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©
Canadian Standards Association
Precast concrete — Materials and construction
November 2009
21.1.2
The placing of concrete shall comply with Clause 7.2 of CSA A23.1 and
Clauses 21.1.1
to
21.4
of this
Standard.
21.2 Placing
21.2.1
The placing of concrete shall be scheduled in such a manner that each layer is placed against concrete that
has not reached its initial set.
21.2.2
The face mix and the backup mix shall be placed in such a manner that the backup mix bonds to but does
not penetrate the face mix.
Notes:
(1)
For the minimum thickness of face mix for precast concrete, see
Clause 16.3.2
.
(2)
The use of separate face and backup mixes or of a uniform concrete mix throughout the element depends on
economics, the type of finish required, the configuration of the element, and the practice of the precasting plant.
Elements with intricate shapes and deep, narrow sections generally require one uniform concrete mix throughout. See
Clause 16.3.1
.
(3)
The second mix should be placed before the evaporation of bleedwater from the first mix unless special bonding
techniques are used.
21.2.3
Concrete coating on reinforcement shall be cleaned off or the backup mix shall be placed before any
concrete on the reinforcement hardens. Alternatively, the placement of the reinforcement may follow the
consolidation of the face mix.
21.2.4
Two-stage precasting, as used for intricate elements such as column covers, corners with large returns, or
elements with abrupt changes in concrete volumes, shall be performed with construction joints that
comply with
Clause 22
.
21.2.5
In the construction of insulated concrete wall panels, the first wythe shall be carefully placed and
consolidated to the correct thickness. The concrete shall be finished with a smooth surface to minimize air
pockets between the insulation and the concrete. Any pockets showing along the edges of the insulation
shall be filled with concrete.
Notes:
(1)
The correct wythe thickness is normally accomplished or checked by a special template riding on the sides of the form.
(2)
Some sandwich wall panels are designed with an uninterrupted air space between the wythe exposed to the weather
and the insulation. This air space should remain completely open to allow water to be drained from the building.
21.3 Consolidation
Surface vibrators shall be of such a type and shape, and applied in such a manner, that suction action is
prevented during use.
Note:
See ACI 309R.
21.4 Removal of paste
Paste adhering to extended reinforcement shall be removed to ensure bonding of bars to concrete in a
later placement.
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