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Contents:
Subgrade Preparation
Subbase Construction
Vapour Barrier - Damp Proof Membrane
Finishing
Levelling and Screeding
Floating and Trowelling
Subgrade Preparation
For most projects, earthworks comprising either excavation
or filling, or a combination of both these operations, will
be necessary to bring the subgrade to the required shape and
level.
The general finished surface level will normally be determined
by drainage requirements and consideration of such factors
as:
- the climatic conditions of the region, particularly rainfall;
- the slope and general level of the existing ground relative
to its surroundings;
- The groundwater level and the extent to which it is influenced
by seasonal, flood or tidal conditions; and
- the soil profile, the nature of the insitu material and
the layer thickness.
The subgrade will generally be constructed to the same shape
as the finished surface of the slab. Thus, at any point, the
subgrade level is equal to the finished level of the slab
minus the total pavement thickness (within the specified tolerance).
When imported fill is required, a selected granular material
should be used, placed in uniform layers and compacted at
or near optimum moisture content to achieve the specified
density. Suitable equipment for compacting granular fill includes
plate type vibrators, pedestrian-operated vibrating rollers
and small tandem rollers (typical examples of which are illustrated
in Figures 1.1 and 1.2).
Layer thicknesses should be chosen
such that compaction occurs over the full layer, and not exceed
150mm, unless heavier compaction equipment than that noted
above is used. Four to eight passes of the equipment will
normally be required. Trucks and tracked or wheeled construction
vehicles that have low contact pressures with the ground are
not suitable for compacting fill.
The strength of the subgrade is not critical, since applied
loads are dispersed over large areas by the concrete pavement
and bearing pressures transmitted to the subgrade are relatively
low. However, it is essential that the upper portion of the
subgrade is of uniform material and density, and provides
uniform support. In order to achieve the desired uniformity,
all top soil should be removed, and soft areas identified
and replaced.
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| Figure 1.1 Subbase being compacted with a pedestrian-operated vibrating
roller. |
Figure 1.2 Subbase being compacted with a tandem roller. |
Subbase Construction
In some circumstances, eg on good quality natural sands or
gravels, it may be possible to build a satisfactory pavement
directly on the subgrade, but a subbase is frequently used
as a levelling course, or as a means of providing a 'working
platform'. Fine-grained subgrade soils in the presence of
free water may be 'pumped' through joints and cracks under
the action of frequent heavy wheel loads. In this case a non-pumping
subbase must be provided.
In constructing the subbase, it is important that the specified
density be achieved to avoid any subsequent problems associated
with consolidation and non-uniform support. Subbases should
be placed in uniform layers, generally not exceeding 150mm
thick, and compacted at or near optimum moisture content using
appropriate equipment.
The subbase should be finished within the required tolerances
to the specified grade and level. In the absence of specified
values, a tolerance of +0, -10mm is considered desirable and
achievable within reasonable standards of construction. Finished
subbase profiles can be achieved by using a scratch template
which operates from the top edge of the levelled side forms.
Accuracy of subbase profile will help ensure that a uniform
concrete layer of the specified thickness is placed.
The use of a blinding layer of fine granular material, eg
sand, may assist in grading to the required level, and will
reduce the risk of perforation or tearing of the vapour barrier
(if used).
Vapour Barrier
Concrete slabs over 100mm in thickness and constructed using
good quality concrete that has been well compacted and cured
are resistant to the passage of water from the ground. However,
concrete slabs, irrespective of their thickness, are not impermeable
to the slow passage of water vapour from the soil beneath.
It is for this reason that a vapour barrier should be placed
under all interior concrete pavements on the ground, particularly
if they are likely to receive an impermeable floor covering,
or are to be used for any purpose where the passage of water
vapour through the pavement is intolerable.
The most common form of vapour barrier is plastic sheeting
(polythene). In order to resist deterioration and punctures
from subsequent construction operations, the polythene should
have a minimum thickness of 0.25mm and be manufactured from
virgin plastic (not from reclaimed scrap polythene).
A vapour barrier placed directly under the concrete also
functions as a slip layer and reduces subgrade drag friction.
With less restraint to slab movement, the extent of cracking
due to volumetric changes of the concrete may well be reduced.
The use of a vapour barrier also prevents the loss of mixing
water from the concrete down in to the subbase or subgrade.
The vapour barrier is placed directly on the subbase (or subgrade
if no subbase course is provided), but if the surface is rough
and likely to perforate the plastic sheeting, a blinding layer
of fine material should be applied. The sheeting should be
continuous under the side forms and lapped at all joints by
a minimum of 150mm. There is no need to seal these joints
with adhesive tape for vapour-proofing purposes as vapour
rises vertically. Furthermore, taping can cause problems by
not allowing the plastic to slip as the concrete is placed.
