Sungnam Hong

Optimal Methods of RTK-GPS/Accelerometer Integration to Monitor the Displacement of Structures

The accurate measurement of diverse displacements of structures is an important index for the evaluation of a structure’s safety. In this study, a comparative analysis was conducted to determine the integrated RTK-GPS/accelerometer method that can provide the most precise structure displacement measurements. For this purpose, three methods of calculating the dynamic displacements from the acceleration data were comparatively analyzed. In addition, two methods of determining dynamic, static, and quasi-static displacements by integrating the displacements measured from the RTK-GPS system and the accelerometer were also comparatively analyzed. To ensure precise comparison results, a cantilever beam was manufactured onto which diverse types of displacements were generated to evaluate the measurement accuracy by method. Linear variable differential transformer (LVDT) measurements were used as references for the evaluation to ensure accuracy. The study results showed that the most suitable method of measuring the dynamic displacement with the accelerometer was to calculate the displacement by filtering and double-integrating the acceleration data using the FIR band-pass filter. The integration method that uses frequency-based displacement extraction was most appropriate for the integrated RTK-GPS/accelerometer method of comprehensively measuring the dynamic, static, and quasi-static displacements.

Experimental Observation on Bond-Slip Behavior between Concrete and CFRP Plate

This paper discusses the failure mode of reinforced concrete beams strengthened with composite materials based on six experimental set-ups to determine the FRP-to-concrete bond strength. Interfacial bond behavior between concrete and CFRP plates was discussed. Shear test were performed with different concrete compressive strengths (21 MPa and 28 MPa) and different bonding length (100 mm, 150 mm, 200 mm, and 250 mm). Shear test results indicate that the effective bond length (the bond length beyond which the ultimate load does not increase) was estimated as through linear regression analysis. Failure mode of specimens occurred due to debonding between concrete and CFRP plates. Maximum bond stress is calculated as about from the relationships between bond stress and slip. Finally, the interfacial bond-slip model between CFRP plates and concrete, which is governed debonding failure, has been estimated from shear tests. Average bond stress was about , the volume of slip between CFRP plate and concrete was about , and the fracture energy was found to be about .

Uniaxial Bond Stress-Slip Relationship of Reinforcing Bars in Concrete

This paper documents a study carried out on the estimation of the bond stress-slip relationship for reinforced concrete members under axial tension loading. An analytical model is proposed that utilizes the conventional bond stress-slip theories as well as the characteristics of deformed bar and concrete cross-sectional area. An equation for the estimation of the bond stress is formulated as the function of nondimensional factors (e.g., bond stress, slip, etc.). The validity, accuracy, and efficiency of the proposed model are established by comparing the analytical results with the experimental data and the JSCE design codes, as well as the analytical models given by Ikki et al. and Shima. The analytical results presented in this paper indicate that the proposed model can effectively estimate the bond stress-slip relationship of reinforced concrete members under axial tension loading.

Behavior of Concrete Columns Repaired with Polymer Mortar and Epoxy Fiber Panel

Underwater structures are not easy to check for the degree of damage or to repair and strengthen damaged regions. Even during repair and strengthening, quality control is very difficult, because the work is done under water. Moreover, underwater structures severely deteriorate, owing to special environmental conditions. If this deterioration continues, the structures face serious structural problems, because of the corrosion of steel rods and the loss of concrete sections. Repairing or strengthening underwater structures requires effective, economic underwater repair and reinforcement techniques that allow the same working conditions as on the ground while maintaining dry condition for the repair sections. However, systematic studies on the repair and strengthening techniques for underwater structures are insufficient. This study proposes a new repair method for underwater structures, which applies epoxy fiber panel forms and shear connectors. To demonstrate the repair effects, this study compared and evaluated the failure modes and repair effects by the surface condition of repair sections, by applying various repair methods, in consideration of the ground and underwater conditions.

Effect of Intermediate Crack Debonding on the Flexural Strength of CFRP-Strengthened RC Beams

The flexural strength of reinforced concrete (RC) beams strengthened with a carbon-fiber-reinforced polymer plate, which fails by intermediate crack debonding, is evaluated. To consider the effect of debonding at the interface between the concrete and the CFRP plate, due to a flexural crack at the midspan, on the flexural strength of the beams, a strength reduction factor is proposed. This factor is derived from the results of flexural tests by using the model of effective strains and is defined as the ratio of the debonding strain to the ultimate strain of the CFRP plate. The validity, accuracy, and efficiency of the factor is verified by comparing analytical results with experimental data. The results of this study revealed that the strength reduction factor proposed can be used to efficiently assess the flexural strength of CFRP-strengthened RC beams with intermediate crack debonding.

