An ultrasonic method for measuring tensile forces in a seven-wire prestressing strand

Initial tensile forces applied to prestressing strands undergo progressive time-dependent losses. These prestress losses, if unaccounted for, may cause catastrophic failures of prestressed concrete structures. The main objective of this study is to develop a nondestructive evaluation method for measuring prestress forces in the seven-wire prestressing strands which are widely used in various types of prestressed concrete structures. Experimental investigation was conducted by measuring the ultrasonic waves propagating through 1/2 inch-diameter seven-wire strands subjected to different levels of tensile forces. Theoretical analysis was conducted by considering the acoustoelastic effect and the dispersive behavior of a longitudinal transient wave traveling through a long, stressed, circular rod. Results indicate that the velocities of each frequency component of the traveling waves can be related to the tensile force levels in the strand. The stress wave signals were processed using the Wigner-Ville Transform to identify the arrival times of different frequency components. The analytical and experimental results correlate well and high measurement accuracy is observed. The results show a promising progress in the field of nondestructive measurement of tensile forces in the seven-wire steel strands for post-tensioning concrete members.