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Note 30 Level 1
38
TheStructuralEngineer
August 2013
Technical
Technical Guidance Note
Ground bearing slabs
Introduction
Ground bearing floor slabs are most commonly found within industrial
installations, such as warehouses or factories. Nevertheless, they can also be
installed in all types of structures where there is no basement, provided that
ground conditions are suitable. The design and detailing of these slabs is an
involved process, as the interaction between the natural and/or made ground
and the rigid concrete floor slab presents challenges with respect to cracking,
movement and flatness. The last aspect is of utmost importance to factory
floors and warehouses where the plant requires the floor to be extremely flat
and level in order to function properly.
ICON
LEGEND
Ground
W bearing slabs
W Applied practice
W Further reading
W Web resources
This Technical Guidance Note provides an introduction to ground bearing
floor slabs, touching on the slabs’ reinforcement by considering both historical
use of mesh as well as current plastic and steel fibre reinforcement methods.
S
Figure 1
Components of a ground bearing slab
E
Figure 2
Components of a two layered ground bearing slab
Wearing surface
Wearing RC slab
Slip membrane
Insulation
Vapour barrier
Levelling screen
Base RC slab
Subbase
Subgrade
Wearing surface
RC slab
Slip membrane
Subbase
Subgrade
Ground bearing slabs
When concrete floor slabs are cast directly
onto the ground they rely on their rigidity
and the interaction between the underlying
soil to perform as intended. For larger floors
there is a need to have joints within them in
order to avoid the development of cracks
within the floor slab. This topic has been
covered before in Technical Guidance Note
26 (Level 1): Cracking in concrete. Another
factor that influences the design and
specification of ground bearing concrete
floors is the nature of the load that they
have to support. If it is non-specific human
occupancy for example, it is assumed that
a nominal load is applied to the floor and
the only criterion is the need to create
a relatively flat surface that rests on the
TSE20_38-40.indd 38
ground. For industrial floors this is not the
case, as they typically have to support
racking and heavy machinery that is moving
or being used to transport items, e.g via a
forklift. In these instances careful provision
needs to be made to accommodate the
high point loads and very flat and level floor
finish. Additionally the location of joints
within the slab impacts on the location of
supports to plant and storage racks.
Floor slab components
A typical ground bearing floor slab has five
components, assuming it is of a single layer
construction. These are listed (from bottom
to top) in Figure 1 and can be summarised as:
• Subgrade – the soil against which the
ground bearing slab construction is placed,
but not cast
• Subbase – a graded material, usually made
from broken masonry and/or rocks
• Slip membrane – provides a plane
against which the concrete slab can flex
independently of the subbase. This can also
act as a against ground-borne gases such
as methane, radon or carbon dioxide and/or
moisture
• Reinforced concrete slab – the floor slab
that contains either a square fabric mesh or
fibres and is sometimes laid onto a blinding
layer of compacted sand
• Wearing surface – the exposed surface of
the floor, which typically receives some sort
of surface treatment, be it a paint, sealant or
a screed
In some instances an insulation layer is
installed. Its location would be similar to that
shown in Figure 2.
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When there is a requirement to have extremely
flat and level floors it is necessary to install a
two layered ground bearing slab. These have
a levelling screed that optimises the ability
to create a flat surface (Fig. 2). The slab’s
components (listed from bottom to top) are:
• Subgrade
• Subbase
• Slip membrane
• Base slab – a reinforced concrete slab on
top of which a levelling screed is placed
• Vapour barrier – to protect a layer of
insulation material
• Insulation – required for thermally sensitive
environments, which typically require ultraflat and level surfaces
• Slip membrane
• Wearing concrete slab – the floor slab that
contains either a square fabric mesh or fibres
• Wearing surface
S
Joints need to be protected from vehicle and
pedestrian movement or else the concrete
will spall and crack around them. Once the
slab has sufficiently dried out and set past
the traditional 28 day limit, a sealant should
be applied to the joint. It is recommended
that any treatment of joints is left as late as
possible within the construction programme
as the concrete continues to set and shrink
many months after it is first cast.
