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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. 

 

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

Direct design of hollow reinforced concrete beams. Part II: experimental investigation

A. S. Alnauimi, Sultan Qaboos University, Sultanate of Oman
P. Bhatt, University of Glasgow, UK

Tests were conducted on eight reinforced concrete hollow beams subjected to combined load of bending, shear and torsion. The beams were designed using the direct design method that was discussed in Part I. All beams had an overall cross-section dimension of 300 300 mm with a wall thickness of 50 mm. The overall length of the beam was 3800 mm. The two main variables in the series were the ratio in the web of the maximum elastic shear stress due to twisting moment to elastic shear stress due to shear force which varied between 0.59 and 6.84, and the ratio of the maximum twisting moment to the bending moment which varied between 0.19 and 2.62. The beams were experimentally tested in the University of Glasgow, Scotland, UK. Good agreement was found between the design and experimental failure loads. All beams failed near the design loads and had undergone ductile behaviour until failure. The results indicate that the direct design method can be successfully used to design reinforced concrete box beams for the combined effect of bending, shear and torsion loads.

Structural Concrete, Vol. 5, no. 3, September 2004

Spatial variability of concrete deterioration and repair strategies

Y. Li, Delft University of Technology, The Netherlands
T. Vrouwenvelder, Delft University of Technology, The Netherlands
G. H. Wijnants, Netherlands Organisation for Applied Scientific Research, Delft, The Netherlands
J. Walraven, Delft University of Technology, The Netherlands

This paper presents an improved and more realistic approach to evaluate the deterioration process and optimise the repair strategy of concrete structures. It is based on the commonly used probabilistic-based reliability analysis methods, but takes into account the spatial variability of concrete properties that has great impact on the design and maintenance decisions of structures. The developed approach is exemplified by the concrete bridge 'Wilpsedijk' in the Netherlands to show the service lifetime prediction based on spatial variability of concrete deterioration including initiation and propagation period. With respect to an established repair criterion, available repair options and the corresponding repair costs, the optimal lifetime repair strategy is determined. In comparison with most of the studies that neglect the variables with random spatial variability, the approach reflects the actual situation more realistically and can produce useful information as the proportion or percentage of the surface area that shows concrete deterioration during the whole period of time. It enables the planning of different repair and maintenance strategies for the structure from a practical point of view. 

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

Direct design of hollow reinforced concrete beams. Part I: design procedure

A. S. Alnauimi, Sultan Qaboos University, Sultanate of Oman
P. Bhatt, University of Glasgow, UK

This paper, Part I, presents a general 'direct design procedure' for the design of reinforced concrete hollow beams subjected to a combined load of bending, shear and torsion. Elastic stress field in conjunction with Nielsen's twodimensional yield criterion for reinforced concrete subjected to in-plane forces were used in the design of reinforcement. This procedure is based on and satisfies the lower bound theorem of the classical theory of plasticity. The main features of this procedure are the precluding of the use of empirical equations as is the case with existing codes of practice and in addition to satisfying the requirements of the theory of plasticity it reduces the ductility demand assumed by this theory. Comparison between the direct design procedure and the truss analogy showed that the direct design procedure leads to steel requirements close to those of the truss analogy when the angle of inclination of the struts is 458. The direct design procedure produced much less reinforcement than the American Concrete Institute and the British Standards Institute codes.

 

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