Transmission of Prestress by Bond - Bruggeling





Preface

Bond between reinforcement and concrete is a key element for the satisfactory performance of structural concrete. This statement applies equally to reinforced concrete and to pretensioned concrete. Though to some lesser degree, it also applies to bonded post-tensioning since bond between prestressing steel and concrete (across grout and duct) is necessary to develop the yield and ultimate strength in tendons at critical sections at ULS. Bond between prestressing steel and grout is also essential for the satisfactory performance of prestressed ground anchors to transfer the load from the anchor through the bond length into the ground.


The mathematical modelling of the bond behaviour of reinforcing and prestressing steel in concrete and grout has made significant progress over the years. An excellent report on this topic has recently been published by a former CEB working group and now fib TG 4.5, see fib Bulletin 10. Comprehensive bond test methods for tendons have been proposed by FIP and RILEM. However, due to their cost these tests are not frequently used. More recently simplified test methods mainly in the form of pull-out tests have been proposed. However, although practical these simplified tests do not represent the actual bond behaviour of prestressed tendons sufficiently well. Hence, designers and users of bonded prestressing steel are left in the somewhat uncomfortable situation that, although we all recognise the importance of bond, we have no practical test procedure available to specify the bond performance as basis for the manufacturing of the prestressing steel nor to perform quality control tests upon receipt of the prestressing steel.


The present report was prepared by Professor Bruggeling. It is a review of selected work of the past and a summary of discussions held in FIP Commission 2 and later fib Commission 9 and TG 9.4. It also includes a proposal for a new bond test which is felt to give more reliable results without the need to perform large and costly test series, and which may also be applicable for quality control testing. This report is hoped: 1. to make designers and users aware of a lack in our specifications for the bond behaviour of prestressing steels, and 2. to serve as basis for future work on the bond specification and testing of prestressing steels by fib and other professional organisations. I extend my sincere thanks to Professor Bruggeling for the preparation of this report.


Hans Rudolf

GANZ Chairman

fib Commission 9


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