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CONTENTS
Volume 15, Number 5, May 2015
 

Abstract
This work deals with a damage model formulation taking into account the unilateral effect of the mechanical behaviour of brittle materials such as concrete. The material is assumed as an initial elastic isotropic medium presenting anisotropy, permanent strains and bimodularity induced by damage evolution. Two damage tensors governing the stiffness in tension or compression regimes are introduced. A new damage tensor in tension regimes is proposed in order to model the diffuse damage originated in prevails compression regimes. Accordingly with micromechanical theory, the constitutive model is validate when dealing with unilateral effect of brittle materials, Finally, the proposed model is applied in the analyses of reinforced concrete framed structures submitted to reversal loading. The numerical results have shown the good performance of the modelling and its potentialities to simulate practical problems in structural engineering.

Key Words
damage mechanics; unilateral effect; concrete structures; structural failure; constitutive model

Address
José J.C. Pituba: Department of Civil Engineering, Campus Catalão, Federal University of Goiás, Av. Dr Lamartine Pinto de Avelar, 1120, Catalão, Goiás 75740-020, Brazil

Abstract
This work deals with unilateral effect of quasi-brittle materials, such as concrete. For this propose, a two-dimensional meso-scale model is presented. The material is considered as a three-phase material consisting of interface zone, matrix and inclusions – each constituent modeled by an appropriate constitutive model. The Representative Volume Element (RVE) consists of inclusions idealized as circular shapes randomly placed into the specimen. The interface zone is modeled by means of cohesive contact finite elements developed here in order to capture the effects of phase debonding and interface crack closure/opening. As an initial approximation, the inclusion is modeled as linear elastic as well as the matrix. Our main goal here is to show a computational homogenization-based approach as an alternative to complex macroscopic constitutive models for the mechanical behavior of the quasi-brittle materials using a finite element procedure within a purely kinematical multi-scale framework. A set of numerical examples, involving the microcracking processes, is provided. It illustrates the performance of the proposed model. In summary, the proposed homogenization-based model is found to be a suitable tool for the identification of macroscopic mechanical behavior of quasi-brittle materials dealing with unilateral effect.

Key Words
multi-scale analysis; constitutive model; quasi-brittle materials; unilateral effect; cohesive contact finite element

Address
José J.C. Pituba: Department of Civil Engineering, Federal University of Goiás, Campus Catalão Av Dr Lamartine Pinto de Avelar, 1120, Setor Universitário, Catalão, Goiás, Brazil, 75740-020
ardo A. Souza Neto: Civil and Computational Engineering Centre, School of Engineering, Swansea University
Singleton Park, Swansea SA2 8PP, United Kingdom

Abstract
In this study, a multi-layer perceptron neural network (MLPNN) prediction model for compressive strength of the cement mortars has been developed. For purpose of constructing this model, 8 different mixes with 240 specimens of the 2, 7, 28, 56 and 90 days compressive strength experimental results of cement mortars containing fly ash (FA), silica fume (SF) and FA+SF used in training and testing for MLPNN system was gathered from the standard cement tests. The data used in the MLPNN model are arranged in a format of four input parameters that cover the FA, SF, FA+SF and age of samples and an output parameter which is compressive strength of cement mortars. In the model, the training and testing results have shown that MLPNN system has strong potential as a feasible tool for predicting 2, 7, 28, 56 and 90 days compressive strength of cement mortars.

Key Words
multi-layer perceptron neural network; cement; silica fume; fly ash; compressive strength

Address
Yilmaz Kocak: Department of Construction, Kutahya Vocational School of Technical Sciences, Dumlupinar University, Kutahya, Turkey
Eyyup Gulbandilar and Muammer Akcay: Department of Computer Engineering, Faculty of Engineering, Dumlupinar University, Kutahya, Turkey

Abstract
This paper proposes an innovative method for selection of measurement sets in static parameter identification of concrete or steel bridges. This method is proved as a systematic tool to address the first steps of Structural System Identification procedures by observability techniques: the selection of adequate measurement sets. The observability trees show graphically how the unknown estimates are successively calculated throughout the recursive process of the observability analysis. The observability trees can be proved as an intuitive and powerful tool for measurement selection in beam bridges that can also be applied in complex structures, such as cable-stayed bridges. Nevertheless, in these structures, the strong link among structural parameters advises to assume a set of simplifications to increase the tree intuitiveness. In addition, a set of guidelines are provided to facilitate the representation of the observability trees in this kind of structures. These guidelines are applied in bridges of growing complexity to explain how the characteristics of the geometry of the structure (e.g. deck inclination, type of pylon-deck connection, or the existence of stay cables) affect the observability trees. The importance of the observability trees is justified by a statistical analysis of measurement sets randomly selected. This study shows that, in the analyzed structure, the probability of selecting an adequate measurement set with a minimum number of measurements at random is practically negligible. Furthermore, even bigger measurement sets might not provide adequate SSI of the unknown parameters. Finally, to show the potential of the observability trees, a large-scale concrete cable-stayed bridge is also analyzed. The comparison with the number of measurements required in the literature shows again the advantages of using the proposed method.

