WycoTool Hydraulic Paving Vibrators - Manual (Page 3)

1
CHAPTER 1
GE ERAL I FORMATIO
a) Purpose of Vibration
The use of internal vibration is required to
improve the workability of low slump con-
crete used in slip form paving. Low slump
concrete reduces edge slump conditions,
which allows the concept of slip form paving
to work. Also, lower slump concrete general-
ly means a lower water to cement ratio, which
increases pavement strength. Vibration
increases the workability of concrete by flu-
idizing it and helps to provide a smooth work-
able surface. Equally important, it consoli-
dates the concrete reducing entrapped air,
honey combing, cold jointing and other inter-
nal flaws which would reduce pavement
strength.
b) Vibrator Positioning and Spacing
Proper positioning and operation of vibrators
in the paving slip form molds is important to
provide proper consolidation of the concrete.
Generally, the distance between vibrators
should be between 12 and 24 inches depend-
ing on mix design, paver speed, vibrator speed
and numerous other factors.
The proper position of the vibrators should be
with their tips angled downward slightly and
as close to the center of the slab as possible.
With the vibrator tip angled down into the
center of the slab, the opening created by the
vibrator will have a tendency to fill with a
more uniform mixture. Also, the highly
vibrated concrete directly next to the vibrator
will be spread over a larger area reducing the
chance of localized over-vibration. The angle
also helps to increase the area vibrated by not
directing vibration straight down into the
subbase.
c) Vibrator Speed (VPM) Vibrations per
Minute
Historically, vibrator speed has been between
5,000 and 10,800 VPM, although in some
areas, the trend is to reduce speeds to
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between 5,000 and 8,000 VPM. As with
vibrator spacing, the optimum vibrator speed
depends on many factors. These include,
but are not limited to mix design, paver speed
and vibrator spacing. Regardless of what
optimum speed is required it is important to
closely monitor vibrator speed to ensure a
consistent quality and more uniformly
consolidated pavement.
d) Paver Travel Speed
Paver travel speed will affect consolidation.
If the paver speed is too high, the vibrators
may not be vibrating in the mix for a long
enough duration to adequately consolidate it.
If the paver travel speed is too slow, over-
vibration will occur, driving entrained air out
and segregating the mix.
e) Mix Design and Slump
Mix design and slump have an influence on
concrete consolidation. A high slump concrete
will consolidate easier than a low slump
concrete. However, if the slump is too high
it will cause edge slump problems and lower
the strength of the concrete. If the mix
consists of fine aggregate and one size of large
coarse aggregate, it will be harder to
consolidate and finish the slab. It is
recommended to use a mix that uses at least
two aggregate sizes, one large and one small.
The smaller aggregate mixed in with the larger
aggregate will fill some of the larger voids,
thus making finishing and consolidation
easier.