Dilation behavior of seven-wire prestressing strand – The Hoyer effect

In prestressed concrete elements, bond between pretensioned strands and the surrounding concrete is attributed to adhesion, mechanical interlock and the friction and ‘wedge-action’ attributable to radial expansion of the strand following release. The latter effect is referred to as the Hoyer effect and is the subject of this paper. The Hoyer effect contributes primarily to the transfer of the initial prestressing force to the concrete thereby affecting the transfer length. The transfer of large prestressing forces, particularly at an early concrete age, can lead to local cracking associated with bursting stresses or splitting associated with transfer of the strand force through bond. An analysis of embedded strand behavior based on fundamental principles of mechanics clearly demonstrates the importance of strand dilation characteristics in the development of concrete stresses and therefore the transfer of prestressing forces. An experimental program demonstrates that the dilation of seven wire strand is affected by not only the Poisson’s ratio for steel but also the ‘tightening’ of the helical wires and effects of bearing between these wires. The resulting dilation of a seven wire strand is larger than Poisson’s ratio for steel and is found to be a function of strand diameter. A discussion of design code provisions for prestressing strand transfer length is included. North American practice, while conservative, does not explicitly address the mechanics of stress transfer. Improvement in this regard could result in more efficient prestressed concrete structures. European practice, on the other hand, appears to reasonably capture the Hoyer effect.