1. No category

INFORMATION BULLETIN: IB 46
Vibration of Concrete
Introduction
The correct placing and compaction of fresh
concrete are probably the most important parts of
the whole sequence of concreting operations.
Success relies on careful planning, the right
manpower and internal equipment.
This
information bulletin discusses various aspects of
the compaction process.
It is pertinent to
remember that the mixing process for concrete
entraps air within the mix. For each 1% of voids left
within the concrete the strength is reduced by
approximately 5-6%. Air entrapped in the concrete
leaving the mixer typically may vary from 5-20%.
Compaction is vital to achieve:
Maximum strength of the placed concrete.
2.
Maximum durability.
3.
Adequate
bond
and
protection
reinforcement in the concrete.
4.
Avoidance or reduction of visual blemishes,
such as honeycombing and blowholes on the
surface of form cast concrete.
for
The ease with which optimum compaction can be
achieved by vibration techniques is related to:
2.
3.
Compactability refers to the ease with which a
concrete can be compacted properly with efficient
removal of entrapped air and the repositioning of
constituent particles into a denser state.
Mobility of mix related to aspects of flow. Internal
cohesion due to frictional effects, surface forces
and the like is an important factor here.
Stability of a mix refers to its resistance to
segregation effects during transporting, handling,
placing and compacting.
1.
1.
There are three inter-related properties that may
influence the behaviour of a concrete mix during
vibration. These are known as compactibility,
mobility and stability. Each is affected by changes
in the physical make-up of the mix, and can control
the degree to which efficient consolidation of the
particles is possible.
Physical properties of the fresh concrete which
in turn depend on the type of aggregate,
constituent particle shapes, and relative mix
proportions. Harsh mixes are more difficult to
consolidate. Mixes high in fines or cement are
"sticky" and may also present problems of
compaction;
Types of vibrators, associated characteristics
and vibration patterns through the concrete;
Techniques in handling vibrators, in particular
spacing and duration of vibration.
Segregation. A significant separation of the course
and fine fractions is highly detrimental to concrete
quality.
The object in vibrating concrete is to mobilise it
sufficiently, so that it becomes plastic enough to
enable air voids to be removed and the aggregate
particles to gravitate together to form a
homogeneous mass. The stiffer the mix and the
larger the aggregate particle sizes, the greater will
be the force required to energise the mix.
Lower water cement ratio concrete has a lower
workability, but becomes a much stronger
compacted concrete. A high degree of compaction
with harsh mixes requires very efficient vibration
both in terms of effectiveness of the applied poker
vibrator and the number of insertions made.
Vibration Mechanisms
The equipment that is used in compacting concrete
IB 46: Vibration of Concrete
Page 1
develops its vibrations by a form of eccentric
rotation. Because of this, the vibrations are
generated in a steady flow of cycles, and are
transmitted into and through the medium in
contact with the vibrator.
..
cycle is given by x = a sin (2πft) where the
maximum acceleration is given by a = 4π2f2s metres
per second2 (figure 1).
The cycle of vibrations travel through the concrete,
transferring their energy to the particles in the mix.
Eventually, at some distance from their source, the
vibrations lose their effectiveness.
As the vibrations pass a certain point, the mix at
that location moves back and forward about its
original point of rest. As this occurs, the entrapped
air is released and moves the surface while
individual particles oscillate about and settle down
into the mix.
The components of the vibration cycle are
amplitude, frequency and acceleration, and these
terms are used to describe the performance
characteristics of vibration equipment. See Table 1.
Table 1:
Vibration
Method
Recommended accelerations and
frequencies of concrete vibration
Recommended
Recommended
Acceleration
Frequency
(without concrete
load)
g
Internal
Hz
vib/min
100-200
150-250
9,000-15,000
Form
5-10
50-200
3,000-12,000
Surface
5-10
50-100
3,000-6,000
Table
5-10
50-100
3,000-6,000
Amplitude is the maximum departure for a point of
rest during a displacement cycle under vibration.
