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CONTENTS
Volume 11, Number 4, April 2013
 

Abstract
In this research, a developed microstructural model of cement particles was presented to describe the cement hydration procedure. To simplify the hydration process, the whole hydration was analyzed in a series of sub-steps. In each step, the hydration degree, as well as the microstructural size of the hydration cell, was calculated as a function of the radius of the unreacted cement particles. With the consideration of the water consumption and the reduction of the interfacial area between water and hydration products, the micro-level expressions of the cement hydration kinetics were established. Then the heat released and temperature history of the concrete was carried out with the hydration degree obtained from each sub-steps. The equivalent age method based on the Arrhenius law was introduced in this research. Based on the equivalent age method, a maturity model was applied to describe the evolution of the mechanical properties of the material during the hydration process. The finite element program ANSYS was used to analyze the temperature field in concrete structures. Then thermal stress field was calculated using the elasticity modulus obtained from code formulate. And the risk of thermal cracking was estimated by the comparison of thermal stress and concrete tensile strength.

Key Words
cement hydration; microstructure; equivalent age; mechanical properties

Address
Ye Tian, Xianyu Jin and Nanguo Jin: College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, China

Abstract
By considering the creep characteristics of concrete core under eccentric compression, a creep model of concrete filled steel tubes (CFT) columns under eccentric compressive loads is proposed based on the concrete creep model B3. In this proposed model, a discrete element method is introduced to transform the eccentric loading into axial loading. The validity of the model is verified by comparing the predicting results with the published creep experiments results on CFT specimens under compressive loading, together with the predicting values based on other concrete creep models, such as ACI209, CEB90, GL2000 and elastic continuation and plastic flow theory. By using the proposed model, a parameters study is carried out to analysis the effects of practical design parameters, such as concrete mix (e.g. water to cement ratio, aggregate to cement ratio), steel ratio and eccentricity ratio, on the creep of CFT columns under eccentric compressive loading.

Key Words
concrete filled steel tubes; creep; eccentric compression; eccentricity ratio; model B3

Address
Bing Han, Yuan-feng Wang, Qian Wang and Dian-jie Zhang: School of Civil Engineering, Beijing Jiaotong University, Beijing, China

Abstract
This paper aims to study the effect of measurement methods and cracking on chloride transport of concrete materials. Three kinds of measurement methods were carried out, including immersion test, rapid migration test and steady-state migration test. All of these measurements of chloride transport show that chloride ion diffusion coefficient decreased with the reduction of water to cement ratio. Results of the immersion test were less than that of rapid migration test and steady-state migration test. For the specimen of lower water to cement ratio, the external electrical field has little effect on chloride binding relatively. Compared with the results obtained by these different measurement methods, the lower water to cement ratio may cause smaller differences among these different methods. The external voltage can reduce chloride binding of concrete, and the higher electrical field made a strong impact on the chloride binding. Considering the effect of high voltage on the specimen, results indicate that results based on the steady-state migration test should be more reasonable. For cracked concrete, cracking can accelerate the chloride ion diffusion.

Key Words
chloride transport; immersion test; rapid migration test; steady-state migration test; cracking

Address
Shiping Zhang and Xiang Dong: Department of Civil Engineering and Architecture, Nanjing Institute of Technology, Nanjing 211167, China; Jinyang Jiang: College of Materials Science and Engineering, Southeast University, Nanjing 210096, China

Abstract
In this study, the pore structure of Portland cement paste is experimentally characterized by MIP (mercury intrusion porosimetry) and nitrogen adsorption, and simulated by a newly developed status-oriented computer model. Cement pastes with w/c=0.3, 0.4 and 0.5 at ages from 1 day to 120 days are comprehensively investigated. It is found that MIP cannot generate valid pore size distribution curves for cement paste. Nevertheless, nitrogen adsorption can give much more realistic pore size distribution curves of small capillary pores, and these curves follow the same distribution mode. While, large capillary pores can be effectively characterized by the newly developed computer model, and the validity of this model has been proved by BSE imaging plus image analysis. Based on the experimental findings and numerical simulation, a hypothesis is proposed to explain the formation mechanism of the capillary pore system, and the realistic representation of the pore structure of hydrated cement paste is established.

Key Words
cement paste; pore structure; MIP; nitrogen adsorption; computer model

Address
Hongyan Ma and Zongjin Li: Department of Civil and Environmental Engineering, the Hong Kong University of Science and Technology, Hong Kong, China

Abstract
The sensitivity of compressive strength of no-slump concrete to its ingredient materials and proportions, necessitate the use of robust models to guarantee both estimation and generalization features. It was known that the problem of compressive strength prediction owes high degree of complexity and uncertainty due to the variable nature of materials, workmanship quality, etc. Moreover, using the chemical and mineral additives, superimposes the problem

Key Words
no-slump concrete; compressive strength; prediction; support vector machine; neural networks; optimization

Address
J. Sobhani: Department of Concrete Technology; Road, Housing & Urban Development Research Center (BHRC), Pas Farhangian St, Sheikh Fazlollah Exp. Way, Tehran , P.O. Box 13145-1696, Iran; M. Khanzadi and A.H. Movahedian: Department of Civil Engineering, Iran University of Science and Technology, Tehran, P.O. Box 16846-13114, Iran

Abstract
This paper presents results from an analytical investigation of the behavior of steel reinforced concrete circular column sections with additional Glass Fiber Reinforced Polymers (GFRP) bars. The primary application of this composite section is to relocate the plastic hinge region from the column-footing joint where repair is difficult and expensive. Mainly, the study focuses on the development of the full nominal moment-axial load (M-P) interaction diagrams for hybrid concrete sections, reinforced with steel bars as primary reinforcement, and GFRP as auxiliary control bars. A large parametric study of circular steel reinforced concrete members were undertaken using a purpose-built MATLAB

Key Words
reinforced concrete; bridge columns and foundations; material constitutive models; glass fiber reinforced polymers (GFRP); plastic hinge

Address
M.S. Shraideh and R.S. Aboutaha: Civil and Environmental Engineering, Syracuse University, Syracuse, USA


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