A boundary detecting method for post-tensioned pre-stressed ducts based on Q-factor analysis

• Proximity capacitive sensor and capacitance measurement system is presented for boundary detecting.
• For the air-water and air-cement models, the method can detect the positions of the boundaries.
• For air-exuded liquid-cement model, a new method for boundary detecting in post-tensioned pre-stressed ducts is proposed.
• Bandwidth analysis for Q-factor is performed and center frequency fc is obtained to detect the boundary.
• The results show its validity and feasibility for identifying the boundary position of three phases.

In this study, a new method to evaluate the quality of post-tensioned pre-stressed ducts based on proximity capacitive sensor is presented. The proximity capacitive sensor is designed according to our previous work. For the air-water (εair = 1, and εwater = 80) and air-cement (εair = 1, and εcement = 4) models, the method based on capacitance measurement is shown to be valid and feasible, and the positions of the boundaries between the two phases can be detected. However, the method is invalid when applied to the water-cement (εwater = 80, and εcement = 4) model, and the results are in conformity with the experience presented in previous studies. In order to resolve this problem, the Q-factor (quality factor) is defined and a frequency sweep of the Q-factor is performed by an impedance analyzer for a three-phase (air-exuded liquid-cement) model. The bandwidth and cutoff points are defined to derive a more stable frequency point, namely center frequency fc. This can be used to identify the boundary position of the three-phase layer, and the boundaries between the air-exuded liquid and exuded liquid-cement can be clearly identified. These results provide potential for multi-phase detection by analyzing the center frequency of the Q-factor.