Most concrete vibrators operate with an amplitude
of 0.5 mm to 2.0 mm.
Frequency (f) is usually described by the number of
vibrations per unit time. 1 Hertz (hz) = 1 vibration
per second, or 60 vibrations per minute. Therefore
200 Hz refers to 12,000 vibrations per minute.
The displacement at any time during a simple sine
wave oscillation is given by the formula x = s sin
(2πft) where s denotes the amplitude.
Similarly, the acceleration (which is the rate at
which the velocity is changing) at any time in the
Figure 1: Sinusoidal vibratory motion.
The maximum acceleration during a vibratory
motion is often expressed as a multiple of the
acceleration due to gravity, g; for example 5 g (50
m/sec2) for table vibration.
From research conducted by Dr L. Forssblad of the
Dynapac organization in Sweden, the interactions
of concrete of properties, frequency and amplitude
of internal vibration are shown in graphs of radius
of action versus frequency, for various amplitudes
and times of operation with a constant mix design.
The studies indicated that there was an optimum
combination of vibratory conditions for the
response of the concrete mix (figure 2, page 3).
During the vibration process the concrete
undergoes three different stages. The first is the
initial rapid collapse of the uncompacted mix. This
requires a large energy usage. If the vibration effort
is too low, the internal resistance of the mix
dampens the motion and the concrete absorbs the
energy without any plastic deformation occurring.
As the force is increased, the mechanical
properties of the mix and its resistance to the
compaction effort falls until the material is
transformed into a liquid. The mass then begins to
flow.
As the concrete then liquefies, de-aeration begins
and most of the entrapped air is released. Finally,
as the number of air bubbles being liberated
IB 46: Vibration of Concrete
Page 2
Figure 2: Graphs showing the correlation between radius of action, frequency, and amplitude for a 60 mm
internal vibrator.
ceases, little energy is required to overcome the
internal friction and damping effect of the concrete
as the mix is behaving nearly as an ideal fluid, and
its surface begins to acquire a glistening smooth
appearance.
Types of Vibrators
The four most commonly used systems for
compacting concrete are internal vibration, table
vibration and surface vibration. With each of these
the mechanism of vibration and the effect of the
formwork on the concrete mix is different.
Internal Vibrators
Internal or poker vibrators are available with a
selection of power sources and types of vibrating
mechanisms. The power source is either electric,
pneumatic, petrol or diesel based. The vibrating
mechanism in the poker head can be driven by a
flexible shaft, motor-in-head or pneumatically.
Pneumatic poker vibrators that operate a rotating
mechanism within the head, are used in areas
where it is convenient to have compressed air
available and when it would be dangerous to use
other types of machines.
Whatever the form of vibrator, the rotating member
in the head produces an eccentric motion that
generates the vibrations. Circular compression
waves are produced in rapid succession. These
travel away from their source and through the
concrete. The further they travel through the
concrete, the amplitude of vibration imparted to
the particles that are met reduces, due not only to
IB 46: Vibration of Concrete
Page 3
the damping effect at the vibrator itself and in the
concrete, but also to the increased length around
the circular wave (figure 3).
head. The diameter of the rotor is smaller than the
inside diameter of this rolling ring, and since the
inside of the tube is scrupulously clean, the rotor
‘grips and skids’ inside the ring four times for every
revolution of the flexible shaft. This system
effectively gears up the 3,000 rpm to produce
12,000 vibrations per minute at the nose cap end.
Since the maximum amplitude is at the nose cap,
the pendulum vibrator when withdrawn from the
concrete compacts the upper surface without
leaving holes or voids.
Developments to the principle have been made
over the years to produce longer lasting and cooler
running vibrators with improved efficiency of
vibration.
These features, coupled with the
availability of waterproof joint extension shafts to
make the flexible shaft up to 11 m long and rubber
covered nose caps to protect formwork, have made
the pendulum vibrator a popular choice for
concrete compaction.
Figure 3: Principle of internal vibration.
