Automatic measurement and warning of tension force reduction in a PT tendon using eddy current sensing

Post-tensioning (PT) using a bundle of pre-stressed strands is a critical process for assembling pre-fabricated and modularized bridge members. However, the tension force gradually diminishes over time due to such factors as corrosion, creep, and steel relaxation. Such changes compromise the overall safety of such structures. In this study, an eddy-current-based tension-force-loss warning (EC-TFLW) technique is proposed to detect and warn automatically of excessive loss of tension force in a PT tendon. A ring-type eddy-current sensor (ECS) is mounted on the outer surface of a wedge holding a tendon, and the level of eddy current measured by the ECS is related to the tension force of the tendon. The advantages of the proposed technique include: (1) low power consumption, (2) low cost, (3) simple installation, and (4) automated warning. The performance of the proposed EC-TFLW technique was validated experimentally in a full-scale lab test of a 3.3-m long, 15.2-mm diameter, mono-strand tendon that was tensioned using a universal testing machine (from 20 to 180 kN). Statistical hypothesis testing using the chi-square distribution was applied to the measured eddy current signals, and if the decline in tension exceeded a certain level, a warning was sent out automatically.