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Structural Concrete, Vol. 7, no. 1, March 2006

Strength efficiency of metakaolin in concrete

K. Ganesh Babu, Indian Institute of Technology, Chennai, India
P. Dinakar, Indian Institute of Technology, Chennai, India

The use of mineral admixtures is widely accepted, because of several improvements possible in concrete composites and the overall economy. Of these metakaolin is one mineral admixture that appears to have significant potential for the production of high-strength and high-performance concretes. Although available information on metakaolin concretes is limited, the present paper attempts to quantify the 28-day cementitious efficiency of metakaolin in concrete at various replacement levels from the data available in the literature. It was seen that the efficiency of metakaolin in concretes can also be defined through a procedure adopted earlier for other cementitious materials like fly ash, silica fume and ground granulated blast-furnace slag (GGBS). The overall strength efficiency was found to be a combination of two parameters, one depending on the age and the other depending on the percentage of replacement, as is the case with other pozzolans. 

Structural Concrete, Vol. 7, no. 2, June 2006

Bond clause proposals for FRP bars/rods in concrete based on CEB/FIP Model Code 90. Part 1: Design bond stress for FRP reinforcing bars

R. Tepfers, Chalmers University of Technology, Göteborg, Sweden

The bond clauses in the CEB/FIP Model Code 1990 (MC90) were written for steel reinforcement in concrete. Since then, fibre reinforced polymer (FRP) reinforcing bars have been introduced as an alternative. Reinforcing steel is a homogeneous material; FRP reinforcements, however, are made of a combination of different materials with various shapes. All these FRPs perform differently under different conditions. For example, in severe environments, where steel is not durable, FRPs may perform well. To make use of fibre composites, it is necessary to bring these materials into codes of practice and the easiest way is to adapt the code clauses for steel reinforced concrete to FRP reinforced concrete. The clauses should include boxed values open for individual coefficients of different FRPs. The necessary coefficients for the code models should be determined using systems of appropriate and coupled test methods. This paper covers a code proposal for the design bond stress. Proposals for the design lengths and tension stiffening code clauses will be presented separately. 

Structural Concrete, Vol. 7, no. 1, March 2006

Failure analysis of a thin-webbed girder of post-tensioned concrete

L. Bing, Shanghai Jiao Tong University, People's Republic of China
L. Guanglu, Tongji University, Shanghai, People's Republic of China
Z. Long, Tianjin Steel Wire and Steel Cable Group Co., Ltd, People's Republic of China

The failure of a thin-webbed girder of post-tensioned concrete was investigated. In a foggy environment containing hydrochloric acid, the girder ruptured in the midspan as a result of steel strand corrosion. The causes and mechanisms of the strand fracture were identified by careful observations of the fracture surface of the strands and corrosion on the surface of the strands. Scanning electron microscopy (SEM) analysis of the fracture morphology of the strands, after mechanical fracture, showed that hydrogen in the environment had an effect on the strands. The investigation revealed that failure of the girder occurred because, while being attacked by the corrosive media, stress accelerated surface corrosion of the strands such that the cross-sections of the wires in the strands were severely weakened, consequently leading to the fracture of the strands. 

Structural Concrete, Vol. 7, no. 1, March 2006

FE modelling of bond interaction of FRP bars to concrete

Z. Achillides, National Technical University of Athens, Greece 
K. Pilakoutas, University of Sheffield, UK

In this paper a computational modelling approach is used to investigate the bond behaviour of fibre-reinforced plastic (FRP) bars in concrete. Two finite element packages (ANSYS and ABAQUS) are used to model the bond interaction of FRP reinforcing bars in cubes and beams. The main purpose of this work is to develop additional understanding of how FRP bars 'cooperate' with concrete to sustain the pullout load. Two modelling approaches are presented. In the first approach, a spring describing the behaviour of short embedment lengths in pullout tests was used for predicting the behaviour of longer embedment lengths. In the second approach, spring characteristics obtained from an experimentally determined bond stress against anchorage length envelope are used in FE modelling of beams. Both approaches showed good agreement between analytical and experimental results. However, further development on the analytical modelling of the bond interaction is required, in order to consider the effect of all parameters that influence bond. 

Structural Concrete, Vol. 7, no. 1, March 2006

Load-deflection behaviour of restrained RC slab strips

K. U. Muthu, M.S. Ramaiah Institute of Technology, Bangalor, India 
K. Amarnath, M.S. Ramaiah Institute of Technology, Bangalor, India 
A. Ibrahim, Universiti Teknologi MARA, Selangor, Malaysia 
H. Mattarneh, Universiti Teknologi MARA, Selangor, Malaysia 

This paper presents results of an experimental investigation into the load-deflection behaviour of restrained reinforced concrete slab strips subjected to uniformly distributed loads beyond the yield load. A method incorporating compressive membrane action is proposed to predict the behaviour. Results indicate that the proposed method can predict the actual trends for certain scenarios. Previously reported methods have been studied and the results compared with those from the current work. 

Structural Concrete, Vol. 6, no. 4, December 2005

On statistical characteristics of the compressive strength of recycled aggregate concrete

Jianzhuang Xiao, Tongi University, P.R. China
Jiabin Li, Tongi University, P.R. China
Chuanzeng Zhang, University of Siegen, Germany

This paper presents a statistical analysis of the compressive strength of recycled aggregate concrete (RAC) with recycled coarse aggregate (RCA) coming from one single source. The statistical analysis includes the mean value, the standard deviation and the coefficient of variation in addition to the probabilistic distribution. In order to obtain the compressive strength variations, 371 cube specimens with different RCA replacement percentages of 0, 30%, 50% and 100%, respectively, were tested under identical laboratory conditions. With the statistical data, both the normal and the lognormal model were fitted to describe the probabilistic distribution of the compressive strength. It was found that the statistical characteristics of the compressive strength of recycled aggregate concrete do not differ very much from those of normal concrete (NC) with similar strength, irrespective of the recycled coarse aggregate replacement percentage.

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