• Proceedings of the Korean Institute of Building Construction Conference
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• 2014.11a
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• pp.44-45
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• 2014

On this study, initial crack index was evaluated by performing FEM analysis to crack propagation from hydration heat for development of precast concrete. On the result, as increased the usage of hardening accelerator, initial compressive strength were improved and setting time also was shortened. Additionally, central temperature of concrete was increased, the reaching time for the highest temperature could be shortened. By the result to assess crack index, there was no problem about crack despite of growth of initial high hydration heating. This result guessed because of small size element when analyzed trough FEM, realization for mass concrete's crack index should be performed.

• Journal of the Korean Society for Railway
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• v.7 no.4
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• pp.312-318
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• 2004

The wall in a subway structure is easily subject to crack occurrence since its expansion and shrinkage associated with hydration heat reaction is constrained by the slab. The greater problem is that the crack in the wall may be developed to pass through thickness and eventually deteriorate the structure due to rusting of reinforced steel. Thus, this study aims at controlling thermal cracks as much as possible and determining an optimized size of concrete placement through hydration heat analysis. For this study, effects of placement height, length, temperature and types of cement on the thermal cracks were evaluated by temperature rise, thermal stress and crack index. As results of parametric study, it was found that placement height and length do not have an effect on the temperature rise but have significant one on thermal stress which relates to direct possibility of thermal crack occurrence. This means that proper selection of size balancing internal constraint with external one is much more important than reducing the placement height and length simply. In order to prevent from thermal cracks most effectively, in addition, it was noted to reduce placement temperature and to use the cement blended with mineral admixture.

• Advances in concrete construction
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• v.12 no.5
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• pp.439-450
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• 2021

In recent years, nano-reinforcing materials are widely utilized in cement composites due to their unique multifunctional properties. This study incorporated multi-walled carbon nanotubes (MWCNTs) into the cementitious composites at ratios of 0.1%, 0.3%, and 0.5%, and investigated their influence on the flowability, mechanical strength, and hydration heat properties. The addition of MWCNTs enhanced the compressive and split tensile strengths approximately by 18-51%. In the semi-adiabatic temperature rise test, the internal hydration heat of the composites reduced by 5%, 9%, and 12% with the increase of MWCNTs in 0.1%, 0.3%, and 0.5%. This study further performed hydration heat analysis and estimated the adiabatic temperature rise, thermal stress, and thermal crack index. The internal hydration heat of the concrete decreased by 5%, 10%, and 13% with the increase of MWCNTs. The thermal stress of the concrete decreased with increase in the addition of MWCNTs, and the obtained temperature crack index was effective in controlling the thermal cracks.

• Journal of the Korea Concrete Institute
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• v.23 no.6
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• pp.773-780
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• 2011

In this study, thermal stress analysis are carried out considering material properties, curing condition, ambient temperature, and casting date of the mass concrete placed in bottom slab and side wall of the in-ground type LNG tank as a super massive structure. Also, based on the numerical results, cracking possibility is predicted and counter measures to prevent the cracking are proposed. For the tasks, two optimum mix proportions were selected. From the results of the thermal stress analysis, the through crack index of 1.2 was satisfied for separately caste concrete lots except for the bottom slab caste in 2 separate sequences. For the double caste bottom slab, it is necessary introduce counter measures such as pre-cooling prior to the site construction. Also, another crack preventive measure is to lower the initial casting temperature by $25^{\circ}C$ or less to satisfy 1.2 through crack index criterion. In the $1^{st}$ and $2^{nd}$ caste bottom slab, the surface crack index was over 1.2. Therefore, the surface cracks can be controlled by implementing the curing conditions proposed in this study. Since the side wall's surface crack index was over 1.0, it is safe to assume that the counter preventive measures can control width and number of cracks.

