Kyoung-Kyu Choi

Lattice Shear Reinforcement for Slab-Column Connections

This study develops a new type of shear reinforcement using lattice bars to increase the punching shear strength and ductility of slab-column connections. To study the structural performance of the lattice shear reinforcement, direct punching shear tests for four specimens with the lattice shear reinforcement and four specimens without the lattice shear reinforcement were performed. The test results showed that the punching shear strength and deformation capacity of the specimens with the lattice shear reinforcement were improved up to 1.4 and 9.2 times those of the specimens without the lattice shear reinforcement, respectively. A method for estimating the shear strengths of slab-column connections with the lattice shear reinforcement was developed using these test results. Although more research is needed, the lattice shear reinforcement is expected to be suitable for the earthquake design of flat plate structure.

Flexural Performance Characteristics of Amorphous Steel Fiber-Reinforced Concrete

In this study, the flexural test of amorphous steel fiber-reinforced concrete was performed according to ASTM C 1609 to investigate its flexural performances. The amorphous steel fibers have different configurations from conventional steel fibers : thinner sections and coarser surfaces. Primary test parameters are fiber type (amorphous and conventional steel fibers), concrete compressive strength (27 and 50 MPa), and fiber volume fraction (0.25, 0.50, and 0.75%). Based on the test results, flexural strength and flexural toughness of the amorphous and conventional steel fiber-reinforced concrete were investigated. The results showed that the addition of the amorphous steel fibers into concrete could enhance both flexural strength and toughness while the addition of the conventional steel fibers into concrete was mainly effective to increase the flexural toughness.

Investigations on Flexural Strength and Stiffness of Hollow Slabs

A new type of hollow slab making use of plastic air balls has been developed, which can effectively reduce the amount of concrete and self weight of concrete structures. In this study, flexural and free vibration tests were carried out to investigate flexural behaviour, including the flexural stiffness, cracking, strength, and ductility. Six test specimens were examined: two conventional RC slabs; two hollow slabs; and two partially precast concrete slabs. The test parameters also included two different types of plastic balls. The test results showed that the hollow slabs yield a satisfactory flexural behaviour (i.e., crack pattern, and flexural strength) which is similar to that of conventional concrete solid slabs up to the ultimate state. It was also found that the current design provisions for concrete solid slabs are applicable to hollow slabs, without significant modification, for the evaluation of flexural strength and initial stiffness essential to the serviceability check.

Creep Effects in Plain and Fiber-Reinforced Polymer-Strengthened Reinforced Concrete Beams

This study considers the potential effects of creep on reinforced concrete (RC) beams strengthened with externally applied fiber reinforced polymer (FRP) strips. The significance of creep in the epoxy adhesive and whether such creep might allow the FRP strips to unload over time is assessed. The long-term deflection behavior of two RC beams with similar dimensions and material properties was monitored. One beam was externally strengthened with fiber-reinforced polymer (FRP) strips, while the other was used as a control specimen. Both beams have been subjected to sustained loading for over 6-1/2 years. Slip movements at the ends of the FRP strips were also monitored. The experimental deflections have been compared to deflection predictions using ACI 209R-92 and CEB-FIP MC 90. The creep deformations of the FRP-strengthened beam are not as predicted from the control beam. A step-by-step in-time analysis and finite element modeling are used in the analysis, and both showed good ability to simulate long-term effects in RC beams strengthened with FRP. Results from both techniques show that creep of the adhesive layer can account for the differences observed between the predicted and actual behaviors of the beam, and that this creep should be included when assessing the long-term effects of strengthening a beam with externally applied FRP.

Minimum Thickness Requirements of Flat Plate Affected by Construction Load

During construction of reinforced concrete building, construction load two times as much as the self weight of a slab, is imposed on the slab, and strength and stiffness of the early-age concrete are not fully developed. As the result, the construction load frequently causes excessive deflection and cracking in the flat plate. The minimum thickness of flat plate specified by the current design codes does not properly address such effect of the construction load. In the present study, a simplified method was developed to calculate the deflection of flat plate affected by the construction load. The proposed method can consider the effects of a variety of design parameters such as the aspect ratio of plate, boundary condition, concrete strength, and construction load. A design equation for the minimum thickness was developed based on the proposed method.

