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CONTENTS
Volume 8, Number 2, April 2011
 

Abstract
In the present paper, a numerical simulation method based on mesoscopic composite structure of concrete, the truss network model, is developed to evaluate the diffusivity of concrete in order to account for the microstructure of concrete, the binding effect of chloride ions and the chloride concentration dependence. In the model, concrete is described as a three-phase composite, consisting of mortar, coarse aggregates and the interfacial transition zones (ITZs) between them. The advantage of the current modelis that it can easily represent the movement of mass (e.g. water or chloride ions) through ITZs or the potential cracks within concrete. An analytical method to estimate the chloride diffusivity of mortar and ITZ, which are both treated as homogenious materials in the model, is introduced in terms of water-tocement ratio (w/c) and sand volume fraction. Using the newly developed approaches, the effect of cracking of concrete on chloride diffusion is reflected by means of the similar process as that in the test. The results of calculation give close match with experimental observations. Furthermore, with consideration of the binding capacity of chloride ions to cement paste and the concentration dependence for diffusivity, the onedimensional nonlinear diffusion equation is established, as well as its finite difference form in terms of the truss network model. A series of numerical analysises performed on the model find that the chloride diffusion is substantially influenced by the binding capacity and concentration dependence, which is same as that revealed in some experimental investigations. This indicates the necessity to take into account the binding capacity and chloride concentration dependence in the durability analysis and service life prediction of concrete structures.

Key Words
mesoscale simulation; chloride diffusion; binding capacity; concentration dependence; truss network model.

Address
Licheng Wang: State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian 116024, China
Tamon Ueda: Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan

Abstract
Advanced material models for concrete are not widely available in general purpose finite element codes. Parameters to define them complicate the implementation because they are case sensitive. In addition to this, their validity under severe shear condition has not been verified. In this article, simple engineering plasticity material models available in a commercial finite element code are used to demonstrate that complicated shear behavior can be calculated with reasonable accuracy. For this purpose dynamic response of a squat shear wall that had been tested on a shaking table as part of an experimental program conducted in Japan is analyzed. Both the finite element and material aspects of the modeling are examined. A corrective artifice for general engineering plasticity models to account for shear effects in concrete is developed. The results of modifications in modeling the concrete in compression are evaluated and compared with experimental response quantities.

Key Words
shear; compression softening; squat wall; plasticity; finite element method; ANSYS.

Address
Ilker Kazaz: Dept. of Civil Engineering, Ataturk University, 25240 Erzurum, Turkey

Abstract
By virtue of chord-length density function from the field of statistical physics, this paper introduced a quantitative approach to estimate the distribution of cement paste thickness between aggregates in concrete. Dynamics mixing method based on molecular dynamics was employed to generate one model structure, then image analysis algorithm was used to obtain the distribution of thickness of cement paste in model structure for the purpose of verification. By comparison of probability density curves and cumulative probability curves of the cement paste thickness among neighboring aggregates, it is found that the theoretical results are consistent with the simulation. Furthermore, for the model mortar and concrete mixtures with practical volume fraction of Fuller-type aggregate, this analytical formula was employed to predict the influence of aggregate volume fraction and aggregate fineness. And evolution of its mean values were also investigated with the variation of volume fraction of aggregate as well as the fineness of aggregates in model mortars and concretes.

Key Words
concrete; cement paste; particle size distribution; aggregate; probability density function; cumulative probability function.

Address
Huisu Chen: Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, P.R.China;
School of Materials Science and Engineering, Southeast University, Nanjing 211189, P.R.China;
Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft 2628CN, The Netherlands
Lambertus Johannes Sluys and Piet Stroeven: Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft 2628CN, The Netherlands
Wei Sun: Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, P.R.China;
School of Materials Science and Engineering, Southeast University, Nanjing 211189, P.R.China

Abstract
This research paper aims at computer based modeling of carbonation induced corrosion under extreme conditions and its experimental verification by incorporating enhanced electrochemical and mass balance equations based on thermo-hygro physics with strong coupling of mass transport and equilibrium in micro-pore structure of carbonated concrete for which the previous research data is limited. In this paper the carbonation induced electrochemical corrosion model is developed and coupled with carbon dioxide transport computational model by the use of a concrete durability computer based model DuCOM developed by our research group at concrete laboratory in the University of Tokyo and its reliability is checked in the light of experiment results of carbonation induced corrosion mass loss obtained in this research. The comparison of model analysis and experiment results shows a fair agreement. The carbonation induced corrosion model computation reasonably predicts the quantitative behavior of corrosion rate for normal air dry relative humidity conditions. The computational model developed also shows fair qualitative corrosion rate simulation and analysis for various pH levels and coupled environmental actions of chloride and carbonation. Detailed verification of the model for the quantitative carbonation induced corrosion rate computation under varying relative conditions, different pH levels and combined effects of carbonation and chloride attack remain as scope for future research.

