Load monitoring in multiwire strands by interwire ultrasonic measurements Ivan Bartoli a , Robert Phillips a , Francesco Lanza di Scalea a,* , Salvatore Salamone a Stefano Coccia a , Charles S. Sikorsky b a NDE & Structural Health Monitoring Laboratory, Dept. of Structural Engineering, Univ. of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0085, USA b Division of Engineering Services, California Department of Transportation 1801 30th Street, P.O. Box 168041, Sacramento, CA 95816-0001 ABSTRACT Nearly 90% of the bridges in California are post-tensioned box-girders. Prestressing (PS) tendons are the main load- carrying components of these and other post-tensioned structures. Despite their criticality, much research is needed to develop and deploy techniques able to provide real-time information on the level of prestress and on the presence of structural defects (e.g. corrosion and broken wires) in the PS tendons. In collaboration with Caltrans, UCSD is investigating the combination of ultrasonic guided waves and embedded sensors as an approach to provide both prestress level monitoring and defect detection capabilities in concrete-embedded PS tendons. This paper will focus on the prestress level monitoring by first discussing the behavior of ultrasonic guided waves propagating in seven-wire, 0.6-in diameter twisted strands typically used in post-tensioned concrete structures. A semi- analytical finite element analysis is used to predict modal and forced wave solutions as a function of the applied prestress level. This analysis accounts for the changing inter-wire contact as a function of applied loads. A feature shown sensitive to load levels is the inter-wire energy leakage. In order to monitor such feature, the method uses low-profile piezoelectric sensors able to probe the individual, 0.2-in wires comprising the strand. Results of load monitoring in free and embedded strands during laboratory tests will be presented. Keywords: Semi-analytical finite element method, Prestressing strands, Structural Health Monitoring. 1. INTRODUCTION Multi-wire steel strands are used in civil engineering as the tensioning components of prestressed concrete structures and in cable systems of cable-stayed and suspension bridges. As documented in several studies 1,2,3,4 , the tendon breakage or the presence of defects in the wires can induce serious consequences for these structures. Many techniques have been applied to the defect detection and load monitoring of prestressing tendons in prestressed concrete structures. The visual inspection is the simplest, oldest and most common form of inspection technique 5 , although it is only efficient when the degradation is visible at the surface of the structure. Global approaches have been employed for the measurement of stay cable loads, which relies on modal-analysis techniques 7,8,9 . In this case the cable natural frequencies are correlated to the level of applied load, following the vibrating chord theory. The technique, unfortunately, is insensitive to small defects such as corrosion and it is not applicable to embedded PS tendons. Radiography 6 , was proved as a very effective technique in many applications for defect detection and stress measurement; however, it encounters several limitations in concrete structures due to the highly scattering nature of concrete and safety issues have dissuaded from its use in open and large areas. For the detection of small defects in stay-cable and suspension bridges cables, the Magnetic Flux Leakage (MFL) method has gained some interest especially in Europe 10 . The MFL method relies on the fact that a flaw in a ferromagnetic object introduces a localized discontinuity of the system magnetic properties. The MFL method cannot be used for continuous health monitoring and it does not provide any information on the level of stress in the cable. An alternative method 11,12 , based on measuring the magnetic permeability of tendons and cables, can * flanza@ucsd.edu ; phone 1 858 822-1458; fax 1 858 534-6373 Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2008, edited by Masayoshi Tomizuka, Proc. of SPIE Vol. 6932, 693209, (2008) 0277-786X/08/$18 · doi: 10.1117/12.775934 Proc. of SPIE Vol. 6932 693209-1 2008 SPIE Digital Library -- Subscriber Archive Copy