison and Green's diagram, regardless of the sample diameters. Also, the 0.5 and 0.25 L/D ratio samples having the diameter of 31.8 and 25.4 mm followed the stress-strain curve. As results, larger samples (31.8 and 25.4 mm) in diameters followed the stress-strain curve regardless of the L/D ratios, whereas the 0.5 and 0.25 L/D ratios of small diameter sample (15.9 and 9.5 mm) did not follow the stress-strain diagram but significantly deviate from the diagram. Our results indicate that the L/D ratio is important determinant in stress-strain responses under the SHPB test when the sample diameter is small relative to the test bar diameter (31.8 mm), but when sample diameter is close to the bar diameter, L/D ratio does not significantly affect the stress-strain responses. This suggests that the areal mismatch (non-contact area of the testing bar) between the sample and the bar can misrepresent mechanical behaviors of the aluminum 7075-T6 at the dynamic loading condition.

• Journal of the Korean Society for Heat Treatment
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• v.14 no.3
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• pp.160-166
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• 2001

The change of ribbon geometry, microstructure and shape recovery with Mn contents, wheel speed and various annealing temperature have been studied in Fe-X%Mn-5Cr-5Co-4Si (X%=15, 20, 24) shape memory alloy (SMA) ribbons rapidly solidfied by single roll chill-block melt-spinning process. The thickness and width of melt-spun ribbons are reduced, results in refining and uniformalizing grains with increasing wheel speed. In the ribbons melt-spun at a wheel speed of 15m/sec, both ${\varepsilon}$ and ${\alpha}^{\prime}$martensites are formed in ribbon 1 (15.5wt%Mn), while only ${\varepsilon}$ martensite is revealed in ribbon 2 (20.2wt%Mn) and ribbon 3 (23.5wt%Mn). The volume fraction of ${\varepsilon}$ martensite is decreased with increasing Mn contents, and those of ${\varepsilon}$ as well ${\alpha}^{\prime}$martensites are increased due to thermal stress relief and grain growth with increasing annealing temperature. Ms temperatures of the ribbons 1, 2 and 3 are fallen with increasing Mn contents. $M_s$ temperatures of the ribbons 1, 2 and 3 annealed at $300^{\circ}C$ for 3 min are risen abruptly, but are nearly constant even at higher annealing temperature, i.e., 400, 500 and $600^{\circ}C$ for 3 min. Shape recovery of the ribbons 1, 2 and 3 increased 30%, 52% and 69% with Mn contents, respectively. Shape recovery of ribbon 1 (15.5wt%Mn) formed ${\varepsilon}$ and ${\alpha}^{\prime}$martensites decreased because of the presence of ${\alpha}^{\prime}$martensite but those of ribbon 2 (20.2wt%Mn) and ribbon 3 (23.5wt%Mn) formed ${\varepsilon}$ martensite increased with increasing annealing temperature.

• The Journal of Engineering Geology
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• v.15 no.3
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• pp.257-268
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• 2005

In situ rock mass is generally heterogeneous and discontinuous, with varying degrees of strength along the planes of weakness. The planes of weakness such as joints, faults, cracks and bedding planes, control the strength and deformation characteristics of the rock mass. Subsequently, the stability of underground opening depends upon the spatial distribution of discontinuities and their mechanical properties in relation with geometrical shape of openins as well as the mechanical properties of intact rock materials. Understanding the behaviour of a discontinuous rock mass remains a key issue for improving excavation design in hiかy stressed environments. Although recent advances in rock mechanics have provided guidelines for the design of underground opening in isotropic rock mass, prediction and control of deformation in discontinuous rock masses are still unclear. In this study, parametric study was performed to investigate the plastic zone size, stress distribution and deformation behavior around underground opening in a discontinuous rock mass using a continuum joint model. The solutions were obtained by an elasto-plastic finite difference analysis, employing the Mohr-Coulomb failure criteria. Non-associated flow rule and perfectly plastic material behavior are also assumed.

