Keywords: prestressed concrete, anchorage, external tendon, post-tensioning, finite element analysis, fatigue.

Loading test and nonlinear analysis results of anchorage blocks for external prestressing tendons are presented and discussed in this paper. External tendons for post-tensioned concrete girders, which are left permanently unbonded to the concrete, have been increasingly applied to new constructions as well as to rehabilitations of existing bridges [1]. The main advantage of the external prestressing technique is the tendon accessibility for inspection, maintenance and replacement. The external tendons are enclosed in ducts located outside of girders and are anchored using special concrete blocks (i.e. anchorage blocks) protruded from the girder cross-section. The anchorage blocks must be properly reinforced to resist tensile and bursting forces induced by the prestressing concentrated loads. Since the anchorage blocks for external tendons are the only means of transmission of the prestressing force to the structure, they are more critical than the corresponding anchorage zones for conventional bonded post-tensioning systems.

In the experimental work, special attention has been given to the durability of anchorage blocks for external tendons. Since structural performance of the anchorage is highly dependent on the reinforcement details, the longitudinal, stirrup and spiral bars of each test specimen were strictly detailed according to the rules of the approval document for the prestressing system. Each specimen had dimensions of 400x200x160 mm. The testing procedure consists of a series of compressive fatigue tests with load ranging between and , where corresponds to the nominal static strength of the tendon.

All of the specimens reached either 2 million or 10 million load cycles in the fatigue test without failure. The static test in compression for each specimen was carried out up to the maximum load of during the first cycle in the fatigue test. After the load cycles of either 2 million or 10 million were reached without failure, the static strength test up to failure was also performed.

The behaviour of the external tendon anchorage is fully three-dimensional, whereas the existing design recommendations for bonded tendon anchorages are based on the two-dimensional methods of analysis [2]. The static, inelastic responses of the tested specimens were simulated using a three-dimensional finite element model. The analysis considers the essential nonlinearties arising in the materials: 1) nonlinear behaviour of concrete under multiaxial stress states; 2) tensile cracking of the concrete; and 3) elasto-plastic behaviour of reinforcing steel bars. Concrete nonlinearity is modeled using a plasticity theory under compression and a smeared crack approach for tensile cracking. To model tension or compression yielding of steel reinforcement, the truss elements provide linear elastic, linear strain-hardening behaviour. Perfect bond is assumed at the reinforcing steel-concrete interface. The numerical model was able to predict adequately the static behaviour and the ultimate load.

The following conclusions are made concerning the behaviour and performance of the external tendon anchorages considered in this research:

1. The rigidity of anchor plate, through which the concentrated prestressing force is transferred to concrete, has significant influence on the structural performance of anchorage blocks. More evenly distributed contact pressure under a thicker, or stiffer, anchor plate enhances the cracking and ultimate loads as well as the durability of anchorages.
2. The spiral steel reinforcement was found to undergo considerable bending as well as confinement-induced tensioning. The confining effect of spiral steel apparently contributes to the increase of the cracking and ultimate loads as well as the ductility of anchorage blocks.
3. Properly designed anchorage blocks for externally prestressed concrete girders can reach 10 million cycles of fatigue loading without failure, keeping the static ultimate loads more than 20 percent higher than the nominal tendon strength.
4. While the first visible cracking of an anchorage block occurred under service load, the development of detrimental cracks was prevented during the fatigue load cycles. The extension of cracks was limited to a depth nearly equal to a wider breadth of anchorage block and their propagating directions almost corresponded to the compressive stress trajectories in the cracked region.

1

L. Huang, H. Hikosaka, Y. Maeda, K. Komine, "Analysis and design of prestressed concrete bridge with corrugated steel web", in: B.I.G. Barr et al. (eds.): Proceedings of the 3rd International Conference on Current and Future Trends in Bridge Design, Construction and Maintenance, ICE, 187-196, 2003.

2

T.J. Ibell, C.J. Burgoyne, "Experimental investigation of behaviour of anchorage zones", Magazine of Concrete Research, 45, 281-291, 1993.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £135 +P&P)