Structural Guidance Note
2018 SGN 01
Ground bearing slabs: good practice
guide
SSN Insight
Issue 1 | 25 January 2018
This report takes into account the particular
instructions and requirements of our client.
It is not intended for and should not be relied
upon by any third party and no responsibility
is undertaken to any third party.
Job number
076084-01
Ove Arup & Partners Ltd
13 Fitzroy Street
London
W1T 4BQ
United Kingdom
www.arup.com
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Ground bearing slabs: good practice guide
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
Structural Guidance Note
2018 SGN 01
Ground bearing slabs: good practice guide
Contents
Page
Introduction
1
1.1
1.2
2
2
Scope
Judgement
Nature of slabs
2
Client expectations
3
General design guidance
4
Conditions below the slab
6
5.1
5.2
5.3
5.4
5.5
Ground conditions
Ground improvement
Foundations and drainage
Sub-base
Slip-membranes
6
6
7
7
9
Design guidance for ground slabs
10
6.1
6.2
6.3
10
23
24
Industrial slabs
Residential
Slabs for other “normal” non-residential uses
Other issues
35
7.1
7.2
7.3
7.4
7.5
35
36
36
36
37
Perimeter cladding support
Under-slab insulation
Underfloor heating
Ground gases and radon
Mesh detailing: flying ends
References
Appendices
Appendix A
Supporting guidance
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Structural Guidance Note
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Ground bearing slabs: good practice guide
Introduction
As a forward-thinking firm, Arup strives to design buildings that perform well,
use best practice and deliver on our client’s expectations.
This Note considers the technical literature and collective Arup experience on
ground bearing slabs, which has developed significantly since the previous SGN
on this subject was written in November 1983.
The guidance should be used in conjunction with and not as a substitute for the
design guides and codes that are referred to within the text. Section 8 contains a
complete list.
This Note generally focusses on the standards, construction methods and design
guidance used in the UK, although much of the guidance in terms of design
principles is equally applicable worldwide.
The majority of published guidance in the UK regarding the design and
construction of ground bearing floor slabs is very specific to industrial type uses.
In practice this guidance is not always applicable to non-industrial, lightly-loaded
ground bearing slabs for use in offices, schools & other institutional type
buildings or even car parks.
These ‘normal’ building types generally do not require slabs to support large
concentrated point loads, or to be built to the tight surface tolerances required for
high bay storage facilities. Furthermore, the surface of the slab is in many cases
concealed beneath floor coverings such as carpet, vinyl or raised access systems
which will be more forgiving to imperfections and narrow cracking than the
exposed surfaces found in industrial facilities.
When faced with such a ‘normal’ slab to design, it is common for engineers to
take one of two approaches, depending on the individual’s inclination;
1.
adapt a similar methodology to one used previously and apply it, often
conservatively, or
2.
ask a colleague with previous experience what they have done in the past.
While there will be numerous examples of slabs designed and built using both of
these methods, without any problems, a more ordered approach would be
beneficial.
This Note sets out the design principles applicable to all ground bearing slabs and
provides such an ordered approach to both industrial and ‘normal’ more lightly
loaded slabs. For residential slabs, a more standardised simplified approach to
BS 8204 [1] [2] and NHBC [3] guidance is outlined.
For industrial warehouses, the floor is often the most valuable asset and yet
insufficient attention is often applied to its design and detailing.
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Ground bearing slabs: good practice guide
Scope
This Note applies to internal slabs in buildings, and different approaches (often
using unreinforced pavements) are used for design of external slabs, which are
beyond the scope of this guide. Section 6 of IGN 01 [4] provides useful
complementary guidance on concrete hardstandings around buildings.
Water-resisting, damp proofing and insulation requirements are not considered
here. Neither does this Note cover raft slabs, suspended slabs or post-tensioned
ground bearing slabs.
1.2
Judgement
Readers should also be aware that a degree of engineering judgement will be
required in applying any recommendations made in this Note, since it is not
possible to consider every variable.
Nature of slabs
A ground bearing slab, sometimes called a ‘solid floor’ or a ‘slab on grade’, is one
of the simplest forms of construction. It is widely used throughout the world and
there are many sources of information on design and construction. Yet, despite
this wealth of experience and availability of information, ground bearing slabs are
a regular source of confusion, queries and differences between expectations and
actual performance.
It may seem that little can go wrong with a slab cast on the ground, but in
problems such as settlement, cracking, surface irregularities and dampness can
arise due to inadequate design, unsuitable detailing, insufficient site preparation,
poorly controlled construction etc. The situation is exacerbated by the growing
tendency to build on sites with natural or made ground of poor loadbearing
qualities, the need in industrial developments to have close tolerance on the
finished level and to accommodate substantial (and ever increasing) storage, plant
or vehicle loads.
Sound guidance on the design, specification and construction of ground bearing
slabs is thus worth reading. This note, which builds on Structural Guidance Note
(SGN) 4.5 from 1983 [5] (now withdrawn but available on request from AT+R),
draws attention to some recommended sources, and current thinking regarding
best practice.
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Client expectations
Every floor should be designed to suit the individual requirements of the end user.
This sounds straightforward, however there is a lot more to consider than just
establishing the floor loadings and then designing the floor to support them
structurally.
It is important to establish what the client’s expectations are at an early stage,
before decisions are made about the form of construction of the floor. Beyond
simply establishing the design loadings, key questions could include:

Is there any requirement for future flexibility (e.g. a mezzanine or additional
plant or storage in the future)?