Figure 3
Crack inducement joint
Saw cut
S
Figure 4
Movement joint in ground bearing slab
Dowel rod
Isolation from vertical elements
Sleeve
Vertical elements within the structure, such
as columns and walls need to be isolated
from the ground bearing slab in order to
allow for the slab to expand against them
without resulting in cracking. By removing
these points of stiffness/restraint the
floor slab is free to move around vertical
elements. The form of isolation is a cast joint
that is filled with a compressible material
and is typically 10-20mm wide, depending
on the predicted movement of the slab.
Layouts of joints around columns are shown
in Figure 5.
S
Figure 5
Isolation joints to columns
Reinforcement bars at
corners to prevent cracking
Both types of floor slab are laid in
accordance with BS 8204: Part 2 – Screeds,
bases and in situ floorings – Concrete
wearing surfaces – Code of practice.
Surface uniformity
Joints
Joints in ground bearing slabs tend to be
present for two reasons: crack control
and buildability. In terms of the latter, most
floor slabs have joints within them due to
the limits of how much concrete can be
poured in a day. Such joints are known
as construction or ‘day’ joints. In terms of
crack control, when slabs are reinforced
with mesh they are cast in strips and have
crack inducing joints that consist of a saw
cut and are typically placed at every 6m.
These joints combat the shrinkage effects
by encouraging the cracks to form in a
controlled manner as the concrete slab
cures. Despite these measures it is almost
impossible to make a ground bearing slab
that is free from cracks, and the only way
to treat them is to encourage their path
into certain locations. These can then be
post-treated with protection measures, such
as folded metal channels or plastic caps
to ensure their presence does not cause
the slab to deteriorate. Joints that receive
these protection systems are known as
armoured joints. See Figure 3 for an example
of a crack induced joint. Where a slab is
reinforced with steel or plastic fibres, the
presence of anti-crack joints is far less likely
due to the increased ductility the fibres
imbue into the concrete.
Another method of crack control is to install
movement joints within the slab. They allow
the slab to shrink unimpeded, by creating a
joint that can contract in one direction yet
maintain lateral resistance along its length.
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S
Figure 6
Effect of irregular floor surface on warehouse plant
Displacement
Actual level
Datum
They can also resist vertical displacements
across the joint and thus reduce the risks of
steps developing (Figure 4).
In instances where the ground slab
is exposed to extreme variations in
temperature, such as a cold-store or an
external slab, it is necessary to install
expansion joints. These joints allow the
slab to expand and contract as it reacts
to thermal effects. Movements due to
temperature variation are not insignificant
and can go beyond the movements of the
slab while it is curing.
The flatness and level of a floor slab is often
one of the most important elements of its
installation. In instances where the slab is
to be installed into a warehouse with a high
ceiling, the plant used to store and access
the items within the building are usually very
tall, making their alignment on the ground
of paramount importance. If the floor slab
is uneven and/or not consistently level, the
use of the plant within the warehouse can
potentially become dangerous (Figure 6).
To overcome this, the specifier of the
floor slab construction needs to establish
reasonable tolerances for the slab
construction based on its end use. This
is normally expressed using two forms of
dimensional control, with flatness across
a length of 300mm and level expressed in
terms of 3m distances between survey points.
The Concrete Society’s Technical Report
No. 34: Concrete Industrial Ground Floors
isolates two properties that floor slabs can
be measured against. These can be used
when determining whether the floor slab
complies with the requirements described
in the specification. Table 1 is a summary of
Table 4.2 of TR34, which defines what the
limits are on ground floor slabs in terms of
flatness and level.
Reading from Table 1, 95% of the surveyed
recordings made on site must fall within
the more stringent criteria, while 100% of
the surveyed samples must fall within the
limiting criteria.
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Note 30 Level 1
40
TheStructuralEngineer
August 2013
In addition it is required that the level of the
floor slab does not deviate beyond 15mm
from the datum.
Time vs. flatness
While a significant amount of effort is put
into creating a floor slab that meets the
criteria described in Table 1, the effects of
time on the flatness of the slab cannot be
ignored. There are three effects: deflection,
unexpected settlement of the bearing soil
and curling of the slab itself. The deflection
can be countered by designing a slab that
has sufficient stiffness to prevent it from
becoming a significant issue. Unexpected
settlements can only be countered by
carrying out sufficiently comprehensive site
investigations prior to constructing the slab.
