114. INFORMATION REQUIRED TO PRESTRESSING OPERATIONS

In addition to the construction drawings, these documents consist mainly of:
• a description of the structure different construction phases,
• the tensioning schedule of tendons, which includes:
– the tensioning sequence,
– the calculation of the respective elongations.
• the grouting sequence of tendons.

114.1 Description of the construction stages

Whenever a structure is not entirely cast in-situ over a general falsework, the consultant specifies for the
contractor, in an accurate manner, the different construction phases so as to prevent any ambiguity on site.
This description must necessarily include the phases which may effect the structure’s stability, either during the
construction, or when in service. It is not always essential to differentiate those relevant to concreting, moul-
ding or tensioning sequences which, as part of the same phase, cause no change to the design.
Special attention must be given when fixing and removing temporary bearings and supports.

114.2 Tensioning schedule of tendons

T

ENSIONING FORCE

A prestressing tendon is tensioned by a special jack, exerting a given force at each end (two live ancho-
rages) or at one end only (one live anchorage and one dead-end anchorage). This force generally equal to
the maximum permissible value according to the specifications is given by the designer.
The application of this force induces the elongation of the tendon. The value of this force can be directly
measured using a special instrument; in practice, it is ascertained by the concordance of the indications of
the jack pressure gauge and the measurement of elongation.

T

ENSIONING SEQUENCE

The tensioning sequence of tendons is an integral part of the design. It must be put into practice after chec-
king the temporary construction phases. Special attention must be paid to the necessary concrete strengths
of the most affected sections of the structure and the tendon anchorage zones.
With a simple structure, the tensioning sequence is generally given on the tendon drawing. In the case of
large structures with several tendon lay-out drawings, or structures built in successive phases, the tensioning
sequence is included in the description of the construction phases.

E

LONGATIONS

The efficiency of the prestressing force is assessed by accurately checking the force (manometric pressure) –
elongation relationship when tensioning the tendon and comparing it with that assumed by the designer. The
elongation measured on site mainly depends on the actual friction coefficients of the tendon on the duct; ho-
wever, it is independent from the modulus of elasticity of the steel batch delivered to the site; as a matter of
fact, this value is measured on a short strand in the laboratory and does not take into account the cluster ef-
fect that actually occurs in the sheath. The determination of elongations, for tendons made up of strands, is
therefore carried out preferably using the following mean modulus: E p = 195 kN/mm

2

.

Exceptionally, this value can be adjusted, if necessary, in the case of accurate site tests. The determination
of the prestressing force’s actual transmission coefficient, from one end of the structure to the other, can be
achieved by the measurement of the hydraulic pressures in two jacks, one active and one passive; it can
also be carried out using recording load cells, installed between the jacks and the anchorage.

PPC

114/

1-

02.95

PAGE 1/3

114. INFORMATION REQUIRED TO PRESTRESSING OPERATIONS

114.3 Permanent protection by cement grout

• The main purpose of grouting the prestressing tendons is to achieve a permanent barrier against corrosion
and, according to the 1990 CEB-FIP Model-Code, also to ensure an efficient bond between the prestressing
tendon and the concrete.

• The durability of the structure depends on these operations, which are as important as those concerning
the tensioning of tendons. Theferore, they must be planned during the initial design when undertaking the
construction drawings and elaborating the construction procedures and, during the construction of the struc-
ture, they must be satisfactorily performed.

• Providing these principles are complied with, prestressed concrete is a resistant, economical and peren-
nial material.

114.31 Grouting of ducts

• The permanent protection of an internal, bonded prestressing tendon (or external, non-dismantable) is
achieved by the grouting of its duct using a cement grout.

• The grout flow inside the duct has a behaviour which has been studied during the grouting of many
models; these tests meant that the ranges of flow speeds for each type of sheath profile could be deter-
mined, as well as the ideal position for the nozzles, draincocks or grouting holes. According to the
1990 CEB-FIP Model Code, the grouting rate may vary, depending on the type of duct profile, from 5
to 15 m/min. This operation is undertaken through points, the location of which are chosen according
to the duct geometry and length. In current cases, there is one grouting hole at each anchorage and at
the peak of each tendon; their main purpose is to allow air or water to be drained off at the peak of the
tendons and to ensure the correct filling of the duct.

For structures built in several phases, when the cable layout is complex, or when the tensioning operations
may stretch over a long period of time, additional intermediate inlets must be provided along with a grou-
ting procedure perfectly suited to all types of duct, in order to prevent any water or air pockets, or to eli-
minate them by re-grouting in the zones presenting such imperfections. When the cable layout is dense,
one should be highly aware of the possibility of communication between ducts and therefore arrange the
appropriate equipment to grout them simultaneously if necessary.

• In certain cases, it is also necessary to create a vacuum in the duct prior to grouting or re-grouting duct, in
order to obtain a perfect fill. The Freyssinet Group, a fore-runner in this field, have shown a good command
of this technique for about twenty years.

