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CONTENTS
Volume 7, Number 5, October 2010
 

Abstract
The bond mechanism for reinforcing bars in concrete is equivalent to the normal contact and friction between the inclined ribs and the surrounding concrete. Based on the contact density model for the computation of shear transfer across cracks, an open-slip coupled model was developed for simulating three-dimensional bond behavior for reinforcing bars in concrete. A parameter study was performed and verified by simulating pull-out experiments of extremely different boundary conditions: short bar embedment with a huge concrete cover, extremely long bar embedment with a huge concrete cover, embedded aluminum bar and short bar embedded length with an insufficient concrete cover. The bar strain effect and splitting of the concrete cover on a local bond can be explained by finite element (FE) analysis. The analysis shows that the strain effect results from a large local slip and the splitting effect of a large opening of the interface. Finally, the sensitivity of rebar geometry was also checked by FE analysis and implies that the open-slip coupled model can be extended to the case of plain bar.

Key Words
bond; open-slip coupled model; strain effect; splitting effect; bar geometry.

Address
Feng Shang, Xuhui An and Seji Kawai: State Key Laboratory of Hydro Science and Engineering, Tsinghua University,
100084-Beijing, China
Tetsuya Mishima: Maeda Corporation, Tokyo, Japan

Abstract
This paper utilizes the modified Davis model and the mode coupling theory, as parts of the electrolyte solution theory, to investigate the diffusivity of the ion in concrete. Firstly, a computational model of the ion diffusion coefficient, which is associated with ion species, pore solution concentration, concrete mix parameters including water-cement ratio and cement volume fraction, and microstructure parameters such as the porosity and tortuosity, is proposed in the saturated concrete. Secondly, the experiments, on which the chloride diffusion coefficient is measured by the rapid chloride penetration test, have been carried out to investigate the validity of the proposed model. The results indicate that the chloride diffusion coefficient obtained by the proposed model is in agreement with the experimental result. Finally, numerical simulation has been completed to investigate the effects of the porosity, tortuosity, water-cement ratio, cement volume fraction and ion concentration in the pore solution on the ion diffusion coefficients. The results show that the ion diffusion coefficient in concrete increases with the porosity, water-cement ratio and cement volume fraction, while we see a decrease with the increasing of tortuosity. Meanwhile, the ion concentration produces more obvious effects on the diffusivity itself, but has almost no effects on the other ions.

Key Words
diffusion coefficient; concrete; ion; model; electrolyte solution theory.

Address
Xiao-Bao Zuo: Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, P.R. China
Department of Civil Engineering, Nanjing University Of Science & Technology, Nanjing 210094, P.R. China
Wei Sun and Cheng Yu: Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, P.R. China
Xu-Rong Wan: Department of Civil Engineering, Nanjing University Of Science & Technology, Nanjing 210094, P.R. China


Abstract
CFRP has been widely used for strengthening reinforced concrete members in last decade. The strain transfer mechanism from concrete face to CFRP is a key factor for rigidity, ductility, energy dissipation and failure modes of concrete members. For these reasons, determination of the effective CFRP bonding length is the most crucial step to achieve effective and economical strengthening. In this paper, generalizations are made on effective bonding length by increasing the amount of test data. For this purpose, ANSYS software is employed, and an experimentally verified nonlinear finite element model is prepared. Special contact elements are utilized along the concrete-CFRP strip interface for investigating stress distribution, load-displacement behavior, and effective bonding length. Then results are compared with the experimental results. The finite element model found consistent results with the experimental findings.

Key Words
effective CFRP length; nonlinear finite element analysis; CFRP-concrete bonding.

Address
Ali Baran Dogan: MITENG Engineering Inc., Eski Guvercinlik Yolu, Gazi Mah., Ankara, Turkey, 06560
Ozgur Anil: Department of Civil Engineering, Gazi University, Maltepe, Ankara, Turkey, 06570

Abstract
This paper presents a prediction and simulation method of tensile stress-strain curves of Engineered Cementitious Composites (ECC). For this purpose, the bridging stress and crack opening relations were obtained by the fiber bridging constitutive law which is quantitatively able to consider the fiber distribution characteristics. And then, a multi-linear model is employed for a simplification of the bridging stress and crack opening relation. In addition, to account the variability of material properties, randomly distributed properties drawn from a normal distribution with 95% confidence are assigned to each element which is determined on the basis of crack spacing. To consider the variation of crack spacing, randomly distributed crack spacing is drawn from the probability density function of fiber inclined angle calculated based on sectional image analysis. An equation for calculation of the crack spacing that takes into quantitative consideration the dimensions and fiber distribution was also derived. Subsequently, a series of simulations of ECC tensile stress-strain curves was performed. The simulation results exhibit obvious strain hardening behavior associated with multiple cracking, which correspond well with test results.

Key Words
ECC; tensile strain hardening; fiber bridging relations; fiber distribution; image analysis; variability of material properties.

Address
Bang Yeon Lee: University of Michigan, Ann Arbor, USA
Jin-Keun Kim: KAIST, Daejeon, South Korea
Yun Yong Kim: Chungnam National University, Daejeon, South Korea

Abstract
This paper investigates the concrete permeability through a numerical and statistical approach. Concrete is considered as a random heterogeneous composite of three phases: aggregates, interfacial transition zones (ITZ) and matrix. The paper begins with some classical bound and estimate theories applied to concrete permeability and the influence of ITZ on these bound and estimate values is discussed. Numerical samples for permeability analysis are established through random aggregate structure (RAS) scheme, each numerical sample containing randomly distributed aggregates coated with ITZ and dispersed in a homogeneous matrix. The volumetric fraction of aggregates is fixed and the size distribution of aggregates observes Fuller

Key Words
concrete; permeability; interfacial transition zone (ITZ); random aggregate structure (RAS); finite element method (FEM); representative volume element (RVE).

Address
Chunsheng Zhou and Kefei Li: Key Laboratory of Structural Engineering and Vibration of China Education Ministry, Civil Engineering Department, Tsinghua University, 100084 Beijing, P.R. China


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