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CONTENTS
Volume 16, Number 5, November 2015
 

Abstract
Degradation of reinforced concrete (RC) structures due to chloride penetration followed by reinforcement corrosion has been a serious problem in civil engineering for many years. The numerical simulation methods at present are mainly finite element method (FEM) and finite difference method (FDM), which are based on mesh. Mesh generation in engineering takes a long time. In the present article, the numerical solution of chloride transport in concrete is analyzed using radial point interpolation method (RPIM) and element-free Galerkin (EFG). They are all meshless methods. RPIM utilizes radial polynomial basis, whereas EFG uses the moving least-square approximation. A Galerkin weak form on global is used to attain the discrete equation, and four different numerical examples are presented. MQ function and appropriate parameters have been proposed in RPIM. Numerical simulation results are compared with those obtained from the finite element method (FEM) and analytical solutions. Two case of chloride transport in full saturated and unsaturated concrete are analyzed to test the practical applicability and performance of the RPIM and EFG. A good agreement is obtained among RPIM, EFG, and the experimental data. It indicates that RPIM and EFG are reliable meshless methods for prediction of chloride concentration in concrete structures.

Key Words
meshless method; chloride diffusion; RPIM; EFG; corrosion

Address
Ling Yao, Ling Zhang and Xiaolu Li: State Key Laboratory for Strength and Vibration Of Mechanical Structures, Xi\'an Jiaotong University,No.28, Xianning West Road, Xi\'an, Shaanxi, 710049, China
Lingling Zhang: School Of Human Settlements and Civil Engineering, Xi\'an Jiaotong University,No.28, Xianning West Road, Xi\'an, Shaanxi, 710049, China

Abstract
When approached using nonlinear finite element (FE) techniques, structural analyses generate,for real RC structures, large complex numerical problems. Damage is a major part of concrete behavior, and the discretization technique is critical to limiting the size of the problem. Based on previous work, the u damage model has been designed to activate the various damage effects correlated with monotonic and cyclic loading, including unilateral effects. Assumptions are formulated to simplify constitutive relationships while still allowing for a correct description of the main nonlinear effects. After presenting classical 2D finite element applications on structural elements, an enhanced simplified FE description including a damage description and based on the use of multi-fiber beam elements is provided. Improvements to this description are introduced both to prevent dependency on mesh size as damage evolves and to take into account specific phenomena (permanent strains and damping, steel-concrete debonding). Applications on RC structures subjected to cyclic loads are discussed, and results lead to justifying the various concepts and assumptions explained.

Key Words
concrete; damage models; cyclic loading; simplified modeling; cracking indicators

Address
Jacky Mazars and Stephane Grange: University Grenoble Alpes, 3SR Laboratory, 38000 Grenoble, France

Abstract
This study investigates the performance of advanced hollow reinforced concrete (RC) bridge column sections with triangular reinforcement details. Hollow column sections are based on economic considerations of cost savings associated with reduced material and design moments, as against increased construction complexity, and hence increased labor costs. The proposed innovative reinforcement details are economically feasible and rational, and facilitate shorter construction periods. We tested a model of advanced hollow column sections under quasi-static monotonic loading. The results showed that the proposed triangular reinforcement details were equal to the existing reinforcement details, in terms of the required performance. We used a computer program, Reinforced Concrete Analysis in Higher Evaluation System Technology (RCAHEST), for analysis of the RC structures; and adopted a modified lateral confining effect model for the advanced hollow bridge column sections. Our study documents the testing of hollow RC bridge column sections with innovative reinforcement details, and presents conclusions based on the experimental and analytical findings. Additional full-scale experimental research is needed to refine and confirm the design details, especially for the actual detailing employed in the field.

Key Words
performance; hollow; column sections; triangular reinforcement details; cost savings;construction periods

Address
T.H. Kim, S.H. Lee: Technology Development Team, Samsung Construction & Trading Corporation,5th Fl., Daerung Gangnam Tower, 826-20 Yeoksam1-dong, Gangnam-gu, Seoul 135-935, Korea

H.Y. Kim, J.H. Lee: Department of Civil Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si,Gyeongsangbuk-do, 712-749, Korea

H.M. Shin: School of Civil and Architectural Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, Korea



Abstract
In this study, the seismic performance of skewed highway bridges has been assessed by using fragility function methodology. Incremental Dynamic Analysis (IDA) has been used to prepare complete information about the different damage states of a 30 degree skewed highway bridge. A three dimensional model of a skewed highway bridge is presented and incremental dynamic analysis has been applied. The details of the full nonlinear procedures have also been presented. Different spectral intensity measures are studied and the effects of the period on the fragility curves are shown in different figures. The efficiency, practicality and proficiency of these different spectral intensity measures are compared. A suite of 20 earthquake ground motions are considered for nonlinear time history analysis. It has been shown that, considering different intensity measures (IM) leads us to overestimate or low estimate the damage probability which has been discussed completely.

