Bond–slip model for estimating the friction coefficient of pretensioned strands considering end slip

Abstract In pretensioned prestressed concrete members, the transfer length of the strand exhibits significant variability, but current design equations do not fully account for the effects of various factors. However, the end slip value of the strand exhibits a strong correlation with the transfer length. In a typical analytical process, the transfer length is determined by assuming a specific friction coefficient between the strand and concrete; thus, analytical models can be used to estimate friction coefficients from experimental results. To address this, a bond–slip model was developed to estimate the friction coefficient that the analytical results match both the measured transfer length and the end slip. Based on the concrete confinement model, the concrete cover was modeled as a confining element for the radial expansion of the strand. The calculated friction coefficients generally ranged from 0.2 to 0.5, with an average value of 0.3. From the estimated friction coefficient, the bond stress exhibited a wave‐shaped distribution in the transfer zone, reaching its maximum at approximately two‐thirds of the transfer length from the free end, minus a constant offset. Since the fracture of the concrete cover in the transfer zone can result in bond failure of the member, confinement reinforcement was determined to be necessary in the cracked region identified through analysis.