Flexible Shaft Poker Vibrators
These vibrators employ two sections for generating
their vibratory output. They are known as “parallel”
or “pendulum” vibrators. The parallel design
embodies an eccentric shaft rotating between
bearings at both ends, whilst the pendulum design
involves the use of a suspended rotor with a self
aligning bearing at the drive end, with the lower
end being allowed to freely orbit when rotated
within the housing.
The vibratory characteristics of the two types are
different, in as much as the vibratory output from
the parallel machine gives a force of equal power
over the length of the tube, while with the
pendulum design the power is at its maximum at
the nose cap end of the vibrator.
The parallel vibrator historically was the preferred
system, but it was found with harsh mixes that
although it effectively compacted the mix, it left a
hole when the poker head was withdrawn.
The pendulum system developed in Sweden in
1936 overcame this problem. In this design the
flexible shaft is driven at 3,000 rpm and is screwed
(via an end shank) to the top end of the solid steel
rotor. The self aligning bearing at this end allows
the other bottom end to float inside a hard steel
ring which is permanently fixed inside the tube
Pneumatic Poker Vibrators
These vibrators generally have an integral oil bottle
and throttle control and a compressed air hose
inside a large diameter exhaust hose to allow used
air to escape by the oil bottle. The poker heads
comprise four basic types. The oldest style uses an
airmotor inside the tube driving an eccentric shaft
between two bearings. Another type uses an
airblown ball bearing in a race to produce the
vibrations. The most common two types in use are
the rotary vane vibrator and the helical rotor
vibrator.
The helical rotor vibrator was a development from
the rotary vane to primarily reduce the weight of the
vibrator which was required in the vane operation.
The spinning rotor is forced outwards by a series of
discretely located airflow paths to produce the
vibration.
This process is repeated over 20,000 times a
minute which reduces to 12,000 times when placed
in the concrete, producing the characteristic rise
and fall droning noise of an air vibrator.
External Vibrator
The selection and application of external vibrators
requires careful consideration. The units are
available in varying output powers, that are defined
IB 46: Vibration of Concrete
Page 4
either by the centrifugal force developed or the
wattage of the motor. The transfer of vibratory
power though the formwork must be considered as
the nature of the mix and density of any
reinforcement within the section (figure 4).
frequencies used range between 3,000 and 12,000
vibrations per minute. The high frequency use
tends to give a better surface appearance. Large
contracts benefit by the use of external vibration
with regard to a lower manpower and reduction of
human error. Far stiffer mixes can also be used.
Vibrating Screeds
Surface vibration is usually accomplished by
comparatively light single or double vibrating
screeds which can compact up to 200 mm thick
layers of flowing to plastic concrete mixtures. For
such screeds, a frequency range of 3,000 to 6,000
vibrations per minute and accelerations to 5-10 g
are customary. The amplitude distribution along
the screed should be reasonably uniform (figure 6).
Figure 4: Principle of form vibration.
Generally, the external vibrator consists of an
electric motor with an unbalanced member to
create the vibration (figure 5).
Figure 6: Principle of surface vibration.
Roller and Laser Screeds
Figure 5: An external vibrator clamped to a form.
The best frequency of vibration depends mainly on
the design of the formwork with high vertical forms
usually requiring the high frequency option.
However, very stiff mixes respond better to high
amplitude and lower frequency. Generally, the
The use of these two methods represents the most
recent advances in the placement and compaction
of concrete. It is important to note that additional
vibration will still be necessary when using these
methods to screed and finish concrete as the
amount of vibration imparted into the concrete by
these two methods may not be enough to achieve
total compaction.
•
Roller Screeds
Roller screeds are sometimes used as a
placement and compaction tool in its own
right, but this is usually only in relatively thin
IB 46: Vibration of Concrete
Page 5
slabs, and with the use of immersion vibrators
around the perimeter forms, where additional
compaction is usually required.
Manufacturers of these screeds claim that the
placement of concrete using roller screeds
leaves more large aggregate at or near the
surface, which enhances the performance of
the slab under harsh operation conditions
(such as is found in warehouses for instance).
