Monitoring the stress of the post-tensioning cable using fiber optic distributed strain sensor

Authors:
Junqi Gao a,*, Bin Shi b, Wei Zhang b, Hong Zhu c
a Department of Civil Engineering, Nanjing University of Aeronautics & Astronautics, 29 Yudao Street, Nanjing 210016, China
b ACEI, Department of Earth Sciences, Nanjing University, 22 Hankou Road, Nanjing 210093, China
c College of Civil Engineering, Southeast University, Si Pai Lou 2, Nanjing 210096, China

Measurement 39 (2006) 420–428

Abstract:
We applied a Brillouin-OTDR, which is a fiber optic distributed strain sensor, to measure the stress of four post-tensioning cables. The fiber optic sensor was bonded to one steel strand and three aramid fiber reinforced plastic (AFRP) cables, which were later installed in three precast beams. The monitoring test was performed both in the tension procedure and the load procedure. The experimental results showed that the fiber optic distributed sensor holds high accuracy, and the relative deviation of the measurement results between fiber optic sensor and strain gauge is under 2.7%. The stress distribution of the post-tensioning cable obtained under all loads can be used to evaluate the health state of the beam. Furthermore, the prestressing loss occurred in post-tensioning cable can be calculated. The initial prestressing loss of steel strand is 8.98% and the initial prestressing loss of AFRP cable is 6.32%. After 30 days, the prestressing loss of steel strand is 8.81% and the prestressing loss of AFRP cable is 9.61%.

Keywords:
BOTDR; Fiber optic distributed sensor; Post-tensioning cable; Stress monitoring; AFRP

1. Introduction

Fiber optic sensors are intrinsic to the optical fiber, i.e., they are physically part of the optical fiber itself, and therefore they possess a great deal of potential for structural monitoring because of their features such as high measurement accuracy, immunity to electrical noise, long-term measurement stability and the resistance to corrosion, which are different from the conventional sensors’. These unique features of fiber optic sensors indicate that they are the important and essential components of the structural health monitoring system in the field of civil engineering.

With the further development of the fiber optic sensing techniques, the applications of fiber optic sensors have been extended from the laboratory test to field experiments. Some kinds of fiber optic sensors have been applied to the health monitoring of bridge and other structures in recent years. Maaskant et al. employed FBG sensors to monitor the strains of both steel and CFRP (carbon fiber reinforced polymer) prestressing tendons [1]. The FBG sensor was embedded in the precast girders of the bridge. Watkins employed the Fabry-Perot interferometric (EFPI) sensor to measure internal strain in the main load-carrying layers [2], i.e., the top and bottom layers of carbon-FRP tubes of a 9 m span all-FRP bridge located on the University of Missouri at Rolla (UMR) campus. Inaudi et al. employed the long-gauge sensors to evaluate the curvature variations and calculate the horizontal and vertical displacements by double integration of the curvatures [3]. Vurpillot et al. employed the SOFO sensors installed in the concrete deck and on the steel girders of an highway bridge to monitor the concreting and thermal shrinkage, load test with heavy trucks and long-term deformations [4]. Being different with the previous researches, this study reported here aims at monitoring the stress of the post-tensioning cables installed in beams that are tested to failure using fiber optic distributed sensors based on BOTDR.

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