Papers by Author: Jong Sung Sim

Abstract: Among various types of fiber reinforced polymer (FRP) materials, sheet-formed materials are commonly used for the strengthening of reinforced concrete structures. Basalt fiber sheet has shown a good durability to a harsh environmental condition comparing to other typical fiber sheets, such as glass and carbon fiber sheets [1]. This study investigates the debonding failure characteristics of the basalt fiber sheet for its application in the field. Fundamental mechanical characteristics were evaluated prior to the bond fracture test. Experimental variables include the number of fiber sheet layers, bonding length and bonding width. Debonding failure mode was typical at the bonded interface and the failure strength increased as the number of layer increased. From the results, the effective bond length and effective bond strength were calculated and compared with the literatures. It was also found that the debonding failure strength was a function of the bonding width rather than the bonding length. Based on the test data, an equation to calculate the effective bonding length and strength was suggested.

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.