• Proceedings of the Korea Concrete Institute Conference
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• 1999.04a
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• pp.543-546
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• 1999

The purpose of the present study is to identify the damage characteristics of concrete structures due to cracking by employing the acoustic emission techniques. A comprehensive experimental study has been done. The cracking damages under tensile and flexural loadings have been identified and the bond damage between steel and concrete have been also characterized. It is seen that the amplitudes and energy level of AE events is found to be smaller for bond cracking damages and larger for tensile cracking damages. The characteristic equations of the AE events for various cracking damages have been proposed based on the present test data. The internal microcracks are progressively developed ahead of a visible actual crack and the present study clearly exhibits thses damage mechanism for various types of cracking in concrete. The present study provides very useful data which can be used to identify the various types of cracking damages in concrete structures. This will allow very efficient maintenance of concrete structures through monitoring of internal cracking based on acoustic emission.

• Journal of the Korea Concrete Institute
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• v.11 no.4
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• pp.107-116
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• 1999

The damage in concrete structures generally starts with microcracking and thus it is important to find and explore these microcracks in concrete in order to ensure appropriate safety and serviceability. The purpose of the present study is to identify the damage characteristics of concrete structures due to cracking by employing the acoustic emission techniques. A comprehensive experimental study has been done. The cracking damage under tensile and flexural loadings have been identified and the bond damage between steel and concrete have been also characterized. It is seen that the amplitudes and energy level of Acoustic Emission(AE) events are found to be relatively small for bond cracking damages and large for tensile cracking damages. The characteristic equations of the AE events for various cracking damages have been proposed based on the present test data. The internal microcracks are progressively developed ahead of a visible actual crack and the present study clearly exhibits these damage mechanism for various types of cracking in concrete. The present study provides useful data which can be used to identify the various types of cracking damages in concrete structures. This will allow efficient maintenance of concrete structures through monitoring of internal cracking based on acoustic emission.

• Journal of the Korea Concrete Institute
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• v.15 no.4
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• pp.625-633
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• 2003

For the construction of open-topped steel box girder bridges, prefabricated concrete slab could offer several advantages over cast-in-situ deck including good quality control, fast construction, and elimination of the formwork for concrete slab casting. However, precast decks without reinforcements at transverse joints between precast slabs should be designed to prevent the initiation of cracking at the joints, because the performance of the joint is especially crucial for the integrity of a structural system. Several prestressing methods are available to introduce proper compression at the joints, such as internal tendons, external tendons and support lowering after shear connection. In this paper, experimental results from a continuous composite bridge model with precast decks are presented. Internal tendons and external tendons were used to prevent cracking at the joints. Judging from the tests, precast decks in negative moment regions have the whole contribution to the flexural stiffness of composite section under service loads if appropriate prestressing is introduced. The validity of the calculation of a cracking load fur serviceability was presented by comparing an observed cracking load and the calculated value. Flexural behavior of the continuous composite beam with external prestressing before and after cracking was discussed by using the deflection and strain data.

• Corrosion Science and Technology
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• v.5 no.2
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• pp.69-76
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• 2006

Premature cracking of the epoxy coatings applied on ship's ballast tanks(BT) can lead to damage of ship's hulls. To avoid this, it's important to have clear understanding of the underlying mechanism and primary factors of the coating crack. In this study, the efforts were made to clarify the integrated effects of main factors, i.e., initial coating shrinkage, thermally induced strain, steel-structural strain and the intrinsic coating flexibility at the initial and after aging, to the early cracking phenomena of epoxy coating in the ship's ballast tank. The coating crack is caused by combination of thermal stress, structural stress, and internal stresses which is closely related to chemical structures of the coatings. On the other hand, thermal stresses and dimensional stabilities would rarely play a major role in coating crack for ballast tank coatings with rather large flexibility. Crack resistance of the coatings at early stages can be estimated roughly by measuring internal stress, FT-IR and $T_g$ value of the coatings. A new screening test method was also proposed in this study, which can be possibly related to the long-term resistance of epoxy-based paints to cracking.

• Corrosion Science and Technology
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• v.7 no.2
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• pp.77-84
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• 2008

Based upon the good compatibility to neutron irradiation and high temperature water environment, austenitic stainless steels are widely used for core internal structural materials of light water reactors. But, recently, intergranular cracking was detected in the stainless steels for the core applications in some commercial PWR plants. Authors studied on the root cause of the intergranular cracking and developed the countermeasure including the alternative materials for these core applications. The intergranular cracking in these core applications are defined as an irradiation assisted mechanical cracking and irradiation assisted stress corrosion cracking. In this paper, the root cause of the intergranular cracking and its countermeasure are summarized and discussed.

