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CONTENTS
Volume 12, Number 4, October 2013
 

Abstract
This paper investigates prediction models estimating the hydration properties of concrete, such as the compressive strength, the splitting tensile strength, the elastic modulus,and the autogenous shrinkage. A prediction model is suggested on the basis of an equation that is formulated to predict the compressive strength. Based on the assumption that the apparent activation energy is a characteristic property of concrete, a prediction model for the compressive strength is applied to hydration-related properties. The hydration properties predicted by the model are compared with experimental results, and it is concluded that the prediction model properly estimates the splitting tensile strength, elastic modulus, and autogenous shrinkage as well as the compressive strength of concrete.

Key Words
temperature; aging; hydration properties; apparent activation energy; prediction model

Address
Inyeop Chu: Civil Engineering Team, Samsung Engineering Co. Ltd., Seoul, Korea
Muhammad Nasir Amin: Civil Engineering Dept., National University of Science and Technology, Islamabad, Pakistan
Jin-Keun Kim: Civil and Environmental Engineering Dept, Korea Advanced Institute of Science & Technology, Daejeon, Korea

Abstract
In the analysis and design of reinforced concrete frames beam-column joints are sometimes assumed as rigid. This simplifying assumption can be unsafe because it is likely to affect the distributions of internal forces and moments, reduce drift and increase the overall load-carrying capacity of the frame. This study is concerned with the relevance of shear deformation of beam-column joints, in particular of exterior ones, on the quasi-static behavior of regular reinforced concrete sway frames. The included parametric studies of a simple sub-frame model reveal that the quasi-static monotonic behavior of unbraced regular reinforced concrete frames is prone to be significantly affected by the deformation of beam-column joints.

Key Words
reinforced concrete beam-column joints; joint shear deformation, quasi-static monotonic behavior; non-linear analysis; sway rc framed structures

Address
Ricardo J.T. Costa, Paulo M.M.P. Providência and Alfredo M.P.G. Dias: Department of Civil Engineering, FCTUC - Polo II, University of Coimbra, Rua Luís Reis Santos,
3030-788 Coimbra, Portugal
Fernando C.T. Gomes: Department of Technology, ESTGOH, Oliveira do Hospital, Portugal
Paulo M.M.P. Providência: NESC Coimbra, Rua Antero de Quental 199, 3000-033 Coimbra, Portugal

Abstract
The primary aim of this study is to develop a three dimensional finite element (FE) model to predict the axial stress-strain relationship and ultimate strength of the FRP-wrapped UHPC columns by comparing experimental results. The reliability of four selected confinement models and three design codes such as ACI-440, CSA-S806-02, and ISIS CANADA is also evaluated in terms of agreement with the experimental results. Totally 6 unconfined and 36 different types of the FRP-wrapped UHPC columns are tested under monotonic axial compression. The values of ultimate strengths of FRP-wrapped UHPC columns obtained from the experimental results are compared and verified with finite element (FE) analysis results and the design codes mentioned above. The concrete damage plasticity model (CDPM) in Abaqus is utilized to represent the confined behavior of the UHPC. The results indicate that agreement between the test results and the non-linear FE analysis results is highly satisfactory. The CSA-S806-02 design code is considered more reliable than the ACI-440 and the ISIS CANADA design codes to calculate the ultimate strength of the FRP-wrapped UHPC columns. None of the selected confinement models that are developed for FRP-wrapped low and normal strength concrete columns can safely predict the ultimate strength of FRP-wrapped UHPC columns.

