Primary Bond Mechanism Behavior of Posttensioned 7-Wire Strand during Load Exposition

Abstract

The three primary bond mechanisms of a 7-wire strand are adhesion, friction, and mechanical interlocking between the concrete surface and the strands’ helical surfaces. Once adhesion fails as a result of micromovements of the strand, it is very difficult to predict the contribution of other two post-initial slip mechanisms to the total bond capacity of the 7-wire strand. The predictions become more difficult when the bond is influenced by the use of oil-based corrosion-protection (OCP) agents. This article discusses the influence of primary bond mechanisms during the loading stages of the posttensioned two-span concrete member and evaluates the contribution of bond mechanisms to the total member resistance and behavior. Elastomagnetic (EM) gauges, which can directly record the force inside the strands during prestressing and loading cycles, were used as one of the primary monitoring mechanisms. Based on these records, it is possible to evaluate force behavior in the strands during successive load stages up to the ultimate limit state (ULS) of the concrete members. The nonlinear bond behavior of the 7-wire strand is presented, including details of its elongation during testing.