Civil Engineering Reference
In-Depth Information
Fig. 11-10. Proper vibration makes possible the placement
of stiff concrete mixtures, even in heavily-reinforced con-
crete members. (55806)
Fig. 11-11. Internal vibrators are commonly used to consol-
idate concrete in walls, columns, beams, and slabs. (69970)
Even in highly reinforced elements, proper mechan-
ical consolidation makes possible the placement of stiff
mixtures with the low water-cementitious materials ratios
and high coarse-aggregate contents associated with high-
quality concrete (Fig. 11-10). Among the mechanical
methods are centrifugation, used to consolidate moderate-
to-high-slump concrete in making pipes, poles, and piles;
shock or drop tables, used to compact very stiff low-slump
concrete in the manufacture of architectural precast units;
and vibration—internal and external.
nected to the shaft rotates at high speed, causing the head
to revolve in a circular orbit. The motor can be powered by
electricity, gasoline, or air. The vibrating head is usually
cylindrical with a diameter ranging from 20 to 180 mm
(
3
⁄
4
to 7 in.). Some vibrators have an electric motor built
right into the head, which is generally at least 50 mm (2 in.)
in diameter. The dimensions of the vibrator head as well as
its frequency and amplitude in conjunction with the work-
ability of the mixture affect the performance of a vibrator.
Small-diameter vibrators have high frequencies
ranging from 160 to 250 Hz (10,000 to 15,000 vpm) and low
amplitudes ranging between 0.4 and 0.8 mm (0.016 and
0.03 in.). As the diameter of the head increases, the fre-
quency decreases and the amplitude increases. The effec-
tive radius of action of a vibrator increases with increasing
diameter. Vibrators with a diameter of 20 to 40 mm (
3
⁄
4
to
1
1
⁄
2
in.) have a radius of action in freshly mixed concrete
ranging between 75 and 150 mm (3 and 6 in.), whereas the
radius of action for vibrators of 50- to 80-mm (2- to 3-in.)
diameter ranges between 180 and 350 mm (7 and 14 in.).
Table 11-1 shows the range of characteristics and applica-
tions for internal vibrators for various applications.
Proper use of internal vibrators is important for best
results. Vibrators should not be used to move concrete
horizontally since this causes segregation. Whenever pos-
sible, the vibrator should be lowered vertically into the
concrete at regularly spaced intervals and allowed to
descend by gravity. It should penetrate to the bottom of
the layer being placed and at least 150 mm (6 in.) into any
previously placed layer. The height of each layer or lift
should be about the length of the vibrator head or gener-
ally a maximum of 500 mm (20 in.) in regular formwork.
In thin slabs, the vibrator should be inserted at an
angle or horizontally in order to keep the vibrator head
completely immersed. However, the vibrator should not
be dragged around randomly in the slab. For slabs on
grade, the vibrator should not make contact with the sub-
Vibration
Vibration, either internal or external, is the most widely
used method for consolidating concrete. When concrete is
vibrated, the internal friction between the aggregate parti-
cles is temporarily destroyed and the concrete behaves
like a liquid; it settles in the forms under the action of
gravity and the large entrapped air voids rise more easily
to the surface. Internal friction is reestablished as soon as
vibration stops.
Vibrators, whether internal or external, are usually
characterized by their frequency of vibration, expressed as
the number of vibrations per second (Hertz), or vibrations
per minute (vpm); they are also designated by the ampli-
tude of vibration, which is the deviation in millimeters
(inches) from the point of rest. The frequency of vibration
can be measured using a vibrating reed tachometer.
When vibration is used to consolidate concrete, a
standby vibrator should be on hand at all times in the
event of a mechanical breakdown.
Internal Vibration.
Internal or immersion-type vibrators,
often called spud or poker vibrators (Figs. 11-10 and 11-11),
are commonly used to consolidate concrete in walls,
columns, beams, and slabs. Flexible-shaft vibrators consist
of a vibrating head connected to a driving motor by a flex-
ible shaft. Inside the head, an unbalanced weight con-
