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CONTENTS
Volume 6, Number 1, February 2009
 

Abstract
Chloride diffusivity of concrete is a crucial material parameter for service life determination and durability designing of marine concrete. Many research works on this issue have been conducted, varying from empirical solutions obtained experimentally to image analysis, based on multi-scale modeling. One of the simple approaches is to express the chloride diffusivity of concrete by a multi-factor function, however, the influences of various factors on the chloride diffusivity are ambiguous. Furthermore, the majority of these research works have not dealt with the carbonation process of concrete, although this process affects the chloride diffusivity of concrete significantly. The purpose of this study is to establish a simple approach to calculate the chloride diffusivity of (non)carbonated concrete. The chloride diffusivity of concrete should be defined, based on engineering and scientific knowledge of cement and concrete materials. In this paper, a lot of parameters affecting the chloride diffusivity, such as the diffusivity in pore solution, tortuosity, micro-structural properties of hardened cement paste, volumetric portion of aggregate, are taken into consideration in the calculation of the chloride diffusivity of noncarbonated concrete. For carbonated concrete, reduced porosity due to carbonation is calculated and used for calculating the chloride diffusivity. The results are compared with experimental data and previous research works.

Key Words
service life; chloride diffusivity; carbonation; porosity; tortuosity.

Address
Induk Institute of Technology, Department of Construction Information Engineering, San76, Wolgye-Dong, Nowon-Gu, Seoul, Korea

Abstract
This paper describes the development of modified heat of hydration and maturity-strength models for concrete containing fly ash and slag. The modified models are developed based on laboratory and literature test results, which include different types of cement, fly ash, and slag. The new models consider cement type, water-to-cementitious material ratio (w/cm), mineral admixture, air content, and curing conditions. The results show that the modified models well predict heat evolution and compressive strength development of concrete made with different cementitious materials. Using the newly developed models, the sensitivity analysis was also performed to study the effect of each parameter on the hydration and strength development. The results illustrate that comparing with other parameters studied, w/cm, air content, fly ash, and slag replacement level have more significantly influence on concrete strength at both early and later age.

Key Words
heat of hydration; strength; modeling; maturity; sensitivity.

Address
Zhi Ge: Dept. of Construction Management and Engineering, 120B CME Building, North Dakota State University, Fargo, ND 58105, USA
Kejin Wang: Dept. Civil, Construction, and Environmental Engineering, 492 Town Engineering, Iowa State University, Ames, IA 50010, USA

Abstract
This paper presents the details of a novel external prestressing technique for strengthening of concrete members. In the proposed technique, transfer of external force is in shear mode on the end block thus creating a complex stress distribution and the required transverse prestressing force is lesser compared to conventional techniques. Steel brackets are provided on either side of the end block for transferring external prestressing force and these are connected to the anchor blocks by expansion type anchor bolts. In order to validate the technique, an experimental investigation has been carried out on post-tensioned end blocks. Performance of the end blocks have been studied for design, cracking and ultimate loads. Slip and slope of steel bracket have been recorded at various stages during the experiment. Finite element analysis has been carried out by simulating the test conditions and the responses have been compared. From the analysis, it has been observed that the computed slope and slip of the steel bracket are in good agreement with the corresponding experimental observations. A simplified analytical model has been proposed to compute load-deformation of the loaded steel bracket with respect to the end block. Yield and ultimate loads have been arrived at based on force/moment equilibrium equations at critical sections. Deformation analysis has been carried out based on the assumption that the ratio of axial deformation to vertical deformation of anchor bolt would follow the same ratio at the corresponding forces such as yield and ultimate. It is observed that the computed forces, slip and slopes are in good agreement with the corresponding experimental observations.

Key Words
prestressed concrete; external prestressing; finite element analysis; analytical model; material nonlinearity; slope; slip.

Address
N. Lakshmanan, S. Saibabu, A. Rama Chandra Murthy, S. Chitra Ganapathi and R. Jayaraman: SERC, CSIR, CSIR Campus, Taramani, Chennai, 600-113, India
R. Senthil: Structural Engineering Department, Anna University, Chennai-34, India

Abstract
Reliability analysis for a proposed limit state bridge design code is performed. In order to introduce reliability concept to design code, the proposed live load model is based on truck weight survey. Test data of domestic material strengths are collected to model statistical properties of member strengths. Sample RC and PSC girder sections are designed following the safety factor format of the proposed code and compared with the current design practice. Reliability indexes are calculated and examined for material and member resistance factor formats and sample calibrations of safety factors are presented. It is concluded that the proposed code provides reasonable level of reliability compared to the international design standards.

Key Words
structural reliability; truck live load; statistical data; resistance factor format; code calibration.

Address
Inyeol Paik: Department of Civil and Enviro. Engineering, Kyungwon University, Songnam, Korea
Eui-Seung Hwang: Department of Civil Engineering, Kyunghee University, Yongin, Korea
Soobong Shin: Department of Civil Engineering, Inha University, Incheon, 402-751, Korea

Abstract
In the present paper, a methodology has been presented for the reliability assessment of concrete barriers that lie at a certain depth in the soil, and a missile (a rigid projectile) impacts the top of the soil cover normally, and subsequently after penetrating the soil cover completely it hits the barrier with certain striking velocity. For this purpose, using expressions available in the literature, striking velocity of missile at any depth of soil has been derived and then expressions for the depths of penetration in crater and tunnel region of concrete barrier have been deduced. These depths of penetration have been employed for the derivation of limit state functions. Using the derived limit state functions reliability assessment of underground concrete barrier has then been carried out through First Order Reliability Method (FORM). To study the influence of various random variables on barrier reliability, sensitivity analysis has also been carried out. In addition, a number of parametric studies is conducted to obtain the results of practical interest.

Key Words
missile penetration; projectiles; concrete barriers; structural reliability; FORM.

Address
N. A. Siddiqui: Dept. of Civil Engineering, King Saud University, Riyadh 11421, Saudi Arabia
F. H. Khan and A. Umar: Dept. of Civil Engineering, Aligarh Muslim University, Aligarh 202002, India


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