Practical Cross-Section Imaging of External Tendons to Reveal Grout Deficiencies Relative to Strand Pattern
Publication: Journal of Bridge Engineering
Volume 25, Issue 11
Abstract
External post-tensioned tendons in concrete segmental bridges have had strand corrosion failures due to grouting deficiencies such as voids or bleed water. Nondestructive assessment of the grout condition is thus often needed, preferably by a cross-section imaging method. Here, a magnetic sensing approach to image the position of the steel strand bundle is combined with an electric impedance method to evaluate the condition of the grout space. Both are embodied in a device that images the tendon’s cross section. The magnetic sensor travels around the circumference of the tendon and measures the force of attraction to the steel strands from which an image of the strand pattern inside the tendon is created. Simultaneously, a traveling plate rotates around the tendon, and variations of the electric impedance between the plate and strands identify grout deficiencies. The impedance and strand position data create a complete color-coded image of the tendon cross-section flagging grout deficiencies. The approach was validated by experiments with tendon specimens with known deficiencies and under field conditions. The method is rapid, economical, easy to replicate, with a small and safe device not requiring specialized operator training.
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Data Availability Statement
Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request. Data: Spreadsheets used to generate figures. Code: List of inputs and outputs. Code itself is subject to intellectual property limitation.
Acknowledgments
These findings would not have been possible without the support and funding of the Florida Department of Transportation. The authors acknowledge the assistance of Michael Hutchison in performing early feasibility measurements, Jordan Riber-Smith in implementing the deconvolution protocol, Dr. Mike Celestin for valuable advice on data acquisition instrumentation, and Dr. Babu Joseph for technical comments on signal processing. The opinions, findings, and conclusions expressed in this publication are those of the authors and not necessarily those of the Florida Department of Transportation or the U.S. Department of Transportation. This paper contains Patent Pending information subject to applicable intellectual property rights by the University of South Florida.
Notation
The following symbols are used in this paper:
- A
- effective sensing area;
- dD
- polymeric duct wall thickness;
- dG
- grout space depth;
- Fa
- corrected force;
- f
- test frequency;
- G (θ)
- distance from inner duct perimeter to strand envelope at a given angular position;
- h
- distance from magnet face to external surface of the duct;
- j
- unit imaginary number;
- k
- constant for computing attractive force between magnet and strands;
- n
- exponent for computing attractive force between magnet and strands;
- w
- magnet thickness;
- Z
- measured impedance;
- ZD
- impedance of polymeric duct;
- ZG
- impedance of grout space;
- |Z|
- modulus of impedance;
- z
- distance between center of magnet and edge of strand;
- ɛG
- dielectric constant of grout;
- ɛ0
- permittivity of vacuum;
- σG
- effective electricalconductivity of grout;
- τ
- characteristic time;
- ϕm
- diameter of magnet;
- ϕs
- diameter of strand; and
- hs
- length of strand.
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Published In
Journal of Bridge Engineering
Volume 25 • Issue 11 • November 2020
Copyright
© 2020 American Society of Civil Engineers.
History
Received: Nov 25, 2019
Accepted: Jun 15, 2020
Published online: Sep 15, 2020
Published in print: Nov 1, 2020
Discussion open until: Feb 15, 2021
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