• Proceedings of the Korea Concrete Institute Conference
• /
• 2008.04a
• /
• pp.997-1000
• /
• 2008

Strength development of concrete subjected to cold weather is generally delayed due to its low temperature. In case of soil nailing method, it is necessary to apply the shotcrete. However, the shotcrete placement under low temperature experiences retardation of strength development due to delayed hydration reaction. Therefore, in this paper, the use of fine particle cement which is produced through particle classification in cement manufacturing process, is discussed to enhance the strength development of the shotcrete under low temperature. According to the results, the concrete containing 100% of fine particle cement had excellent strength development even at $-9^{\circ}C$ of temperature and at 5days, it reached design strength with PE film curing. It is thought that more than 70% of fine particle cement can secure required strength of the shotcrete even at low temperature condition.

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

• Journal of the Korean GEO-environmental Society
• /
• v.15 no.9
• /
• pp.19-28
• /
• 2014

Recently, the proportion of long tunnel has increased for efficient use of land due to limited land area, driving convenience and high speed. RMR and Q-System of empirical methods has been mainly used for tunnel support design in domestic. Although shotcrete is the key to NATM tunnel, the related studies are insufficient. So, steel fiber reinforced shotcrete is applied to II~V grade rocks on domestic and foreign applications. And same amount of shotcrete is equally applied to tunnel roof and wall regardless of the applied rock load. Shotcrete is applied excessively rather than the original proposed value of RMR and Q-System. Thus, this study is to reevaluate the application part of plain shotcrete and steel fiber reinforced shotcrete of tunnel in Daebo granite, and to propose the reasonable application method of steel fiber reinforced shotcrete. Field test and numerical back analysis using measurements were performed to verify stability. According to results, if RMR values are the upper class in the III grade, it can be designed in accordance with upper grade. In addition, if rock condition is good as a mountain tunnel in Daebo granite, it can be applied for plain shotcrete to III grade rocks because there is also no stability problems. And although steel fiber reinforced shotcrete is applied only crown of the tunnel in IV grade rocks, it is possible to secure stability for falling rock by key-block.

• Tunnel and Underground Space
• /
• v.18 no.3
• /
• pp.226-238
• /
• 2008

The steel ribs which are used to enhance the supporting capability of the shotcrete are estimated to be very effective, but their characteristics depending on the types of steel support are not well understood enough to be considered in the design stage. This paper describes the behavior of the shotcrete reinforced by various types of steel supports. Through flexural toughness test, major strength parameters such as flexural tensile strength, equivalent flexural tensile strength and residual tensile strength were obtained and used in the numerical analyses. Test results show that steel rebar was not as dependable as H-beam or lattice girder but close examination of the test results revealed that the specimen was failed in shear because of the shorter span than desired. Therefore tests on the properly dimensioned specimens are necessary for valid evaluation of the steel rebar reinforced shotcrete. In the first set of numerical stability analyses, shotcrete and steel supports were modelled separately. Then compared with the second set of analyses in which shotcrete and steel supports were regarded as a composite material. The two results coincided reasonably and this equivalent model turned out to be useful.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.34 no.3
• /
• pp.919-930
• /
• 2014

In this study, the load capacities and behaviors of a shotcrete member with steel supports, as composite member, are investigated numerically by using a fiber section element. The cross section of a shotcrete lining with steel support is divided into a bundle of fibers, which are allocated nonlinear stress-strain relations and used for determining internal forces. To verify the used approach of the finite element method for shotcrete with steel supports, the load-displacement relations of shotcrete lining obtained by numerical analysis are compared with existing experimental results and are analyzed with the stress distribution of the shotcrete and steel support obtained numerically. As a result, it is shown that the proposed approach can predict the load capacities of each material and the overall nonlinear behavior of shotcrete lining with steel supports. The change of location of the neutral axis and the flexural resistance ratio of each material are also derived from the stress distribution of the cross section of the shotcrete lining with steel supports. From the results, it is concluded that the flexural resistance performance of steel support should be considered in shotcrete lining design.

• Journal of the Korean Geotechnical Society
• /
• v.22 no.10
• /
• pp.77-91
• /
• 2006

Domestic practices in shotcrete use have developed in many respects even now, but it still has issues about material, construction, quality standard and so on. In overseas, the construction using high strength shotcrete with $39.2{\sim}58.8 MPa$ of compressive strength is becoming common based on the shotcrete technology of high strength and durability. However, domestic shotcrete design strength is low at around 20.6 MPa of compressive strength and a long term durability is also insufficient. In this paper, field tests using high-quality additives and accelerators were performed to obtain the improvement of shotcrete strength and EFNARC standard was used to evaluate the field test results. In addition, deterioration test combined with the freezing-thawing and carbonation was also performed in order to investigate a long-term durability of high-strength shotcrete. As a result of the field test, the promotion ratio of early strength was $90{\sim}97%$ in case of using alkali-free accelerators. And the compressive strength of the shotcrete using Micro-silica fume was $45.2{\sim}55.8MPa$ and flexible strength was $5.01{\sim}6.66MPa$, so the promotion ratio of strength was $37{\sim}79%$ and $17{\sim}61%$ respectively. The promotion effect of strength by silica fine additives ratio of $7.5{\sim}10%$ for cement mass was much superior to the other cases. It was especially examined that using Micro-silica fume reduced deterioration due to mixed steel fiber and improved a long-term durability of shotcrete.