• Journal of Ocean Engineering and Technology
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• v.16 no.5
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• pp.73-79
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• 2002

In this paper, the effect of the compressive residual stresses was obtained at the test conditions of the higher temperature than the ambient temperature. The examination was performed with the CT specimen result of the material(JISG SUP9) which is being commonly used for the marine engine parts and the ocean structures. As a result, the test conditions at the higher temperature were acquired considering the peak values of the compressive residual stresses of the specimens and the effect on the fatigue crack propagation speed da／dN in stage II and the threshold stress intensity factor range Δth in stage I. Also the material constant C and the fatigue crack propagation index m in the formula of Paris Law da／dN＝C (ΔK)$^{m}$ were suggested to estimate the dependence on the test temperature.

• Korean Journal of Materials Research
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• v.10 no.4
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• pp.305-311
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• 2000

Applicability of crack arrest load measured from the Charpy V-notch impact test has been investigated to predict the fracture toughness of nuclear reactor pressure vessel (RPV) steels (ASME SA508 Cl.3). The temperature dependence of the crack arrest load was well described by the type of exponential function characterized by an index temperature at which the crack arrest load is 2kN. The specific index temperature, which also well correlated with $T_{NDT}\;and\;T_{41J}$ is expected to be representative index temperature characterizing the crack arrest fracture toughness of RPV steels. Also, the crack arrest load correlated well with the stable crack length measured from the fracture surface. From the measurements of the crack arrest load and the stable crack length, the lower bound fracture toughness, $K_{Ia}$ of RPV steels could be predicted with sufficient accuracy.

• Proceedings of the Korea Concrete Institute Conference
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• 2002.10a
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• pp.375-380
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• 2002

Tong-young LNG tank is a LNG storage tank of 140,000 kl, and it is composed of Bottom Slab(Annular, Center), Outer Wall, Ring Beam and Roof. Generally, when concrete temperature arise, the complex thermal stress of inner and outer part can cause serious thermal crack and damage at structure. So in this paper, for the control of this thermal crack, we did the concrete mix design with the base of fly-ash 30% substitute at binder, and through the computer modelling at Bottom Slab(Annular, Center), Outer Wall, Ring Beam and Roof, we studied the probability of thermal crack by thermal crack index.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.11a
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• pp.385-390
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• 2001

The thermal crack in mass concrete is mainly due to the difference of concrete temperature, which is generated by hydration heat of cement. As the thickness of mat foundation increases, the difference of temperature becomes bigger. The purpose of this study is to estimate the optimum placing depth. The temperature of real mat foundation was observed and the thermal analysis by Finite Element Method was executed. Finally, the crack index according to the placing depth was estimated.

• Journal of the Korea Computer Industry Society
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• v.9 no.3
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• pp.99-104
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• 2008

The thermal stresses due to hydration heat in massive concrete structures are affected by ambient temperature and placing concrete temperature. It is needed to predict the thermal stresses considering ambient temperature and placing concrete temperature. In this study, hydration heat analyses of coping were carried out. After the maximum tensile stress was occurred at 2,75 days the crack index was increased. Therefore the possibility of crack occurrence was rare. The possibility of crack occurrence can be reduced by placing concrete temperature drop. Therefore some method to drop the placing concrete temperature may be effective to reduce the possibility of crack occurrence.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.5
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• pp.134-140
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• 2017

In this study, the flexural behavior of high strength concrete members with different curing condition of 90 MPa of compressive strength was investigated. Experimental parameters included normal and low temperature curing conditions, tensile steel amount and concrete compressive strength. 8 beam members were fabricated and flexural tests were carried out. Crack spacing, load-deflection relation, load-strain relation and ductility index were determined. Experimental results show that as the amount of rebar increases, the number of cracks increases and the crack spacing decreases. The higher the concrete strength, the smaller the number of cracks, but the effect is significantly smaller than the amount of rebar. As a result of comparison with the proposed average crack spacing in the design criteria, the experimental results are slightly larger than the results of the proposed formula, but the proposed formula does not reflect the concrete strength and curing conditions. The ductility index of normal temperature cured members was 3.36~6.74 and the ductility index of low temperature cured members was 1.51~2.82. The behavior of low temperature cured members was found to be lower than that of normal temperature cured members. As a result of comparing the ductility index with the existing studies similar to the experimental members, the ductility index of the high strength concrete member was larger than the ductility index of the ordinary strength concrete of the previous study. Further research is needed to understand more specific results.