Behavior of Post-Tensioned Girder Anchorage Zones

Several large, thin-webbed box girder bridges, with post-tensioned anchorage zones designed in accordance with AASHTO and ACI requirements, have experienced large cracks along the tendon path in the anchorage zones at the design stressing load. Cracking of this nature provides a path for penetration of moisture and salts and thus presents a potential corrosion and frost damage threat. In addition, such cracking negates a major reason for the use of prestressed concrete, the minimization of service load cracking.

This report summarizes the observation noted in an extensive physical test program. A simplififed test specimen was developed to accurately simulate the behavior of the post-tensioned box girder web. The experimental program investigated the primary variables affecting the formation of the tendon path crack: tendon inclination and eccentricity, section height and width, tensile splitting strength of the concrete, anchor width and geometry, and the effect of supplementary anchorage zone reinforcement, both active and passive. Behavioral trends are presented as determined from three sources. These include physical tests of 40 quarter-scale microconcrete models, physical tests of 9 full-scale prototype box girder web sections, and an extensive serires of three-dimensional linear elastic finite element computer analyses.