Deuck Hang Lee

Revision on Material Strength of Steel Fiber-Reinforced Concrete

Many studies have been performed on steel fiber-reinforced normal/high-strength concrete (SFRC, SFRHC) for years, which is to improve some of the weak material properties of concrete. Most of equations for material strengths of SFRHC, however, were proposed based on relatively limited test results. In this research, therefore, the material test results of SFR(H)C were extensively collected from literature, and material tests have conducted on SFR(H)C; compressive strength tests, splitting tensile tests, and modulus of rupture tests. Based on the extensive test data obtained from previous studies and this research, a database of SFR(H)C material strengths has been established, and improved equations for material strengths of SFR(H)C were also proposed. Test results showed that both the splitting tensile strength and the modulus of rupture of SFR(H)C increased as the volume fraction of steel fiber increased, while the effect of the steel fiber volume fraction on the compressive strength of SFR(H)C were not clearly observed. The proposed equations for the splitting tensile strength and the modulus of rupture of SFR(H)C showed better results than the previous equations examined in this study in terms of not only accuracy but also safety/reliability.

Application of Hydrophilic Silanol-Based Chemical Grout for Strengthening Damaged Reinforced Concrete Flexural Members

In this study, hydrophilic chemical grout using silanol (HCGS) was adopted to overcome the performance limitations of epoxy materials used for strengthening existing buildings and civil engineering structures. The enhanced material performances of HCGS were introduced, and applied to the section enlargement method, which is one of the typical structural strengthening methods used in practice. To evaluate the excellent structural strengthening performance of the HCGS, structural tests were conducted on reinforced concrete beams, and analyses on the flexural behaviors of test specimens were performed by modified partial interaction theory (PIT). In particular, to improve the constructability of the section enlargement method, an advanced strengthening method was proposed, in which the precast panel was directly attached to the bottom of the damaged structural member by HCGS, and the degree of connection of the test specimens, strengthened by the section enlargement method, were quantitatively evaluated by PIT-based analysis.

Shear Behavior Models of Steel Fiber Reinforced Concrete Beams Modifying Softened Truss Model Approaches

Recognizing that steel fibers can supplement the brittle tensile characteristics of concrete, many studies have been conducted on the shear performance of steel fiber reinforced concrete (SFRC) members. However, previous studies were mostly focused on the shear strength and proposed empirical shear strength equations based on their experimental results. Thus, this study attempts to estimate the strains and stresses in steel fibers by considering the detailed characteristics of steel fibers in SFRC members, from which more accurate estimation on the shear behavior and strength of SFRC members is possible, and the failure mode of steel fibers can be also identified. Four shear behavior models for SFRC members have been proposed, which have been modified from the softened truss models for reinforced concrete members, and they can estimate the contribution of steel fibers to the total shear strength of the SFRC member. The performances of all the models proposed in this study were also evaluated by a large number of test results. The contribution of steel fibers to the shear strength varied from 5% to 50% according to their amount, and the most optimized volume fraction of steel fibers was estimated as 1%–1.5%, in terms of shear performance.

Flexural Behavior of Prestressed Steel-Concrete Composite Members with Discontinuous Webs

The corrugated webbed prestressed (CWPS) composite member was developed to improve the efficiency of the prestress introduced into the steel beam, and experimental studies were performed to examine its structural behavior. Additionally, a unified analysis model that can estimate the nonlinear flexural behavior of the composite member and the accordion effect of the corrugated webbed steel beams at the prestressing stage was proposed. As a consecutive experimental series, in this study, the discontinuous-webbed prestressed (DWPS) composite member was developed, and this innovative composite member can reduce the amount of steel materials used, compared with the former CWPS composite member. Flexural tests were carried out to investigate their structural performances, and their behaviors were analyzed in detail by a nonlinear finite element analysis.

Evaluation of Horizontal Shear Strength of Prestressed Hollow-Core Slabs with Cast-in-Place Topping Concrete

Prestressed hollow-core (PHC) slabs are structurally-optimized lightweight precast floor members for long-span concrete structures, which are widely used in construction markets. In Korea, the PHC slabs have been often used with cast-in-place (CIP) topping concrete as a composite slab system. However, the PHC slab members produced by extrusion method use concrete having very low slump, and it is very difficult to make sufficient roughness on the surface as well as to provide shear connectors. In this study, a large number of push-off tests was conducted to evaluate interfacial shear strengths between PHC slabs and CIP topping concrete with the key variable of surface roughness. In addition, the horizontal shear strengths specified in the various design codes were evaluated by comparing to the test results that were collected from literature.

Numerical Investigation of Drilled Shafts near an Embankment Slope under Combined Torque-Lateral-Load Scenario

Generally, cantilevered structure-foundation systems supporting highway signs, signals, and luminaires in the areas exposed to severe wind loadings (e.g., hurricane) have been designed under coupled torsion and lateral load scenario. Especially, mast arm cantilevered structures constructed near or on an embankment slope may have more concerns on the torsional and lateral resistance of the foundation. However, most research works have merely considered drilled shaft foundations under torsion-lateral load case with an embankment in proximity. In this study, a numerical study is performed with different soil layers to: (1) understand the combined torque-lateral load behavior of drilled shafts near an embankment slope; (2) examine the effect of both the torsion and lateral resistances in the proximity of an embankment slope. It was found that torsional stiffness decreases with increase in slope angles. Finally, design criteria (e.g., minimum allowable distance from the embankment, maximum allowable point load near the embankment) of the mast arm assembly and loads are provided.