• Journal of the Korea institute for structural maintenance and inspection
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• v.11 no.1
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• pp.57-67
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• 2007

The service life of concrete structure is to a great extent influenced by crack developed at early ages of concrete material. Especially, hydration heat is a main cause of thermal cracking at mass concrete structures. The thermal cracking of massive structure is analyzed of the thermal cracking index which was presented Concrete Standard Specifications. The thesis analyzed the thermal cracking index which considered various variable (cement type, height of casting, curing condition, concrete mixing temperature, the unit cement content) at internal restricted mass concrete. The analysis result is denoted increase and decrease rate of thermal cracking index whenever the variables change. The results is helped to understand thermal cracking every time structures is designed and constructed. And I think that it is useful economic and stable design of mass concrete structures.

• Journal of the Korea Concrete Institute
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• v.22 no.1
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• pp.69-76
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• 2010

The majority of research that has been performed on cracking potential of concrete by shrinkage has assumed that concrete acts as a homogeneous material. However, with this approach, it is not able to evaluate the micro-cracking behavior in concrete due to autogenous shrinkage under unrestrained boundary condition (free boundary condition) nor to understand the cracking behavior properly because of the heterogeneous nature of concrete. To better understand the micro-cracking behavior of concrete induced by autogenous shrinkage, series of experiments were performed measuring the length change and acoustic emission energy. As an analytical approach, this research uses an object oriented finite element analysis code (OOF code) to simulate the behavior of the concrete on a meso-scale. The concrete images used in the simulations were directly obtained from mortar samples. From the experiments and simulation results, it was able to better understand the micro-cracking behaviour of concrete due to shrinking of paste phase and internal restraint by aggregates.

• Proceedings of the KSR Conference
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• 2008.11b
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• pp.1065-1068
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• 2008

Concrete is still one most common construction materials even in railway structures. As structures become massive and mega-sized, the importance of early age concrete quality control becomes more significant. Among various factors, relative humidity and temperature are the primary factors governing the early age quality. Temperature raise due to cement hydration causes stress, which can develop to cracking with internal and/or external restraints. Exposure conditions including ambient temperature, humidity and wind also significantly affect the cracking behavior of early age concrete. Among many of studies on the early age concrete behavior, investigation on the variation of temperature and relative humidity internal of concrete is not common. That is in part because the difficulties in measuring the relative humidity and temperature inside the concrete. This study used a digital sensor with an appropriate logger to measure internal temperature and relative humidity. This direct measuring method is expected to provide more reliable and comprehensive data acquisition on the early age behavior of concrete.

• Structural Engineering and Mechanics
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• v.64 no.4
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• pp.427-436
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• 2017

Concrete technologists have used ultrasonic pulse velocity test for decades to evaluate the properties of concrete. However, the presented research work focuses on the use of ultrasonic pulse velocity test to study the degradation in steel-concrete bond subjected to increasing loading. A detailed experimental investigation was conducted by testing five identical beam specimens under increasing loading. The loading was increased from zero till failure in equal increments. From the experimentation, it was found that as the reinforced concrete beams were stressed from control unloaded condition till complete failure, the propagating ultrasonic wave velocity reduced. This reduction in wave velocity is attributed to the initiation, development, and propagation of internal cracking in the concrete surrounding the steel reinforcement. Using both direct and semidirect methods of testing, results of reduction in wave velocity with evidence of internal cracking at steel-concrete interface are presented. From the presented results and discussion, it can be concluded that the UPV test method can be successfully employed to identify zones of poor bonding along the length of reinforced concrete beam. The information gathered by such testing can be used by engineers for localizing repairs thereby leading to saving of time, labor and cost of repairs. Furthermore, the implementation strategy along with real-world challenges associated with the application of the proposed technique and area of future development have also been presented.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.11a
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• pp.683-686
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• 2004

In recent years, some attention was particularly given to cracking sensitivity of high performance concrete. It has been argued and demonstrated experimentally that such concrete undergoes autogenous shrinkage due to self-desiccation at early age, and, as a result, internal tensile stress may develop, leading to micro cracking and macro cracking. One possible method to reduce cracking due to autogenous shrinkage is the addition of expansive additive. Tests conducted by many researches have shown the beneficial effects of addition of expansive additive for reducing the risk of shrinkage-introduced cracking. However, the research on hydration model of expansion additive has been hardly researched up to now. This paper presents a study of the hydration model of Ettringite-Gypsum type expansive additive. Result of comparing forecast values with experiment value, proposed model is shown to expressible of hydration of expansive additive.