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

Written on 19 August 2011.

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

Written on 19 August 2011.

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

Written on 19 August 2011.

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

Written on 19 August 2011.

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

Written on 19 August 2011.

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

Written on 19 August 2011.

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.