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CONTENTS
Volume 18, Number 4, October 2016
 

Abstract
High-strength prestressing tendons with a tensile strength of 2400 MPa have been recently developed in the steel industry of South Korea, and is currently being mass produced for common commercial applications in structural engineering practices. Accordingly, since most of the existing transfer length estimation models were derived based on the test results of the specimens having the prestressing tendons with a tensile strength of 1860 MPa or less, modifications of the transfer length models are now required to consider the effect of the enhanced tensile strength of the prestressing tendons. In this study, six pretensioned concrete specimens reinforced with 2400 MPa tensile strength prestressing tendons were fabricated and tested to investigate their transfer lengths. In addition, a simplified design equation for the transfer length was developed based on the assumption of the linear strain profile curves of the prestressing tendon and surrounding concrete in transfer length zone. The accuracy of the proposed equation was verified in detail by comparing a total of 215 transfer length test results with analysis results. The simplified design equation provided very accurate results on the transfer lengths of all the test specimens, regardless of the tensile strength grades of prestressing tendons.

Key Words
transfer length; 2400 MPa; high-strength; prestressed concrete; pretensioned concrete; slip

Address
Sun-Jin Han,Jae-Yuel Oh, Kang Su Kim: 1Department of Architectural Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504 Republic of Korea

Deuck Hang Lee: Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign,
205 N. Mathews Ave. Urbana, IL 61801 USA

Seong-Tae Yi: Department of Civil and Environmental Engineering, Inha Technical College, 100 Inha-ro, Nam-gu,
Incheon 22212 Republic of Korea

Abstract
The present study is an exploratory research that appraises and compares the performance of four rapid tests for assessing the chloride penetration of concrete; Rapid Chloride Permeability Test (RCPT), Rapid Chloride Migration Test (RCMT), Surface Electrical Resistivity (SR) and Modified Rapid Chloride Permeability Test (MRCPT). Due to the concern raised by some researchers on that the conductivity of the pore solution impinges upon the results on some of these rapid tests, silica fume was used as supplementary cementitious material in proportions of 7.5% and 15%. All four methods exhibited substantial reduction in the chloride permeability of concrete mixtures containing silica fume compared to the control mixture at the ages of 28 and 90 days. The variations in performance caused by silica fume usage were significant for RCPT and SR methods, moderate for RCMT and marginal for MRCPT. Results show that the variation in the conductivity of the pore solution significantly alters the results of SR and RCPT tests, and marginally alters those of MRCPT.

Key Words
concrete; chloride permeability; conductivity; pore solution

Address
Amirreza Pilvar, Ali Akbar Ramezanianpour and S. Mahmoud Motahari Karein: Department of Civil & Environmental Engineering, Amirkabir University of Technology, Tehran, Iran

Hosein Rajaie: Concrete Technology and Durability Research Center, Amirkabir University of Technology, Tehran, Iran

Abstract
Terrorist attacks have used conventional high explosives. Such attacks need the development of blast resistant construction. Most of sandwich panels used in protection purpose have the capability of dissipating energy by large plastic deformation under blast loading. These sandwich panels could not resist repetitive explosions. This panel should be replaced with new one to resist another explosion event. The aim of this study is to use double reinforced concrete panels to achieve high resistance against different explosion charges and to resist repetitive explosion shots. This sandwich panel is composed of two reinforced concrete slabs connected by number of helical springs. The blast field test is carried out. The finite element analysis (FEA) is also used to model the double reinforced concrete panels under shock wave. The performance of the double reinforced concrete panel is studied based on detonating different TNT explosive charges. There is a good agreement between the results obtained by both the field blast test and the numerical simulation. The spring improves the double reinforced concrete panel performance under the blast wave propagation.

