Byung Hwan Oh

Nonlinear Analysis of Temperature and Moisture Distributions in Early-Age Concrete Structures Based on Degree of Hydration

In this paper, adequate modeling of early-age concrete is used to realistically analyze the actual concrete structures at early ages. The models are incorporated into a 3-D finite element procedure. Mathematical formulation of the degree of hydration is based on the combination of 3 rate functions of reaction that represent effects of moisture condition as well as temperature. In moisture transfer, moisture transport coefficient, capacity, and sink are strongly dependent on the degree of hydration as well as the water-cement (w/c) ratio. The realistic models and finite element program developed in this work provide fairly good results on the temperature and moisture distribution for early-age concrete and correlate very well with actual test data.

Development of high-performance concrete having high resistance to chloride penetration

The resistance to chloride penetration is one of the simplest measures to determine the durability of concrete, e.g. resistance to freezing and thawing, corrosion of steel in concrete and other chemical attacks. Thus, high-performance concrete may be defined as the concrete having high resistance to chloride penetration as well as high strength. The purpose of this paper is to investigate the resistance to chloride penetration of different types of concrete and to develop high-performance concrete that has very high resistance to chloride penetration, and thus, can guarantee high durability. A large number of concrete specimens have been tested by the rapid chloride permeability test method as designated in AASHTO T 277 and ASTM C 1202. The major test variables include water-to-binder ratios, type of cement, type and amount of mineral admixtures (silica fume, fly ash and blast-furnace slag), maximum size of aggregates and air-entrainment. Test results show that concrete containing optimal amount of silica fume shows very high resistance to chloride penetration, and high-performance concrete developed in this study can be efficiently employed to enhance the durability of concrete structures in severe environments such as nuclear power plants, water-retaining structures and other offshore structures.

Critical Corrosion Amount to Cause Cracking of Reinforced Concrete Structures

Tensile stresses can be caused in surrounding concrete by steel bar corrosion in concrete through expansion pressure induction. Serious concrete cover cracking may result, which greatly affects concrete structure serviceability and durability. Realistic determination of the critical corrosion amount (CCA) that causes concrete cover cracking initiation is therefore necessary. The studys purpose is exploration and determination of CCA causing concrete cover surface cracking. A comprehensive experimental study was performed to this end. Concrete strength and cover thickness were among major test variables. The concrete steel bar corrosion tests have been conducted, and concrete cover surface strains have been measured according to various steel corrosion amounts. The present test results determined the CCA, which caused cracking initiation on concrete cover surface. It was seen that with an increase in concrete cover thickness, there was a great increase in CCA. CCA is indicated by the present study to increase approximately proportionately to concrete cover thickness square. CCA is also affected by concrete strength. The study shows that with a compressive strength increase, there is also a CCA increase, especially in the case of normally used large and medium cover thicknesses, that, when a cover thickness in actual concrete structures exceeds 40 mm (1.6 in.c) The study found that compressive strength effect on CCA is not large for small cover thicknesses, such as 20 mm (0.8). A good base may be provided by the present study for future development of concrete structure service-life assessment and realistic durability design.

Chloride Diffusion Analysis of Concrete Structures Considering Effects of Reinforcements

Reinforced concrete structures exposed to sea environments suffer from corrosion of steel bars due to chloride ingress. Chloride penetration into concrete is influenced by many parameters such as cement type, mixture proportions, and presence of reinforcing bars. Conventional diffusion analyses have neglected the presence of steel bars in concrete. The aim of this paper is, therefore, to study the effects of reinforcement on chloride diffusion in concrete structures by incorporating realistic diffusion models. To this end, the nonlinear binding isotherm that includes the effects of cement types and mixture proportion is introduced in the chloride diffusion analysis. The effects of reinforcement on the chloride penetration are analyzed by finite element method. This study indicates that the chlorides are accumulated in front of a reinforcing bar and the accumulation of chlorides is much more pronounced for the case of larger-size bars. The higher accumulation of chlorides at bar location causes faster corrosion of reinforcing bars. The corrosion initiation time reduces by about 30-40% when the existence of reinforcing bars is considered in the chloride diffusion analysis. The effects of reinforcements must be properly considered in the chloride diffusion analysis to obtain more accurate and realistic estimation for the service of concrete structures.

Modeling Mechanical Behavior of Reinforced Concrete due to Corrosion of Steel Bar

Expansion of a steel bar due to corrosion causes tensile stresses in concrete around the reinforcing bar and may induce cracking through the concrete cover. The purpose of the present study is to explore realistic mechanical properties of the corrosion layer, including the pressure-free corrosion strain and the stiffness of the corrosion layer. The corrosion products that penetrate into the pores and cracks around a steel bar have been considered in the calculation of expansive pressure due to steel corrosion. A realistic relationship between the expansive pressure and the average strain of the corrosion product layer has been derived and the representative stiffness of the corrosion layer was also determined. A concept of free-expansion (pressure-free) strain of the corrosion product layer was newly introduced to describe the relationship between the expansive pressure and the corrosion layer strain. The comparisons were made between the theory and test data on the surface strains of the concrete cover according to the corrosion amount for various concrete strengths and cover thicknesses. The proposed theory reasonably agrees with experimental data and may be a good base for the realistic durability analysis/design of concrete structures.