Key Words
ultra-high performance concrete; fiber reinforced polymer; finite element analysis; ultimate strength; design codes

Address
Soner Guler and Alperen Çopur and Metin Aydogan: Faculty of Civil Engineering, Istanbul Technical University, 34469, Istanbul, Turkey

Abstract
This study attempted to find a proper method applicable to simulating practical equifield lines of two-dimensional Accelerate Lithium Migration Technique (ALMT), and evaluate the feasibility of using the theoretical ion migration model of one-dimensional ALMT to predict the ion migration behavior of two-dimensional ALMT. The result showed that the electrolyte or carbon plate can be used as matrix to draw equifield line graph similar to that by using mortar as matrix. Using electrolyte electrode module for simulation has advantages of simple production, easy measurement, rapidness, and economy. The electrolyte module can be used to simulate the equifield line distribution diagram in practical two-dimensional electrode configuration firstly. Then, several equifield line zones were marked, and several subzones under one-dimensional ALMT were separated from various equifield line zones. The theoretical free content distribution of alkali in concrete under two-dimensional electric field effect could be obtained from duration analysis.

Key Words
electrochemical; alkali-silica reaction; migration; electrode

Address
Chih-Chien Liu and Wen-Ten Kuo: Department of Civil Engineering, ROC Military Academy, No.1, Wei-Wu Rd., Fengshan District, Kaohsiung 83059, Taiwan, R.O.C.
Chun-Yao Huang: Department of Civil Engineering, National Kaohsiung University of Applied Sciences, No. 415, Chien-Kung Rd., Sanmin District, Kaohsiung 80778, Taiwan, R.O.C.

Abstract
A detailed finite element modeling is presented for the simulation of the nonlinear behavior of reinforced concrete structures which manages to predict the nonlinear behavior of four different experimental setups with computational efficiency, robustness and accuracy. The proposed modeling method uses 8-node hexahedral isoparametric elements for the discretization of concrete. Steel rebars may have any orientation inside the solid concrete elements allowing the simulation of longitudinal as well as transverse reinforcement. Concrete cracking is treated with the smeared crack approach, while steel reinforcement is modeled with the natural beam-column flexibility-based element that takes into consideration shear and bending stiffness. The performance of the proposed modeling is demonstrated by comparing the numerical predictions with existing experimental and numerical results in the literature as well as with those of a commercial code. The results show that the proposed refined simulation predicts accurately the nonlinear inelastic behavior of reinforced concrete structures achieving numerical robustness and computational efficiency.

Key Words
reinforced concrete; smeared crack; embedded reinforcement; natural beam-column element,; flexibility element

Address
George Markou: Alhosn University, Department of Civil Engineering, P.O.Box 38772, Abu Dhabi, UAE
Manolis Papadrakakis: Institute of Structural Analysis & Seismic Research, National Technical University of Athens, 9 Iroon Polytechniou Str., Zografou Campus,GR-15780 Athens, Greece

Abstract
Fiber reinforced polymer (FRP) jackets have been widely used as an effective tool for the strengthening and rehabilitation of concrete structures, especially damaged concrete columns. Therefore, a clear understanding of the compressive behavior of FRP-confined concrete is essential. The objective of this paper is to develop a simple efficient method for predicting the compressive strength, the axial strain at the peak stress, and the stress-strain relationship of FRP-confined concrete. In this method, a compressive strength model is established based on Jefferson\'failure surface. With the proposed strength model, the strength of FRP-confined concrete can be estimated more precisely. The axial strain at the peak stress is then evaluated using a damage-based formula. Finally, a modified stress-strain relationship is derived based on Lam and Teng\'s model. The validity of the proposed compressive strength and strain models and the modified stress-strain relationship is verified with a wide range of experimental results collected from the research literature and obtained from the self-conducted test. It can be concluded that, as a competitive alternative, the proposed method can be used to predict the compressive behavior of FRP-confined concrete with reasonable accuracy.

Key Words
FRP-confined concrete; strength model; strain model; stress-strain relationship

Address
Xinglang Fan and Zhimin Wu: State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian , P. R. China
Yufei Wu: Department of Civil and Architectural Engineering, City University of Hong Kong, Kowloon, Hong Kong
Jianjun Zheng: School of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou, P. R. China

Abstract
This paper presents a mixed formulation frame element with the assumptions of the Timoshenko shear beam theory for displacement field and that accounts for interaction between shear and normal stress at material level. Nonlinear response of the element is obtained by integration of section response, which in turn is obtained by integration of material response. Satisfaction of transverse equilibrium equations at section includes the interaction between concrete and transverse reinforcing steel. A 3d plastic damage model is implemented to describe the hysteretic behavior of concrete. Comparisons with available experimental data on RC structural walls confirm the accuracy of proposed method.