•
Laser Screeds
Laser screeds are used primarily in large floor
pours where slab flatness is of prime concern.
While thinner slabs will only require
supplemental vibration around the slab
perimeter, thicker slabs are generally placed
using a combination of immersion vibration
and the vibrating laser screed. (This approach
is the recommended method to follow).
Table vibrators can give less consistent results
even with careful operation. The compaction effect
is determined by the acceleration of the table.
Accelerations of about 5-10 g before the forms are
placed on the table, and 2-4 g during vibration, are
required.
For table vibration the optimum frequency range is
fairly low, 3,000-6,000 vibrations per minute.
Comparatively large amplitudes are generally
needed for efficient and rapid consolidation.
The location of the vibrators and direction of
rotation is important since it effects the primary
direction of the vibration which may be a rotational
motion or uni-directional.
Compaction Methods
Table Vibration
The characteristics of concrete, effects of vibration
and equipment available, have been discussed in
the previous sections.
This section deals
specifically in turn with the practical consideration
in the workplace of using vibrators. Often the
cause of many problems of faults in concrete is
directly traceable to the failure to ensure adequate
vibration.
Vibrating table techniques are usually restricted to
precasting operations. On a vibrating table, the
forms as well as the concrete can move during
vibration and resonance may occur. Also reflection
of the pressure waves against the concrete surface
will influence the amplitude distribution (figure 7).
The main feature of construction work tends to be a
lack of sufficient vibration to the concrete in terms
of providing manpower and equipment to match
the placing rate of the concrete. When placement
is by concrete pump considerable resources are
needed if full compaction is to be achieved.
Both methods will produce a slab with surface
levelness and flatness tolerances that are much
better than free screeding techniques.
Internal Vibrators
Figure 7: Principle of table vibration.
Most concrete is compacted by means of
immersion or poker vibrators. This method is
considered the most satisfactory because the
poker works directly on the concrete and can be
moved from one position to another easily and
quickly. For most reinforced concrete work, pokers
of diameters from 25 mm up to 75 mm are used.
Diameters up to 100 and 150 mm are available, but
their use is mainly restricted to mass concrete in
heavy civil engineering works like dam
construction. Due to their weight, these large
pokers usually need two people to handle and
operate them. For efficient compaction, the largest
diameter that the complexity of formwork and
reinforcement will allow should be used. Table 2
gives an indication of poker sizes and applications.
IB 46: Vibration of Concrete
Page 6
Although the table indicates the radius of action for
various poker diameters, the actual effectiveness of
a particular poker in a specific situation depends
on the workability of the concrete and the
characteristics of the poker itself. Generally, the
larger the diameter and the higher the frequency,
the greater will be the radius of action, but in
practice it is best to judge by eye the actual radius
for a particular situation.
This radius will determine the spacing and pattern
of insertions of the poker. For example, if the
radius of action is about 200 mm, insertions will
need to be about 300 mm apart and to a
predetermined pattern if all the concrete is to be
fully compacted.
As a guide, a spacing of about 450 mm (250 mm
radius of action) may be assumed for a 60 mm
diameter poker with concrete of medium
workability.
from a number of sites have shown that they are
often running wastefully, or at a reduced efficiency,
for about 70% of their operating time:
•
•
•
This means that the poker is doing useful work for
only 30% of the time, which is why it is necessary
to plan the compaction, placing method and
technique in advance, so that both operations are
carried out as economically and quickly as
possible.
The following guidelines are helpful to ensure a
well compacted mix (see also figures 8, 9 and 10):
1.
Make sure the operator can see the concrete
surface.
2.
When inserting the poker, allow it to penetrate
to the bottom of the layer as quickly as
possible under its own weight. If done slowly,
the top part of the layer will be compacted
first, making it more difficult for the entrapped
air in the lower part to escape the surface.
3.
Leave the poker in the concrete for about 10
seconds and withdraw it slowly ensuring that
the hole made by the poker is closed up. If a
hole is left (and it is often difficult to prevent if
the concrete is very stiff), replace the poker
near enough to the hole for the next spell of
vibration to close it up. For the final insertion,
withdraw the poker even more slowly and
wiggle it about to ensure that the hole closes
up properly.
