Papers by Author: Cheol Woo Park

Abstract: Various types and forms of FRP materials have been applied for structural strengthening of reinforced concrete (RC) beams. When CFRP plates are used, however, a premature failure used to occur before strengthening effect appears adequately. This is primarily due to the rip-off of CFRP plate attached on RC beams. Despite of numerous studies on the rip-off failure of externally strengthened RC beams, the failure mechanism is not clearly explained yet. Investigations from the literatures have shown that the rip-off failure is dependant on vertical and shear stresses at the level of main reinforcements in RC beams. This study suggests an analytical model to investigate the ripoff failure load based on the stresses at the level of main reinforcements. The proposed model is relatively simple and produces very comparable results to the test data. Therefore, it is anticipated that the proposed model can be successfully used to provide further information on the rip-off failure mechanisms and its prevention.

Abstract: This study investigates the punching shear strength of concrete bridge decks strengthened with various fiber reinforced polymer (FRP) materials, carbon fiber sheet (CFS), glass fiber sheet (GFS) and carbon fiber grid (CFG). This study performed fatigue loading tests on the strengthened bridge decks with different fatigue loading levels. Based on the experimental results, a damage index was determined considering the damage mechanics and was applied to the plastic punching shear strength model for the evaluation of the punching shear strength with respect to the number of fatigue loading cycles. The developed model seems to successfully estimate the punching shear strength of damaged bridge decks with sufficient reliability. It is anticipated, therefore, that the developed model may help improving the design of strengthening of damaged bridge deck panels.

Abstract: Traditional concrete structures with steel reinforcing bars shall gradually deteriorate owing to external loadings and environmental attacks. Fiber reinforced polymer (FRP) is one of the most attractive alternative material for steel since it provides excellent tensile strength and much higher corrosion resistance as well as lower self-weight. This study utilized a newly developed FRP rebar that uses glass fibers in core and chopped glass fibers to make rips on the surface of rebar. Flexural test was performed on concrete beam specimens reinforced with the developed GFRP rebar at various reinforcement ratios. The stiffness of the beams reinforced with GFRP was lower than those with steel rebar. The ultimate strength, however, was improved by using the GFRP rather than the steel rebar. The rip-shaped surface provided better bonding between the GFRP rods and concrete and no significant slip/debonding was observed. In addition, the load and deflection increased gradually until the complete failure without apparent yielding. The current equations for estimating the ultimate moment was too conservative by underestimating values. However, as the reinforcement ratio increased, the difference calculated values became closer to the measured.

Abstract: Even though the application of fiber reinforced polymer (FRP) as a concrete reinforcement becomes more common with various advantages, one of the inherent shortcomings may include its brittleness and on-site fabrication and handling. Therefore, the shape of FRP products has been limited only to a straight bar or sheet type. This study suggests a new technique to use glass fiber reinforced polymer (GFRP) bars for the shear reinforcement in concrete beams, and investigates its applicability. The developed GFRP stirrup was used in the concrete instead of ordinary steel stirrups. The experimental program herein evaluates the effectiveness of the GFRP stirrups with respect to different shear reinforcing ratios under three different shear span-to-depth testing schemes. At the same shear reinforcing ratio, the ultimate loads of the beams were similar regardless the shear reinforcing materials. Once a major crack occurs in concrete, however, the failure modes seemed to be relatively brittle with GFRP stirrups. From the measured strains on the surface of concrete, the shear stresses sustained by the stirrups were calculated and the efficiency of the GFRP stirrups was shown to be 91% to 106% depending on the shear span-to-depth ratio.

Abstract: Fracture of bonded cement based materials is complicated due to not only bonding itself but also heterogeneous genuine nature of cement-based materials. This study investigates the fracture parameters that obtained from the mechanical fracture tests and the post-peak behavior of bonded cement-based materials. Fracture parameters were analyzed such as the critical stress intensity factor, the critical crack length and the critical crack tip opening displacement. In addition, this study defines a new fracture parameter, the critical crack opening angle, which describes a crack opening resistance. In order to evaluate the fracture energy of quasi-brittle materials, it is typical to use the non-linear elastic fracture mechanics approaches. From the test results, however, it is known that the toughening action at the fracture process zone of the bonded interface has been significantly diminished because of the brittle fracture and the pre-determined weak crack path. Therefore, the post-peak behavior could be successfully estimated by using the suggested model that considered only the elastic deformations.

Abstract: In steel structure systems such as plate girder bridges and framed structures, fatigue damage used to occur at welded areas rather than primary structural members. These damages and behaviors of the welded attachments need to be extensively investigated so that the fatigue design criterion can effectively control the fatigue damage of steel structure systems. This study utilized a full-scale plate girder on which various welding attachments were mounted. The welded attachments investigated herein included flange gussets, web gussets, vertical stiffeners, and cover plates. The fatigue cracks initiated at the longitudinal end of joint area of the weld bead and the parent metal where stress was significantly concentrated. The initiated fatigue cracks developed along the weld path and then, propagated to the parent metal in the direction perpendicular to the principal stress. The fatigue cracks developed even under a compressive stress when a significant residual stress was experienced from the welding. The fatigue strengths of the each welded attachment were evaluated and compared with the current fatigue design specifications in AASHTO [1] and JSSC [2].