Civil Engineering Reference
In-Depth Information
their exposed portions. This practice eliminates air and
water trapped under the reinforcing bars and increases
bond between the bars and surrounding concrete; use this
method only if the concrete is still workable under the
action of vibration. Internal vibrators should not be
attached to reinforcing bars for this purpose because the
vibrators may be damaged.
Revibration of previously compacted concrete can be
done to both fresh concrete as well as any underlying
layer that has partially hardened. Revibration is used to
improve bond between concrete and reinforcing steel,
release water trapped under horizontal reinforcing bars,
and remove additional entrapped air voids. In general, if
concrete becomes workable under revibration, the prac-
tice is not harmful and may be beneficial.
External Vibration.
External vibrators can be form vibra-
tors, vibrating tables, or surface vibrators such as vibra-
tory screeds, plate vibrators, vibratory roller screeds, or
vibratory hand floats or trowels. Form vibrators, designed
to be securely attached to the outside of the forms, are
especially useful (1) for consolidating concrete in mem-
bers that are very thin or congested with reinforcement,
(2) to supplement internal vibration, and (3) for stiff mixes
where internal vibrators cannot be used.
Attaching a form vibrator directly to the form gener-
ally is unsatisfactory. Rather, the vibrator should be
attached to a steel plate that in turn is attached to steel
I-beams or channels passing through the form stiffeners
themselves in a continuous run. Loose attachments can
result in significant vibration energy losses and inade-
quate consolidation.
Form vibrators can be either electrically or pneumat-
ically operated. They should be spaced to distribute the
intensity of vibration uniformly over the form; optimum
spacing is best found by experimentation. Sometimes it
may be necessary to operate some of the form vibrators
at a different frequency for better results; therefore, it is
recommended that form vibrators be equipped with con-
trols to regulate their frequency and amplitude. Duration
of external vibration is considerably longer than for
internal vibration—generally between 1 and 2 minutes.
A reed tachometer can not only determine frequency of
vibration, but also give a rough estimate of amplitude of
vibration by noting the oscillation of the reed at various
points along the forms. This will assist in identifying
dead spots or weak areas of vibration. A vibrograph
could be used if more reliable measurements of fre-
quency and amplitude are needed.
Form vibrators should not be applied within the top
meter (yard) of vertical forms. Vibration of the top of the
form, particularly if the form is thin or inadequately stiff-
ened, causes an in-and-out movement that can create a
gap between the concrete and the form. Internal vibrators
are recommended for use in this area of vertical forms.
Vibrating tables are used in precasting plants. They
should be equipped with controls so that the frequency
and amplitude can be varied according to the size of the
element to be cast and the consistency of the concrete.
Stiffer mixtures generally require lower frequencies
(below 6000 vpm) and higher amplitudes (over 0.13 mm
[0.005 in.]) than more workable mixtures. Increasing the
frequency and decreasing the amplitude as vibration pro-
gresses will improve consolidation.
Surface vibrators, such as vibratory screeds (Figs.
11-12, 11-13, and 11-14), are used to consolidate concrete in
floors and other flatwork. Vibratory screeds give positive
control of the strikeoff operation and save a great deal of
labor. When using this equipment, concrete need not have
slumps in excess of 75 mm (3 in.). For greater than 75 mm
slumps, care should be taken because surface vibration of
such concrete will result in an excessive accumulation of
mortar and fine material on the surface; this may reduce
wear resistance. For the same reason, surface vibrators
Fig. 11-12. Vibratory screeds such as this truss-type unit
reduce the work of strikeoff while consolidating the con-
crete. (55801)
Fig. 11-13. Where floor tolerances are not critical, an exper-
ienced operator using this vibratory screed does not need
screed poles supported by chairs to guide the screed. In-
stead, he visually matches elevations to forms or previous
passes. The process is called wet screeding. (69938)