Key Words
nonlinear time history analysis; fragility function methodology; intensity measures

Address
M. Bayat and F. Daneshjoo: Department of Civil and Environmental Engineering,Tarbiat Modares University, Tehran, Iran

Abstract
In the last decade, several modeling approaches have been proposed and applied to estimate the high-performance concrete (HPC) slump flow. While HPC is a highly complex material, modeling its behavior is a very difficult issue. Thus, the selection and application of proper modeling methods remain therefore a crucial task. Like many other applications, HPC slump flow prediction suffers from noise which negatively affects the prediction accuracy and increases the variance. In the recent years,ensemble learning methods have introduced to optimize the prediction accuracy and reduce the prediction error. This study investigates the potential usage of bagging (Bag), which is among the most popular ensemble learning methods, in building ensemble models. Four well-known artificial intelligence models (i.e., classification and regression trees CART, support vector machines SVM, multilayer perceptron MLP and radial basis function neural networks RBF) are deployed as base learner. As a result of this study, bagging ensemble models (i.e., Bag-SVM, Bag-RT, Bag-MLP and Bag-RBF) are found superior to their base learners (i.e., SVM, CART, MLP and RBF) and bagging could noticeable optimize prediction accuracy and reduce the prediction error of proposed predictive models.

Key Words
bagging (bootstrap aggregating); classification and regression trees; ensemble learning; multilayer perceptron; support vector machines

Address
Hacer Yumurtaci Aydogmus: Department of Industrial Engineering, Alanya Alaaddin Keykubat University,07450, Alanya-Antalya, Turkey
Halil Ibrahim Erdal: Turkish Cooperation and Coordination Agency (TİKA), Ataturk Bulvar No:15 Ulus-Ankara, Turkey

Abstract
This study simulates the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams reinforced with steel and glass fiber-reinforced polymer (GFRP) rebars. For this, micromechanics-based modeling was first carried out on the basis of single fiber pullout models considering inclination angle. Two different tension-softening curves (TSCs) with the assumptions of 2-dimensional (2-D) and 3-dimensional (3-D) random fiber orientations were obtained from the micromechanics-based modeling, and linear elastic compressive and tensile models before the occurrence of cracks were obtained from the mechanical tests and rule of mixture. Finite element analysis incorporating smeared crack model was used due to the multiple cracking behaviors of structural UHPFRC beams, and the characteristic length of two times the element width (or two times the average crack spacing at the peak load) was suggested as a result of parametric study. Analytical results showed that the assumption of 2-D random fiber orientation is appropriate to a non-reinforced UHPFRC beam, whereas the assumption of 3-D random fiber orientation is suitable for UHPFRC beams reinforced with steel and GFRP rebars due to disorder of fiber alignment from the internal reinforcements. The micromechanics-based finite element analysis also well predicted the serviceability deflections of UHPFRC beams with GFRP rebars and hybrid reinforcements.

Key Words
ultra-high-performance fiber-reinforced concrete; flexure; micromechanics; fiber orientation; reinforcement; finite element analysis

Address
Doo-Yeol Yooa and Nemkumar Banthia: Department of Civil Engineering, The University of British Columbia, 6250 Applied Science Lane, Vancouver, BC V6T 1Z4, Canada


Abstract
Moisture transport in concrete in atmospheric environment was studied in this paper. Based on the simplified formula of the thickness of the adsorbed layer, the pore-size distribution function of cement paste was calculated utilizing the water adsorption isotherms. Taking into consideration of the hysteresis effect in cement paste, the moisture diffusivity of cement paste was obtained by the integration of the pore-size distribution. Concrete is regarded as a two-phase composite with cement paste and aggregate, neglecting the moisture diffusivity of aggregate, then moisture diffusivity of concrete was evaluated using the composite theory. Finally, numerical simulation of humidity response during both wetting and drying process was carried out by the finite difference method of partialdifferential equation for moisture transport, and the numerical results well capture the trend of the measured data.

Key Words
concrete; moisture transport; atmospheric environment; pore-size distribution

Address
Weiping Zhang, Fei Tong, Xianglin Gu and Yunping Xi: Department of Structural Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China

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