Effects of Carbonation on Chloride Penetration in Concrete

Previous studies were confined mostly to the deterioration of concrete structures under a single deteriorating factor, such as chloride ingress only or carbonation only, although the real environment may be a combination of such factors. Therefore, the purpose of this study is to explore the influence of carbonation on chloride penetration in concrete structures. Several series of concrete specimens were tested. The test results indicate that the chloride penetration is more pronounced when the carbonation process is combined with the chloride ingress. It is also shown that the ratio of water-soluble chloride to the acid-soluble chloride content is higher for the case of the carbonated test series than the case of the normal noncarbonated test series. This may cause a more vulnerable situation to the corrosion of steel bars when chloride ingress is combined with carbonation. This study allows for a more realistic assessment of durability for concrete structures that are subjected to a combined environment of chlorides and carbonation.

Fracture behavior of concrete under high rates of loading

Abstract Fracture properties and fracture behavior of concrete under dynamic loadings are investigated. Several series of concrete beams were tested under various rates of loadings. The relative initial notch depths of test beams were varied from zero to 0.75. The load deflection curves and the corresponding fracture energies are determined for various cases. The variation of fracture energy with notch depth is established. The fracture energy of concrete is found to increase with an increase of loading rate. A rate-dependent expression to predict the dynamic fracture energy of concrete is proposed. The nominal failure stresses for various notched beams are also studied and found to depend on the rates of loading. The increase of nominal failure stress due to dynamic loading is formulated.

Shear Behavior of Full-Scale Post-Tensioned Prestressed Concrete Bridge Girders

Test data on shear behavior of full-scale posttensioned prestressed concrete girders (PPCGs) is very limited. Therefore, this study aimed to experimentally explore the shear behavior of PPCGs. Large-scale PPCGs were designed, fabricated, and tested on 1.2 m deep x 10.6 m long PPCGs with medium-high and high-strength concretes. The girders were tested to failure while deflections, steel stirrup strains, cracking pattern, and average strains in the web were monitored. The stirrup strains showed a sudden increase immediately after cracking and continue to grow as the load increases. To measure the average strains in 3 directions, triangular grids of linear variable differential transducers (LVDTs) were also placed at the shear regions of the girders. These LVDTs enable measurement of average strains in the vertical, longitudinal, and diagonal directions even after the diagonal cracking has occurred. Shear strains, principal strains, and principal directions were calculated from these measured average strains. It was found that the angle of principal strain direction decreases as the applied load increases and that it approaches ~23-25 deg at ultimate load.

Measurement Device and Characteristics of Diffusion Coefficient of Carbon Dioxide in Concrete

The durability of concrete structure is greatly affected by the carbonation of concrete. The rate of carbonation depends largely on the diffusivity of carbon dioxide in concrete. The purpose of this study is to: (1) develop a measurement device for the carbon dioxide diffusion coefficients in concrete; and (2) identify the diffusion coefficients of carbon dioxide for various concrete materials. Several series of tests were planned and conducted. It was shown that the diffusion of carbon dioxide reached a steady state within approximately 5 hours after exposure. The current test results indicate that the diffusion coefficient increases with an increase of the water-cement ratio (w/c) and decreases with an increase of relative humidity (RH) at the same w/c. The content of aggregates also influences the diffusivity of carbon dioxide in concrete. The study found that the diffusion coefficient of cement paste is much larger than that of concrete or mortar. The quantitative values of the diffusivity of carbon dioxide for various materials are presented. This study allows for a more realistic assessment of the carbonation process and carbonation depth in concrete structures. This is very important for the durability assessment and service-life prediction of concrete structures.

Consistent numerical formulation of eccentric follower loads applied to shell structures Part II: Application to the analysis of prestressed concrete shell structures

In the preceding companion paper, a more advanced theory for the geometrically nonlinear finite element(FE) analysis of the shell structures under eccentric follower loads has been carefully established. It is expected that the derived formulation can be applied realistically to wide range of practical problems. As an important application of the present theories, the numerical procedures for FE analysis of prestressed concrete(PSC) shell structures considering tendon-induced nonconservative loads are presented in this paper. The equivalent load approach is employed to implement the effect of prestressing tendon. The characteristics of the equivalent external load by the tendon are discussed and rigorously formulated into the load correction stiffness matrix(LCSM). It is found that the present numerical procedures can predict correctly the geometrically nonlinear response of the PSC shell structures up to the large deformations with the apparent contribution of the derived tangent stiffness matrices. Numerical examples of PSC shell structures are presented to demonstrate the applicability and validity of the proposed method. The present study allows more realistic and accurate analysis of shell structures which are subjected to nonconservative follower loads by exhibiting much faster convergence even for the relatively high load factors compared to the conventional method without the LCSM.