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©
Canadian Standards Association
Precast concrete — Materials and construction
November 2009
19.2 Air content — Frequency and number of tests
19.2.1
The air content of each concrete mix shall be tested at the start of production and periodically during daily
operation if the variation in slump exceeds 30 mm or the temperature of the concrete varies by more than
5 °C.
Note:
When the air content is critical, it should be checked before the concrete is placed and again when strength samples
are taken during the casting.
19.2.2
When special consolidation methods make it difficult to predict the air content in the hardened concrete,
the air content shall be verified by a test of the hardened concrete in accordance with ASTM C457. The
test shall be performed periodically, whenever the placing or consolidation methods are modified and
whenever the concrete materials are changed.
Note:
Manufacturing methods (e.g., machine-cast extrusion techniques) and some consolidation methods (e.g., use of
spinning machines or impact tables) cause loss of air. This may be compensated for by increasing the initial air dosage unless
other tests prove, to the satisfaction of the owner, that the additional strength and density eliminate the need for additional
air.
19.3 Compressive strength
19.3.1 Frequency and number of tests
For prestressed concrete production, the following minimum number of cylinders shall be taken each time
a bed is used:
(a) two for determining compliance with the specified compressive strengths;
(b) two for determining compliance with the release strength at time of transfer. These cylinders shall be
made from concrete placed during the latter half of bed casting; and
(c) one for each intermediate strength level, as necessary.
Note:
Where several beds are cast with the same concrete mix during one working day, the testing for the specified
compressive strength may be reduced to a minimum of two beds.
19.3.2 Test methods
For architectural and structural precast concrete production, compliance with the specified compressive
strength shall be determined by testing a minimum of two cylinders from each type of concrete mix each
day or a minimum of two cylinders for each 30 m
3
of any one mix when daily production exceeds 30 m
3
.
When the daily quantities are small, one test shall be performed for every 3 m
3
. When the daily quantities
are smaller than 3 m
3
, two tests shall be performed each week.
Cylinders for determining handling strengths shall be made from each new mix until the plant has
correlated the cylinder strength to the maturity of the concrete in accordance with ASTM C1074. If the
plant has not conducted correlation tests, cylinders shall be made.
Note:
If more than one mix is in use, the cylinder tests may be performed on a rotating basis.
19.3.3 Test cylinders
Standard 100 × 200 mm test cylinders shall be used.
19.3.4 In-house testing
When concrete cylinders are made for the purpose of in-house testing, methods other than those
described in Test Method A23.2-9C in CSA A23.2 may be used, if acceptable to the owner.
19.3.5 Unbonded caps
Unbonded caps, if used, shall comply with ASTM C1231/C1231M. The number of uses of unbonded caps
shall not be limited if the manufacturer can demonstrate that the strength results will not be affected by
the reuse of the unbonded caps. Unbonded caps may be used to determine strength levels at any age.
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