Weiher and Zilch - Lehrstuhl für Massivbau

Weiher and Zilch 2008 CBC
is at 45% for the not exchangeable type (Fig. 4c, Table 1). Presently used external tendons
can also be arranged inside of the concrete section but they are clearly less compact. For pt of
slabs the deviation radii usually are quite large and the tendons small – transverse loading of
the steel to the ducts is of less importance. This is quite different when using loop-anchors
with small radii of curvature. There you also have to proof the durability respectively the
resistance of the duct against the loading by the steel members.
a)
b)
c) d)
Fig. 4 Internal tendon types, cross section at high points of the curvature 13
Table 1 Filling ratio of steel for tendons with a prestressing force of about 3 MN
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Post-tensioning system (F ≈ 3 MN) Ap / Atotal
SUSPA CD-66 (Fig. 3b) 27 %
VBF-CMM D (Fig. 3a) 28 %
VBF compact, one PE-duct (Fig. 4c) 45 %
VBF compact, double PE-duct (Fig 4d) 29 %
VT-CMM D (similar to Fig. 3a) 23 %
VT-CMM KD (similar to Fig. 3a) 25 %
Features of unbonded pt and consequences for design and construction
Unbonded tendon systems basically differ from bonded systems by their corrosion protection
of the steel. Unbonded tendons are only protected by the components of the tendon:
corrosion protection mass, PE-HD sheathing(s). Bonded tendons using corrugated steel ducts
need the surrounding concrete cover to protect the steel. Mainly, thin concrete cover (e.g. RC
structures built before implementation of the national guideline ZTV-K 80 in 1980 19 ) caused
corrosion of the prestressing steel in the past 1,11 .
BOND / REINFORCEMENT
Because the steel is not bonded to the concrete there is a higher need of reinforcing steel, as
an increase of loading just leads to a small increase of steel stress – but over the total length
of the tendon. In addition to that, the prestressing steel cannot be counted as steel that is
necessary for limitation of crack width. Generally, more reinforcement makes the structure
robust but also leads to higher costs.
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