Models for flexural cracking in concrete: the state of the art
A. Borosnyói, Budapest University of Technology and Economics, Hungary G. L. Balázs, Budapest University of Technology and Economics, Hungary
Crack formation presents a complex mechanical and geometrical question to be modelled. The available crack width formulations are often based on simplifications. A rigorous formulation of crack widths should be based on the integration of strain differences of reinforcement and concrete between cracks, due to the accumulated slips. In this paper an extensive literature review on crack widths and crack spacing is presented. The basic intention of the present paper is to summarise the development of flexural crack models and collect the most relevant formulae for crack spacing and crack width. It reports not only the possible improvement of design or research equations but also the appearance of new types of reinforcements with different characteristics from those of steel reinforcements. This state-of-the-art Report is a contribution to the work of fib TG 4.1 'Serviceability Models'.
Experimental study of the shear strength of precast segmental beams with external prestressing
T. Wakasa, Structure Division, New Structural Engineering Ltd, Japan H. Otsuka, Graduate School of Civil Engineering, Kyushu University, Japan W. Yabuki, Graduate School of Civil Engineering, Kyushu University, Japan
In order to grasp the shear strength of a precast segment structure with external tendons, shear tests were carried out on nine different cantilever beams. The parameters were in situ concrete or precast segment, internal and external prestressing, and shear keys. This paper presents the results of these tests and proposes a new formulation to estimate the shear strength of a precast segment beam using external prestressing.
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.
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.
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.
