Velocity Constants for Ultrasonic Stress Measurement in Prestressing Tendons

Abstract

There is currently widespread use of high-strength steel tendons for prestressing and post-tensioning of concrete structures and as suspension cables for long-span bridges. Tendons normally consist of one or more seven-wire, helically wound steel strands or solid rods. There are more than 130,000 prestressed bridges in the United States that contain these tendons, and approximately 3,000 new bridges are constructed each year. The prestressing tendons are critical structural elements because the forces in the tendons counteract tensile stresses in the concrete that result from loads acting on a bridge. The tendons are frequently inaccessible for visual inspections and there is currently no accepted nondestructive evaluation technique to assess the condition of these tendons. The goal of this research is to examine ultrasonic stress measurement techniques for the condition assessment of prestressing tendons. This information could be used to compare the actual force in the tendon with its design values to determine if the tendon is performing below expectations. The focus of this paper is the characterization of the relationship between ultrasonic wave velocity and stress level in prestressing tendons. Measurements were made to determine constants that relate the change in ultrasonic velocity to the change in stress. The effects of dispersion in prestressing tendons, which act as circular waveguides for ultrasonic waves, are evaluated.