What is the nature of the loading, e.g. significant point, patch, pattern, uniform
loads; is the load static or not?

Is the floor trafficked (i.e. materials handling equipment, MHE)?

What is the precise nature of the trafficking / MHE?

What finishes are proposed? Will there be any brittle or bonded finishes?

Will any screeds or levelling compounds be used? Are there any raised floor
areas?

What are the surface finish requirements, e.g. flatness and regularity
(particularly for exposed slabs)?

Are there joint and crack requirements, and does the client understand the
implications of the different approaches to managing joints and cracks?

Is underfloor heating proposed? Is insulation required?

Are any recessed areas required (i.e. for showers, revolving doors etc. A
75mm finish zone is unlikely to be sufficient)?

What are the expectations with respect to maintenance?

Other considerations e.g. ground gas, radon, damp or water proofing.
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Ground bearing slabs: good practice guide
General design guidance
The design of ground bearing floor slabs is commonly referred to as a “black art”.
Indeed, George Garber wrote jokingly in Design and Construction of Concrete
Floors [6], that
“Slab thickness = Westergaard, Kelley, Pickett and all the other standard
formulae × 0 + 6 in”
Many ground bearing slabs have been designed based purely on an engineer’s
previous experience, and many of these have performed adequately. Since much
of the guidance in codes was based on site testing, this is perhaps not so
unexpected. However, the trend toward tighter required tolerances, and the fact
that disputes as a consequence of different expectations about ground slab
performance are common, makes the clear justification of the design essential.
Section 6 of this SGN is split into three sub-sections, covering the most common
uses for slabs, as the detailing and design considerations differ to a degree for
each of these uses:

Industrial slabs. These slabs have the largest amount of published design
guidance. Although all slabs may experience issues in service, the
consequences in a busy industrial or warehouse facility can be greater, and the
ways in which such slabs are used and loaded means that there are particular
aspects such as joint durability, flatness, surface regularity and resistance to
crack formation that require a higher level of consideration. These slabs are
often extensive, and there is a desire to construct them in larger pours, both for
programme considerations and to minimise joints.
Warehouse slabs will often have storage racking, MHE etc. requiring very
tight tolerances. Manufacturing and other industrial facilities will usually not
have the same strict tolerance needs, however in either case the requirements
should be discussed at an early stage with the Client.

Residential slabs. These have the least published design guidance. They will
generally have an applied finish, are poured in smaller areas and are lightly
loaded.

Slabs for other ‘normal’ non-residential uses, e.g. institutional buildings,
offices, schools etc. This Note sets out the approaches when using standard
fabric (mesh) reinforcement for these slab types.
Two distinct approaches are in use in the firm for such ‘normal’ situations: a
jointed solution with planned construction joints and saw cut joints, and a
largely unjointed solution developed by Arup in Bristol. Both approaches
have been successful, and a decision on which to adopt will depend on
knowledge of issues such as finishes, contractor preference, client
expectations and workmanship considerations.
If adopting an unjointed (or predominantly unjointed) approach, attention to
details (i.e. particularly re-entrant corners, restraint, slip membranes etc.)
becomes even more critical, and these aspects must be carefully considered
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and reviewed by an experienced engineer familiar with the potential pitfalls of
eliminating joints.
Further discussion on jointed and unjointed solutions is given in section 6.3.2
of this Note, and general notes intended for design documents are included in
Appendix A.
Guidance on industrial slabs can be applied in less onerous situations, but needs to
be considered carefully as to appropriateness to achieve an economic design.
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Conditions below the slab
5.1
Ground conditions
The structural integrity of the material below a ground bearing slab is of vital
importance to the long term performance of the slab. This will need to be
established by a geotechnical site investigation at the earliest possible opportunity.
When designing an industrial floor slab using the methodology outlined in
TR34 [7] and section 6.1 of this SGN, one of the input parameters used is the
modulus of subgrade reaction, k. It should be noted that the design is not acutely
sensitive to the value of k, and this is even more so the case for more lightly and
evenly loaded slabs, so it is normally sufficient for lightly loaded slabs to
categorise the ground conditions using descriptions, such as those used in [8]
Where formation is a good (modulus of subgrade reaction greater than 0.05-0.06
N/mm3 which would typically correlate to a California Bearing Ratio CBR of 815%), consistently firm subgrade, a slab subjected to modest and evenly
distributed loads should only need to be reinforced for thermal and shrinkage
affects. In these situations, it could be argued that a thinner section will perform
better (since the smaller cross sectional area means a lower build-up of stresses,
fewer through thickness effects, less distance for free water not used in hydration
to migrate to the top surface and reduced heat of hydration).
Where ground conditions are expected to be variable, there may be a risk of soft
spots causing local depressions and the slab should be sized appropriately to span
over these features. A method for assessing the size of soft spots and capacity of
the slab can be found in reference [8], although it should be noted that a degree of
engineering judgement is required when following this approach.
5.2
Ground improvement
Various ground improvement techniques may be employed to improve the
subgrade of poor or brownfield sites.
These may include:

