Keun-Joo Byun

Finite element failure analysis of reinforced concrete T-girder bridges

Failure behaviors of in-situ deteriorated reinforced concrete T-girder bridges under cyclic loading is experimentally observed and a finite element modeling technique to predict the behaviors is developed. In this study, full-scale destructive tests of in-situ bridges are performed by applying cyclic loads up to failure, and modeling techniques for the non-linear finite element analysis of in-situ deteriorated reinforced concrete T-girder bridge are presented. Two in-situ reinforced concrete T-girder bridges were selected for the failure tests and the analysis, one a symmetrically loaded bridge and the other a non-symmetrically loaded bridge. Path-dependent in-plane constitutive laws of cracked reinforced concrete were utilized for material modeling of the analysis. An RC zoning method was applied to two-dimensional finite element modeling of the symmetrically loaded bridge and a combination of frame elements utilizing the fiber technique and layered shell elements were used for three-dimensional modeling of the non-symmetrically loaded bridge. Experimental results indicate that significant load carrying capacity is retained in old reinforced concrete bridges and analysis results show that the manner of modeling of degraded support conditions significantly affects the predicted responses of the capacity as well as the stiffness of the bridges. This significant effect of support conditions of the deteriorated RC bridges is verified and a simple modeling technique for the support condition is proposed to consider the degradation of supports. By applying the proposed modeling to the boundary condition of the bridges, a finite element failure analysis is carried out for the bridges subjected to cyclic loading. Then, the analytical results are compared with full-scale failure test results. The comparison shows that the finite element analysis technique along with the proposed boundary condition can be effectively applied to the failure analysis of in-situ reinforced concrete T-girder bridges subjected to cyclic loading.

Analytical Evaluations of the Retrofit Performances of Concrete Wall Structures Subjected to Blast Load

In case of retrofitting a concrete structure subjected to blast load by using retrofit materials such as FRP (fiber-reinforced polymer), appropriate ductility as well as raising stiffness must be obtained. But the previous approximate and simplified models, which have been generally used in the design and analysis of structures subjected to blast load, cannot accurately consider effects on retrofit materials. Problems on the accuracy and reliability of analysis results have also been pointed out. In addition, as the response of concrete and reinforcement on dynamic load is different from that on static load, it is not appropriate to use material properties defined in the previous static or quasi-static conditions to in calculating the response on the blast load. In this study, therefore, an accurate HFPB (high fidelity physics based) finite element analysis technique, which includes material models considering strength increase, and strain rate effect on blast load with very fast loading velocity, has been suggested using LS-DYNA, an explicit analysis program. Through the suggested analysis technique, the behavior on the blast load of retrofitted concrete walls using CFRP (carbon fiber-reinforced polymer) and GFRP (glass fiber-reinforced polymer) have been analyzed, and the retrofit capacity analysis has also been carried out by comparing with the analysis results of a wall without retrofit. As a result of the analysis, the retrofit capacity showing an approximate reduction of maximum deflection, according to the retrofit, was confirmed, and it is judged ate suggested analysis technique can be effectively applicable in evaluating effectiveness of retrofit materials and techniques.

Creep prediction of concrete for reactor containment structures

Since the biggest time-dependent prestress loss of a prestressed concrete nuclear reactor containment structure is due to the creep of concrete, creep is one of the most important structural factors to be considered for the safety of a reactor containment structure during design, construction and maintenance. Creep in concrete has also recently been considered in evaluation of the crack resistance of concrete at an early-age in the durability examination of massive concrete structures like reactor containment structures. Existing empirical formulas on creep prediction show errors in their predictions due to simplified consideration of mixture proportions, and they also show large discrepancy among their predictions. In addition, they do not consider early-age behaviors of concrete and thus are mainly for the prediction of long-term creep at hardened concrete. In this paper, the creep characteristics of the reactors both early-age and hardened reactor concrete made of type V cement are examined by carrying out both early-age and long-term creep tests. Then, the creep of the reactor concrete is predicted by using major creep-prediction equations of the AASHTO LRFD design specification, the Japanese standard specification for concrete structure, the ACI Committee 209 and the CEB/FIP model code and the Bazant and Panulas model, and the predicted results are compared with the test results. From the comparison, the applicability of the creep-prediction equations for the concrete of a reactor containment structure at both early-age and hardened stages is discussed.

