Evaluation of Electro-Mechanical Impedance (EMI) for Structural Health Monitoring (SHM) of Post-Tensioned Ground Anchors

Abstract

The US Army Corps of Engineers has demonstrated a need to monitor ground anchors within their large civil works portfolio in order to avoid structural damage to critical infrastructure. Ground anchors (ie, “tiebacks”) are used for securing loose soil or rock by placing a tendon (such as seven wire strand) into a predrilled hole, pulling it into tension against an anchor plate or block, then typically grouting it into place. A common failure mode of these ground anchors is tendon degradation due to corrosion resulting in tension loss. However, the tendons are difficult to monitor due to embedment in material limiting visual inspection to exposed anchor plates, wedges, and strand ends. Furthermore, retrofitting sensors onto existing anchor installations is limited to the exposed components. Therefore, instrumentation is desired to detect and quantify corrosion and/or tension loss of the embedded tendon as part of a comprehensive Structural Health Monitoring (SHM) plan. An effort to evaluate instrumentation to this end was undertaken by the authors. A process for inducing accelerated corrosion of cable sections using additively manufactured corrosion cells was developed in order to facilitate a more time-efficient evaluation of sensing modalities’ ability to detect corrosion induced material loss. Literature reveals that electro-mechanical impedance (EMI) using PZT (Lead zirconate titanate) patches (i.e. piezoelectric transducers) has been previously experimentally evaluated for both corrosion monitoring of steel components and cable tension respectively, although not concurrently. This paper presents an experimental methodology and laboratory results of corrosion detection and tension monitoring using PZT patches affixed to seven strand steel wire rope. Preliminary results from separate experiments indicate an ability to detect material loss due to corrosion and changes in tension using EMI.