Ha-Won Song

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.

Analysis of corrosion resistance behavior of inhibitors in concrete using electrochemical techniques

Reinforced concrete is one of the most durable and cost effective construction materials. However, in high chloride environments, it can suffer from corrosion due to chloride induced breakdown of the normal passive layer protecting the reinforcing steel bars inside concrete. One means of protecting embedded steel reinforcement from chloride induced corrosion is the addition of corrosion inhibiting admixtures. In the present investigation, various inhibitors such as sodium nitrite, zinc oxide, mono ethanol amine, diethanolamine, and triethanol amine have been used in concrete in different percentages. Their effectiveness was then studied using various electrochemical techniques such as rapid chloride ion penetration test, open circuit potential measurement, electrochemical impedance measurement, potentiodynamic polarization measurement, and gravimetric weight loss measurement. The results thus obtained indicate that the addition of inhibitors enhances the corrosion resistance properties.

Chloride Penetration Resistance of Ternary Blended Concrete and Discussion for Durability

Mineral admixtures are used to improve the quality of concrete and to develop sustainability of concrete structures. Supplementary cementitious materials (SCM), such as silica fume (SF), granulated blast furnace slag (GGBS) and pulverized fly ash (PFA), are gradually recognized as useful mineral admixture for producing high performance concrete. The study on ternary blended concrete utilizing mainly three major mineral admixtures is limited and the study on durability and chloride induced corrosion resistance of ternary blended concrete is very few. This study examines the durability characteristics of the ternary blended concrete composed of different amount of the SCM with ordinary Portland concrete and the study experimentally focuses on corrosion resistance evaluation of ternary blended concrete subjected to chloride attack. In this study, 50% replacement ratio of mineral admixture to OPC was used, while series of combination of GGBS, SF and PFA binder were used for chloride corrosion resistance test. This study concerned the durability properties of the ternary blended concrete including the corrosion resistance, chloride binding, chloride transport and acid neutralization capacity. It was found that the ternary blended concrete utilizing the SCM densified the pore structures to lower the rate of chloride transport. Also, increased chloride binding and buffering to acid were observed for the ternary blended concrete with chlorides in cast.

Vorhersage der Lebensdauer von Betontragwerken im Seeklima unter Berücksichtigung gekoppelter zerstörender Vorgänge / Service Life Prediction of Concrete Structures under Marine Environment Considering Coupled Deterioration

The durability limit state of concrete structures under marine environment is defined as a time for the corrosion initiation, i.e., the time to reach the chloride threshold value at the surface of the rebar in reinforced concrete structures. In order to predict the service life of marine concrete structures under combined mechanical and climate loads, it is necessary to develop an analytical approach to predict the time and space dependent deterioration of concrete structures up to the durability limit state and additional deterioration like cracking of cover concrete due to corrosion. For more accurate prediction of the service life, the pre-existing cracks in concrete and coupled deterioration should be considered. In this paper, a service life prediction of marine concrete structures subjected to chloride attack and carbonation is carried out by using a micro-mechanics based deterioration analysis tool. In the tool, the pre-existing cracks and the coupled deterioration, i.e., chloride attack at carbonated concrete, in reinforced concrete structures are additionally considered. The prediction approach can evaluate the initiation of corrosion, corrosion rate and cracking of cover concrete. For the verification of the analysis tool, service life predictions for two major reinforced concrete bridges and tunnel designed for 100 years of service life in marine environment were carried out. The effect of crack widths and the coupled deterioration on the service life of reinforced concrete structures are also analyzed and discussed.

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.

Influence of Chemistry of Chloride Ions in Cement Matrix on Corrosion of Steel

Mortar specimens were manufactured with a centrally located steel reinforcing bar in order to assess the resistance of steel against corrosion, . The mortars were formulated by combining ordinary Portland cement (OPC) with a single supplementary cementitious material. The proportions were 30% pulverized fuel ash (PFA), 60% ground granulated blast-furnace slag (GGBS), or 10% silica fume (SF) by weight. In addition, a pure OPC mortar was prepared. The mortar specimens contained chlorides from 0.0 to 3.0% by weight of binder to accelerate the corrosion process. Then, the corrosion rate was measured by a polarization technique to determine the chloride threshold level for corrosion in concrete. The buffering capacity of the cement matrix to a pH reduction was quantified by measuring the acid neutralization capacity. The suspension consisting of distilled water and paste powder was exposed to nitric acid to characterize a decrease in the pH. As a result, the chloride threshold level for corrosion was dependent on binders, with values of 0.74 to 0.93%, 0.48 to 0.59%, 0.29 to 0.44%, and 0.63 to 0.78% by weight of binder for OPC, 30% PFA, 60% GGBS, and 10% SF, respectively. It was found that an increase in the buffering capacity resulted in an increased threshold value, of which characteristics were used for a new representation of the chloride threshold. The threshold ratio of [Cl–]:[H+] for acidification to a pH value of 10 at corrosion accounted for 0.0063 and 0.0082, irrespective of binder type, at 1.0 and 2.0 mA/m2 (6.50 × 10–4 and 1.29 × 10–3 mA/in.2) of the corrosion rate, respectively.

