Strand Debonding for Prestressed Concrete Girders to Control End Horizontal Web Cracks Based on a Modified G-S Model

Abstract

Horizontal web cracks are frequently observed at the ends of pretensioned concrete members (I girders or Bulb Tee girders) at the time of prestress transfer. Using a modified Gergely–Sozen (mG-S) model to calculate the spalling forces of prestressed concrete girder ends was illustrated step by step. A new design procedure based on the mG-S model was proposed with the aim to control horizontal web cracks by debonding a number of strands. Three different criteria were introduced to determine the suitable debonding ratios and debonding patterns of the girder ends. Prestressed Standard Wisconsin Wide-Flange girders (WF54) and Standard Nebraska I girders (NU1100) were chosen for case studies, in which several debonding ratios and debonding patterns were analyzed to check the reasonability and effectiveness of the current limits in the standards. The analysis results obtained from the mG-S model were validated by the finite-element analysis (FEA) method, which proved that 25% debonding can greatly reduce the concrete tension strains in the web but cannot eliminate the web crack, and it can satisfy the code requirements on the vertical reinforcement stresses and crack width. Four percent of the total prestressing force is enough to be used to design the vertical reinforcement at the girder ends. The recommendations on debonding patterns are also presented.