Effect of prestress levels on flexural and debonding behavior of reinforced concrete beams strengthened with prestressed carbon fiber reinforced polymer plates

This study is focused on the effect of the prestress level of fiber-reinforced polymer reinforcement on the flexural and debonding behavior of strengthened beams. A total of seven reinforced concrete beams were used in the bending test. A carbon fiber reinforced polymer plate was used as the fiber-reinforced polymer reinforcement. Test variables included various prestress levels and the use of anchorage systems. The specimens were comprised of one control beam, one bonded non-prestressed carbon fiber reinforced polymer-strengthened beam, and five bonded prestressed carbon fiber reinforced polymer-strengthened beams. The deflection-controlled three-point bending test was conducted on all beams until failure. The failure of the beam with an externally bonded carbon fiber reinforced polymer plate was attributed to the debonding of the carbon fiber reinforced polymer plate. The failure mode of beams with bonded prestressed carbon fiber reinforced polymer plates was a two-stage debonding, which was followed by the rupture of the carbon fiber reinforced polymer plates; thus, the behavior of the prestressed beams changed to the unbonded state because of the anchorage system. The cracking, yield, and debonding loads of prestressed beams increased as the prestress level of the carbon fiber reinforced polymer plate increased. However, the effect on the deflection corresponding to these loads was not significant. Also, the ultimate load was constant regardless of the prestress level, except for the case of the 70% prestressed beam, but the effect on ultimate deflection was significant. The recommended appropriate prestress level in securing the ductility of a strengthened beam was 40% or less of the tensile strength of the carbon fiber reinforced polymer plate.

New Analytical Approach of Flexural Crack Width Estimation for Reinforced Concrete Beams

This paper deals with the prediction of crack width for reinforced concrete structural members under pure bending. An analytical method is proposed to estimate the flexural crack width for reinforced concrete members. The proposed method utilizes conventional crack theory. Also, to consider stress conditions in tensile zone reinforced concrete members, a bond-slip relationship developed from axial tension tests was used. An analytical equation for the estimation of maximum crack width is formulated as a function of maximum bond stress. The validity, accuracy and efficiency of the proposed method are obtained by comparing the analytical results with the experimental data, and major design codes, as well as a number of analytical solutions. The analytical results presented in this paper indicate that the proposed method can effectively estimate the flexural crack width of reinforced concrete members under bending.

RC Slabs Repaired and Strengthened by Alumina/Polymer Mortar and Prestressing Strands in the Tension Zone: Experimental Investigation Under Static and Fatigue Loadings

While the extent of repair and rehabilitation of existing old concrete structures is rapidly increasing, a vast number of repaired and rehabilitated structures do not function properly during their remaining service life. Especially in the case of using heterogeneous repair materials, it is very important to maintain the bonding performance between materials and to prevent the interface failure under static and fatigue loads. This paper focuses on the experimental investigation of reinforced concrete (RC) slabs, repaired and reinforced with an alumina/polymer (AP) mortar and a prestressing (PS) strand in the tension zone, under static and fatigue loadings. The variables in this experimental study were the space of strengthening, the number of strands, and AP mortar thickness. Attention is concentrated on the overall load-carrying capacity, deflection, strains of reinforcing bars, and the efficiency of repaired and reinforced RC slabs. Test results showed that the deflection of the repaired and reinforced RC slabs was approximately 40% lower than that of control RC slabs. The initial and horizontal cracking loads of a RC slab with an AP mortar of thickness 30 mm in the static test were approximately the same as those of a RC slab with a 20-mm-thick one. In the fatigue test, the deflection, the strain of reinforcing bars at the midspan, and the maximum shear stress of the repaired and strengthened RC slab were about 40~70% lower than those of the control RC slab. Therefore, it can be concluded that RC slabs with an AP mortar and PS strands have a good strengthening efficiency under both static and fatigue loadings, thanks to the high bonding capacity of the AP mortar.

An Experimantal Study on Flexural Behavior of RC Beams Strengthened with Near Surface Mounted Prestressed FRP

Strengthening concrete structures with FRP (fiber reinforced polymer) have grown to be a widely used method over most parts of the world today, which FRP was developed in 1960s. A method to apply prestressing force to FRP is developed newly in these days, which can use the maximum performance of FRP materials. This study investigated the flexural behavior of simply supported Reinforced Concrete( R/C) beams strengthened with Prestressed Near Surface Mounted (NSM) CFRP (carbon fiber reinforced polymer) . CFRP plate and rod were used for flexural strengthening. Prestressing level changed from 0 % of CFRP tensile strength to 50 %. Any mechanical device has not been used to maintain the prestress during testing. Static four point loading tests are conducted for eleven R/C beams strengthened with Prestressed Near Surface Mounted (NSM) CFRP and Non-prestressed NSM) CFRP. The test shows that the beams with prestressed NSM CFRP exhibited a higher yielding load and a higher ultimate load, compared to the beams with non-prestressed NSM CFRP and the control beams. Beams strengthened by CFRP rod failed due to fiber rupture of the FRP in the groove, but beams strengthened by CFRP plate failed due to concrete cover separation.

Behavior of Concrete Beams with Peel-Plied Aramid-Fiber-Reinforced Polymer Plates

The effect of fiber-reinforced polymer (FRP) plates, to which a peel-ply was fastened to increase their bonding area, on the behavior of strengthened concrete beams was investigated. A total of six concrete beams were tested. For the FRP plates, aramid-fiber-reinforced polymer (AFRP) ones were used. The test variables included their surface treatment (smooth and deformed), the depth of removal of concrete cover (0 and 10 mm), and the number of the plates. Each beam was tested in four-point bending under displacement control up to failure. Based on the experimental results obtained, the effect of the peel-plied AFRP plates on the flexural behavior of the concrete beams was evaluated.