• Journal of the Korean Ceramic Society
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• v.27 no.3
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• pp.329-336
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• 1990

In this investigation, bar-shaped specimens which consisted of three layers are prepared to study the effects of residual compressive stress on the mechanical properties in ZTA. The outer layers contained Al2O3 and unstabilized ZrO2 and the central layer contained Al2O3 and stabilized ZrO2(with 5.10wt% Y2O3). When cooled from the sintering temperature, some of zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation which induces to dilatational expansion led to the estabilishmenet of compressive stress in the outer layers and balances tensile stress in the central layer. Decrease of outer layer thickness(for a fixed total thickness)increases residual compressive stress. Because of residual compressive stress in the outer layers, the fracture toughness of outer layers of 3-layers composite is 10.21 Mpam1/2, which is increased to 25% above in comparison with 1-layer specimens in ZTA. Also, the 3-layers composite is believed to exhibite greater fracture resistance in contact damage environment from thermal shock test.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.186-191
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• 2001

Tensile residual stress happen by difference of coefficients of thermal expansion between fiber and matrix is one of the serious problems in metal matrix composite(MMC). In this study, TiNi alloy fiber was used to solve the problem of the tensile residual stress as the reinforced material. TiNi alloy fiber improves the tensile strength of composite by occurring compressive residual stress in matrix using shape memory effect of it. Pre-strain was added to generate compressive residual stress inside TiNi/A16061 shape memory alloy(SMA) composite. It was also evaluated the effect of compressive residual stress corresponding to pre-strains variation and volume fraction of TiNi alloy. AE technique was used to clarify the microscopic damage behavior at high temperature and the effect of pre-strain difference of TiNi/A16061 SMA composite. In addition, two dimensional AE source location technique was applied to inspect the crack initiation and propagation in composite.

• Structural Engineering and Mechanics
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• v.52 no.4
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• pp.701-721
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• 2014

In the assessment of existing RC buildings, the reliable appraisal of the compressive strength of in-situ concrete is a fundamental step. Unfortunately, the data that can be obtained by the available testing methods are typically affected by a high level of uncertainty. Moreover, in order to derive indications about the degradation and ageing of the materials by on site tests, it is necessary to have the proper terms of comparison, that is to say, to know the reference data measured during the construction phases, that are often unavailable when the building is old. In the cases when such a comparison can be done, the in situ strength values typically turn out to be lower than the reference strength values (tests performed on taken samples during the construction). At this point, it is crucial to discern and quantify the specific effect induced by different factors: ageing of the materials; poor quality of the placement, consolidation or cure of the concrete during the construction phases; damage due to drilling. This paper presents a procedure for correlating the destructive compressive tests and non-destructive tests (ultrasonic pulse velocity tests) with the data documenting the compressive strength tested during the construction phases. The research work is aimed at identifying the factors that induce the difference between the in-situ strength and cubes taken from the concrete casting, and providing, so, useful information for the assessment procedure of the building.

• Computers and Concrete
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• v.21 no.4
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• pp.377-384
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• 2018

The accelerated increase of the population growth rate in the world and the current lifestyle based on consumerism considerably increased the amount of waste generated by the human activity. Specifically, e-plastic waste causes significant damage to the environment because of its difficult degradation process. This paper aims to establish the feasibility of using e-plastic waste in concrete as a partial replacement of coarse mineral aggregate. Considering a control mix without e-plastic waste designed for a compressive strength of 21 MPa, tests on concrete mixes with 40, 50 and 60% of e-plastic waste aggregate to determine the fresh and hardened properties were carried out. A reduction in the compressive strength as the percentage of e-plastic waste increases was observed, the maximum reduction being 44% with respect to the control mix. In addition, a significant reduction as much as 22% in the density of the concrete mixes with e-plastic waste was recorded, which means that lighter elements can be produced with this type of concrete. Two new equations based on regression analysis of the experimental data from this study were proposed. These equations estimate the reduction in the compressive strength of concrete mixes with e-plastic waste aggregate at 14 and 28 days. A cost analysis and a practical alternative to introduce this waste material into the market are also presented.

• International conference on construction engineering and project management
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• 2009.05a
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• pp.794-799
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• 2009

The temperature control of in-place concrete is the most important factor for an early age of curing concrete. Heat stress of mass concrete caused by the heat of hydration can induce the crack of concrete, and a frost damage from cold weather casting concrete results defect on compressive strength and degradation of durability. Therefore, success and failure of concrete work is dependant on the measurement and control of concrete temperature. In addition, the compressive strength assessment of in-place concrete obtained from the maturity calculated from the history of temperature make a reduction of construction cycle time, possible. For that purpose, wireless temperature measuring system was developed to control temperature and assess strength of concrete. And, it was possible to monitor the temperature of concrete over 1km apart from site office and to take a proper measure; mesh-type network was developed for wireless sensor. Furthermore, curing control system that contains the program capable to calculate the maturity of concrete from the history of temperature and to assess the compressive strength of concrete was established. In this study, organization and practical method of developed curing control system are presented; base on in-place application case.

