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Structural Concrete, Vol. 6, no. 2, June 2005

Prediction of the bond capacity of bars cast under drilling fluids

A. Jones, Arup Research and Development, London, UK

This paper discusses the influence that two types of drilling fluid, bentonite and a polymer, have on the bond capacity of reinforcement bars that are cast in concrete placed under them. Test results from both laboratory specimens and site tests are discussed and the results compared to capacities predicted by various codes. It is shown that Eurocode 2 predicts the bond capacity of bars in concrete cast under bentonite well, providing the assumption of poor bond conditions is made. The results for bars in concrete cast under polymers are less clear and there appear to be significant differences between the performance on site and that in the laboratory. 

Structural Concrete, Vol. 6, no. 1, March 2005

Punching resistances of unbonded post-tensioned slabs by decompression methods

R. J. Carvalho Silva, University of Brasilia, Brazil
P. E. Regan, University of Westminster, UK
G. S. S. A. Melo, University of Brasilia, Brazil

This paper presents the direct decompression method for calculating the punching strengths of post-tensioned slabs. It is a method already used for determining the shear strengths of beams. In many respects it is similar to the Fédération Internationale de la Précontrainte (FIP) treatment of punching in post-tensioned slabs, but it is simpler to use. The predictions of two variants of the direct decompression approach, and of the FIP method, are compared with the results of tests of slabs with various arrangements and profiles of tendons. It is shown that the direct method reduces the scatter of ratios of experimental and calculated strengths although all three approaches provide reasonable results. 

Structural Concrete, Vol. 6, no. 2, June 2005

Experimental testing of helically confined high-strength concrete beams

N. Elbasha, School of Civil, Mining and Environmental Engineering, University of Wollongong, Australia
M. N. S. Hadi, School of Civil, Mining and Environmental Engineering, University of Wollongong, Australia

The strength and ductility of high-strength concrete (HSC) beams are enhanced through the application of helical reinforcement located in the compression region of the beams. The pitch of the helix is an important parameter controlling the level of strength and ductility enhancement of over-reinforced HSC beams. This paper presents an experimental investigation of the effect of helix pitch on the beam behaviour by testing five helically confined, full-scale beams. The helix pitches were 25, 50, 75, 100 and 160 mm. The cross-section of the beams was 200 300 mm, and with a length of 4 m and a clear span of 3.6 m subjected to four-point loading, with emphasis placed on the midspan deflection. The main results indicate that the helix had negligible effect when the helical pitch was 160 mm (helix diameter), the concrete cover spalling-off load increased linearly as the helical pitch increased, and the ultimate load decreased as the helical pitch increased. 

Structural Concrete, Vol. 6, no. 1, March 2005

Compressive behaviour of steel fibre reinforced concrete

R. D. Neves, Concrete Division, LNEC, Portugal
J. C. O. Fernandes de Almeida, Civil Engineering Dept., Instituto Superior Tecnico, Portugal

An experimental study to investigate the influence of matrix strength, fibre content and diameter on the compressive behaviour of steel fibre reinforced concrete is presented. Two types of matrix and fibres were tested. Concrete compressive strengths of 35 and 60 MPa, 0.38 and 0.55 mm fibre diameter, and 30 mm fibre length, were considered. The volume of fibre in the concrete was varied up to 1.5%. Test results indicated that the addition of fibres to concrete enhances its toughness and strain at peak stress, but can slightly reduce the Young's modulus. Simple expressions are proposed to estimate the Young's modulus and the strain at peak stress, from the compressive strength results, knowing fibre volume, length and diameter. An analytical model to predict the stress-strain relationship for steel fibre concrete in compression is also proposed. The model results are compared with experimental stress-strain curves. 

Structural Concrete, Vol. 5, no. 4, December 2004

Analyses of hollow core floors subjected to shear and torsion

K. Lundgren, Chalmers University of Technology, Göteborg, Sweden
H. Broo, Chalmers University of Technology, Göteborg, Sweden
B. Engström, Chalmers University of Technology, Göteborg, Sweden

Hollow core units are commonly subjected to shear and torsion, for example when placed in floors with openings or skew ends. Present design codes give rough estimations for how the torsional moment can be estimated. The aim of this work was to increase the understanding of torsion in hollow core floors, and to develop a modelling strategy suited to model complete hollow core floors subjected to shear and torsion, using the non-linear finite element method. In a simplified global model, the cross-section of each hollow core unit was represented by one beam element, and the neighbouring hollow core units were coupled by means of slave nodes in the corners, allowing compression but not tension. Comparisons with test results showed that the simplified global model can, with reasonable accuracy, describe the real behaviour of hollow core floors. Furthermore, the simplified global model was used together with solid elements in a part of a hollow core unit, to enable modelling of a shear and torsion failure. Good agreement with test results was obtained concerning failure mode, crack pattern, maximum load, and displacements. Thus, the modelling technique used appears to describe the actual situation in a good way. 

Structural Concrete, Vol. 5, no. 4, December 2004

Analyses of hollow core floors subjected to shear and torsion

K. Lundgren, Chalmers University of Technology, Göteborg, Sweden
H. Broo, Chalmers University of Technology, Göteborg, Sweden
B. Engström, Chalmers University of Technology, Göteborg, Sweden

Hollow core units are commonly subjected to shear and torsion, for example when placed in floors with openings or skew ends. Present design codes give rough estimations for how the torsional moment can be estimated. The aim of this work was to increase the understanding of torsion in hollow core floors, and to develop a modelling strategy suited to model complete hollow core floors subjected to shear and torsion, using the non-linear finite element method. In a simplified global model, the cross-section of each hollow core unit was represented by one beam element, and the neighbouring hollow core units were coupled by means of slave nodes in the corners, allowing compression but not tension. Comparisons with test results showed that the simplified global model can, with reasonable accuracy, describe the real behaviour of hollow core floors. Furthermore, the simplified global model was used together with solid elements in a part of a hollow core unit, to enable modelling of a shear and torsion failure. Good agreement with test results was obtained concerning failure mode, crack pattern, maximum load, and displacements. Thus, the modelling technique used appears to describe the actual situation in a good way. 

 

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