Key Words
carbonation; corrosion rate; reinforced concrete; mass transport; computational modeling; computer applications in concrete.

Address
Raja Rizwan Hussain: Center of Excellence for Concrete Research, Civil Engineering Department, College of Engineering, King Saud University Riyadh, 11421, Saudi Arabia

Abstract
The use of Carbon Fiber Reinforced Polymers (CFRP) to strengthen reinforced concrete beams under bending and shear has gained rapid growth in recent years. The performance of shear strengthened beams with externally bonded CFRP laminate or fabric strips is raising many concerns when the beam is loaded under cyclic loading. Such concerns warrant experimental, analytical and numerical investigation of such beams under cyclic loading. To date, limited investigations have been carried out to address this concern. This paper presents a numerical investigation by developing a nonlinear finite element (FE) model to study the response of a cantilever reinforced concrete T-beam strengthened in shear with side bonded CFRP fabric strips and subjected to cyclic loading. A detailed 3D nonlinear finite element model that takes into account the orthotropic nature of the polymer

Key Words
cyclic loading; nonlinear analysis; CFRP; reinforced concrete; side bonded.

Address
Rami A. Hawileh and Jamal A. Abdalla: Dept. of Civil Engineering, American University of Sharjah, Sharjah, UAE
Murat H. Tanarslan: Dept. of Civil Engineering, Dokuz Eylul University, Buca, Izmir 35160, Turkey
Mohannad Z. Naser: Dept. of Civil Engineering, American University of Sharjah, Sharjah, UAE

Abstract
In flexural strength design of unconfined reinforced concrete (RC) members, the concrete compressive stress-strain curve is scaled down from the uni-axial stress-strain curve such that the maximum concrete stress adopted in design is less than the uni-axial strength to account for the strain gradient effect. It has been found that the use of this smaller maximum concrete stress will underestimate the flexural strength of unconfined RC members although the safety factors for materials are taken as unity. Herein, in order to investigate the effect of strain gradient on the maximum concrete stress that can be developed in unconfined flexural RC members, several pairs of plain concrete (PC) and RC inverted T-shaped specimens were fabricated and tested under concentric and eccentric loads. From the test results, the maximum concrete stress developed in the eccentric specimens under strain gradient is determined by the modified concrete stress-strain curve obtained from the counterpart concentric specimens based on axial load and moment equilibriums. Based on that, a pair of equivalent rectangular concrete stress block parameters for the purpose of flexural strength design of unconfined RC members is determined.

Key Words
rectangular stress block parameters; reinforced concrete; strain gradient; uni-axial concrete stress.

Address
J.C.M. Ho, H.J. Pam and J. Peng: Dept. of Civil Engineering, The University of Hong Kong, Hong Kong
Y.L. Wong: Dept. of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong

Abstract
In this work, cement paste samples with 1% (by cement mass) of a conductive carbon fiber admixture have been studied under uniaxial compression. Three different arrangements were used to measure the resistivity of the samples. According to the results obtained, the resistance should be measured using the four wire method in order to obtain good sensitivity and repeatability. The effect of the load value and the load rate on the fractional change of the volume resistivity has been determined. It has been observed that the gage factor (fractional change in resistance respect to strain) increases when the maximum load is increased, and the loading rate does not affect significantly this parameter. The effect of the sample ambient humidity on the material piezoresistivity has also been studied, showing that the response of the composite is highly affected by this parameter.

Key Words
carbon fiber; cement; piezoresistivity; sensor; strain; stress.

Address
F.J. Baeza, E. Zornoza, L.G. Andion, S. Ivorra and P. Garces: Dpto. de Ingenieria de la Construccion, Universidad de Alicante, Spain


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