Length of Head
Because it is only the head itself which is vibrating,
the concrete layer should not be deeper than the
head length, otherwise there is a danger that the
top part will not be fully compacted. For most
pokers within the range of diameters given in table
2, the poker head is likely to be between 350 and
600 mm long.
Using a Poker Vibrator
Pokers are often used inefficiently. Observations
15% out of the concrete and running,
35% wrongly positioned in the concrete,
20% vibrating already compacted concrete.
Table 2: Poker sizes and applications
Diameter
of head
(mm)
Radius of
action
(mm)
Appropriate rate of
compaction, assuming
rapid placing (m3/h)
20-30
(Needle)
80-150
0.8-2
50 mm slump and above in very thin sections and
confined places. May be needed in conjunction with
larger vibrators where reinforcement, ducts and other
obstructions cause congestion.
35-40
130-250
2-4
50 mm slump and above in thin columns and walls and
confined places.
50-75
180-350
3-8
25 mm slump and above in general construction free from
restrictions and congestion.
Application
IB 46: Vibration of Concrete
Page 7
4.
5.
Replace the poker in the concrete to correct
spacing.
Avoid touching the formwork face with the
poker as this will leave a ‘poker burn’ on the
formwork and a resulting mark will be left on
the finished concrete surface. To be on the
safe side, keep the vibrator about 75-100 mm
away from the formwork.
Avoid touching the reinforcement with the
poker, although, provided that all the concrete
is still fresh, vibrating the reinforcement
should not do any harm and could improve
the bond. The danger lies in the vibrations in
the reinforcement being transmitted into parts
of the section where the concrete may have
stiffened, in which case the bond may be
affected.
there is a risk of the bearings overheating.
6.
Avoid sharp bends in flexible drives and do
not move the vibrator by pulling on the flexible
drive.
7.
Remember that where finish is important, a
little extra vibration can reduce the number of
blowholes.
8.
Make sure the driver motor will not vibrate
itself off the stagings, and when finished clean
all the equipment thoroughly.
Avoid using the poker to make the concrete
flow and never use it to flatten a heap.
Instead, insert the poker carefully around the
perimeter which will avoid segregation,
remembering that compaction starts only after
the heap has been flattened.
7.
Make sure that the poker extends about 150
mm into any previous layer of concrete and
put the whole length of the poker head into
the concrete. This is essential to keep the
bearings cool. Avoid leaving the poker running when it is not in the concrete, otherwise
Figure 8: Diagram showing incorrect and correct
placing of poker in concrete.
Figure 9: Use of poker vibrator.
IB 46: Vibration of Concrete
Page 8
1.
Length of Time Required for Full
Compaction
The length of time a poker has to be in the concrete
at any one position in order to fully compact the
surrounding concrete cannot be precisely stated
since it depends both on the workability of the mix
and on the size of the poker itself. The duration
will vary between 5 and 15 seconds for concrete
with a slump of 25-75 mm, so practically, a time of
around 10 seconds in the concrete should be
satisfactory.
Being able to tell when concrete is fully compacted
is a matter of experience. With a poker, one soon
gets the feel of it and can judge the right amount of
vibration to give. The following will help:
1.
Initial consolidation is rapid and the level of
the concrete drops quickly but the entrapped
air has still to be removed.
2.
As the concrete is vibrated, air bubbles come
to the surface. When the bubbles stop, it can
be taken as a sign that not much more useful
work can be done on the concrete. The
distance of the bubbles from the poker is also
a useful guide to its radius of action.
3.
Sometimes the sound can be a helpful guide.
When the poker is inserted there is usually a
dropping off in frequency, and when the pitch
(whine) becomes constant the concrete is free
from entrapped air.
4.
The surface appearance also gives an
indication of whether or not compaction is
complete. A thin film of glistening mortar on
the surface is a sign that the concrete is
compacted, as is cement paste showing at the
junction of the concrete and formwork.
