Rotation-affected bond strength model between steel strand and concrete

Bond between seven-wire steel strand and concrete is critical for prestressed concrete structures. In a well-confined condition, the strand pull-out specimen tends to fail with strand rotation instead of directly pulling out from concrete due to its helical-shaped exterior wires. An analytical model, which considers the helical structural features of strand, is proposed to model bond strength for the pull-out failure case including strand rotation. Factors that have been experimentally verified to have influences on bond strength, such as concrete compressive strength, strand diameter and friction coefficient, are reflected and applied into the model through theoretical analysis. Comparison of results between the prediction and experimental results shows that the proposed model can be used to reasonably evaluate the bond strength. For practical design purposes, a simplified equation for transfer length of prestressing strand was proposed indirectly and compared with the current ACI 318 criterion. The proposed transfer length model shows higher safety margin in predicting the experimental results when comparing with that of ACI 318-14 for both 12.7 mm and 15.2 mm prestressing strands.