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VI. Vibrating Wet Concrete
Concrete Consolidation by vibration is achieved by
adding a mechanically-induced combination of force
and frequency allowing those forces to act on the
freshly-poured concrete within the vibrator's area
of inuence. The proper combination of force and
frequency will ensure that the concrete will retain
its homogeneity while allowing the entrapped air to
escape through the surface. A frequency of 9,000
– 12,000 vibrations per minute initiates resonance
in (or “excites”) the cement, causing the ultra-ne
cement particles to release the extremely small
air pockets that have adhered to the cement, so
that the mortar lls in all of the spaces around the
aggregate. Vibration evenly distributes the cement
and aggregates resulting in a more dense and a
smoother concrete nish.
When enough of the entrapped air has risen to the
surface to achieve concrete consistent with the
intended strength of the mixture, the vibration is
complete. In practical terms, it is neither possible
nor necessary to remove all of the entrapped air in
the consolidation process.
How to Select an External Vibrator
There is not an exact method or science when using
external vibrators for concrete consolidation. Mixes
vary, and therefore, consolidation procedures and
preferred vibrator styles vary. “Experts” become
experts through trial and error. What makes every
application different is that the mixes will vary due
to the slump, any chemical additives, aggregate
sizes/shape, cement content, consistency of the
mixture, weather conditions, and even the type of
form work used. Following these general rules may
be helpful in selecting the number and placement
of external vibrators for effective concrete consoli-
dation.
In selecting External Vibrators, the contractor
should initially consider the workability of the
concrete and the rigidity of the forms. Plastic
concrete (slump > 3") responds better to high
frequency vibration, while stiffer mixtures
(slump < 3") require higher amplitude vibration to
initiate uidization. While properly-sized external
vibrators can be successfully used on virtually any
concrete formwork, using a too-powerful vibrator
on lightweight forms can cause damage to the
formwork. This has caused some contractors to
mistakenly believe that external vibrators cannot
be used on lightweight concrete formwork.
Vibrator Selection
Depending on specic conditions, external vibrators
with speeds between 3,000 and 12,000 rpm may be
suitable for form vibration. However, because the
natural resonant frequency of Portland Cement is
between 9,000 – 12,000 rpm, pneumatic vibrators
are often the only equipment capable of delivering
this necessary frequency.
After determining the approximate combined
weight of the formwork and concrete to be vibrated,
the contractor should select a vibrator producing the
amount of force specied by the table below. It will
often be necessary to use more than one vibrator
to produce the total amount of force required.
Note: If specific density of the concrete is
unavailable, use a standardized weight of 150 pounds
per cubic foot (2,400 kg / m3) for approximation.
Vibrator Selection
for Concrete Consolidation
Consistency Slump Vibrator Selection
Very stiff
concrete <0.5"
Vibrator force output
should be equal to
200-300% of the total
weight of the concrete
and form.
Stiff
or
stiff plastic
concrete
0.5" - 2.0"
Vibrator force output
should be equal to
130-150% of the total
weight of the concrete
and form.
Plastic
or
owing
concrete
>2.0"
Vibrator force output
should be equal to
the total weight of the
concrete and form.
