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CONTENTS
Volume 15, Number 4, April 2015
 

Abstract
Cooling by the flow of water through an embedded cooling pipe has become a common and effective artificial thermal control measure for massive concrete structures. However, an extreme thermal gradient induces significant thermal stress, resulting in thermal cracking. Using a mesoscopic finite-element (FE) mesh, three-phase composites of concrete namely aggregate, mortar matrix and interfacial transition zone (ITZ) are modeled. An equivalent probabilistic model is presented for failure study of concrete by assuming that the material properties conform to the Weibull distribution law. Meanwhile, the correlation coefficient introduced by the statistical method is incorporated into the Weibull distribution formula. Subsequently, a series of numerical analyses are used for investigating the influence of the correlation coefficient on tensile strength and the failure process of concrete based on the equivalent probabilistic model. Finally, as an engineering application, damage and failure behavior of concrete cracks induced by a water-cooling pipe are analyzed in-depth by the presented model. Results show that the random distribution of concrete mechanical parameters and the temperature gradient near water-cooling pipe have a significant influence on the pattern and failure progress of temperature-induced micro-cracking in concrete.

Key Words
concrete; mesoscopic simulation; equivalent probabilistic model; thermal cracking; micro-cracking

Address
Chao Zhang, Wei Zhou, Gang Ma, Chao Hu and Shaolin Li: State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, 430072, Wuhan, Hubei Province, People

Abstract
Fibre-added concretes are frequently used in large site applications such as slab and airports as well as in bearing system elements or prefabricated elements. It is very difficult to determine the mechanical properties of the fibre-added concretes by experimental methods in situ. The purpose of this study is to develop an artificial neural network (ANN) model in order to predict the compressive and bending strengths of hybrid fibre-added and non-added concretes. The strengths have been predicted by means of the data that has been obtained from destructive (DT) and non-destructive tests (NDT) on the samples. NDTs are ultrasonic pulse velocity (UPV) and Rebound Hammer Tests (RH). 105 pieces of cylinder samples with a dimension of 150 X 300 mm, 105 pieces of bending samples with a dimension of 100x100x400 mm have been manufactured. The first set has been manufactured without fibre addition, the second set with the addition of %0.5 polypropylene and %0.5 steel fibre in terms of volume, and the third set with the addition of %0.5 polypropylene, %1 steel fibre. The water/cement (w/c) ratio of samples parametrically varies between 0.3-0.9. The experimentally measured compressive and bending strengths have been compared with predicted results by use of ANN method.

Key Words
fibre-added concrete; hybrid fibre; compressive; bending, non-destructive test; artificial neural network

Address
Ali Demi: Department of Civil Engineering, Celal Bayar University, P.O. 45140, Muradiye, Manisa, Turkey

Abstract
The paper presents results of FE simulations of the strain-rate sensitive concrete behaviour under dynamic loading at the macroscopic level. To take the loading velocity effect into account, viscosity, stress modifications and inertial effects were included into a rate-independent elasto-plastic formulation. In addition, a decrease of the material stiffness was considered for a very high loading velocity to simulate fragmentation. In order to ensure the mesh-independence and to properly reproduce strain localization in the entire range of loading velocities, a constitutive formulation was enhanced by a characteristic length of micro-structure using a non-local theory. Numerical results were compared with corresponding laboratory tests and available analytical formulae.

Key Words
concrete; characteristic length; dynamic loading; elasto-plasticity; visco-plasticity; non-local theory; strain localization; viscosity

Address
Ireneusz Marzec and Jacek Tejchman: Faculty of Civil and Environmental Engineering, Gdańsk University of Technology,
Narutowicza Street 11/12, 80-233 Gdańsk, Poland


Andrzej Winnicki: nstitute of Building Materials and Structures, Cracow University of Technology,
Warszawska Street 24, 31-155 Cracow, Poland

Abstract
A two-dimensional multi-layered finite elements modeling of reinforced concrete structures at non-linear behaviour under monotonic and cyclical loading is presented. The non-linearity material is characterized by several phenomena such as: the physical non-linearity of the concrete and steels materials, the behaviour of cracked concrete and the interaction effect between materials represented by the post-cracking filled. These parameters are taken into consideration in this paper to examine the response of the reinforced concrete structures at the non-linear behaviour. Four examples of application are presented. The numerical results obtained, are in a very good agreement with available experimental data and other numerical models of the literature.