In any case, the dangers from under-compaction
are far greater than those from over-compaction, so
if there is any doubt don’t be in a hurry to stop
vibrating. Too much is better than too little, since it
is virtually impossible to over-vibrate a properly
designed mix.
Figure 10:
Sequence of the stages that occur
during vibration of a heap of concrete. The
photographs show the spacings of the vibrator
insertions and the glistening appearance that is
given to the surface.
IB 46: Vibration of Concrete
Page 9
The result of over-vibrating badly designed mixes,
such as those prone to segregation and lacking
cohesiveness or containing too much water, is at
the worst, only likely to cause an excess of laitance
on the surface, and it is better to have to remove
this laitance than risk under-vibrating the mix.
With columns and wall tops, this removal is not
difficult and usually has to be done before the next
lift is placed. However with slabs, laitance removal
is impossible, and it is therefore essential to make
sure that the mix is designed to reduce bleeding to
a minimum, and that the surface is not overworked.
Concrete can be placed and compacted at any time
after mixing provided that it is still workable by the
compacting method available, even if some loss of
workability has taken place. For example, if a
poker will sink into the concrete under its own
weight and the hole closes up as the poker is
withdrawn, then that concrete can still be
compacted.
No fixed time limit can be applied to all concreting
operations because the actual time will depend on
the stiffening of the mix which in turn depends on
the richness, on the temperature (both ambient
and of the concrete itself), and on whether a
retarder has been used. On cool, damp days, most
concrete is still workable 3-4 hours after mixing,
whereas on warm dry days, and especially with rich
mixes, 30 minutes may be the limit.
Revibration
Provided that it is still workable, compacted
concrete will not be harmed if it is revibrated. In
fact, tests have shown that the strength is
increased slightly if it is revibrated some time after
the initial compaction.
On columns and walls where surface finish is of
importance, there is sometimes a tendency for
blowholes to occur in the top 600 mm of a lift;
because unlike the lower layers, this top layer does
not have the advantage of the weight of additional
concrete on top to increase the compaction. It can
often help to revibrate the top 600 mm or so some
thirty minutes to one hour after the initial
compaction.
In thick sections of slabs and beams, and
particularly with mixes that are prone to bleeding,
there is a danger of plastic settlement cracks
appearing over the line of top reinforcement. These
cracks generally form about 1-2 hours after
compaction and if they are noticed within this time,
and provided the concrete is still workable,
revibration of the top 75-100 mm can close them up
again.
Care and Maintenance of Poker
Vibrators
Whatever the type of vibrator, it must be treated
with care and properly maintained if breakdowns
are to be avoided. Obtain the manufacturer’s
instruction booklet and follow its recommendations
for both operation and maintenance. Some general
points of care and maintenance are given below:
1.
With electrically operated machines, check the
voltage and frequency before connection to
any power supply, ensure that the equipment
has a good earth connection and see that all
joints are adequately protected.
2.
With a petrol or diesel engine, periodically
check that it is running at the speed
recommended by the vibrator manufacturer. If
it isn’t, the frequency developed in the poker
head won’t be correct either, and compaction
of the concrete won’t be as quick and efficient
as it should be.
3.
Always avoid sharp bends in drive shafts,
particularly when in use.
4.
Regularly check all equipment for signs of
wear and get any faults seen to.
5.
Never engage a poker drive to a motor that is
running. Many accidents have happened
because the operator didn’t bother to switch
off the motor or, if it was fitted with a
centrifugal clutch, didn’t throttle it back.
6.
Ensure there is enough grease in the bearings,
for example, the vibrator tube may start to
twist and jump about. If this happens, stop
the vibrator, examine the bearings, and grease
them if necessary.
7.
Avoid leaving pokers in the same place for
long periods when vibrating concrete.
8.
Don’t leave pokers running while waiting for
fresh supplies of concrete.
IB 46: Vibration of Concrete
Page 10
9.
