123. FREYSSINET C AND K SYSTEMS ANCHORAGES

123.1 Internal prestressing

Internal prestressing is divided into two techniques:
• bonded prestressing;
• unbonded prestressing.
All the anchoring systems mentioned in the previous section are to be found in use in one or other of these
techniques.

• Internal, bonded prestressing

– This traditional system uses the live anchorage A’ (C System) or A and A’ (K System). The tendons, made
up of bare strands, are threaded into steel ducts, embedded in the concrete; the ducts (see paras. 122-3)
generally consist of a steel sheath made up of cold rolled sheet metal, or of HDPE when there are impor-
tant corrosion factors. Cement grouting ensures the permanent protection of the sheaths against corrosion;
bond of the tensioned tendons to the surrounding concrete is ensured by their special profile, which has
transverse crimps.
When the tensioning of the tendon can only be carried out at one end, anchorages A’ or A become dead-
end anchorages; they may be replaced by fixed anchorages, type B’ or B and by those relevant to types
NE, NE’, G or V.
Internal, bonded prestressing also resorts to type C’ or C couplers, with notched rim and threaded sleeves,
as well as those consisting of automatic locking monostrand couplers, type C’I (with tromplate) or CI (with
plate and trumpet).
According to the French regulation, Fascicule Spécial N° 86-19bis, the couplers were not designed to with-
stand a rise of the primary live anchorage block from its bearing plate (tromplate). Such a rise occurs during the
tensioning of the secondary tendon, when the force transmitted to the coupler is greater than the primary ten-
don’s anchored force; due to varying individual tensile forces in the strands of the two segments of tendon and
since their real geometric axis does not always exactly coincide, the coupled pieces rotate and an angular kink
appears in the strands. This phenomenon is less significant for the C’I or C I type couplers, since the tightening
force of the jaws decreases in accordance with their unwedging. Furthermore, the decompression of the pri-
mary anchorage zone results in shear stresses which must be dealt with using longitudinal reinforcing bars.
– End to end joining devices, of the R type, can also be considered.

NOTE
Grouting with a soft product (grease or petroleum wax), instead of cement grouting, transforms this system into a dismantable, recheckable, re-
tensionable, unbonded internal prestressing.

In addition to traditional internal prestressing, there is now a new type of prestressing, achieved with an an-
chorage system recently patented by Freyssinet:

• The recheckable, re-tensionable, unbonded internal prestressing

– This system uses similar anchorages to those of traditional external prestressing, but needs no overall ducting.
Contrary to all other systems, the inside of the tromplate is embedded in concrete; this refers to the A’D type models
for C System, or those relevant to versions N°1 and N°2 of K System external prestressing (units 27K15 and
37K15 are actually make of a cast iron plate and trumpet, type A). These tromplates (or trumpets) do not include
any internal HDPE trumpet, or steel tube sheath coupling. The tendon is made up of bundles of monostrands or mul-
tistrands, unbonded, arranged in one or several bundles, depending on the type of structure, following the vertical
or the horizontal plane. When the concrete is being poured, a fixing device secured to the formwork ensures the
strand correct geometric position. The absence of a single sheath regrouping the bundle of strands, offers a signi-
ficant advantage in many eases.
All the other particular anchoring dispositions used in traditional bonded, internal prestressing (embedded
anchorages, couplers, etc…) may also be considered (kindly refer to our technical literature).

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