Special care should be taken to avoid damage to the vapour
barrier prior to and during concreting, and any tears or perforations
should be patched immediately. Placing the sheeting as late
as possible will assist in avoiding damage.
Figure 1.3 Typical values for 'µ' coefficient of friction
Finishing
Many of the problems associated with the performance of concrete
pavements are caused by poor finishing procedures. During
the compacting, levelling and power floating of a pavement,
a layer of cement-rich mortar is inevitably brought to the
surface. This surface laitance should not be allowed to become
too thick by excessive working of over-wet concrete. A slab
with a thick layer of surface laitance will wear rapidly,
possibly craze, and dust badly. The use of fully compacted,
low-slump concrete followed by the floating and trowelling
operations at the correct times will avoid the production
of an excessively thick layer of laitance, and result in a
durable pavement surface.
It is essential in the direct finishing of concrete pavements
that no floating or trowelling operations be commenced while
bleed water continues to rise or remains on the surface. The
incorporation of bleed water into the surface layer will significantly
increase the water-cement ratio of the concrete in that surface
layer, resulting in a weakened surface prone to dusting. The
use of a mixture of cement and stone dust (known as driers)
to absorb bleed water will also produce a very poor wearing
surface, and this practice should be banned for industrial
pavements
Levelling
It is important that the concrete surface be brought to the
final specified level prior to the commencement of any finishing
operations, and this will generally be achieved by one or
two passes of the vibrating beam. Floating and trowelling
should not be considered as methods of correcting inaccuracies
in level or profile.
Screeding
Where a pavement is to be finished by power floating and trowelling,
the surface left by the double-beam vibrating screed will
be level enough to be followed by initial power floating after
a suitable delay.
Figure 1.4 Function of a double vibrating beam
Floating and
Trowelling
General floating and trowelling for large pavement areas is
normally undertaken using powered equipment. Power floating
and trowelling will not necessarily achieve a better quality
of surface finish than good hand floating and trowelling,
but will be more economical.
A power-trowelled pavement finish is obtained in two stages:
STAGE 1:
Power-floating the stiffened concrete to even out any slight
irregularities left by the vibrating beam.
A power float is a machine with large horizontal steel rotating
blades, used for the initial floating operations only.
STAGE 2:
Final power-trowelling to close the surface, making it smooth
and dense. A power trowel is the same or similar machine to
a power float, but fitted with small individual steel trowel
blades that can be progressively tilted during the trowelling
operations. The power-trowel should be used only for the final
trowelling operation.
Power-floating
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It is important that power-floating is not begun until the
concrete has stiffened sufficiently. The time interval before
the initial power floating can commence depends on the concrete
mix and the temperature. In cold weather it may be three hours
or more after the concrete is placed. In hot weather the concrete
may stiffen rapidly, and it is then important that concrete
is not placed faster than it can be properly power-floated
and trowelled with the available resources.
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Figure 1.5 Slab surface being power floated |
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As a general guide, when an average-weight man can stand
on the surface and leave footprints not more than about 3mm
deep, the surface is ready to power float. The power-float should be systematically operated over the
concrete in a regular pattern leaving a matt finish (see Figure
1.5).
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Concrete close to obstructions or in panel corners that cannot
be reached with a power-float must be manually floated before
any power-floating starts.
A steel hand-trowel may be used to give an improved finish
near the panel edges (see Figure 1.6). The concrete must
always be kept level with the side forms. |
| Figure 1.6 Finish near slab being improved with a steel
hand trowel |
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Power-trowelling
If power-trowelling is started too early, the trowel blades
will leave ridges. Power-trowelling should be commenced when
most of the moisture brought to the surface by the initial
power-floating has disappeared and the concrete has lost its
stickiness. Whilst high concrete strength assists in providing
surface abrasion, resistance power trowelling also increases
surface abrasion.
A practical test to check the readiness for each trowelling
operation is to press the palm of the hand onto the concrete
surface. If mortar sticks to the palm when the hand is taken
away from the surface, the pavement is not yet ready for trowelling.
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Power-trowelling of the full pavement bay is undertaken in
a systematic pattern with the trowel blades set at a slight
angle; the angle depends on the concrete stiffness but should
be as steep as possible for the particular surface (see Figure
1.7). If the tilt on the blades is too great, the concrete
surface will be marked.
Where a second power-trowelling is specified, it should not
be commenced until the excess moisture brought to the surface
during the initial trowelling has disappeared.
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Figure 1.7 Close up view of a trowel blade
tilted during final stages of power trowelling |
Again, the
practical test described above may be used. The tilt of the
trowel blade should be gradually increased to match the concrete
stiffness. |
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