Weiher and Zilch - Lehrstuhl für Massivbau

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Weiher and Zilch 2008 CBC Here it is commendable to use jacks for monostrands. At abutments enough space for handling has to be kept free or the backfill has to be removed. Both are very complex and its application mainly for short-span pt bridges is not economical. At multispan bridges most tendons are anchored at the diaphragms and thus are easy to access. If restressing is done, one has to bear in mind that the additional stressing length is large enough so that the “new” imprints of the wedges into the steel are not at the same place than the “old” imprints. Therefore, depending on the stress increase, a certain minimum length of the tendon is required - otherwise the tendon has to be exchanged. An exchange also is required if the tendon is damaged (e.g. by fire or vandalism). PRODUCT QUALITY Unbonded tendons are often fabricated completely in a factory. Therefore, grouting on site can be avoided. The quality of works and the schedule is not influenced by weather conditions (e.g. frost). With unbonded tendons the installation and stressing of the tendons can be performed almost entirely independent from the reinforcing and pouring of the concrete. When using internal tendons the quick assembly can be assured by preparing few supports. Between these supports the tendon finds its final vertical position due to its stiffness (“free tendon curvature” 29 ). In Germany, the arrangement of tendons in the webs of boxes has been forbidden in 1999. For T-beams this is not valid. Compact shaped cross sections are advantageous because they weaken the cross section of the RC less. The fabrication of unbonded tendons off-site not only guarantees a high quality of the products but moreover accelerates the construction progress. The tendons can be shipped to the site briefly before it will be installed. With mass products (e.g. automobile industry) this Just-in-Sequence-concept (JIS) led to remarkable time and cost savings 30 . To prevent damages of the soft PE-ducts there are higher demands on the accuracy of installation and handling on-site. Deviation along sharp edges must be avoided which might lead to cutting of the steel through the polyethylene. INDICATION OF FAILURE Simple indication of tendon failures can be achieved by using unbonded tendons. They consist of prestressing steel, a wax-like corrosion protection mass and polyethylene sheathing. Fig. 5 (left) shows a two span girder with a bonded tendon (top) and a girder with an unbonded tendon (bottom), both with failed tendons close to the left support, e.g. due to stress corrosion. For the girder with the unbonded tendon, a local tendon failure is equivalent to a total failure along the total tendon length since there is no reintroduction of the force due to bonding. Therefore, the prestressing is also reduced in the areas of maximum bending moments (mid-support, mid-span) so that the girder will crack in these areas. Concrete cracking can be observed by many monitoring systems and can be verified numerically as an indication of tendon failure. The cracks are indicating the failure of the tendon; see Fig. 5 (left). Zilch, Hennecke and Gläser have shown in their research, that cracking occurs far from the ultimate limit state of the structure 26 . In Fig. 5 (right) they illustrate their results in which a rather long part of a span will show cracks at rates of prestressing losses that do not harm 6
Weiher and Zilch 2008 CBC the load capacity (e.g. 25% loss). Hence, a sudden collapse of the structure (e.g. a bridge) can be avoided by using unbonded tendons. x L L 7 100% Fig. 5 Early indication of failure for unbonded post-tensioning EXAMPLES / PILOT PROJECTS length of cracked zone / span (x/L) 80% 60% 40% 20% fctk,0.05 fctm permanent 0% 0,0% 12,5% 25,0% 37,5% 50,0% loss of prestressing Already since the 90ies of the last century external tendons have been widespread in Germany. For road bridges with box girders the combination with internal bonded tendons (only in deck and bottom slab) has shown to be better. The pros of unbonded post-tensioning (product quality, structural safety, restressability and exchangability etc.) can be combined with the pros of bonded post-tensioning (larger lever arm, bigger steel strength for ULS, continuous tendon curvature). However, the structure cannot be seen as an RC member any more because of the bonded tendons and thus has to fulfil more strict design criteria in SLS. Since few years unbonded tendons have been uses as internal tendons. Pilot projectrs have been realized, like Mühlenberg Viaduct (North Rhine-Westphalia) or Roßriether Graben (Bavaria). For tank construction the advantages of internal guided unbonded post-tensioning can be extensively used, e.g. for the four digestion tanks of the sewage water treatment plant “Gut Großlappen” near Munich, Germany. One example of the vast application opportunities of unbonded post-tensioning is demonstrated at a pedestrian bridge with a granite slab. MÜHLENBERG VIADUCT The Mühlenberg Viaduct at Arnsberg (B 229n) spans with six fields a distance of 255m 31 . The box is 2,45m in height (see Fig. 6a). The structure was built stepwise. According to Eisler 31 twelve external tendons à 3 MN and two precaution tendons would have been necessary if not using internal tendons. This would have led to problems of space and uneconomic diaphragms. For internal tendons unbonded ones have been chosen. Fig. 6b shows the pt concept. The internal tendons are arranged as additional tendons spanwise in the bottom slab for the pier regions and in the deck slab for mid-span. The external tendons are deviated at all diaphragms (pier, approaching span and twice in the inner spans). These diaphragms or concrete deviator are densely reinforced concrete members serve both for deviation and for anchoring (see Fig. 6c). Despite the higher prices for unbonded tendons in
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Weiher and Zilch - Lehrstuhl für Massivbau

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