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.
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.
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.
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.
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.
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.