If a pendulum-type poker fails to vibrate when
switched on, it can often be started by rattling
the head and giving the nose cap a smart rap
(but don’t bang it hard). If this doesn’t work,
switch it off and check the motor coupling.
Don’t go on using the machine if it is still
faulty.
2.
With shaft driven machines, the drive shaft or
drive pin may have failed. With electric
machines, it could be a switch, fuse or a break
in the wiring; it could even be a complete
motor burn-out.
Make sure that the vibrators are firmly
clamped or bolted to the brackets, and keep a
constant eye on them during use to see that
they don’t loosen; otherwise the full vibrations
won’t be transmitted to the formwork and the
concrete.
3.
Feed the concrete into the section in small
quantities so that it is placed uniformly in
layers about 150 mm thick. This will prevent
air being trapped as the lift is built up.
4.
Keep a continuous watch on all fixing (which
should be screwed rather than nailed),
especially on nuts of through-bolts which can
easily work loose under intense vibration.
Also watch out for grout loss, plugging leaks
whenever you can.
5.
If possible, compact the top 600 mm of
concrete in a wall or column with a poker. If
this isn’t feasible, compact the top 600 mm by
hand-rodding and spading down the face of
the formwork. External vibrators tend to create
a gap between the formwork and the concrete.
In the lower lifts this gap is closed by the
weight of the subsequent layers of concrete,
but in the open layer it can remain to disfigure
the surface.
10. When using a pneumatically driven vibrator,
clear the air line of moisture before coupling it
up. Also check that there are no leaking lines
or connections otherwise the vibrator will not
be operating at full power.
External or Clamp-on Vibrators
and grout will find its way through the smallest
of openings.
External vibration systems are available with
different frequencies and centrifugal forces.
The external or clamp-on vibrator consists of an
electric motor with an unbalanced member. It is
fixed to the formwork so that the vibrations are
transmitted through the formwork into the
concrete. Although their use is mainly in precast
concrete, they may sometimes be necessary for
insitu construction when it is not possible to insert
a poker, as in very narrow sections or where there
is congested reinforcement. They will only compact
concrete in sections up to 400 mm thick. Where it
is possible to fit vibrators on either side of
formwork even greater thicknesses of concrete can
be compacted.
When external vibrators are used, the formwork has
to be designed and constructed to stand up to the
repeated reversals of stress, and to be capable of
spreading the vibrations uniformly over a
considerable area. Specially designed brackets
must be fixed to the formwork to hold the vibrators.
Since vibrators are usually moved up or along as
the forms are filled, the number of brackets may be
greater than the number of vibrators available.
Numbers and Spacing of External
Vibrators
Because of the variables involved, such as rigidity
of the formwork, the quality of the concrete and the
effective range of vibrators available, there are no
hard and fast rules about the number of vibrators
required and their most suitable arrangement. The
following points are suggested as guides.
1.
The positions should generally be not more
than 1.0 m apart in any direction when using
small external vibrators with low centrifugal
force. In some instances, they may need to be
closer. More powerful vibrators can be spaced
up to 2.0 m apart.
2.
At intersections and angles, the distance over
which they are effective is reduced; they
should therefore be positioned about 0.5 m
The following points should be noted:
1.
Ensure that all joints, both within and between
panels, are tight and sealed. The formwork
moves more than it does with poker vibration
IB 46: Vibration of Concrete
Page 11
from corners and intersections.
3.
For walls and columns no more than about
1.0 m high, a single row of vibrating
positions about mid-height will usually be
sufficient.
4.
For heights greater than 1.0 m, the lowest row
should be fixed about 0.5 m above the
bottom, with subsequent rows at 1.0 m
spacings vertically. Once each 1.0 m lift of
concrete has been placed and compacted, the
lower row of vibrators can be switched off, the
next higher row being switched on until the
next layer has been compacted, and so on. If
there aren’t enough vibrators for the full
height, the vibrators will have to be raised as
concrete progresses.
With modern equipment it is possible to have
quick release systems.
This allows the
movement of vibrators either along or up a
shutter as the pour progresses.