Key Words
displacements; finite element analysis; field blast test; double reinforced concrete panels; springs; TNT explosive charge

Address
Mostafa M. A. Wahab and Sherif A. Mazek: Civil Engineering Department, Military Technical College, Cairo, Egypt

Abstract
In this paper, it is aimed to evaluate the structural behavior of box girder bridge model under undamaged and damaged conditions considering time effect between June 2009 and February 2015 using ambient vibration tests. For this purpose, a scaled bridge model is constructed and experimental measurements are performed to determine the damage and time effect on dynamic characteristics such as natural frequencies, mode shapes and damping ratios. In the ambient vibration tests, natural excitations are provided and the response of the bridge model is measured. The signals collected from the tests are processed by Operational Modal Analysis; and the dynamic characteristics of the bridge model are estimated using Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods. Measurement time, frequency span and effective mode number are selected by considering similar studies found in the literature. To expose the damage and time effect on dynamic characteristics, four experimental measurement cases are considered between 2009 and 2015. The first measurement case is conducted on June 2009 under undamaged conditions. The second and third measurement cases are performed on October 2010 under undamaged and damaged conditions to emerge the damage and time effect, respectively. The fourth measurement tests are carried out on February 2015 to display the time effect on the dynamic characteristics considering same damage condition in third measurements. At the end of the study, experimentally identified dynamic characteristics are compared with each other to investigate the damage and time effects. It can be stated that the both of Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods are very useful to identify the dynamic characteristics of the bridge model. Maximum differences obtained as 11.98% and 112.24% between Case 1 and Case 2, 10.30% and 26.24% between Case 2 and Case 3, 12.36% and 401.50% between Case 3 and Case 4 for natural frequencies and damping ratios, respectively. It is seen that damages and environmental conditions affect the structural behavior of concrete bridges, substantially. Also, time dependent environmental conditions such as temperature, humidity and ageing are as important as cracks and damages.

Key Words
ambient vibration; bridge model; damage effect; dynamic characteristic; enhanced frequency domain decomposition; operational modal analysis; stochastic subspace identification; time effect

Address
Ahmet Can ALTUNIŞIK: Karadeniz Technical University, Department of Civil Engineering, 61080, Trabzon, Turkey

Abstract
This study is based on extending the previous research work entitled Fiber reinforced polymers (FRP) confined columns that published by the authors. The modeling characteristics for plastic hinges of RC columns have been determined in FEMA 356. However, for evaluating a retrofitted member, there are no parameters for one. This issue is an important deficiency of the mentioned code. The main purpose of this study is to introduce the plastic hinge parameters for the RC rectangular columns that are retrofitted by FRP. These characteristics of plastic hinges can be used in a nonlinear static analysis, instead of nonlinear dynamic one. In order to analytical simulation of RC column behavior and also accuracy of acquired results, at first using LS-DYNA software including 3D nonlinear finite element modeling, a RC column studied in the literature has been verified. The obtained results are showed a good match between both Finite element model and experimental test and there are reasonable correlations between ones. Moreover in the next stage, in order to evaluate the robustness of the proposed method 20 reinforced concrete columns which supposed to be fixed at one end, have been retrofitted using FRP in the plastic hinge zone. The columns have been simulated under a constant compressive load and lateral cyclic displacement that the hysteresis curves have been drawn for obtaining the plastic hinge parameters. Numerical results demonstrate that the plastic hinge parameters have averagely been increased after retrofitting with a thickness of CFRP about 0.165 mm.

Key Words
FRP; RC column; plastic hinge; retrofitting; cyclic loading; finite element analysis

Address
B. Mohebi and S.M. Hosseinifard: Faculty of Engineering, Imam Khomeini International University, Qazvin, Iran

