A Smart Steel Strand for the Evaluation of Prestress Loss Distribution in Post-tensioned Concrete Structures

Prestress loss adversely affects the behavior of in-service post-tensioned structures in terms of deflection/camber, cracking, and ultimate capacity. It is thus important to determine the level of prestressing force at various loading stages from the initial prestressing force transfer to the structure, through different in-service loads, to the ultimate load of the structure. Prestress loss is difficult to evaluate due to several intertwined factors such as creep, shrinkage, relaxation, geometric configuration, distributed friction, and slippage of post-tensioned strands. Till date, there is no cost-effective and reliable sensor and installation technique for the long-term monitoring and evaluation of prestress loss. In this study, a smart fiber-reinforced polymer (FRP) rebar with an embedded novel optical fiber (OF) is developed for the distributed strain of post-tensioned strands. The new OF is an integrated global and local monitoring technology developed by combining the Brillouin optical time domain analysis/refectory sensor and the optical fiber Bragg grating into one single fiber. The FRP rebar and six steel wires were bundled together to form a seven-wire steel strand for the post-tensioning and monitoring of concrete structures. The performances of the smart rebar and strand were validated with static tests of a prestressed steel frame structure and a post-tensioned concrete beam. The smart steel strand can accurately measure the prestress loss at each loading stage, which agrees well with that measured by a pressure loading cell and predicted by a design code.