Identification of the governing mechanism of haunch cracking in post-tensioned prestressed concrete box girders using a three-dimensional bursting stress model

Haunch cracking in prestressed concrete (PC) box girders is a practical damage problem that may facilitate moisture ingress and accelerate durability deterioration. However, the cracking mechanism in the haunch zone remains insufficiently understood, particularly when prestressing tendons are anchored along the girder width and the associated bursting stress cannot be accurately quantified. In the haunch zone, anchorage-induced bursting stress is an important source of local tensile stress. To solve this problem, this study establishes a three-dimensional (3D) bursting-stress model and applies it to identify the governing mechanical contributor of construction-stage haunch cracking in PC box girders. First, the analytical solution for 3D bursting stress is derived, and the relationship between tendon anchorage orientation and the direction of the maximum bursting stress is clarified with the aid of finite element analysis. On this basis, the governing cracking mechanism is identified by correlating crack morphology in the haunch zone with the 3D bursting-stress field under different tendon anchorage orientations. The proposed method is then validated through actual bridge cases of haunch cracking. The results show that the theoretical calculations of the maximum bursting stress agree well with the finite element results, with a maximum error of less than 8%. For height-anchored tendons, the maximum bursting stress is mainly distributed along the girder-width direction, whereas for width-anchored tendons, it is mainly distributed along the girder-height direction. The agreement among the theoretical calculations, finite element results, and field-observed crack morphology indicates that excessive bursting stress is the governing mechanical contributor of haunch cracking in PC box girders during construction.