• Journal of Korean Tunnelling and Underground Space Association
• /
• v.10 no.3
• /
• pp.269-282
• /
• 2008

In general, the effects of steel ribs are not considered in the numerical analysis of tunnel design. However, attempts have been increased recently to consider these effects in the analysis of shallow tunnels in soft ground, based on the fact that the steel ribs embedded in the shotcrete take a role to support some portion of the redistributed load due to excavation. In such analyses, the steel ribs can be considered in four different methods: (1) a conventional method where the steel ribs are not considered, (2) a method using the equivalent composite cross section in which the bending moment of shotcrete is not considered, (3) a method using the equivalent composite cross section in which both the compressive stress and the bending moment for the shotcrete and steel rib are considered, and (4) a method using beam elements for the shotcrete and the steel rib, respectively. These methods are adopted in the numerical analysis using FLAC 2D to investigate stresses of both the shotcrete and the steel rib. The overall results show that the analyses are more practical and economical when the effects of steel rib are considered fer the methods (2), (3), and (4). Since the results of those analyses considering steel rib capacity may be different according to the ground condition, it will be necessary to consider the appropriate method among them in accordance with design conditions.

• Computers and Concrete
• /
• v.33 no.2
• /
• pp.121-136
• /
• 2024

In recent times, there has been a growing need to retrofit and strengthen reinforced concrete (RC) structures that have been damaged. Numerous studies have explored various methods for strengthening RC beams. However, there is a significant dearth of research investigating the utilization of ultra-high-performance concrete (UHPC) for retrofitting damaged RC beams within a concrete frame. This study aims to develop a finite element (FE) model capable of accurately simulating the nonlinear behavior of RC beams and subsequently implementing it in an RC concrete frame. The RC frame is subjected to loading until failure at two distinct degrees, followed by retrofitting and strengthening using Ultra high performance shotcrete (UHPS) through two different methods. The results indicate the successful simulation of the load-displacement curve and crack patterns by the FE model, aligning well with experimental observations. Novel techniques for reinforcing deteriorated concrete frame structures through ABAQUS are introduced. The second strengthening method notably improves both the load-carrying capacity and initial stiffness of the load-displacement curve. By incorporating embedded rebars in the frame's columns, the beam's load-carrying capacity is enhanced by up to 31% compared to cases without embedding. These findings indicate the potential for improving the design of strengthening methods for damaged RC beams and utilizing the FE model to predict the strengthening capacity of UHPS for damaged concrete structures.

• Proceedings of the KSR Conference
• /
• 2004.10a
• /
• pp.1077-1082
• /
• 2004

With increasing the number of tunnel constructions, more reliable analysis methods for tunnel excavation is needed to accomplish technically sound design, and stable and economical constructions. For this purpose, a series of construction procedures, which include excavation and support stages of tunneling, need to be considered. In this study, therefore, rock-support response behavior due to railway tunnel construction has been examined by using analytic methods and numerical calculations. For examining rock-support response behavior, the effects of shotcrete, thickness and time of installation have been considered. Through analytic and numerical calculations, it is shown that support pressure becomes higher with increasing the shotcrete thickness and stiffness, and hence the tunnel deformation tends to be stable. It is also important to notice that there is a significant effect of shotcrete installation time on the tunnel deformation, although no significant change in support pressure is observed.

• Proceedings of the Korean Society for Rock Mechanics Conference
• /
• 1995.03a
• /
• pp.1-26
• /
• 1995

Key aspects of the Norwegian Method of Tunnelling (NMT) are reviewed. These include a predictive method of support design using the six-parameter Q-system of rock mass characterisation. The rock mass rating or Q-value is updated during tunnel driving. The designed tunnel support generally consists of wet process, steel fibre reinforced shotcrete combined with fully grouted, untensioned rock bolts, Even in poor rock conditions S(fr) + B usually acts as the final rock reinforcement and tunnel lining. Since it is a drained lining, it is very economic compared to cast concrete with membranes. Light, free-standing steel liners are used to prevent water affecting the runnel environment. Rock mass conditions, and hence lining design and cost estimation can be assessed by careful use of seismic surveys. Relationships between the P-wave velocity, the rock mass deformation modulus and the Q-value have recently been established, where tunnel depth, rock porosity and the uniaxial compression strength of the rock are important variables. The rock mass modulus estimate, and simple index testing of the joints, provide the key input which joints are discretely represented (either in two dimensions with the UDEC code or in three dimensions with the 3DEC code) is generally favoured compared to continuum analysis. The latter may give a misleading impression of uniformity and deformations tend to be understimated. Q-system NMT designs of S(fr) + B (fibre reinforced shotcrete and bolting) are numerically checked and adjustments made to bolt capacities and shotcrete thickness if overloading is evident around the modelled profile.