Detecting Damage in Grouted Tension Elements


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Title: Detecting Damage in Grouted Tension Elements

Author(s): Sharon L. Wood

Publication: Special Publication

Volume: 296


Appears on pages(s): 1-16

Keywords: cable stays, post-tensioned tendons, residual tensile strength, damage detection, natural frequencies, redundancy

Date: 3/6/2014

Prestressing steel is used as the primary tension element in stay cables and external, post-tensioned tendons on long-span bridges throughout the US. The steel is often encased in plastic duct or pipe, which is filled with grout, to provide two layers of corrosion protection. Nondestructive methods have been used successfully to detect severely damaged tension elements on several bridges, but the reliability of these methods to detect the onset of structural damage has not been documented. The results of laboratory tests are used to demonstrate that the residual tensile strength of a grouted tension element decreases more rapidly with increasing levels of damage than the tensile force under service loads. The grout and duct provide a mechanism for fractured strands to re-anchor along the length. Nondestructive methods that detect damage by approximating changes in the service-level tensile force will underestimate the level of structural damage and overestimate the residual tensile strength of a damaged cable or tendon if this inherent redundancy is not considered. Continuous monitoring with an acoustic system provided a reliable means of tracking damage in near real time for the laboratory specimens, but only damage that occurs while the system is operational can be detected.