Bond-Slip Model of Interface between CFRP Sheets and Concrete Beams Strengthened with CFRP

External bonding of carbon fiber reinforced plastic sheets has recently emerged as a popular method for strengthening reinforced concrete structures. The behavior of CFRP-strengthened RC structure is often controlled by the behavior of the interface between CFRP sheets and concrete. In this study, a review of models on bond strength, bond-slip, and interfacial stresses has been first carried out. Then a new bond-slip model is proposed. The proposed bond-slip model has bilinear ascending regions and exponential descending region derived from modifications mode on the conventional bilinear bond-slip model. The comparison of the results with those of existing experiment researches on bond-slip models indicate good agreements.

Development of Underwater Adhesive, Epoxy, and FRP Composite for Repair and Strengthening of Underwater Structure

Recently, numerous construction techniques for repairing and strengthening methods for above ground or air exposed concrete structure have been developed. However repairing and strengthening methods for underwater structural members under continuous loading, such as piers and steel piles need the further development. Therefore, this study develops an aqua epoxy, which can be used for repairing and strengthening of structural members located underwater. Moreover, using the epoxy material and strengthening fibers, a fiber reinforced composite sheet called Aqua Advanced FRP (AAF) for underwater usage is developed. To verify and to obtain properties of the material and the performance of AAF, several tests such as pull-off strength test, bond shear strength test, and chemical resistance test, were carried out. The results showed that the developed aqua epoxy does not easily dissolve in wet conditions and does not create any residual particle during hardening. In spite of underwater conditions, it showed the superior workability, because of the high viscosity over 30,000 cps and adhesion capacity over 2 MPa, which are nearly equivalent to those used in dry conditions. In case of the chemical resistance test, the developed aqua epoxy and composite showed the weight change of about 0.5~1.0%, which verifies the superior chemical resistance.

Seismic Analysis of RC Subway Station Structures Using Finite Element Method

Even though a lot of advanced researches on analysis, design, and performance evaluation of reinforced concrete (RC) under seismic action have been carried out, there has been only a few study on seismic analysis of underground RC structures surrounding soil medium. Since the underground RC structures interact with surrounding soil medium, a path-dependent soil model which can predict the soil response is necessary for analyzing behavior of the structure inside soil medium. The behavior of interfacial zone between the RC structure and the surrounding medium should be also considered for more accurate seismic analysis of the RC structure. In this paper, an averaged constitutive model of concrete and reinforcing bars for RC structure and path-dependent Ohsakis model for soil are applied, and an elasto-plastic interface model having thickness is proposed for seismic analysis of underground RC structures. A finite element analysis technique is developed by applying aforementioned constitutive equations and is verified by predicting both static and dynamic behaviors of RC structures. Then, failure mechanisms of underground RC structure under seismic action are numerically derived through seismic analysis of underground RC station structure under different seismic forces. Finally, the changes of failure mode and the damage level of the structures are also analytically derived for different design cases of underground RC structures.

Microscopic analysis of 50% axially strained cementitious materials

Abstract The microscopic behavior of 50% axially strained cementitious materials is studied. The microscopic behavior of concrete under the triaxial stress condition must be fully understood to explain the additional concrete ductility that comes from lateral confinement and a microstructural mechanism which has been subjected to extreme strain. The so-called “tube-squash” test was used to achieve 50% axial strain and over 70° of shear angle change in a concrete under extremely high pressure, without visible cracks. Then, microscopic analysis focusing on hydration and the microstructure of a 50% axially strained cement paste were performed on cored-out deformed and virgin (undeformed) cement paste specimens; the first specimen was 40 days old and the second one was 1 year old. Field emission scanning electronic microscope (FESEM) analysis was performed to compare the microstructures of the 40-day-old and 1-year-old specimens applied with finite strain. On the 1-year-old specimens, X-ray diffractometer (XRD), energy dispersive X-ray spectrometer (EDS), and differential thermal analysis/thermo-gravity (DTA/TG) analyses were also performed to study the change of hydration and microstructures due to 50% axial strain. The results and implications from these various analyses are discussed.