two hours or until the concrete begins to lose its plasticity. Concrete
for beams, girders, brackets, column capitals, haunches, and drop panels
shall be placed at the same time as concrete for adjoining slabs.
3.8.2
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NOTE: For large jobs, this paragraph may be
expanded. Consolidation equipment and procedures
are described in detail in ACI 309.
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Immediately after placing, each layer of concrete shall be consolidated by
internal vibrators, except for slabs 100 mm 4 inches thick or less. The
vibrators shall at all times be adequate in effectiveness and number to
properly consolidate the concrete; a spare vibrator shall be kept at the
jobsite during all concrete placing operations. The vibrators shall have a
frequency of not less than 10,000 vibrations per minute, an amplitude of at
least 0.6 mm 0.025 inch, and the head diameter shall be appropriate for the
structural member and the concrete mixture being placed. Vibrators shall
be inserted vertically at uniform spacing over the area of placement. The
distance between insertions shall be approximately 1.5 times the radius of
action of the vibrator so that the area being vibrated will overlap the
adjacent just-vibrated area by a reasonable amount. The vibrator shall
penetrate rapidly to the bottom of the layer and at least 150 mm 6 inches
into the preceding layer if there is such. Vibrator shall be held
stationary until the concrete is consolidated and then vertically withdrawn
slowly while operating. Form vibrators shall not be used unless
specifically approved and unless forms are constructed to withstand their
use. Vibrators shall not be used to move concrete within the forms. Slabs
100 mm 4 inches and less in thickness shall be consolidated by properly
designed vibrating screeds or other approved technique. Excessive
vibration of lightweight concrete resulting in segration or flotation of
coarse aggregate shall be prevented. Frequency and amplitude of vibrators
shall be determined in accordance with COE CRD-C 521. Grate tampers
("jitterbugs") shall not be used.
3.8.3
Cold Weather Requirements
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NOTE: When the designer is especially concerned
about corrosion of reinforcing steel or embedded
items, or possibility of sulfate attack,
(particularly to prestressing steel) the percentage
of chloride ion in the mixture should be limited.
See ACI Committee 201 report "Guide to Durable
Concrete" and ACI Committee 222 report "Corrosion of
Metals in Concrete" for guidance on control of
chloride ion.
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Special protection measures, approved by the Contracting Officer, shall be
used if freezing temperatures are anticipated before the expiration of the
specified curing period. The ambient temperature of the air where concrete
is to be placed and the temperature of surfaces to receive concrete shall
be not less than 5 degrees C 40 degrees F. The temperature of the concrete
when placed shall be not less than 10 degrees C 50 degrees F nor more than
25 degrees C 75 degrees F. Heating of the mixing water or aggregates will
be required to regulate the concrete placing temperature. Materials
SECTION 03300A
Page 42
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