• Journal of the Korea Concrete Institute
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• v.12 no.3
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• pp.47-55
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• 2000

Protections from the frost damage at early ages are one of the serious problems to be considered in cold weather concreting. Frost damage at early ages brings about the harmful influences on the concrete structures such as surface cracks and declination of strength development. Therefore, in this paper, protecting periods of frost damage at early ages according to the standard specifications provided in KCI(Korean Concrete Institute) are suggested by appling logistic curve, which evaluates the strength development of concrete with maturity. W/B, kinds of cement and curing temperatures are selected as test parameters. According to the results, the estimation of strength development by logistic curve has a good agreement between calculated values and measured values. As W/B and compressive strength for protecting from frost damages at early ages increase, it is prolonged. It shows that the protecting periods of FAC(Fly Ash Cement) and BSC(Blast-furnace Slag Cement) concrete are longer than those of OPC(Ordinary Portland Cement) concrete. The protecting peridos from frost damage at early age by JASS are somewhat shorter than those by this paper.

• Proceedings of the KSME Conference
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• 2000.11a
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• pp.316-321
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• 2000

The role of fatigue on the surface damage of 5% chrome cold milling work roll is evaluated. Uniaxial and torsional fatigue tests are carried out, and the baseline data for fatigue life evaluation are established. An appropriate multiaxial fatigue parameter is developed from the fatigue data. Fatigue tests are also performed under compressive mean stresses, and a mean stress model is formulated. A computer program is developed to assess the interaction of fatigue and grinding of the roll. The fatigue damage is computed for selected servicing conditions. It is found that the fatigue damage can be an important issue when the effect of mean stress is ignored, however the fatigue damage is negligibly small when the effect of mean stress including the residual stresses currently used is fully accounted. The result indicates that spalling due to the growth of thermal shock cracks is more important than fatigue damage in roll surface failure.

• The Journal of Engineering Geology
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• v.4 no.2
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• pp.231-243
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• 1994

To better understand the fundamental problems of the true micro-damage in medium-grained granite under uniaxial compressive stress, micro-damage localization, initiation and propagation have been observed in a great detail in contact portion of two grains such as quartz and feldspar. For this purpose, new experimental system allowing us to observe the micro-damaging process continuously was developed. Earlier studies used the specimens of unloaded state and it is difficult to visualize stress-induced microcracks under unloading state. Thus, direct observation under loading state is very important for understanding the true micro-damage process. The results explain well the mechanism of micro-damage at two grains, and mechanics of the micro-damage is clarified well by Hertzian fracture mechanics.

• Advances in concrete construction
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• v.5 no.5
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• pp.539-561
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• 2017

In this study, the effects of initial damage of concrete columns on the post-repair performance of reinforced concrete (RC) columns strengthened with carbon-fiber-reinforced polymer (CFRP) composite are investigated experimentally. Four kinds of compression-damaged RC cylinders were reinforced using external CFRP composite wraps, and the stress-strain behavior of the composite/concrete system was investigated. These concrete cylinders were compressed to four pre-damaged states including low -level, medium -level, high -level and total damage states. The percentages of the stress levels of pre-damage were, respectively, 40, 60, 80, and 100% of that of the control RC cylinder. These damaged concrete cylinders simulate bridge piers or building columns subjected to different magnitudes of stress, or at various stages in long-term behavior. Experimental data, as well as a stress-strain model proposed for the behavior of damaged and undamaged concrete strengthened by external CFRP composite sheets are presented. The experimental data shows that external confinement of concrete by CFRP composite wrap significantly improves both compressive strength and ductility of concrete, though the improvement is inversely proportional to the initial degree of damage to the concrete. The failure modes of the composite/damaged concrete systems were examined to evaluate the benefit of this reinforcing methodology. Results predicted by the model showed very good agreement with those of the current experimental program.

• Advances in concrete construction
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• v.8 no.1
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• pp.9-19
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• 2019

This paper describes an experimental investigation on hybrid fiber reinforced concrete (HYFRC) beams. And the main aim of this present paper is to examine the dynamic characteristics and damage evaluation of undamaged and damaged HYFRC beams under free-free constraints. In this experimental work, totally four RC beams were cast and analyzed in order to evaluate the dynamic behavior as well as static load behavior of HYFRCs. Hybrid fiber reinforced concrete beams have been cast by incorporating two different fibers such as steel and polypropylene (PP). Damage of HYFRC beams was obtained by cracking of concrete for one of the beams in each set under four-point bending tests with different percentage variation of damage levels as 50%, 70% and 90% of maximum ultimate load. And the main dynamic characteristics such as damping, fundamental natural frequencies, mode shapes and frequency response function at each and every damage level has been assessed by means of non-destructive technique (NDT) with hammer excitation. The fundamental natural frequency and damping values obtained through dynamic tests for HYFRC beams were compared with control (reference) RC beam at each level of damage which has been acquired through static tests. The static experimental test results emphasize that the HYFRC beam has attained higher ultimate load as compared with control reinforced concrete beam.