• Proceedings of the Korea Concrete Institute Conference
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• 1999.04a
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• pp.239-244
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• 1999

The reduction phenomena of the compressive strength of concrete with respect to the size of specimens have been extensively investigated. However, adequate analysis technique have not been developed until now. Existing researches have shown that the larger member size, the smaller the strength. This indicated the necessity of nonlinear fracture mechanics theory in order to analyze the fracture behaviors of concrete. The are some models that predict the size effect of compressive strength of cylindrical specimens. Theses equations, however, are developed not considering the difference of fracturing mechanism which depends on both geometry of specimen and the strength level of concrete. In this paper, a model to predict compressive strength of cylindrical concrete specimens with respect to diameters, h/d ratios, and the strength level of concrete, is suggested. For this purpose, theoretical and statistical analyses are conducted. Experimental constants used in the model of new size effect are formulated in terms of strength levels of concrete based on existing experimental data.

• Structural Engineering and Mechanics
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• v.90 no.5
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• pp.481-492
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• 2024

This research examined the mechanical behavior of fiber-reinforced concrete at unstable environmental conditions. Concrete composites with varying percentages of steel and glass fibers were analyzed. Compressive, indirect tensile, and fracture toughness properties were evaluated using the Edge Notched Disc Bend (ENDB) test under freezing-thawing and acidic environments and the results were compared with normal conditions. Steel fibers decreased the strength in the specified cycles, while glass fibers showed a normal strength trend. The compressive, tensile and fracture toughness of the samples containing 1.5 vol.% fibers showed a 1.28-, 2.13- and 4.5-fold enhancement compared to samples without fibers, after 300 freezing-thawing cycles, respectively.

• Structural Engineering and Mechanics
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• v.84 no.2
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• pp.285-293
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• 2022

Lumped damage mechanics (LDM) is a recent nonlinear theory with several applications to civil engineering structures, such as reinforced concrete and steel buildings. LDM apply key concepts of classic fracture and damage mechanics on plastic hinges. Therefore, the lumped damage models are quite successful in reproduce actual structural behaviour using concepts well-known by engineers in practice, such as ultimate moment and first cracking moment of reinforced concrete elements. So far, lumped damage models are based in the strain energy equivalence hypothesis, which is one of the fictitious states where the intact material behaviour depends on a damage variable. However, there are other possibilities, such as the energy equivalence hypothesis. Such possibilities should be explored, in order to pursue unique advantages as well as extend the LDM framework. Therewith, a lumped damage model based on the energy equivalence hypothesis is proposed in this paper. The proposed model was idealised for reinforced concrete structures, where a damage variable accounts for concrete cracking and the plastic rotation represents reinforcement yielding. The obtained results show that the proposed model is quite accurate compared to experimental responses.

• Magazine of the Korea Concrete Institute
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• v.7 no.4
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• pp.149-156
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• 1995

The fracture process zone in concrete is a region ahead of a traction-free crack, in which two major mechanisms, microcracking and bridging, play important roles. The toughness due to bridging is dominant compared to toughness induced by microcracking, so that the bridging is dominani: mechanism governing the fracture process of concrete. Fracture mechanics does work for concrete provided that the fracture process zone is being considered, so that the development of model for the fracture process zone is most important to describe fracture phenomena in concrete. In this paper the bridging zone, which is a part of extended rnacrocrack with stresses transmitted by aggregates in concrete, is modelled by a Dugdale-Barenblatt type model with linear tension-softening curve. Two finite element techniques are shown for the analysis of progressive cracking in concrete based on the discrete crack approach: one with crack element, the other without crack element. The advantage of the technique with crack element is that it dees not need to update the mesh topology to follow the progressive cracking. Numerical results by the techniques are demonstrated.

• Structural Engineering and Mechanics
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• v.19 no.5
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• pp.567-580
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• 2005

Numerous tests on concrete structure members under local pressure demonstrated that the compressive strength of concrete at the loaded surface is increased by the confinement effect provided by the enveloping concrete. Even though most design codes propose specific criteria for preventing bearing failure, they do not take into consideration size effect which is an important phenomenon in the fracture mechanics of concrete/reinforced concrete. In this paper, six series of square prism concrete blocks with three different depths (size range = 1:4) and two different height/depth ratios of 2 and 3 are tested under concentrated load. Ultimate loads obtained from the test results are analysed by means of the modified size effect law (MSEL). Then, a prediction formula, which considers effect of both depth and height on size effect, is proposed. The developed formula is compared with experimental data existing in the literature. It is concluded that the observed size effect is in good agreement with the MSEL.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.13 no.1
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• pp.25-37
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• 1993

