Finite Element Bond Models for Seven-Wire Prestressing Strands in Concrete Crossties

Seven-wire strands are commonly used in pretensioned concrete ties, but its bonding mechanism with concrete needs further examination to provide a better understanding of some concrete tie failure modes. As a key component in the finite element (FE) analysis of concrete crossties, macro-scale or phenomenological FE bond models are developed for seven-wire strands in this paper. The strand-concrete interfaces are homogenized with a thin layer of cohesive elements applied between the strand and concrete elements. Further, the cohesive elements are assigned traction-displacement constitutive or bond relations that are defined in terms of normal and shear stresses versus interfacial dilation and slip. The bond relations are developed within an elasto-plastic framework that characterizes the adhesive, frictional and/or dilatational bonding mechanisms in the interface. The yield functions and plastic flow rules specific for the seven-wire strands are presented. The bond parameters are calibrated from untensioned pullout tests and pretensioned prism tests conducted on concrete specimens. The bond models are then verified with (1) the surface strain data measured on actual concrete crossties made at a tie manufacturing plant, and (2) the force-displacement relation obtained in a center negative moment test conducted also on concrete crossties.