• Coupled systems mechanics
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• v.9 no.2
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• pp.163-182
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• 2020

Coupled thermo-mechanical analysis of reinforced concrete slab at elevated temperatures from a fire accounting for nonlinear thermal parameters is carried out. The main focus of the paper is put on a one-way continuous reinforced concrete slab exposed to fire from the single (bottom) side as the most typical working condition under fire loading. Although contemporary techniques alongside the fire protection measures are in constant development, in most cases it is not possible to avoid the material deterioration particularly nearby the exposed surface from a fire. Thereby the structural fire resistance of reinforced concrete slabs is mostly influenced by a relative distance between reinforcement and the exposed surface. A parametric study with variable concrete cover ranging from 15 mm to 35 mm is performed. As the first part of a one-way coupled thermo-mechanical analysis, transient nonlinear heat transfer analysis is performed by applying the net heat flux on the exposed surface. The solution of proposed heat analysis is obtained at certain time steps of interest by α-method using the explicit Euler time-integration scheme. Spatial discretization is done by the finite element method using a 1D 2-noded truss element with the temperature nodal values as unknowns. The obtained results in terms of temperature field inside the element are compared with available numerical and experimental results. A high level of agreement can be observed, implying the proposed model capable of describing the temperature field during a fire. Accompanying thermal analysis, mechanical analysis is performed in two ways. Firstly, using the guidelines given in Eurocode 2 - Part 1-2 resulting in the fire resistance rating for the aforementioned concrete cover values. The second way is a fully numerical coupled analysis carried out in general-purpose finite element software DIANA FEA. Both approaches indicate structural fire behavior similar to those observed in large-scale fire tests.

• Journal of the Korea institute for structural maintenance and inspection
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• v.16 no.2
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• pp.40-48
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• 2012

Steel corrosion in concrete is a main cause of deterioration and early failure of concrete structures. A novel integration of electromagnetic heat induction and infrared (IR) thermography is proposed for nondestructive detection of steel corrosion in concrete, by taking advantage of the difference in thermal characteristics of corroded and non-corroded steel. This paper focuses on experimental investigation of the concept. An inductive heater is developed to remotely heat the embedded steel from concrete surface, which is integrated with an IR camera. Concrete samples with different cover depths are prepared. Each sample is embedded with a single rebar in the middle, resulting an identical cover depth from the front and the back surfaces, which enable heat induction from one surface and IR imaging from the other simultaneously. The impressed current (IC) method is adopted to induce accelerated corrosion on the rebar. IR video images are recorded during the entire heating and cooling periods. The test results demonstrate a clear difference in thermal characteristics between corroded and non-corroded samples. The corroded sample shows higher rates of heating and cooling than those of the non-corroded sample. This study demonstrates a potential for nondestructive detection of rebar corrosion in concrete.

• Steel and Composite Structures
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• v.38 no.2
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• pp.121-139
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• 2021

Ultra-high strength concrete (UHSC) encased steel columns are receiving growing interest in high-rise buildings owing to their economic and architectural advantages. However, UHSC encased steel columns are not covered by the modern fire safety design code. A total of 14 fire tests are conducted on UHSC (120 MPa) encased steel columns under constant axial loads and exposed to ISO-834 standard fire. The effect of load ratio, slenderness, stirrup spacing, cross-section size and concrete cover to core steel on the fire resistance and failure mode of the specimens are investigated. The applicability of the tabulated method in EC4 (EN 1994-1-2-2005) and regression formula in Chinese code (DBJ/T 15-81-2011) to fire resistance of UHSC encased steel columns are checked. Generally, the test results reveal that the vertical displacement-heating time curves can be divided into two phases, i.e. thermal expansion and shortening to failure. It is found that the fire resistance of column specimens increases with the increase of the cross-section size and concrete cover to core steel, but decreases with the increase of the load ratio and slenderness. The EC4 method overestimates the fire resistance up to 186% (220 min), while the Chinese code underestimates it down to 49%. The Chinese code has a better agreement than EC4 with the test results since the former considers the effect of the load ratio, slenderness, cross section size directly in its empirical formula. To estimate the fire resistance precisely can improve the economy of structural fire design of ultra-high strength concrete encased steel columns.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.543-546
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• 2002

This paper discussed micro - structure of skin concrete to understand transport properties from surface and seek thickness from surface which is seriously influenced on durability. Concrete at nearer surface has high porosity relative to inner concrete. The porosity of concrete and ISAT value at region from surface to 20 mm depth is decreased with depth. On the other hand, according to the result of ASTM C 1202 with specimen thickness, critical depth which affects fast ionic penetration through interfacial transition zone (ITZ) equals 35mm and the critical depth would be directly influenced by the effects of ITZ on chloride diffusion unrelated with W/C ratio.

