Development of an embedded wireless sensor node for post-tensioning tendon force monitoring inside pre-stressed concrete

Abstract

This study proposes an eddy-current-based tension-reduction-detection (ETRD) technique for a post-tensioning (PT) tendon in a pre-stressed concrete (PSC), and develops a ETRD wireless sensor node (ETRD-WSN) to achieve (1) automatic detection of tension reduction using low cost and low power consuming eddy current sensor (ECS), and (2) semi-permanent monitoring during the whole life-span of a bridge without direct access of human resource. An ECS is attached on PT anchorage holding a PT tendon, and the stress alteration on the PT anchorage is evaluated by investigating the eddy current response varying with its stress condition due to the magnetoelastic effect in a ferromagnetic material. The developed ETRD-WSN consumes only 6 W (12V, 500 mA), and its stable operation is possible under embedded condition by adopting wireless power and data transfer (WPDT) module. The performance of the developed ETRD-WSN is experimentally validated. In laboratory environment, a full-scale mono-strand PT tendon and a jacking instrument were prepared, and it successfully detects an excessive tension reduction over 20 % of initially-introduced tension. Then, the test results in detecting the complete disconnection of a 4.4 m-long bonded external PT tendon system demonstrated the successful performance of the ETRD-WSN. In addition, the actual field application to a 35 m-long PSC girder validated the feasibility of the ETRD-WSN.