Time Dependent Chloride Transport Evaluation of Concrete Structures Exposed to Marine Environment

This paper presents a model for durability evaluation of concrete structures exposed to marine environment, considering mainly a build-up of surface chloride as well as diffusion coefficient (D) and chloride threshold level . In this study, time dependency of and D were extensively studied for more accurate evaluation of service life of concrete structures. An analytical solution to the Fick`s second law was presented for prediction of chloride ingress for time varying . For the time varying , a refined model using a logarithm function for time dependent was proposed by the regression analysis, and averaging integrated values of the D with time over exposed duration were calculated and then used for prediction of the chloride ingress to consider time dependency of D. Durability design was also carried out for railway concrete structures exposed to marine environment to ensure 100 years of service life by using the proposed models along with the standard specification on durability in Korea. The proposed model was verified by the so-called performance-based durability design, which is widely used in Europe. Results show that the standard specification underestimates durability performances of concrete structures exposed to marine environment, so the cover depth design using current durability evaluation in the standard specifications is very much conservative. Therefore, it is found that utilizing proposed models considering time dependent characteristics of and D can evaluate service lift of concrete structures in marine environment more accurately.

A Study on Chloride Threshold Level of Blended Cement Mortar Using Polarization Resistance Method

The importance of chloride ions in the corrosion of steel in concrete has led to the concept for chloride threshold level (CTL). The CTL can be defined as the content of chlorides at the steel depth that is necessary to sustain local passive film breakdown and hence initiate the corrosion process. Despite the importance of the CTL, due to the uncertainty determining the actual limits in various environments for chloride-induced corrosion, conservative values such as 0.4% by weight of cement or 1.2 kg in 1m concrete have been used in predicting the corrosion-free service life of reinforced concrete structures. The paper stud- ies the CTL for blended cement concrete by comparing the resistance of cementitious binder to the onset of chloride-induced cor- rosion of steel. Mortar specimens were cast with centrally located steel rebar of 10 mm in diameter using cementitious mortars with ordinary Portland cement (OPC) and mixed mortars replaced with 30% pulverized fuel ash (PFA), 60% ground granulated blast furnace slag (GGBS) and 10% silica fume (SF), respectively, at 0.4 of a free W/B ratio. Chlorides were admixed in mixing water ranging 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 2.5 and 3.0% by weight of binder(Based on Cl ). Specimens were curd 28 days at the room temperature, wrapped in polyethylene film to avoid leaching out of chloride and hydroxyl ions. Then the corrosion rate was measured using the polarization resistance method and the order of CTL for binder was determined. Thus, CTL of OPC, 60%GGBS, 30%PFA and 10%SF were determined by 1.6%, 0.45%, 0.8% and 2.15%, respectively.

Material Characteristic of POFA Concrete and Its Application to Corrosion Resistance Evaluation

In this study, corrosion resistance of palm oil fuel ash (POFA) concrete as a blended concrete is evaluated by using electrochemical technique. The POFA is an industrial byproduct obtained from fuel ash after extracting palm oil from palm-tree. In order to obtain basic material characteristics of the POFA concrete, tests on compressive strength, slump, weight loss, bleeding and expansion ratio were carried out the early-aged POFA concrete. On the other hand, durability characteristics, both chloride penetration and carbonation depth test, were also conducted. Finally, corrosion resistance were evaluated by applying electro-chemical artificial crack healing technique, and the tests on the impressed voltage characteristic, galvanic current and linear polarization resistance. From the experimental results, it was found that long-term strength, bleeding, lower slump ratio, expansion ratio, chloride penetration, carbonation and corrosion resistance were improved by using the POFA due to activated pozzolanic reaction. It can be also mentioned that POFA concrete has a potential to be used as a cementitious binder for green-recycling resources.

Experimental Study on Artificial Crack Healing for Concrete Using Electrochemical Deposition Method

In this study, autogenous crack healing and artificial crack healing using electrochemical electro deposition method were conducted to compare in the aspects of corrosion monitoring. Furthermore, the analysis of impressed voltage characteristics, galvanic current and linear polarization resistance comparison, and photo image processing technique were performed for quan- titative comparisons of healing ratio. As a result, it was found that, in view of impressed voltage of artificial crack healing, the measured voltage was increased as time goes by. From the galvanic test results of artificial crack healing, the current vs. potential distribution value were formed widely in comparison with autogenous crack healing. In this point, it was shown that artificial crack healing has more eleatic resistance capacity than autogenous crack healing technique. Finally, it was found that artificial crack heal- ing was 1.63 times higher than autogenous healing in view of crack healing ratio.