• Proceedings of the Korea Concrete Institute Conference
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• 1999.10a
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• pp.613-616
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• 1999

The bond between reinforcing bar and concrete is a significant factor to confirm that they behave uniformly in the reinforced concrete. Thus, the studies on this field have been conducted by many researchers. But for the high strength lightweight concrete few studies have been done. In this study, the steel fiber reinforced high strength lightweight concrete developed to complement the brittleness of the high strength lightweight concrete was studied experimentally to find the local bond stress. Total 20 specimens were tested and the measured test values were compared with those calculated according to ACI 318-95 code and CEB-FIP code, respectively. The results indicate that the maximum bond stress has been influenced by increment of volume fracture of steel fiber, compressive strength and cover, Especially steel fiber caused not only increment of bond strength but also ductile behaviro.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.05a
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• pp.653-658
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• 2001

Recent long-span bridges, such as Kwang-Ahn Grand bridge, Seo-Hae Grand Bridge, Young-Jong Grand Bridge, etc, have been designed and constructed near the shore. Thus, it needs to analyze the durability of marine concrete structures which are exposed to severe chloride environments. It is well known that corrosion of reinforcement steel in concrete is one of the major factors for the durability of concrete structures. The objective of this experimental study is to investigate the performance of impressed current system and corrosion inhibitors for the corrosion protection of reinforced concrete structures. Concrete test specimens were made with various test parameters, such as cover depth, steel diameter, compressive strength, direction and frequency of notch. For the efficient evaluation of these corrosion protection systems, these tests have been carried out in the shore.

• Proceedings of the Korea Concrete Institute Conference
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• 1998.10c
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• pp.161-166
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• 1998

The concrete structures designed and constructed by conventional design concept based on structural performance consideration show sometimes serious durability problem when the structures are exposed to aggressive environment. Because present design system focuses on the structure safety and considers durability indirectly by the concrete mix design and cover depth, the durability of concrete structure cannot be ensured. As the first step to develope the durability design for concrete structure, durability index which represents internal concrete resistance and environment index which represents external environmental exposure are derived quantitatively. In the next step, the durability design system is developed by checking durability limit state with computed two indexes under service life condition by considering of the reliability of structure. Finally, the proposed system is verified with a model problem.

• Proceedings of the Korea Concrete Institute Conference
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• 2000.10a
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• pp.461-466
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• 2000

To improve the quality of concrete, we usually consider the reduction of water/cement ratio, the increase of concrete cover depth and the use of mineral admixtures. Reportedly, the use of admixtures make concrete more durable and tighten against water. But, it is needed to study more about the relationship between the admixtures and the chloride ion diffusion. Therefore, in this study, we focused on the chloride ion diffusion properties of the pozzolanic admixtures such as fly-ash, slag and silica fume which are known as being useful on chloride attack resistance when mixed into mortar or concrete. Furthermore, we treed to analyze the correlation between mortar and concrete using the admixture, which is useful for analyzing chloride ion diffusion mechanism.

• Journal of the Korea Concrete Institute
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• v.12 no.3
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• pp.21-30
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• 2000

Recently, reinforced concrete structures exposed to severe enviroment are increased in metropolitan area. The acid rain and CO2 penetrated towad rebar, thus rebar corrosion occurred. The corrosion of rebar in concrete is, as in most corrosion processes, an electrochemical nature. The corrosion may severely affect on durability and service life of such a concrete structures. This study was performed for the purpose of acquiring data about corrosion condition and considering a countermeasure to prevent rebar from corroding due to carbonation of concrete. An accelerated car bonation testing procedure was applied to measure the evolution of carbonation and rebar corrosion with time for various water-binder ratios and cement types.

• Proceedings of the Korea Concrete Institute Conference
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• 2006.05b
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• pp.165-168
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• 2006

The major cause of deterioration for the bare concrete bridge decks exposed to de-icing chemicals would be chloride-induced reinforcement corrosion. Thus, in this paper, in order to predict time to corrode for concrete bridge decks on highways, the chloride concentration was measured with depth from the surface. Then, the surface chloride concentration and apparent diffusion coefficient were calculated by regression. The premature failure of bare concrete decks were mostly related with thin cover depth and poor concrete property(high permeability). The good protection of deck surface might contribute to the prolongation of the service life of bare concrete bridge decks.