Rheological modeling and finite element simulation of epoxy adhesive creep in FRP-strengthened RC beams

We have shown that a significant creep occurs at the concrete–fiber reinforced polymer (FRP) interface based on double shear long-term test. The primary test parameters were the shear stress to ultimate shear strength ratio, the epoxy curing time before loading as well as the epoxy thickness. The test results showed that when the epoxy curing time before loading was earlier than seven days the shear stress level significantly affected the long-term behavior of epoxy at the interfaces, and in particular the combined effect of high shear stress and thick epoxy adhesive can result in interfacial failure if subjected to high-sustained stresses. In this paper, based on the previous experimental observations, an improved rheological model was developed to simulate the long-term behavior of epoxy adhesive at the concrete–FRP interfaces. Furthermore, the newly developed rheological creep model was incorporated in finite element (FE) modeling of a reinforced concrete (RC) beam strengthened with FRP sheets. The use of rheological model in FE setting provides the opportunity to conduct a parametric investigation on the behavior of RC beams strengthened with FRP. It is demonstrated that creep of epoxy at the concrete–FRP interfaces increases the beam deflection. It is also shown that consideration of creep of epoxy is essential if part or the entire load supported by FRP is to be sustained.

Strength Model for Punching Shear of Flat Plate-Column Connections

A number of experiments were performed to investigate the punching shear strength of flat plate-column connections. According to the experiments, the punching shear strength varies significantly with design parameters such as the column size of the connection, reinforcement ratio, and boundary condition. However, current design methods do not properly address the effects of such design parameters. In the present study, a theoratical approach using Rankines failure cirterion was attempted to define the failure mechanism of the punching shear According to the study, the failure mechanism can be classified into the compression-controlled and the tension-controlled, depending on the amount of bottom re-bars placed at the connection, and the punching shear strength is also significantly affected by the flexural damage of slab. Based on the finding, a new strength model of punching shear was developed, and verified by the comparisons with existing experiments and nonlinear finite element analyses. The comparisons show that the proposed strength model addressing the effects of various design parameters can predict accurately the punching shear strength, compared to the existing strength models.

Crack modeling of steel–carbon hybrid FRCCs

An experimental study to investigate nonlinear material behavior in tension of steel–carbon hybrid fiber reinforced cementitious composites (FRCC) was performed. In the tests, tensile strength, toughness index, and fracture properties, including fracture energy and crack opening or elongation, were measured. Based on the test results, a fictitious crack model for steel–carbon hybrid FRCC was developed. The proposed crack model was verified by comparing its predictions with the test results. Acceptable accuracy in simulating the tensile behavior of steel–carbon hybrid FRCC was obtained. For convenience in design and structure analysis, simplified equations to predict tensile strength and fracture energy in hybrid FRCC were developed.

Shear Strength Model for Interior Flat Plate-Column Connections

An alternative design method for interior flat plate-column connections subjected to punching shear and unbalanced moment was developed. Since the slab-column connections are severely damaged by flexural cracking before punching shear failure, punching shear was assumed to be resisted mainly by the compression zone of the slab critical section. Considering the interaction with the flexural moment of the slab, the punching shear strength of the compression zone was evaluated based on the material failure criteria of concrete subjected to multiple stresses. The punching shear strength was also used to evaluate the unbalanced moment capacity of the slab-column connections. For verification, the proposed strength model was applied to existing test specimens subjected to direct punching shear or combined punching shear and unbalanced moment. The results showed that the proposed method predicted the strengths of the test specimens better than current design methods in ACI 318 and Eurocode 2.

Shear Strength Model for Slab-Column Connections

Dept. of Architecture, Seoul National University, Seoul 151-744, KoreaABSTRACT On the basis of the strain-based shear strength model developed in the previous study, a strength model was devel-oped to predict the direct punching shear capacity and unbalanced moment-carrying capacity of interior and exterior slab-columnconnections. Since the connections are severely damaged by flexural cracking, punching shear was assumed to be resisted mainlyby the compression zone of the slab critical section. Considering the interaction with the compressive normal stress developed bythe flexural moment, the shear strength of the compression zone was derived on the basis of the material failure criteria of concretesubjected to multiple stresses. As a result, shear capacity of the critical section was defined according to the degree of flexural dam-age. Since the exterior slab-column connections have unsymmertical critical sections, the unbalanced moment-carrying capacitywas defined according to the direction of unbalanced moment. The proposed strength model was applied to existing test specimens.The results showed that the proposed method predicted the strengths of the test specimens better than current design methods. Keywords : slab, slab-column connections, eccentric shear, unbalanced moment