Experimental Study on Reinforced Concrete Column Incased in Prefabricated Permanent Thin-Walled Steel Form

Conventional construction methods of reinforced concrete (RC) structures generally require a long construction period and high costs due to many on-site temporary form works. In this study, a prefabricated permanent thin-walled steel form integrated with reinforcement cage (PPSFRC) was developed, and it makes for a fast-built construction by reducing the temporary form works. Axial compression tests were conducted on a total of 9 test specimens to investigate the structural performances of the newly developed columns. The proposed column construction method utilized relatively thinner steel plates compared to conventional concrete-filled tube (CFT) columns, but it was designed to have sufficient resistance performances against the lateral pressure of fresh concrete and to prevent the buckling of the thin plates by utilizing the steel angles and channel stiffeners prefabricated in the permanent thin-walled steel form. The experimental results showed that the column specimens fabricated by the PPSFRC method had better local buckling resistance and behaved in a more ductile manner compared to the conventional CFT columns. In addition, the axial strengths of the test specimens were compared with those estimated by design provisions, and the flexural moments induced by initial imperfection or accidental eccentricity of axial loads were also discussed in detail.

Experimental Study on RC Frame Structures with Non-Seismic Details Strengthened by Externally-Anchored Precast Wall-Panel Method (EPWM)

The infill-wall strengthening method has been widely used for the seismic performance enhancement of the conventional reinforced concrete (RC) frame structures with non-seismic detail, which is one of the promising techniques to secure the high resisting capacity against lateral forces induced by earthquake. During the application of the infill-wall strengthening method, however, it often restricts the use of the structure. In addition, it is difficult to cast the connection part between the wall and the frame, and also difficult to ensure the shear resistance performances along the connection. In this study, an advanced strengthening method using the externally-anchored precast wall-panel (EPCW) was proposed to overcome the disadvantages of the conventional infill-wall strengthening method. The one-third scaled four RC frame specimens were fabricated, and the cyclic loading tests were conducted to verify the EPCW strengthening method. The test results showed that the strength, lateral stiffness, energy dissipation capacity of the RC frame structures strengthened by the proposed EPCW method were significantly improved compared to the control test specimen.

An Evaluation on Fire Resistance Performances of CFT Columns According to Conditions Exposed to Fire

CFT 기둥은 강재튜브 내에 콘크리트를 타설한 부재로서, 강재튜브에 의한 구속력으로 인하여 내부 콘크리트의 강도증 진 효과가 발휘되며, 별도의 철근보강 없이도 우수한 구조적 성능을 가지기 때문에 국내-외의 대형 건축물에 널리 사용되 고 있다. CFT 기둥은 강재튜브가 외부로 노출되어 있기 때문 에 필요한 내화성능을 확보하기 위해서는 일반적으로 적절한 내화처리가 필요하다. 그러나, 내화도료 또는 뿜칠 등의 시공 이 CFT기둥의 경제성을 저감시키기 때문에 무피복 CFT기둥 의 적용을 위하여 이에 대한 다양한 연구가 진행되어 왔다 (Lie, and Chabot, 1992; Chabot, and Lie, 1992; Wang, 2002; Kodur, 1999). 국내에서도 Ahn, and Lee, 2014 및 Choi et al., 2014에 의하여 무피복 각형 CFT기둥에 대한 실험적 연구 가 수행되었다. 이때, 모든 실험체는 4면이 모두 화재에 노출 된 조건으로 수행되었으며, 표준화재곡선(ISO 834, 1999; KS Abstract

Consideration on punching shear strength of steel-fiber-reinforced concrete slabs

The flat plate slab system is widely used in construction fields due to its excellent constructability and savings in story height compared to the conventional beam-column moment-resisting system. Many researchers are, however, concerned about the punching shear failure that can happen in a two-way flat plate slab system, for which many shear-strength-enhancement techniques have been suggested. One of the effective alternatives is the application of steel-fiber-reinforced concrete. However, most previous studies on the punching shear strength of steel-fiber-reinforced concrete flat slabs had presented empirical formulas based on experimental results. On the other hand, theoretical models proposed in previous studies are difficult to be applied to practical situations. Therefore, in this study, a punching shear strength model of the steel-fiber-reinforced concrete two-way flat slab is proposed. In this model, the total shear resistance of the steel-fiber-reinforced concrete flat slab is expressed by sum of the shear contribution of steel fibers in the cracked tension region and that of intact concrete in the compression zone. A total of 91 shear test data on steel-fiber-reinforced concrete slab–column connection were compared to the analysis results, and the proposed model provided a good accuracy on estimating the punching shear strength of the steel-fiber-reinforced concrete flat slabs.