stabilization with cement and/or lime

excavation and compaction of material in layers

use of dynamic compaction (with an impact roller or similar)

use of techniques such as ‘stone columns’ or ‘concrete modulus columns’
(CMCs) to enhance the subgrade stiffness (effectively making the slab partly
suspended)

replacement of a depth of subgrade with an engineered fill.
There is a need for careful discussion between the slab designer, geotechnical
engineer, main contractor and subcontractor when these techniques are used to
ensure that the parameters used in the slab design are appropriate for the method
of ground improvement selected and the method of testing of the finished
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improved subgrade. A set of plate bearing tests on the top of the improved
subgrade is the most appropriate way to determine an appropriate stiffness (k
value) for slab design, but this needs to be considered alongside other
geotechnical parameters and testing to assess the overall settlement potential.
5.3
Foundations and drainage
The type of foundations, and their location relative to the ground slab will have an
influence on other considerations, such as whether isolation joints are included
around columns.
Table 1 gives some pros and cons for the two approaches to setting out the
superstructure foundations: setting them below or flush with the underside of the
slab.
Table 1. Design considerations for different superstructure foundation positions.
Top of foundations set below slab
Top of foundations flush with underside of
slab
Allows flexibility for below-slab drainage. In
normal circumstances 600 to 650mm from top
of slab (TOS) is required.
Below-slab drainage, picking up down pipes
adjacent to columns, will need to be cast in to
the foundations.
A degree of vertical isolation of the slab from
the column can be achieved:
 > 200mm of stone sub-base between
foundation and slab will act to reduce
‘hard spots’ forming
No vertical isolation between foundations and
ground floor slab. Foundations will form ‘hard
spots’, which the slab should be checked for.
The slab may require local thickening and/or
additional reinforcement to increase its
bending moment capacity.

< 200mm of stone between foundation
and slab will be difficult to compact.
Less excavation (particularly significant for a
hard or near surface rock site).
Possible details for both situations listed in Table 1 are shown in section 6.3.6 of
this Note.
5.4
Sub-base
A sub-base (usually a compacted layer of Type 11 graded granular material) is
normally provided over the subgrade2, and contributes to the platform stiffness.
Some points:

If the finish slab surface requires a tight flatness and regularity tolerance, then
the level of the underlying subgrade will also need closer tolerance. This
should be specified and highlighted to the, often different, subcontractor.
‘Type 1’ is a classification defined in the Specification for Highway Works [9] which has been
revised in accordance with BS EN 13285 [10] and BS EN 13242 [11].
2
‘Subgrade’ and ‘sub-base’ should not be confused: the subgrade is the ground material below the
sub-base.
1
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TR34 specifies a subgrade tolerance, generally +0mm/-25mm (consistent also
with BS 8204). In other words, the sub-base must not be high, such that it
reduces the slab thickness. We should adopt this requirement in our
specifications and general notes drawings. It is also possible to specify tighter
tolerances when ensuring flatness for warehouse and industrial slabs (for
instance +0mm/-10mm). The need to do this should be carefully considered
as it will require much tighter control by the Contractor, and may therefore
affect costs, and could be seen as overly onerous, depending on the situation.
The choice of sub-base should be made in consultation with the geotechnical
engineer to suit the ground (subgrade) conditions. The sub-base needs to fulfil
three primary functions:
1.
increase the stiffness and compaction of the ground immediately below the
slab and take out any unevenness in the ground conditions
2.
provide a stable platform for construction traffic
3.
receive the slip membrane / damp proof membrane (DPM).
Most sub-bases comprise of crushed stone, which is compacted in layers. The
specification of the type of stone varies, but most commonly used in the UK are
Type 1 or possibly 6F1/6F2 (or site won equivalent) in accordance with [9].
Type 1 is more expensive, but provides a more even grading which will provide a
better finish surface than 6F1/6F2 which is much lumpier, but cheaper. 6F1/6F2 is
easier to lay in wet conditions. It is recommended that the contractor is involved
in the decision making process, especially if the sub-base is required to act as a
construction works platform or piling mat. When the sub-base is trafficked it will
become worn and uneven. It is common practice to specify that the top 50 to
75mm is scraped off and disposed of before a light sand dusting/blinding is
applied and rolled to closure.
The stone layer is finished (rolled to closure) with a layer of fines to close the
surface. Sand is most commonly used, although a lean concrete mix is an
alternative which may be advisable for heavily reinforced slabs.
The blinding helps ensure that the slab is free to move horizontally due to thermal
and early age effects, minimising restraint that can lead to cracking. Sand blinding
also reduces the risk of puncturing the slip membrane.
Sand blinding should be specified as “sand blinded and rolled to closure”, rather
than “min X mm of sand blinding”, since any thickness of sand above the stone
will be easily displaced and reinforcement spacers will sink into it under foot
traffic. Having a surface that is not closed can lead to surface rutting, creasing of
the membrane and increased restraint, which is especially critical where a very
high tolerance for subbase flatness has been specified.
Reference [9] contains guidance on the layering, compaction and minimum
thicknesses to be used, and should be referenced in our documentation.
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Ground bearing slabs: good practice guide
Slip-membranes
A slip membrane (typically 1200 gauge
Waterproofing and gas
polythene) is usually placed over the sub-base. As
protection are not
the name implies, the slip membrane reduces
considered
in this Note
friction between the underside of the slab and the
sub-base below. It also helps prevent loss of
cement fines from fresh concrete, protects the slab from ground borne sulphates
(alongside appropriate concrete mix specification) and can be designed and
detailed to act as a damp proof membrane (DPM).
Note that a polythene membrane may not provide adequate resistance to water
vapour, so this final point shouldn’t be taken for granted. Its adequacy in this
regard will depend on the use of the building.
Slip membranes will need suitable detailing at joints to provide adequate slip and
damp proofing. Typically, this would be by lapping the membrane sheets,
commonly using a 300mm minimum lap (although taped joints may be a
requirement depending on conditions, and whether the membrane also has a
vapour/gas resistance requirement, in which case specialist manufacturers
recommendations should be followed).
If gas protection is a requirement, membrane systems (membranes and joint tapes
etc.) are available with various grades. These membranes are generally of a
different colour to distinguish them from polythene membranes that only provide
a slip membrane and DPM function.
Specification of membranes as DPM’s or for gas protection is beyond the
scope of this SGN.
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Design guidance for ground slabs
6.1
Industrial slabs
Introduction
Much has changed since the issue of SGN 4.5 – the main change being that the
Concrete Society (formerly published by the Cement & Concrete Association C&CA) Technical Report 34 (commonly referred to as TR34) has been released
and has gone through several updates (at the time of writing the current edition is
the 4th, published in August 2013, revised October 2013 [7]). The design guidance
in TR34 on assessing structural performance is also available as a Tekla Tedds
calculation. TR34 is only applicable to design of internal slabs.
Though the ‘computerisation’ of the calculation is to be welcomed, there is
concern that it is sometimes used without adequate understanding. Figure 1 shows
the iterations for an industrial slab design:
Determine
loadings
Review soil
parameters
Estimate slab &
sub-base
thickness,
reinforcement etc
Sketch out a plan
of proposed
jointing - there
will probably be
several types
Determine joint
reinforcement
Check point loads
at joints - there
may be several
cases
Check internal
area of slab using
TR34
Calculate mid
slab panel design
Check joint
widths, fillers,
ariss details
Finishes
Figure 1. Design iteration for an industrial ground bearing slab.
Determine loadings
General loadings are taken from Eurocodes, British
Standards, and the like. There are also loads from
specialist suppliers to take into account such as shelving
& racking, plant loads, forklift trucks (FLT’s) and other
MHE, mezzanines etc.
Talk to the client
and user about
loading in the
facility
Global area floor loads are seldom the governing design case for the slab in
industrial buildings but may be critical for settlement calculations, so pass these to
the geotechnical engineer. Point loads (from racking leg loads, mezzanines and
materials handling equipment MHE) generally dominate the slab design, and may
need to be considered in combination.
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As clients or project managers may not have an understanding of the loading of
plant, it is worth spending some time considering loading and clarifying
assumptions and risks. Points for consideration include:

Installation loads:



UDLs, line loads, point loads, wheel types, intended access routes
dynamic loading and the transmission of vibration
age at loading.

In-service loads; similar issues to those outlined above for installation loads
apply, plus handling equipment and stock loads.