• The cement grout is a suspension of cement in water, achieved using admixtures to improve the perfor-
mance of the grout. The characteristics of the components must meet the requirements of current stan-
dards, especially with regard to the content of aggressive ions (Cement: chlorine
Cl

–

< 0.05 %, sulphur ion S

2–

= 0 ; Water: Cl

–

≤

500 mg/l, SO

4

2–

≤

400 mg/l ;

Grout: Cl

–

and SO

4

2–

≤

0.1 % of the cement mass, traces of S

2–

). According to the French code, the ce-

ment must be a CPA Portland cement, class 45 or 55, in compliance with standard NFP 15-301 (equi-
valent types: ASTM C-150-1984-Portland, symbol 1, or DIN 1164 - 1978 - Portland PZ, Class 45, or
BS 12 - 1978 - OP or RHP). The other characteristics of the actual grout must fulfil the following require-
ments: exudation (2% max.), mechanical strength at 28 days (compressive strength > 30 MPa, flexural
strength > 4 MPa measured on 40 x 40 x 160 mm samples), setting achieved within 24 hours and lastly,
capillary absorption < 0.01 g/mm

2

.

PPC

114/

1-

02.95

PAGE 2/3

114. INFORMATION REQUIRED TO PRESTRESSING OPERATIONS

Several types of grout may be chosen, depending on the geometry of the ducting, the characteristics of the
structure and the intended duration of the grouting (effective groutability period). The choice can be made
from slightly retarded grouts (1/2 h to 1 h) for short tendons, semi-retarded (1 h to 5 h) for medium and long
tendons and retarded (> 5 h) in the case of complex structures.

• The grouting periods may be established using the following considerations:
– in simple structures, where tendons can be tensioned in two or three phases, at two or three week inter-
vals, all the tendons are grouted during a single operation, as soon as the tensionings are completed;
– in structures cast in several spread out phases, the grouting must be carried out taking into account the
maximum permissible intervals between tensioning and grouting (generally 4 weeks).

• The permissible intervals between tensioning and grouting can be increased beyond 4 weeks if all precau-
tions have been taken to protect steel against corrosion, by spraying soluble oil, or applying VPI powder in
order to create a basic environment resulting from the chemical reaction with the air inside the sheath. One
should also see to it that, during the waiting period, no water penetrates through the stressing heads or inlets,
especially when frost is likely. Therefore, it is necessary to close all the inlets tightly and place the grouting
caps over the stressing heads.

114.32 Protection of anchorages

The cachetage of the tendon anchorages can be carried out in three different manners:
• permanent concrete cachetage;
• temporary cachetage with reusable cast iron cap;
• permanent cachetage with cast iron or mechanically-welded permanent cap.

• P

ERMANENT CONCRETE CACHETAGE

The permanent concrete cachetage, with no cover previously installed over the stressing head, is a solution
which can be implemented when the construction schedule allows for the concrete to reach a sufficient
strength to withstand the grouting pressure. One must ensure that the new concrete adheres to the structural
concrete, for example by bush-hammering and cleaning the faces, then applying a bonding agent, as well
as curing the concrete to prevent shrinkage cracks.
This care is essential in preventing water ingress, or the exposure of anchorages to aggressive agents such
as de-icing salts, or ambient sea air.

• T

EMPORARY CACHETAGE WITH REUSABLE CAST IRON CAP

This type of encasement permits the realization of grouting immediately after tensioning. The cover ensures
the temporary watertightness during grouting; it is withdrawn after hardening. The use of mould oil or
grease, on the inner face of the cap, necessitates the removal of the grout of the stressing head, prior to the
completion of the permanent concrete encasement (see recommendations above).

• P

ERMANENT CACHETAGE WITH CAST IRON OR MECHANICALLY

-

WELDED PERMANENT CAP

This is a permanent protection of the stressing head and the tendon extremity; the cap is fitted with leak-proof
joints in order to ensure a total protection against aggressive agents; it is externally protected against corro-
sion using a paint or an anodic coating. It makes the permanent concrete cachetage unnecessary. It is filled
with grease or petroleum wax, when the prestressing tendon is adjustable, the overlengths being preserved.
When the prestressing is replaceable but non adjustable and therefore with no overlength, the temporary
type is used, with a corrosion protection coating.
Prior to filling the duct with cement grout, the surroundings of each jaw are filled with grease or wax, in
order to allow the anchorage to work correctly, after hardening of the grout. In the case of tendons made up
of unbonded strands, the cap are filled with soft products such as: • INJECTELF wax • CONDAT grease,
type TRACTA N° 1983 • VISCONORUST grease N° 3166 • MOBILPLEX grease N° 47 • TOTAL grease
N° 3412.

PPC

114/

1-

02.95

PAGE 3/3