Key Words
bridge; structural system identification; observability trees; damage detection; measurementset

Address
Jose Antonio Lozano-Galant: Department of Civil Engineering, University of Castilla-La Mancha, Ciudad Real, Spain
Maria Nogal: Department of Civil, Structural and Environmental Engineering, Trinity College, Dublin, Ireland
Jose Turmo: Department of Construction Engineering,
Universitat Politècnica de Catalunya BarcelonaTECH, Barcelona, Spain
Enrique Castillo: Department of Applied Mathematics and Computational Sciences, University of Cantabria, Santander, Spain

Abstract
In this study, dynamic modulus of elasticity of self-consolidating rubberized concrete is evaluated by using results of ultrasonic pulse velocity and resonance frequency tests. Additionally, correlation between dynamic modulus of elasticity and compressive strength results is compared. For evaluating the dynamic modulus of elasticity of self-consolidating rubberized concrete, prismatic specimens having 100 x 100 x 500 mm dimensions are prepared. Dynamic modulus of elasticity values obtained by non-destructive measurements techniques are well agreed with those given in the literature.

Key Words
rubberized concrete; ultrasonic measurement; resonance frequency; dynamic modulus

Address
Mehmet Emiroğlu: Department of Civil Engineering, Düzce University Technology, Düzce, Turkey
Servet Yildiz: Department of Civil Engineering, Firat University Technology, Elazig
Halidun Keleştemur: School of Engineering Department of Mechanical Engineering, Meliksah University, Kayseri, Turkey

Abstract
In the previous analytical studies on 2D reinforced concrete (RC) beam-column joints, the modified compression field theory (MCFT) and the strut-and-tie method (STM) are usually employed. In this paper, the limitations of these analytical models for RC joint applications are reviewed. Essentially for predictions of RC joint shear behaviour, the MCFT is not applicable, while the STM can only predict the ultimate shear strength. To eliminate these limitations, an improved STM is derived and applied to some commonly encountered 2D joints, viz., interior and exterior joints, subjected to monotonic loading. Compared with the other STMs, the most attracting novelty of the proposed improved STM is that all critical stages of the shear stress-strain relationships for RC joints can be predicted, which cover the stages characterized by concrete cracking, transverse reinforcement yielding and concrete strut crushing. For validation and demonstration of superiority, the shear stress-strain relationships of interior and exterior RC beam-column joints from published experimental studies are employed and compared with the predictions by the proposed improved STM and other widely-used analytical models, such as the MCFT and STM.

Key Words
improved strut-and-tie method; 2D RC beam-column joint; shear stress-strain relationship; monotonic loading

Address
Xu Long: School of Mechanics, Civil Engineering and Architecture, Northwestern Polytechnical University, 710072,
Xi\'an, Shaanxi, China
Chi King Lee: School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

Abstract
In the world total boron reserve rating, Turkey is taken place on the first rank, meeting the demand of refined mineral and main boron chemicals. Development of the new boron products and production technologies, spreading the using area of the boron are the study topics which must be finically discussed. In this study, with the help of colemanite taken in ratio as (0%, 7.5%, 12.5%, and 17.5%) by being mixed by the cement, surfaces of the pumice aggregates have been covered. Permeability of the samples has been investigated by producing lightweight concrete with 400 dose with the help of aggregates covered with colemanite. For this, the experiments of water absorption, capillary water absorption, depth of penetration of water under pressure and rapid chloride permeability have been performed. In addition, analyses of the thin section of covered and uncovered pumice aggregates and SEM (Scanning Electron Microscope) have been investigated. When the control samples produced with the covered aggregates and concretes produced with colemanite covered aggregates are compared each other, it has been determined that special lightweight concretes whose values of capillary water absorption experiment, depth of penetration of water under pressure experiment and rapid chloride permeability are low can be produced.

Key Words
aggregates; lightweight aggregate concrete (LWA); durability; concrete products; construction materials

Address
Özlem Salli Bideci and Alper Bideci: Department of Architecture, Duzce University, Art, Design and Architecture Faculty, 81600, Düzce, Turkey
Sabit Oymael: Department of Architecture, Istanbul Arel University, Faculty of Engineering and Architecture,
34537 Istanbul, Turkey
Ali Haydar Gültekin: Department of Geological Engineering, Istanbul Technical University, Faculty of Mines, 34469 Istanbul, Turkey
Hasan Yildirim: Department of Civil Engineering, Istanbul Technical University, Faculty of Civil Engineering,
34469 Istanbul, Turkey


Abstract
In this paper, a reaction-diffusion model of carbonation process in self-compacting concrete (SCC) was realized with a consideration of multi-field couplings. Various effects from environmental conditions, e.g. ambient temperature, relative humidity, carbonation reaction, were incorporated into a numerical simulation proposed by ANSYS. In addition, the carbonation process of SCC was experimentally investigated and compared with a conventionally vibrated concrete (CVC). It is found that SCC has a higher carbonation resistance than CVC with a comparable compressive strength. The numerical solution analysis agrees well with the test results, indicating that the proposed model is appropriate to calculate and predict the carbonation process in SCC. The parameters sensitivity analysis also shows that the carbon dioxide diffusion coefficient and moisture field are essentially crucial to the carbonation process in SCC.

Key Words
SCC; carbonation process; numerical simulation; reaction-diffusion modeling

Address
Chuanqing Fu: College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, 310058, China;
College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou, 310014, China;
Zhejiang Key Laboratory of Civil Engineering Structures & Disaster Prevention and Mitigation Technology, Hangzhou, 310014, China
Hailong Ye: Department of Civil and Environmental Engineering, The Pennsylvania State University, 3127 Research Drive, PA, 16801, USA
Xianyu Jin, Nanguo Jin and Lingli Gong: College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, 310058, China


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