Measurement of tensile forces in a seven-wire prestressing strand using stress waves

This paper presents a nondestructive, stress wave technique to evaluate the prestress levels in posttensioned seven-wire strands. Seven-wire steel strands are widely used in various types of prestressed concrete structures. In this study both experimental measurements and theoretical analysis were conducted. A commonly used 12.7 mm (1/2-in.) diameter seven-wire prestressing strand was studied. Tensile forces up to 111.2 kN (25 kips) were applied to the strand. A numerical analysis was conducted by accounting for acoustoelasticity effects and the dispersion of waves to calculate the prestress levels in the strands. This analysis provides a successful description of the behavior of a longitudinal transient wave traveling through a long prestressed, circular strand. The calculation correlates well with the experimental observation. The results indicate that the time of flight of the stress waves can be related to the tension level in the strand. Change of wave velocities at different stress levels was evaluated and the accuracy of this technique is discussed. This study proposes a technique that can provide an efficient nondestructive measurement of stress retension levels in posttensioned steel strands.