Research on Inductance–capacitance electromagnetic resonance measurement of prestressed concrete beam

•A load/stress-frequency function model of unbonded prestressed steel strands based on electromagnetic resonance is established.•A vertical load test based on the LC resonance frequency detection system was carried out on the prestressed concrete beam.•According to the measured data of three vertical loading experiments, the functional relationship between the increase in electromagnetic resonance frequency and the increase in stress is fitted.•The three load experimental data measured by the LC electromagnetic resonance method circuit reflect the changes of real stress and frequency. Effective prestress change detection has a guiding effect on the safety of prestressed concrete structures. The measurement method of inductance–capacitance (LC) electromagnetic oscillation circuit can directly reflect the correlation between the stress of the steel strands and the electromagnetic resonance frequency. In this paper, the post-tensioned unbonded prestressed steel strands inside the structure are treated as a helical coil. Combined with the inductance characteristics of the helical coil in the resonance circuit and the characteristics of stress in the prestressed structure, the relationship between the load and the stress of the structure under vertical loading is analyzed. A function model of the stress and resonance frequency in the LC oscillation circuit is established. Then a 4.8 m long unbonded prestressed concrete beam is used for 0–55 kN vertical loading experiments and the electromagnetic resonance circuit is completed to detect the change of the resonance frequency increment of the steel strands with the measured stress increment. Theoretical analysis and experimental results show that the electromagnetic resonance circuit can effectively detect structural load stress changes that are positively related to the electromagnetic resonance frequency of the steel strands.