Key Words
modelling; multi-layers finite elements; damage; non-linear behaviour; unilateral model

Address
Khebizi Mourad and Guenfoud mohamed: Department of Civil Engineering, University of Guelma,LP 401 Guelma, Algeria

Abstract
Piers are the most vulnerable part of a bridge structure during an earthquake event. During Kobe earthquake in 1995, several bridge piers of the Hanshin Expressway collapsed for more than 600m of the bridge length. In this paper, the most important results of an experimental and analytical investigation of ten reinforced concrete bridge piers specimens with the same cross section subjected to constant axial (or variable) load and reversed (or one direction) cycling loading are presented. The objective was to investigate the main parameters influencing the seismic performance of reinforced concrete bridge piers. It was found that loading history and axial load intensity had a great influence on the performance of piers, especially concerning strength and stiffness degradation as well as the energy dissipation. Controlling these parameters is one of the keys for an ideal seismic performance for a given structure during an eventual seismic event. Numerical models for the tested specimens were developed and analyzed using SeismoStruct software. The analytical results show reasonable agreement with the experimental ones. The analysis not only correctly predicted the stiffness, load, and deformation at the peak, but also captured the post-peak softening as well. The analytical results showed that, in all cases, the ratio, experimental peak strength to the analytical one, was greater than 0.95.

Key Words
bridge piers; reinforced concrete; performance; cyclic loading; damage; energy dissipation

Address
Fouad Kehila and Hakim Bechtoula: National Earthquake Engineering Research Center CGS, 01 Rue Kaddour RAHIM, BP 252, Hussein Dey, Algiers, Algeria


Djillali Benaouar: Bab Ezzouar University of Science & Technology, USTHB BP32, El Alia, Bab Ezzouar, Algiers, Algeria

Abstract
It is well-documented that the major deterioration of coastal RC structures is chloride-induced corrosion. Therefore, regional investigations are necessary for durability based design and evaluation of the proposed service life prdiction models. In this paper, four reinforced concrete jetties exposed to severe marine environment were monitored to assess the long term chloride penetration at 6 months to 96 months. Also, some accelerated durability tests were performed on standard samples in laboratory. As a result, two time-dependent equations are proposed for basic parameters of chloride diffusion into concrete and then the corrosion initiation time is estimated by a developed probabilistic service life model Also, two famous service life prediction models are compared using chloride profiles obtained from structures after about 40 years in the tidal exposure conditions. The results confirm that the influence of concrete quality on diffusion coefficients is related to the concrete pore structure and the time dependence is due to chemical reactions of sea water ions with hydration products which lead a reduction in pore structure. Also, proper attention to the durability properties of concrete may extend the service life of marine structures greater than fifty years, even in harsh environments.

Key Words
durability; chloride penetration; service life prediction models; silica fume; tidal zone

Address
Majid Safehian and Ali Akbar Ramezanianpour: Concrete Technology and Durability Research Center, Department of Civil and Environment Engineering, Amirkabir University of Technology, Tehran, Iran

Abstract
The stress-strain relationship of concrete in flexure is one of the essential parameters in assessing the flexural strength and ductility of reinforced concrete (RC) columns. An overview of previous research studies revealed that the presence of strain gradient would affect the maximum concrete stress developed in flexure. However, no quantitative model was available to evaluate the strain gradient effect on concrete under flexure. Previously, the authors have conducted experimental studies to investigate the strain gradient effect on maximum concrete stress and respective strain and developed two strain-gradient-dependent factors k3 and ko for modifying the flexural concrete stress-strain curve. As a continued study, the authors herein will extend the investigation of strain gradient effects on flexural strength and ductility of RC columns to concrete strength up to 100 MPa by employing the strain-gradient-dependent concrete stress-strain curve using nonlinear moment-curvature analysis. It was evident from the results that both the flexural strength and ductility of RC columns are improved under strain gradient effect. Lastly, for practical engineering design purpose, a new equivalent rectangular concrete stress block incorporating the combined effects of strain gradient and concrete strength was proposed and validated. Design formulas and charts have also been presented for flexural strength and ductility of RC columns.

