The successful placing and compaction of concrete can be achieved only if there has been careful forethought and planning. Because they are done almost simultaneously, placing and compaction are interdependent, and the two operations need to be considered together. The rate of delivery of the concrete should match the rate at which the concrete can be both placed and compacted.
The consistence of the concrete at the point of placing needs to suit both the placing technique and the means of compaction available.
Placing
Before concrete placing begins, the insides of the forms should be inspected to make sure they are clean and have been treated with release agent.
If the forms are deep, temporary openings and
lighting may have to be provided for this inspection. Rubbish, such
as
sawdust, shavings and reinforcement tying wire, should be blown out with compressed air. Any rainwater at the bottom of the form should be removed. Similarly, the reinforcement should be
inspected to see that it complies with the drawings,
and that the
correct spacers have been used and there are enough of them.
The main objective with placing is to deposit the concrete as close
as
possible to its final position,
quickly and efficiently and in such a
way
that segregation is avoided (Figure 17). Moving the concrete with poker vibrators should generally be avoided.
Particular care is necessary
when using a skip for placing in thin
walls and other narrow sections in order to avoid heaps and sloping layers. The skip discharge needs to be carefully controlled and the skip moved so that a ribbon of concrete is placed. The
concrete should be placed in uniform layers not more than 500 mm thick or less, depending on the length of the poker blade. Otherwise compaction may be impeded by the weight of concrete
on
top. Provided that the concrete has been well designed
and proportioned and is sufficiently cohesive, there is generally no need to restrict the height
from
which the concrete
is dropped. This assumes that
the
concrete is unimpeded and does not
ricochet off formwork or reinforcement, which may cause
segregation of the mix.
In deep lifts of columns and walls, delays and interruptions should be avoided to prevent colour variations
on the surfaces; the rate
should exceed 2 m height per hour. The correct rate of rise will
have
been calculated by the temporary
works coordinator and
must be observed in the interests of avoiding excessive formwork
pressure and achieving satisfactory surface
finish. On columns and
walls, care should be taken so that
the
concrete does not strike the
face
of the formwork, otherwise the surface
finish may be affected;
care also needs to be taken to avoid displacing
reinforcement or ducts and to ensure that the correct cover is maintained.
Compaction
After concrete has been mixed, transported and placed,
it contains entrapped
air in the form of large voids. The object of compaction is to get rid of as much of this air as possible.
Before compaction, concrete
of consistence class S2 may contain 5% entrapped air,
while
concrete of S1 consistence class may contain as much as 20%.
If this entrapped air is not removed by proper compaction the
presence of these large voids will:
- Reduce the strength of the concrete - more than 5% loss of strength for every 1 % air
- Increase the permeability and hence reduce the durability and protection to the reinforcement
- Reduce the bond between concrete and reinforcement
- Result in visual blemishes such as excessive blowholes and honeycombing on formed surfaces
- Fully compacted concrete will be dense, strong, impermeable and durable.
Vibration
Most concrete is compacted by means of internal poker vibrators that 'fluidize' the concrete and permit the entrapped air to rise to
the surface.
Pokers vary in size, usually from 25 - 75 mm in diameter. Table 13 gives a broad indication of poker sizes, and their characteristics and typical applications.
The radius of action will determine
the spacing and pattern of
insertions. As a guide, a spacing up to 500 mm centres is about right for a 60 mm diameter poker with concrete of medium
consistence (see Table).
The poker should be inserted vertically and quickly and should
penetrate some 100 mm into any previous layer; thereby stitching
the two layers together. It should remain in the concrete until the
air
bubbles cease to come to the surface.
Figures 18, 19 and 20 illustrate the process.
Being able to judge when the concrete has been fully compacted is
largely a matter of experience. Sometimes the sound can be a useful indicator, in that the pitch (whine) becomes constant when
the concrete
is compacted. In addition, 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 between
the concrete and the formwork.
The poker should be withdrawn slowly so that the
concrete can flow back into the space occupied
by the poker.
External vibrators are occasionally used, but their usefulness is
limited on site by the heavy formwork needed to resist the stresses and shaking they produce. Their use is mainly confined to precast concrete elements, but they may be necessary for heavily reinforced walls and the webs of deep beams where it is difficult
or impossible to insert a poker.
Table : Characteristics and uses of internal poker vibrators.
|
Diameter of head
(mm)
|
Radius of action
(mm)
|
Approximate rate of compaction, assuming rapid placing
(m3/h)
|
Uses
|
|
20-3 0 (needle)
|
80 - 150
|
0.8- 2
|
Concrete with class S3 consistence 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
|
|
35-4 0
|
130-25 0
|
2- 4
|
Concrete with S2 consistence and above
in slender columns
and walls and confined places
|
|
50-7 5
|
180-35 0
|
3- 8
|
Concrete with class
S1 consistence and above in general
construction free from restrictions and congestion
|
Slabs are best consolidated by vibrating beam compactors. These combine the action of a screed and a vibrator,
but they are only
effective for a limited depth. In general,
a slab more than 150 mm thick should be compacted with poker vibrators
and finished with
a vibrating beam. The edges of all slabs butting up to side forms
should always be poker vibrated. Construction joints need particular attention (see the next section).
Over-vibration
The dangers and problems
arising from under-vibration are far
greater than any supposedly
arising from over-vibration, since it is
virtually impossible to over-vibrate a properly designed and
proportioned concrete.
Re-vibration
Provided that it is still workable,
no harm will be done if concrete
that
has already been compacted
is re-vibrated. In fact, tests have shown
that the strength is likely
to
be slightly increased.
Re-vibration of the top 75 - 100 mm of deep sections can
minimize plastic settlement cracks or close them if they have been seen to develop.
Similarly, the re-vibration of the tops of columns and walls can often reduce the tendency of blow-holes to occur in the top 600 mm or so.