Key Words
shear behavior; finite element method; mixed formulation; frame finite element; shear walls

Address
Afsin Saritas: Department of Civil Engineering, Middle East Technical University, Ankara 06800, Turkey
Filip C. Filippou: Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720, USA

Abstract
Computation of flexural ductility of reinforced concrete beam sections has been proposed by taking into account strain rate sensitive constitutive behavior of concrete and steel, confinement of core concrete and degradation of cover concrete during load reversal under earthquake loading. The estimate of flexural ductility of reinforced concrete rectangular sections has been made for a wide range of tension and compression steel ratios for confined and unconfined concrete at a strain rate varying from 3.3 x 10-5 to 1.0/sec encountered during normal and earthquake loading. The parametric studies indicated that flexural ductility factor decreases at increasing strain rates. Percentage decrease is more for a richer mix concrete with the similar reinforcement. The confinement effect has marked influence on flexural ductility and increase in ductility is more than twice for confined concrete (0.6 percent volumetric ratio of transverse steel) compared to unconfined concrete. The provisions in various codes for achieving ductility in moment resisting frames have been discussed.

Key Words
strain rate, seismic behavior; constitutive relationships; reinforced concrete frames; confinement, ductile detailing

Address
Akhilesh K. Pandey: CSIR-Central Building Research Institute, Roorkee, 247667, Uttarakhand, India

Abstract
The present study aims to show how the problem of reproducing, as closely as possible, binders of historic mortars by mixing raw materials which are commercially available, can be formulated as a linear optimization problem. The study points out that by mixing five standard raw materials (end-members) it is possible to obtain mortar binders with the almost same chemical compositions of those determined on the historic and archaeological mortar samples studied in some recent scientific papers. An advanced function of the Microsoft Excel spreadsheet, the Solver add-in, was used for the calculation of the right amount of each raw material to be mixed for producing the new binders. This approach could be useful to provide an optimal solution in the process of restoration of ancient monuments, where it is necessary to replace the historic mortars with new highly compatible repair mortars.

Key Words
mortar; concrete; binder; aggregate; lump; mixing; optimization problem; excel; solver; restoration

Address
D. Miriello, A. Bloise, C. Apollaro and G.M. Crisci: DiBEST Dipartimento di Biologia, Ecologia e Scienze della Terra, Università della Calabria, Italy
M. Lezzerini: Dipartimento di Scienze della Terra, Università di Pisa, Italy
F. Chiaravalloti: Dipartimento di Fisica, Università della Calabria, Italy

Abstract
Existing numerical models for strain-hardening cement-based composites (SHCC) are short of providing sufficiently accurate solutions to the failure patterns of coupling beams of different designs. The objective of this study is to develop an effective model that is capable of simulating the nonlinear behavior of SHCC coupling beams subjected to cyclic loading. The beam model proposed in this study is a macro-scale plane stress model. The effects of cracks on the macro-scale behavior of SHCC coupling beams are smeared in an anisotropic model. In particular, the influence of the defined crack orientations on the simulation accuracy is explored. Extensive experimental data from coupling beams with different failure patterns are employed to evaluate the validity of the proposed SHCC coupling beam models. The results show that the use of the suggested shear stiffness retention factor for damaged SHCC coupling beams is able to effectively enhance the simulation accuracy, especially for shear-critical SHCC coupling beams. In addition, the definition of crack orientation for damaged coupling beams is found to be a critical factor influencing the simulation accuracy.

Key Words
strain-hardening cement-based composites; coupling beams; numerical modeling; cyclic loading; hysteretic responses; crack patterns

Address
Chung-Chan Hungand Yen-Fang Su: Civil Engineering, National Central University, Taoyuan County, 32011,Taiwan


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