Key Words
columns; ductility; flexural strength; strain gradient; stress block parameters

Address
M.T. Chen: Department of Civil Engineering, The University of Hong Kong, Hong Kong

J.C.M. Ho: School of Civil Engineering, The University of Queensland, QLD 4072, Australia

Abstract
Chloride induced reinforcement corrosion is widely accepted to be the most frequent mechanism causing premature degradation of reinforced concrete members, whose economic and social consequences are growing up continuously. Prevention of these phenomena has a great importance in structural design, and modern Codes and Standards impose prescriptions concerning design details and concrete mix proportion for structures exposed to different external aggressive conditions, grouped in environmental classes. This paper focuses on reinforced concrete column section load carrying capacity degradation over time due to chloride induced steel pitting corrosion. The structural element is considered to be exposed to marine environment and the effects of corrosion are described by the time degradation of the axial-bending interaction diagram. Because chlorides ingress and consequent pitting corrosion propagation are both time-dependent mechanisms, the study adopts a time-variant predictive approach to evaluate residual strength of corroded reinforced concrete columns at different lifetimes. Corrosion initiation and propagation process is modelled by taking into account all the parameters, such as external environmental conditions, concrete mix proportion, concrete cover and so on, which influence the time evolution of the corrosion phenomenon and its effects on the residual strength of reinforced concrete columns sections.

Key Words
pitting corrosion, strength deterioration, diffusion coefficient, surface chloride concentration, concrete structures, marine environment

Address
Giuseppe Carlo Marano: DICATECH, Department of Civil Engineering, Environmental, Territory, Building and Chemical, Technical University of Bari, via Orabona 4 - 70125, Bari -Italy

Rita Greco: DICAR, Department of civil Engineering and Architecture, Technical University of Bari, via Orabona 4 - 70125, Bari -Italy

Abstract
R-curve based on the size effect law previously developed for geometrically similar specimens (geometry type III) is extended to geometries with variable depth (geometry type I) as well as with variable notch (geometry type II), where the R-curve is defined as the envelope of the family of critical strain energy release rates from specimens of different sizes. The results show that the extended R-curve for type I tends to be the same for different specimen configurations, while it is greatly dependent on specimen geometry in terms of the initial crack length. Furthermore, the predicted load-deflection responses from the suggested R-curve are found to agree well with the testing results on concrete and rock materials. Besides, maximum loads for type II specimen are predicted well from the extended R-curve.

Key Words
fracture mechanics; size effect law; energy release rate; R-curve

Address
Yan-hua Zhao and Jian-mei Chang: State Key Laboratory of Coastal and Offshore Engineering,
Dalian University of Technology, Dalian, China 116024

Jian-mei Chang : Transportation Institute, Inner Mongolia University, Hohhot, China 010070

Hong-bo Gao: College of Civil Engineering and Architecture, Hainan University, Haikou, China 570228

Abstract
A finite element computer code for short-term analysis of steel-concrete composite structures is extended to study long-term effects under service loads, in the present work. Long-term effects are important in engineering design because they influence stress and strain distribution of the structural system and therefore contribute to the increment of deflections in these structures. For creep analysis, a rheological model based on a Kelvin chain, with elements placed in series, was employed. The parameters of the Kelvin chain were obtained using Dirichlet series. Creep and shrinkage models, proposed by the CEB FIP 90, were used. The shear-lag phenomenon that takes place at the concrete slab is usually neglected or not properly taken into account in the formulation of beam-column finite elements. Therefore, in this work, a three-dimensional numerical model based on the assemblage of shell finite elements for representing the steel beam and the concrete slab is used. Stud shear connectors are represented for special beam-column elements to simulate the partial interaction at the slab-beam interface. The two-dimensional representation of the concrete slab permits to capture the non-uniform shear stress distribution in the horizontal plane of the slab due to shear-lag phenomenon. The model is validated with experimental results of two full-scale continuous composite beams previously studied by other authors. Results are given in terms of displacements, bending moments and cracking patterns in order to shown the influence of long-term effects in the structural response and also the potentiality of the present numerical code.

Key Words
steel-concrete composite beams; viscoelasticity; creep; shrinkage; finite elements

Address
Maiga M. Dias, Jorge L.P. Tamayo , Inácio B. Morsch and Armando M. Awruch: Department of Civil Engineering, Engineering School, Federal University of Rio Grande do Sul, Av. Osvaldo Aranha, 9999-3oFloor, 90035-190, Porto Alegre, RS, Brazil


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