Curling is a well-known phenomenon that
occurs at the corners and edges of slabs
as they are curing, which results in the slab
rotating upwards, sometimes up to two years
after it has been cast. It is difficult to counter
curling, with the only primary method being
restraining the concrete from the effects of
shrinkage. This is done by ensuring that it
is thick enough to prevent it from lifting and
that the membrane is installed correctly.
Impact on subgrade material
There are various types of subbase, which
are dependent on their composition and
resistance to compressive forces. The soil
that the subbase is placed upon also plays a
role in determining the capacity of the slab.
One of the key variables is the Californian
Bearing Ratio or CBR. This is further
explained in Technical Guidance Note No. 20
(Level 1): Site investigations.
When designing a ground bearing slab, the
impact on the subgrade material should
not be underestimated. This material will
deflect when placed under load and as such
its make-up determines the performance
of the ground bearing slab. There are two
ways to model how the soil interacts: one
assumes it deflects proportionally when
Technical
Technical Guidance Note
loaded in a similar fashion to dense liquid,
while the other has the soil acting elastically
by causing the slab to deflect continuously
as it is exposed to load. The most accurate
scenario is somewhere between these two
assumptions, which makes it very difficult
to model. The current school of thought is
that the first model, commonly known as the
‘Winkler Model’ is preferred when designing
ground bearing slabs. By applying a
resilience modulus k to the soil, it is possible
to determine the required thickness of the
ground bearing slab. The value of k varies
from 0.015 to 0.3 N/mm3 and is dependent
upon the soil’s resistance to compression.
applied directly to the concrete, which has
been levelled using a power-float method.
In such instances the timing of the float is
important. If carried out too early, cracks
will appear in the surface of the concrete.
Applying the finish too late however, will
result in the float having no effect on the
floor slab. This can result in an uneven
floor finish, which can only be corrected by
grinding it until it is level.
Steel and plastic reinforcement
and fibres
BS EN 1992-1-1 Eurocode 2: Design of
Concrete Structures – Part 1-1: General Rules
for Buildings
Steel reinforcement within ground bearing
slabs typically consists of a square mesh
that is placed in the bottom of the slab with
50mm cover. It increases the ductility of the
ground slab as well as provides a tie through
crack-induced joints and thus shares the
load between segments of a slab. It does not
provide much in the way of enhancement
for resistance to the effects of shrinkage
during curing. It is also possible to use fibre
reinforcement to create ‘jointless’ slabs,
provided certain conditions are met, such as
size of slab and employment of unique curing
procedures that reduce the risk of cracking.
Instead of mesh reinforcement, it is possible
to place steel or plastic fibres within the mix
of the slab to provide increased load bearing
capacity as well as resistance to the effects
of shrinkage. These fibres contain hooks
to increase the bond to the concrete. The
ductility of the concrete is dependent upon
the volume of fibres introduced into it as it is
being mixed.
Applied practice
BS EN 1992-1-1 UK National Annex to
Eurocode 2: Design of Concrete Structures
– Part 1-1: General Rules for Buildings
BS 8204: Part 2 – Screeds, bases and
in situ floorings – Concrete wearing surfaces
– Code of practice
Glossary and
further reading
Crack-induced joint – A cut in a ground
bearing slab that is made soon after it
solidifies to reduce shrinkage effects.
Expansion joint – A joint that allows both
expansion and contraction.
Fibres – Small pieces of plastic or steel
added to the concrete mix, replacing the
need for mesh reinforcement.
Finishes
The finishes that are applied to the floor slab
vary from a simple paint finish, to a screed,
to some form of tiling. In many cases the
finish is little more than a sealant that is
Flatness – Surface regularity of the slab,
not to be confused with ‘level’.
Further Reading
The Concrete Society (2003) TR34
Concrete industrial ground floors (3rd ed.)
Camberley, Surrey: The Concrete Society
Table 1: Limits on deviation to floor flatness and level
Flatness (mm)
Level (mm)
Floor use
95%
100%
95%
100%
Other installations requiring
extremely flat and level floors
2.5
4
4.5
7
Warehouses
>8m high
3.5
5.5
8
12
Warehouses
<8m high
5.0
7.5
10.0
15.0
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Eurocode 0.
The Concrete Society (2007) TR63
Guidance for the design of steel-fibrereinforced concrete Camberley, Surrey: The
Concrete Society
Eurocode 0.
Web resources
The Concrete Society:
www.concrete.org.uk/
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