• Journal of the Korea institute for structural maintenance and inspection
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• v.22 no.1
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• pp.167-175
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• 2018

This research deals, The characteristics of mechanics and behavior of the tube structural systems, It has been investigated and considered conventional theory and case models, It has shown the suitability, The best location, And optimal shape of the unit module system, Considered variables materials of stiffness increase and decrease in hybrid tube structural systems this study carried out adapting analysis of statistical concepts. In a concrete way, This study exams the effect of reducing horizontal displacement and the shear lag phenomenon, Also, The purpose of this study is to utilize the basic data on the design and study of future high-rise hybrid structural system using this research. As a result, The framed- tube structural system does not effectively cope with horizontal behavior of high-rise buildings, The results of using varying material tested resistance factors and lateral loads in hybrid tube structural system, When each material is compared Bracing material is identified as a key factor in lateral behavior. In a ratio of material quantity framed-tube structural system, The level of sensitivity affecting the horizontal displacement is greater then the beam's column, In case of braced tube structural system, Braced appeared to be most sensitive in comparison of material quantity ratio in columns and beams.

• Tunnel and Underground Space
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• v.28 no.6
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• pp.584-595
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• 2018

This study conducted direct shear test on about 290 sorts of materials such as sandy soil, clayey soil and gravely soil to present proper standard on shear strength of soil. Shear strength of soil in large scale tends to show that angle of internal friction increases as sand contents grow and it ranges $23.5^{\circ}{\sim}34.9^{\circ}C$ with cohesion of 2.0 kPa~15.7 kPa. Elastic modulus was visibly distinct by load, and which increased approximately 80% as vertical load grows. Angle of internal friction arranging $15.0^{\circ}{\sim}28.6^{\circ}$ on clayey soil decreased as clay contents rises and cohesion increase in regular scale. Elastic modulus tends to increase initial elastic modulus with almost same growing rate. While angle of internal friction on gravely soil indicates $29.9^{\circ}{\sim}36.7^{\circ}$ which hardly shows distinctive features. According to test in detail, cohesion of SW (well-graded sand), SP (poorly-graded sand), SC (clayey sand) and SM (silty sand) indicates value by 94%, 78% and 59% comparing to SC, SW and SP respectively. Angle of internal friction of ML (low-liquid limit silt) and CL (low-liquid limit clay) appears almost same features, and MH (high-liquid limit silt) despite of 88% value of ML. Cohesion among them varies with similar growing rate.

• Journal of the Korea Concrete Institute
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• v.17 no.5 s.89
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• pp.709-716
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• 2005

The objective of this study is to develop a high ductile fiber reinforced mortar, ECC(Engineered Cementitious Composite) with using raw material commercially available in Korea. A single fiber pullout test and a wedge splitting test were employed to measure the bond properties in a matrix and the fracture toughness of mortar matrix respectively, which are used for designing mix proportion suitable for achieving strain-hardening behavior at a composite level. Test results showed that the properties tended to increase with decreasing water-cement ratio. A high ductile fiber reinforced mortar has been developed by employing micromechanics-based design procedure. Micromechanical analysis was initially peformed to properly select water-cement ratio, and then basic mixture proportion range was determined based on workability considerations, including desirable fiber dispersion without segregation. Subsequent direct tensile tests were performed on the composites with W/C's of 47.5% and 60% at 28 days that the fiber reinforced mortar exhibited high ductile uniaxial tension property, represented by a maximum strain capacity of 2.2%, which is around 100 times the strain capacity of normal concrete. Also, compressive tests were performed to examine high ductile fiber reinforced mortar under the compression. The test results showed that the measured value of compressive strength was from 26MPa to 34 MPa which comes under the strength of normal concrete at 28 days.

• Tunnel and Underground Space
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• v.32 no.2
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• pp.160-172
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• 2022

The design regulations for simple explosive storage in Korea only stipulate standards for the materials and thickness of the wall of the structure because the amount of explosives that can be stored is small. There is concern about secondary damage during an internal explosion in a simple storage facility, and it is necessary to reexamine the current standards. The numerical analysis for the TNT 15 kg explosion inside the simple storage was carried out by setting the factors using the robust experimental design method. The displacement of the structure generated under the same time condition was analyzed, and the contribution was evaluated. The contribution of concrete thickness was the highest, and the contribution of concrete strength and rebar arrangement was lower than that of concrete thickness. The reinforcement diameter contributed extremely little to the displacement. The structural standards of the simple storage that are currently applied are insufficient on blast resistance, and it is necessary to present new design standards. Therefore, the design factor to be applied later analysis and actual experiments were taken into consideration. For the design variables, the thickness of the concrete was 15 cm considering the displacement, the concrete strength was selected as general concrete considering the inlet discharge pressure, the factor with the lowest average displacement was selected for the reinforcement arrangement and the diameter of the reinforcement, the factor with the smallest level was selected in consideration of economic feasibility because the difference in displacement was low.