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©
Canadian Standards Association
Precast concrete — Materials and construction
November 2009
28.2.5.3.4
Where rods under tension are used in multiple-tendon tensioning, an allowance shall be made for the
elongation of the rods.
28.2.5.3.5
Elongation calculations shall take into account the effect of temperature variations if the temperature of
the steel at the time it is stressed and at the time the concrete starts to set differs by more than 15 °C. The
thermal coefficient of the expansion of steel shall be taken as 12 × 10
–6
/°C.
28.2.5.4 Measurement of elongation
28.2.5.4.1
After completion of initial stressing, reference marks shall be established from which elongations resulting
from final tensioning forces can be measured.
Reference marks shall be visible during the entire final stressing operation.
28.2.5.4.2
Elongation shall be measured and recorded to the nearest millimetre.
Note:
Stressing procedures and calculations should be the responsibility of the manufacturer’s engineer.
28.2.5.4.3
Where there are no intermediate bulkheads or friction points, elongation and tendon slippage shall be
measured on the first and last tendon stressed and on at least 10% of the remaining tendons.
28.2.5.5 Force corrections
The following shall be taken into account in calculating the indicated hydraulic pressure:
(a) friction in the jacking system;
Note:
Because of the large jacking ram and the heavy sliding or rolling anchorage to which the tendons are attached,
multiple-tendon tensioning methods usually used on post-tensioned construction are subject to substantial variations
in friction that need to be overcome by the jack. Friction in single-tendon tensioning is small and can usually be
ignored.
(b) temperature variations (allowance shall be made in the stressing force for temperature variations of
15 °C or more between the ambient temperature and the concrete temperature at the time of
placing); and
(c) live end seating (allowance shall be made for loss of force as the tendon seats into the anchor).
Note:
See PCI MNL-116 for assistance in stress corrections.
28.2.5.6 Final stressing of straight tendons
After application of the initial load in single-tendon tensioning and the establishment of reference marks
for measuring elongation and slippage, the full tendon load shall be applied and checked by at least two
control measures. The frequency of elongation and tendon slippage measurement shall be as specified in
Clause 28.2.5.4.3
.
Note:
For multiple-tendon tensioning, see PCI MNL-116.
28.2.5.7 Final stressing of draped tendons
28.2.5.7.1
After application of the initial load and the establishment of the reference mark for measuring elongation
and slippage, the full tendon load shall be applied and checked by at least two control measures, as
specified in
Clause 28.2.5.6
.
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