Performance evaluations of post tensioned anchorage zone in ultra-high performance concrete

Abstract

Behavior of a post-tensioned anchorage zone was investigated in this paper with three-dimensional finite element (FE) analyses model. The FE model was verified through experiments with failure pattern and load-displacement relation. Based on the verified FE model, ultimate strength and bursting force in the anchorage zone are evaluated through the parametric studies.

Since the failure in the anchorage zone is usually initiated from the local zone, the placement of supplementary reinforcements can effectively be used to enhance the resisting capacity and ductility for the local zone within the anchorage block. Considering the advanced mechanical properties of UHPC, parametric analyses were carried out to show the influence of the spiral reinforcement on the ultimate strength of the UHPC anchorage zone. Based on the FE results, it figured out that the design guidelines in NCHRP Report 356 can be used to evaluate the ultimate strength of UHPC anchorage zone. Finally, improved design guidelines have been suggested to cover the design of the anchorage zone made of UHPC and a comparison with the design guidelines have been made.

Since the anchorage zone failure by bursting stress has usually been initiated along the tendon, the placement of supplementary reinforcements at the local zone can effectively be used to prevent the tensile crack. Accordingly, exact evaluation of bursting force is important to determine the amount of additional reinforcement. In this paper, the bursting stress was investigated through the linear elastic finite element analyses upon the consideration of the change in design parameters such as the bearing plate size, the eccentricity, and the tendon inclination. Moreover, the consideration of the duct hole, which causes an increase of the bursting stress with the change in its distribution along the anchorage zone as well, was emphasized. It means that an exact prediction of the bursting force will be the primary interest in design practice. Parametric analyses were followed to evaluate the relative contribution of all design parameters in determining the bursting force, and a comparison with the design guidelines mentioned in AASHTO-LRFD has been made. Finally, an improved design guideline which takes into account the influence by the duct hole is suggested.

Even though the design code based on the normal strength concrete can be used in the UHPC anchorage zone design, using this design code for the design of UHPC anchorage zone may lead to the inefficient design. This is because the design codes are based on the normal strength concrete and stress-strain relations of UHPC are different from NSC. Therefore additional analyses were conducted to evaluate the safety of the UHPC anchorage zone without reinforcements. The results showed that UHPC anchorage zone without spiral confinement reinforcements satisfy the acceptance criteria of the guideline. In contrast, the UHPC anchorage zone needs to be placed the additional reinforcements to resist the bursting stress without cracking. In conclusion, based on the finite element analyses, the design recommendation of UHPC anchorage zone without the placement of the spiral confinement reinforcements is proposed.