This paper describes the use of the J-integral, a one parameter of the non-linear fracture mechanics(NLFM), as a means to measure toughness of steel fiber reinforced concrete. This parameter can be conveniently evaluated from experimentally determined load-deflection curves from flexural tests when a maximum-load failure criterion is employed. And, for high strength concrete which was reinforced steel fiber, with two different fiber length in the form of notched beams, were tested under 3-point bending, and $J_{IC}$, as well as the linear elastic fracture mechanics(LEFM) parameters $K_{IC}$ and $G_{IC}$ were evaluated. The results suggest that $J_{IC}$ is a promising fracture criterion for all of these. while $K_{IC}$(or $G_{IC}$) almost certainly are not. Also it was found that a fiber addition of less than 0.5% did not improve the fracture toughness of the high strength concrete. However, at more than 1.0% in fiber contents, $J_{IC}$ showed significant increases. reflecting the changed character of the concrete; $K_{IC}$ and $G_{IC}$ did not.

• Magazine of the Korea Concrete Institute
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• v.6 no.3
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• pp.201-214
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• 1994

The fracture process zone in concrete is a region ahead of a traction-free crack, in which two major mechanisms, microcracking and bridging, play important roles. The toughness due to bridging is dominant compared to toughness induced by microcracking, so that the bridging is dominani: mechanism governing the fracture process of concrete. Fracture mechanics does work for concrete provided that the fracture process zone is being considered, so that the development of model for the fracture process zone is most important to describe fracture phenomena in concrete. In this paper the bridging zone, which is a part of extended rnacrocrack with stresses transmitted by aggregates in concrete, is modelled by a Dugdale-Barenblatt type model with linear tension-softening curve. Two finite element techniques are shown for the analysis of progressive cracking in concrete based on the discrete crack approach: one with crack element, the other without crack element. The advantage of the technique with crack element is that it dees not need to update the mesh topology to follow the progressive cracking. Numerical results by the techniques are demonstrated.

• Magazine of the Korea Concrete Institute
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• v.11 no.2
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• pp.95-103
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• 1999

The reduction phenomena of concrete compressive strength with the size of specimens have been extensively investigated, but till now the adequate analysis technique is not fixed. The existing research results show that the bigger the member size, the smaller the strength. This means the nonlinear fracture mechanics theory is needed in order to analyze the fracture behaviors of concrete and the size effect. There is a few model equations that is to predict the size effect of compressive strength of standard and non-standard cylindrical specimen. However, theses equations did not considered the difference of fracturing mechanism which depends on the strength level. In this paper, model equations to predict compressive strength of concrete considering the size effect and strength level are suggested. The size effect model suggested in this paper shows good prediction compared with the existing test data of various concrete size and strength level.

• Journal of the Korea Concrete Institute
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• v.14 no.1
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• pp.58-66
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• 2002

The objective of this study is to examine the fracture characteristics of concrete at early ages such as critical stress intensity factor, critical crack-tip opening displacement, fracture energy, and bilinear softening curve based on the concepts of the effective-elastic crack model and the cohesive crack model. A wedge splitting test for Mode I was performed on cubic wedge specimens with a notch at the edge. By taking various strengths and ages, load-crack mouth opening displacement curves were obtained, and the results were analyzed by linear elastic fracture mechanics and the finite element method. The results from the test and analysis showed that critical stress intensity factor and fracture energy increased, and critical crack-tip opening displacement decreased with concrete ages from 1 day to 28 days. By numerical analysis four parameters of bilinear softening curve from 1 day to 28 days were obtained. The obtained fracture parameters and bilinear softening curves at early ages may be used as a fracture criterion and an input data for finite element analysis of concrete at early ages.

• Journal of the Korea institute for structural maintenance and inspection
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• v.2 no.4
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• pp.159-171
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• 1998

This study attempts to develop a rational approach to flexural concrete beam analysis with crack growth. In order to develope analytical solutions, several simplification and assumption are made and the Hillerborg fictitious crack model is adapted for new rational approach to the flexural concrete beam. To provide desired results, the concrete beams with various conditions(more than 126 beam conditions) are analyzed. Before producing the results, these assumptions are founded to be justified by comparison with a FE analysis. The results for each condition of the beams are presented in terms of crack lengths, the strength and cracking stability of concrete beams. And also size effects in a flexural concrete beam is studied using a new flexural cracking model.