Many
concrete works use only three or four units
over a much larger number of bracket mounts.
5.
Before concreting begins, the effectiveness of
the arrangement of vibrators can be roughly
checked by switching them on and moving a
hand over the formwork to feel the vibrations
and see whether there are distinct strong,
weak or ‘dead’ areas. It may be necessary to
adjust the positions of the vibrators to obtain
uniform vibrations over the whole area.
4.
Make sure however that the beam itself is
riding on the side forms and not riding up on
the concrete forced on to the side.
5.
Keep beams moving evenly when the vibrator
is running.
6.
Turn vibrator off every time the beam stops.
Table Vibration
This requires special design consideration since
every application is likely to be different.
Summary
Vibrating Screed
These can be used for compacting slabs up to 200
mm in thickness. The following points should be
noted.
1.
The vibrating beams should be run over as
long a length of slab as possible in one pass.
One well controlled pass of a double beam
should be adequate. A second faster pass of
the double beam may be necessary in some
cases to improve the finish on the concrete.
2.
Too many passes of the beam will bring
unwanted excess mortar to the surface.
3.
Figure 11: The roll of concrete maintained in front
of leading beam of double vibrating beam.
A surcharge of concrete is required to be
maintained ahead of the beam (see figure 11).
Optimum compaction of concrete must be achieved
if the concrete is going to achieve its strength and
durability requirements.
Modern day methods of mechanical vibration
provide the most economical means of compacting
concrete in most construction situations. They
cannot however make up for human deficiencies in
the handling of the equipment which usually
relates to having insufficient manpower and
equipment available to match the speed of
concrete placing that can be achieved.
Further Reading
Cable, J.K., McDaniel, L., Schlorholtz, S., Redmond,
IB 46: Vibration of Concrete
Page 12
D., & Rabe, K. (2000). Evaluation of vibrator
performance vs. concrete consolidation & air void
system (Research and Development Information
2398). Skokie, Ill.: Portland Cement Association.
Chan, Y-W., Chen, Y-G., & Liu, Y-S. (2003). Effect of
consolidation on bond of reinforcement in concrete
of different workabilities. ACI Materials Journal,
100(4), 294-301.
Compaction of concrete using immersion and
surface vibrators (Current Practice Note 33). (2002).
North Sydney, N.S.W.: Concrete Institute of
Australia.
Ford, J.H. (2003). Internal or external vibration.
Concrete Construction. [Online]. Retrieved April 1,
2005
from
ftp://imgs.ebuild.com/woc/C03A084.pdf
Forssblad, Lars.
Rheology and mechanism of
concrete vibration – Solna: Dynpac Research 1980
– (Research Bulletin No. 8023 Eng. February 1980).
Harding, M.A. (1995). Vibrating concrete in wall
forms: use proper internal vibrating techniques to
ensure
adequate
consolidation.
Concrete
Construction. [Online]. Retrieved April 1, 2005 from
ftp://imgs.ebuild.com/woc/C950180.pdf.
Koski, J.A. (1994). Using Internal concrete vibrators.
Concrete Producer. [Online]. Retrieved April 1, 2005
from ftp://imgs.ebuild.com/woc/J941010.pdf.
New technologies for improving the consolidation
of concrete (Technical Report CPAR-SL 97-2). (1997).
Vicksberg, Miss: United States Army Corps of
Engineers.
Pneumatic external vibrators. (1997). Concrete
Producer [Online]. Retrieved April 1, 2005 from
ftp://imgs.ebuild.com/woc/J970690.pdf.
ISSN 0114-8826
© Revised Edition March 2005. Cement & Concrete Association of New Zealand, Level 6, 142 Featherston Street, PO Box 448, Wellington, telephone
(04) 499-8820, fax (04) 499-7760, e-mail [email protected], www.cca.org.nz.
Since the information in the bulletin is for general guidance only and in no way replaces the services of professional consultants on particular projects,
no liability can be accepted by the Association by its use.
IB 46: Vibration of Concrete
Page 13