Structural Concrete, Vol. 6, no. 3, September 2005

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A study of the influence of environmental effects on the behaviour of a pre-stressed concrete viaduct

R. W. Howells, Cardiff School of Engineering, Cardiff University, UK
R. J. Lark, Cardiff School of Engineering, Cardiff University, UK
B. I. G. Barr, Cardiff School of Engineering, Cardiff University, UK

This paper addresses the influence that changing environmental effects have on the strains developed in a major pre-stressed concrete viaduct. The changes in strain, resulting from variations in temperature and relative humidity, were analysed over a period of one year. It was found that changes in temperature had a greater influence on the strain behaviour of the structure than changes in relative humidity, with strain variations of up to 75 microstrain (με) being recorded. A further study was conducted on the effect that changing temperature has on the strain behaviour of the viaduct during each of the four seasons. It was observed that the variation of temperature over the course of a week resulted in strains of up to 15 με. Finally, in both studies, a linear relationship was observed between strain and temperature, although these relationships differ slightly depending on the season, and the segment location within the span.

Structural Concrete, Vol. 6, no. 3, September 2005

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Long-term monitoring of electrically isolated post-tensioning tendons

B. Elsener, Institute of Materials Chemistry and Corrosion, ETH Zurich, Switzerland

Electrically isolated tendons with plastic ducts for internal grouted post-tensioning were developed about 15 years ago. This new generation of tendons offers enhanced corrosion protection of the steel strands and the possibility to monitor the corrosion protection by simple non-destructive measurements (electrical impedance). This paper reports practical experience on quality control and long-term monitoring of two flyovers with electrically isolated tendons in Switzerland. The results of impedance measurements are rationalised on the basis of a simple model of a capacitance C (of the polymer duct) in parallel to a resistance R (leaks, defects) that both have a clear physical meaning and depend on the length of the tendon. The penetration of (chloride-containing) water at defects of the duct will lead to a decrease of the resistance R of that tendon. Thus for the first time the corrosion protection of the structurally important post-tensioned tendons can be monitored during the whole service life of the structure. 

Structural Concrete, Vol. 6, no. 3, September 2005

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Influence of the shear force and transverse reinforcement ratio on plastic rotation capacity

R. do Carmo, Instituto Superior de Engenharia de Coimbra, Coimbra, Portugal
S. M. R. Lopes, Universidade de Coimbra, Coimbra, Portugal

Ductility, particularly the plastic rotation capacity of critical regions, conditions the available degree of moment redistribution and the ability to exploit the additional resistance of hyperstatic structures. A theoretical model for calculating plastic rotation capacity, considering the influence of the main factors, is presented. Special attention has been paid to the influence of the tensile reinforcement ratio, the shear force and the confinement of compressed concrete on plastic rotation capacity. Theoretical results are compared with those obtained using an experimental programme designed to study the influence of these factors. Some extrapolations are made on the basis of the model, and conclusions are drawn.

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

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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'.

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

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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. 

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

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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. 

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