M. Bastami: International Institute of Seismology and Earthquake Engineering, Tehran, Iran

Abstract
In this paper, a new numerical model for the evaluation of the seismic behaviour of existing reinforced concrete (RC) structure with considering the effect of variation of applied axial load on columns is presented. Focus is given on developing accurate and practical models for simulating nonlinearities in joint core as well as column under varying axial load. For exterior as well as interior joints, according to experimental and finite element results, principal tensile stresses versus shear deformation relations in joint core are recommended. Moreover, to consider the effects of stirrup on the principal tensile stress-shear deformation relation, an incremental procedure is proposed based on the Mohr theory. According to the proposed numerical model, complex nonlinear behaviour of joint core is simulated by two diagonal axial springs so that the effect of variation of axial load is also considered. The properties of these springs are determined based on the proposed principal tensile stress-shear deformation relation in the joint core. Moreover, the shear and flexural nonlinear behaviour of RC beams and columns is also simulated by rotational springs. A simplified methodology is developed to consider the effects of axial load variation on shear and flexural nonlinear behaviour of RC columns. To demonstrate the capability of the proposed numerical model at structural level, two RC frames with various failure modes are investigated. The results confirm the ability of the model in predicting the nonlinear behaviour of the frame, which can provide an alternative method for engineers in practice.

Key Words
beam-column joints; numerical model; shear effect; nonlinear analysis; principal tensile stresses

Address
J.Shayanfar and H. Akbarzadeh Bengar: Department of Civil Engineering, University of Mazandaran, Babolsar, Iran

Abstract
The purpose of this paper is to investigate the feasibility of using artificial neural network programming for the prediction of the fresh properties of self-compacting concrete. The input parameters of the neural network were the mix composition influencing the fresh properties of self-compacting concrete namely, the cement content, the dosages of limestone powder and water, fine aggregate, coarse aggregate, and superplasticizer, and other parameter of time of testing (5, 30 and 60 minutes after addition of water). The model is based on a multilayer feed forward neural network. The details of the proposed ANN with its architecture, training and validation are presented in this paper. Six outputs of the ANN models related to the fresh properties were the slump flow, T50, T60, V-funnel flow time, Orimet flow time, and blocking ratio (L-box). The effectiveness of the trained ANN is evaluated by comparing its responses with the experimental data that were used in the training process. The dosage of water was varied from 188 to 208 L/m3, the dosage of SP from 3.8 to 5.8 kg/m3, and the volume of coarse aggregates from 220 to 360 L/m3 (587 to 961 kg/m3). In total twenty mixes were used to measure the fresh properties with different mix compositions. ANN performed well and provided very good correlation coefficients (R2) above 0.957 for slump flow, T50, V-funnel flow time, Orimet flow time, and L-box blocking ratio. The predicting results for T60 was slightly lower (R2=0.823). With the calculated models these properties of new mixes within the practical range of the input variables used in the training can be predicted with an absolute error for slump flow, T50, T60, V-funnel flow time, Orimet flow time, and L-box ratio of 3.3%, 13%, 16%, 14%, 15%, and 22%, respectively. The results show that the ANN model can predict accurately the fresh properties of SCC.

Key Words
artificial neural network; blocking ratio of L-box test; orimet; prediction model; slump flow; V-funnel

Address
Mohammed Sonebi: School of Planning, Architecture and Civil Engineering, Queen

Abstract
The correlation between the characteristics of acoustic emission signals and the strength parameters of concretes is investigated by combining the acoustic emission (AE) technique and steel fiber reinforced concrete (SFRC). By means of AE detection system, three kinds of steel fibers of varying content were used as reinforcement in concrete and their influence on the fracture process and the acoustic activity was considered in this paper. Analysis of the AE test results revealed that for different steel fiber contents, the role of steel fiber is different. Concave-convex steel fiber has an inhibitory effect on the control of crack expansion. Bow steel fiber has a better effect on the control of crack propagation when the fiber content is lower. The compressive strength of SFRC decrease when bow steel fiber content rises. Ultra-short can prevent early cracks of concrete.

Key Words
acoustic emission; steel fiberreinforced concrete; steel fiber content; AE events column graph

Address
Xiong Zhou, Yuyou Yang, Xiangqian Li and Guoqing Zhao: School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 10083, China


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