Maintenance and repairs. How practical is it to carry out work on the slabs
once the slab is in use? For example, if a damaged joint arris may affect a
strategic forklift route, it may be worth considering armouring the joints.
TR34 has a check-list in Appendix A that may help steer a discussion.
Design modulus of subgrade reaction
The modulus of subgrade reaction, k, is one of the input parameters in the
calculation of required slab thickness – it should be noted that the design is not
acutely sensitive to the value of k. However, it is vital that the degree of
consolidation of the subgrade is evaluated under sustained loading.
The modulus of subgrade reaction is a measure
It is vital that a
of the stiffness (deformation under load) of the
geotechnical
review of the
underlying earthworks platform. It can be
underlying ground is
determined by testing during ground
undertaken. The
investigation and is related to the CBR
structural design does not
(California Bearing Ratio) test value for the
subgrade. The most accurate way to determine
cover ground settlement /
the k value is to use a series of standard plate
consolidation
bearing tests. The tests should be conducted on
the subgrade rather than the subbase surface and the thickness design of the slab
based on the k value so determined. The fourth edition of TR34 recommends that
k values are always determined from plate bearing tests rather than relying on
correlation to CBR test values.
Where limited data exists on ground conditions, for instance when carrying out
initial assessments of requirements prior to undertaking a fully detailed ground
investigation (GI), a reasonable likely ‘worst case’ assumption for many sites
would be k = 0.03 N/mm3. This would approximately correspond to a CBR test
value of just above 2%. It should not be assumed without carrying out proper GI
and desk study that a ground bearing solution will always be possible however,
and proper geotechnical assessment must form part of the decision making on
whether a suspended or ground bearing slab is adopted.
Note that the ground bearing slab design does not look at ground settlement /
consolidation. For this, a geotechnical engineer should review the ground as a
separate study.
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Estimate slab parameters
Table 2 presents an extract of Table 5 from TR550 [12] which gives a good initial
estimate of the slab thickness for industrial purposes. The full table also sets out
critical aisle widths and mezzanine loads.
Slab depth (mm)
Table 2. Estimate of slab thickness (extract from Table 5 of TR550).
Fork Lift Trucks
Back to back racks
Uniform
distributed
Loading
(kN/m2)
Leg load (tonnes)
Fork lift capacity
(tonnes)
Width of racks (m)
A
B
C
D
0.9
1.5
2.1
2.7
3.3
3.9
4.5
A
B
150
2.4
2.3
2.2
2.1
1.5
1.6
1.6
1.6
1.7
1.7
1.7
41
88
175
3.5
3.3
3.2
3.0
2.0
2.0
2.1
2.1
2.1
2.2
2.2
44
94
200
4.8
4.6
4.4
4.2
2.5
2.6
2.6
2.7
2.7
2.8
2.8
47
101
225
6.2
5.9
5.6
5.3
3.0
3.2
3.2
3.3
3.3
3.5
3.5
50
108
250
8.4
7.9
7.5
7.0
3.6
3.8
3.9
4.0
4.0
4.2
4.2
53
115
275
11.0
10.4
9.9
9.3
4.2
4.6
4.7
4.8
4.8
4.9
5.0
56
122
300
13.4
12.6
12.0
11.4
4.9
5.4
5.5
5.6
5.7
5.8
5.9
59
130
Tabulated values are based on:
Concrete grade (fcu) = 30N/mm2, (i.e. C30/40)
Modulus of subgrade reaction:
Good = 54MN/m3 (equivalent to a CBR of 10%)
Poor = 13MN/m3 (equivalent to a CBR of less than 2%)
Load assumed to transfer between adjacent bays of the floor.
A: 100,000 load repetitions
Area of loading (100mm x 100mm) = 0.01m2
B: 200,000 load repetitions
A: Poor
subgrade
B: Good
subgrade
C: 300,000 load repetitions
D: any number of load rep’s
Forklifts assumed to have pneumatic
tyres
Modification factors (to be applied to loading) to accommodate variation in parameters:
fcu = 40  1.37
fcu = 40  1.23
Poor subgrade  0.76
Poor subgrade  0.88
fcu = 40  1.23
Area of loading (150 x 150mm) = 1.11
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Table 2 predates the 3rd Edition of TR34 and so has somewhat more conservative
values than the current best practice, but is useful for obtaining an initial estimate
of required thickness.
The loading repetitions in the table are based on pneumatic tyres, and so may not
necessarily be entirely representative of modern MHE which predominantly use
small plastic wheels.
An alternative ‘rule of thumb’ approach to initial sizing of ground supported mesh
fabric reinforced slabs is to take the maximum rack leg load in kN and add
100mm to give an initial thickness estimate (i.e. 75kN on 175mm floor, 100kN on
200mm floor).
The most commonly used slab thickness in
medium to heavy duty industrial applications is
175mm. Generally, it takes quite onerous nonstandard loading to be applied (or other
influencing factors such as a required
minimum thickness for embedded items)
before a typical slab needs to exceed 200mm in
thickness.
The most common slab
thickness in typical
medium to heavy duty
industrial applications is
175mm.
Note that 150mm thick slabs in industrial applications can be problematic as any
loss of thickness due to workmanship or construction issues brings them into the
range (100mm – 125mm) where curling and other effects are problematic. For this
reason it is recommended that 150mm slabs are only used for very lightly loaded
situations with no dynamic loading, and that slabs of less than 150mm thickness
should be avoided in industrial applications. TR34 includes a requirement for a
minimum thickness of 150mm
Plan the joint locations
The key to successful design for an industrial floor slab is to minimise
uncontrolled cracking. Joint layout and detailing at areas which can lead to
restraint are critical to achieving this objective.
Since concrete shrinks as it cures and undergoes thermally induced strains, joints
are used to ‘collect’ the shrinkage and ‘store’ it in defined locations. If joints
weren’t provided, there would be
Cracks are generally caused by:
uncontrolled cracking over the slab.
Joints are also required to define the
end of concrete pours and to provide a
separation at structures that pass
through the slab (columns, walls etc.).
Joints are created in two ways – by
forming or by sawing. There are a
multitude of names and descriptions
for the different types of joints. TR34
defines the following:




Shrinkage
(autogenous, plastic, drying)
Thermal (early age and
ambient)
Settlement
Overstressing (overloading)
All of these issues should be
addressed in the design
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


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Isolation joints – these formed joints have no restraint. They are used around
columns and walls or adjacent to plinths supporting vibrating machinery and
allow both vertical and horizontal movement.
Free-movement joints – aim to cause minimum restraint to horizontal
movement, but restrict vertical movement. They can be either sawn or formed.
Restrained-movement joints – allow limited (1-2mm) horizontal movement
and can be either sawn or formed.
Tied joints – are formed joints used to provide a break in construction and
have enough reinforcement to prevent opening.
TR34 describes these joints in detail. But in general:

Restrained-movement joints should be installed at around 6m centres. 8m or
above is probably too great a spacing.

Where there are re-entrant corners, odd shapes etc., consider providing
additional joints. The smaller the section, the less the risk of cracking.

Joints should be aligned with one another where crossing a perpendicular
joint.

Panels must not exceed 1.5:1 ratio of sides. If they do, a crack across the
middle of the panel is very likely.
Calculate mid panel design
Using TR34, or the Tedds calculation, an internal slab panel area can be checked.
This case is normally trivial as the critical area for design is normally adjacent to
joints, with application of point loads.
Reinforcement
Reinforcement is required in order to use the design methods presented in TR34,
as TR34 does not cover unreinforced slabs. (The TR34 calculations are based on
plastic analysis and reinforcement is required to ensure that the concrete remains
ductile.) Reinforcement can be either loose bar, fabric or fibres.
The most common types of fibre reinforcement are steel and plastics. Plastic
fibres come in two distinct forms often referred to as macro and micro synthetic
fibres.
Fibres should conform to BS EN 14889. Micro synthetic fibres may affect the
properties of the plastic concrete but will have a negligible effect in hardened
concrete. They are sometimes used in combination with other steel or plastic
macro synthetic fibres. Steel fibres and macro synthetic fibres may impart
residual or post cracking strength to the concrete matrix.
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Bar reinforcement (fabric or loose bars)
In TR34, fabric (or less commonly, loose bars) are
Health & Safety
placed a cover distance from the bottom of the slab.
Consider bar spacing
Since fabric reinforcement is placed near the bottom
– a spacing of
of the section, it will not contribute significantly to
150mm or less is
crack control at the surface. Reinforcement placed at
safer to walk on.
the top surface may reduce the risk of significant
surface cracks by controlling crack widths and
distributing cracks (dissipating strain over a greater number of narrower cracks).
The quantities of reinforcement that would be required to prevent cracks forming
is significantly higher than that commonly used in ground bearing floor slabs, or
specified in TR34.
Fibre reinforcement
Fibres are distributed through the whole section as they are mixed in with the
concrete. Fibres are generally a proprietary item, supplied by a specialist provider
who has carried out their own testing – for example Dramix by Bekaert.
It should be noted that generally synthetic (plastic) fibres do not provide reliable
post crack ductility and so are not considered in strength design calculations.
If using fibres, it is usual to get the proprietary supplier to carry out a design. The
suppliers will provide mix requirements for input into the concrete specification.
Proprietary designs, the properties of fibre reinforced concrete and the testing
work used to substantiate the design values should be reviewed by someone
familiar with the relevant test standards and design methods.
Mix design is crucial with fibre reinforced concretes to ensure that the mix
remains adequately workable.
It is unlikely in the UK that fibres would be added at the batching plant, and there
is therefore frequently a manual handling operation of adding fibres to the
concrete at the site.
Quality control and monitoring is particularly important to ensure that the fibres
are consistently mixed, distributed evenly through the concrete and are not
‘balling’ within the mix.
Fibres have a health and safety advantage over fabric or loose bar. They:


doesn’t need to be placed, reducing manual installation work
do not present a trip hazard.
However, fibres may be seen at the surface, even when the surface is power
floated. The fibres can be removed after curing with a heat tool, but care should be
taken not to ‘burn’ the concrete surface.
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Detailing at internal corners
At corners or at columns full depth isolation
joints should generally be specified to
minimise restraint and reduce uncontrolled
cracking risk.
Additional loose bars at re-entrant corners and
adjacent to columns, even where fibre
reinforcement is provided, helps control
cracking, and reduces crack widths, distributing
any cracking which does occur.
Figure 2: Internal Column Isolation
Joint Detail (Detail G041 from [13]).
Concrete
Although higher strength concretes give higher compressive and flexural tensile
strengths, and higher cement content may be a requirement for factors such as
better chemical resistance etc., they can also lead to increased shrinkage
movement.
BS 8500-1 [14], Table A.13 recommends three designated concrete mixes for
floors:

Wearing surface: light foot and trolley traffic: RC25/30

Wearing surface: general industrial: RC32/40

Wearing surface: heavy industrial: RC40/50
As TR34 notes, however, “all practical steps should be taken to minimise
shrinkage” and a lower cement content may be advantageous. The concrete will
also need to address durability, abrasion resistance and process requirements.
TR34 recommends a maximum water cement ratio of 0.55. For a typical C32/40
mix or less, tighter water cement ratios should be readily achievable and 0.5 is
recommended.
Shrinkage is not directly related to cement content and it is also advantageous to
limit the free water content of the concrete to 160 kg/m3. When specifying the free
water content, a designed concrete mix will need to be used.
Note that the compatibility of any admixtures with any topping or finish should be
reviewed as bonding may be impaired by their use.
Consideration of 40mm max aggregate size for economy should be made for
thicker (~250mm+) slabs.
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Calculate joint design
The CSC Tedds calculation tool allows a variety of adjustment of bar size,
spacing and bar length for each of the joint types. However, since much of the
design is based on observation of test slabs, it is unwise to get bogged down in the
calculation detail or over rely on the maths.
The following points should be borne in mind:

For joints that open, dowel bars should be smooth, normally mild steel & sawn
(not cropped, as this can distort the section and prevent free movement). Mild
steel is increasingly hard to obtain and there may be a cost pressure to use high
yield bars. However, high yield bars tend to be ribbed which would reduce the
joints ability to open. Proprietary sleeves are available which if correctly used
and installed, ensure that one end of the bar is able to slip without any
significant restraint.

Installing dowel bars that are sufficiently robustly fixed to avoid being
displaced during a pour, and are carefully positioned parallel to one another
and perpendicular to the joint, is a difficult exercise for the Contractor. There
are proprietary methods of transferring loads at joints – for example Permaban
supply a square plate (diamond dowel)3. The faster installation of these
systems may offset their increased cost.
Where joints will be trafficked by MHE consideration should be given to use of
an armoured joint solution, which provide protection to the arrises at the joint,
usually with a steel plate edge. A number of proprietary systems are available.
Joint details are shown in Series G of the SSN UK typical details [13].
Joints and joint widths
Concrete slabs contract and expand under the thermal and curing related effects.
Typically the majority of shrinkage occurs over a period of one or two years. In
addition slabs have to accommodate seasonal expansion and contraction.
Induced joints
These are relatively narrow – normally formed or a saw cut (normally 5mm wide
and the greater of 0.3 × slab thickness or 50mm deep). The joints will open as the
concrete moves, and could close if there is expansion in the slab. Saw cut joints
are introduced soon after the concrete is poured to accommodate the contraction
associated with the early thermal effects of cement hydration.
Crack inducers placed at the base of the slab during construction should not be
used as these can promote uncontrolled cracks forming prior to saw cutting.
3
http://www.permaban.com/products/dowel-systems/diamond-dowel-1
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Formed joints
Joints can comprise a sealant
and a filler board. The filler
should be compressible to
accommodate some movement
while also being sufficiently
rigid to support the joint filler
during construction and in
service. The sealant prevents
water / dirt / ice getting into a
joint and damaging it, and should Figure 3. Formed joint (Detail G026 from [13]).
support the arris. It can affect the [Compressible Filler should not usually be used!]
appearance of the floor.
It is relatively unusual to include formed expansion joints (i.e. with a
compressible filler) into a warehouse floor slab. The early thermal contraction and
contraction at formed joints is generally of far greater magnitude than any
subsequent expansion, so that such joints are generally unnecessary.
Using a compressible material opens the joint leaving arrises more vulnerable to
trafficking
If an expansion joint closes up, the filler board is unlikely to fully expand again
and this can lead to contraction manifesting itself at other joints, making them
more susceptible to damage.
Where a design requirement for such a joint is identified, it is recommended that a
proprietary armoured joint solution is adopted, with steel plates reinforcing the
arrises etc.
Proprietary armoured joints are also preferred for formed contraction joints,
particularly where there is heavy MHE use.
Joint Fillers and Sealants
The amount of movement at a joint should be calculated to determine the type of
sealant and filler used. As noted, since the majority of shrinkage occurs in the first
two years, it is suggested that a soft sealant is installed to start with and changed
after this period for a harder and more durable (and therefore less compressible)
sealant. This requirement needs to be specified / advised to the client. Discuss
with the client whether installation of the joint sealants can be delayed to avoid
the need to install and then remove the softer sealant. If harder materials are
installed immediately, they can break the concrete arris as the slabs shrink away
from the joint. This is covered in some detail within TR34.
A joint sealant technical information will state
the amount the material can change in width.
For example, Sikaflex Pro has a 25% movement
capability. For a 5mm movement (2.5mm either
side), the joint would need to be 20mm wide.
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Specify the sealant or state
the amount of movement at
the joint in the contract
information.
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In the UK sealants are specified by Shore A hardness rating, which is a recognised
specification criterion, familiar to sealant suppliers and installers. The initial
sealant should have a Shore A hardness of 40, and should be changed after about
2 years with a sealant with a Shore A hardness of 80.
Also, when choosing a sealant, consider compatibility with any surface finish,
chemical resistance, durability and colour.
Specify a bond breaker or backer rod in the bottom of the joint. This prevents the
sealant from sagging into the joint and adhering to the bottom of the joint, which
allows the sealant to stretch on both the top and the bottom.
A backer rod in the joint also keeps the sealer plug thinner, reducing the amount
of sealant needed.
The sealant should be recessed where traffic is high; where the traffic is low but
the surface needs to be kept clean (e.g. food factories), the sealant should be flush
with the surface of the slab.
Refer to the sealant manufacturer's installation instructions in the contract
information.
Figure 4. Sealant and backer.
Finish
As with all concrete, curing is critical to achieving a satisfactory finish. A suitable
curing agent and curing regime (often using spray on acrylic resin type products)
should be used.
The slab finish should consider:

