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©
Canadian Standards Association
Precast concrete — Materials and construction
November 2009
28.2.5.10.2
There may be a maximum of one splice per tendon.
28.2.5.10.3
The ends of tendons to be spliced shall be cut by shears or abrasive grinders.
28.2.5.11 Wire failure in strands
Failure of one wire in a seven-wire strand shall be acceptable if the 5% allowable variation in prestress force
for all strands in the entire element is not exceeded and the total area of wire failure is not more than 2.5%
of the total area of tendons in any member.
28.2.6 Detensioning
28.2.6.1
Before detensioning, the concrete strength shall be determined in accordance with
Clause 19.8.2
.
28.2.6.2
Where concrete curing has been accelerated, the detensioning shall be performed immediately following
the curing period, while the concrete is still warm and moist.
Note:
If concrete is allowed to dry and cool before detensioning, volume changes can cause tension stresses and cracking in
the concrete.
28.2.6.3
Forms, ties, inserts, hold-downs, or other devices that would restrict longitudinal movement of the
members along the bed shall be removed or loosened, or, alternatively, detensioning shall be performed in
such a manner and sequence that longitudinal movement is precluded.
28.2.6.4
The prestressing forces shall be kept nearly symmetrical around the vertical axis of the element and shall
be applied in a manner that will minimize sudden loading.
28.2.6.5
For draped tendons, the sequence of release of hold-down and anchorage forces shall be established to
minimize cracking of the element.
Notes:
(1)
Cracks resulting from initial release of hold-downs will generally close upon release of anchorage forces; however,
cracking of concrete members is undesirable and should be avoided.
(2)
Computation of the hold-down forces and comparison with the weight of the member should always be performed if
hold-downs are to be released before release of anchorage forces. Release of hold-downs without release of anchorage
forces can result in concentrated vertical loads that can crack the top of the member.
(3)
In general, if the sum of hold-down forces is not more than one-half the weight of the member, it is safe to release
hold-downs before releasing anchorage forces. If the hold-down forces are less than three-fourths of the weight of the
element and nominal reinforcing steel is contained in the top flange or slab, cracking is unlikely. If the hold-down forces
approach the weight of the element, some cracking of upper surfaces immediately above the hold-downs is inevitable if
hold-downs are released before the anchorage.
(4)
Preloading of the element with weights or additional vertical restraint can minimize the effect of releasing hold-down
forces.
28.2.6.6
Tendons shall be detensioned by hydraulic jack or by heat applied to the tendons at both ends
simultaneously to produce a gradual release of stress before cutting.
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