Surface regularity. This is a major factor in the design of high bay warehouses.
TR34 defines floor classes to help specify the correct surface tolerances. There
are two types of classification depending on the proposed use, which
distinguish between defined movement and free movement areas. Standards
for defined movement are more onerous. The definitions are covered in detail
in TR34.

Dusting. Sealants are available and should be specified as required. A well
cured and finished internal ground bearing slab should not experience
significant dusting even without a sealant.

Abrasion resistance. Sprinkle or dry shake finishes are often an effective and
cost efficient way to increase the abrasion resistance as they improve the
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resistance just where it is needed. Increasing the concrete strength can also
give a higher resistance, however dry shakes are more effective than
increasing concrete strength. Dry shakes are commonly used with fibres to
reduce incidence of fibres becoming exposed on the surface. Refer to TR34
and suppliers information for more guidance. A well cured and well finished
slab should have sufficient abrasion resistance to pass the usual tests.
Some sprinkle finishes are applied using spreaders which can create
cementitious dust and so can present a health and safety concern.
Delamination problems can also arise. Care is therefore needed in selecting an
appropriate product, not specifying such products unless there is a specific
requirement, and in following manufacturer’s recommendations for safe
application.

Direct finish, e.g. power floating etc. Floors are typically floated to rough
level and then power trowelled and/or power floated to give the smooth
polished finish associated with industrial/warehouse floors. Power
trowelling/floating will give a denser finish and also a smooth finish. Brush
finishes can give grip in wet conditions (a requirement for such finishes is
unusual in internal slabs with generally dry conditions). A Skip finish may be
suitable under applied finishes.

Painting. Painting can be used to define bays or access routes, alter appearance
and give resistance or grip.

Chemical resistance. This will depend on the application of the floor slab and
is normally provided by applying an appropriately resistant finish. Concrete is
generally resistant to many liquids used in industry, but chemical resistance is
a consideration for some aggressive substances (acids for instance).

Use for cold storage. Refer to specialist supplier details & TR34 for further
information
Common issues – cracking
Drying shrinkage cracking is a relatively common issue on industrial slabs.
Factors such as high wind velocity, low relative humidity, high ambient
temperature and poor curing can increase the rate of loss of water from the
concrete surface.
Rapid loss of water from the surface can cause curling of slab corners after the
concrete has hardened, particularly on thinner slabs on a more flexible sub-base
and this can cause corner cracking under load.
Rapid loss of water from the surface can also cause plastic shrinkage cracks.
Synthetic fibre reinforcement can help to resist this, as can dampening the
subgrade as well as reducing or avoiding the factors above. Evaporation retarders
are available but they are not curing agents, per se, and normal curing will still be
required. If plastic shrinkage cracks do appear, the finisher may be able to close
them during refinishing.
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Fatigue effects can occur through frequent trafficking with heavy MHE and this
needs special consideration in heavy warehouses, paper stores etc.
Cracking can have an adverse effect on the performance of MHE, creating
‘bumps’ which the operator must slow down to cross and reducing operational
efficiency. This is a relatively common operational concern where excessive
cracking occurs in a new facility.
We should consider the following actions:

Advise the client that it is not practical to expect a totally uncracked slab.

Advise that occasional thin cracks generally perform no worse structurally
than designed joints (as they have aggregate interlock and mesh reinforcement
crossing the crack) but they may require more on-going maintenance.

Provide a note on our drawings regarding repair requirements for any cracks
that do develop above a width of, say, 0.5mm. This should be aligned with the
Clients requirements and expectations. Such cracks may not perform well in
service under MHE trafficking and required ongoing maintenance.

Make sure that our specification emphasises the need for good mix design (to
reduce cement and free water content) and that curing is done properly with
the slab protected from the elements during curing.

Make sure that we detail suitable additional reinforcement in critical areas (i.e.
diagonal bars at columns and re-entrant corners). Such reinforcement will not
prevent cracking entirely but will control any cracks that develop.

Pay attention to panel aspect ratios, panel sizes and areas which may cause
restraint.

Be wary if the contractor says the general ground worker will cast the
industrial slab. There are flooring contractors who specialise in industrial
floors, high tolerances, etc., and we should look to make sure that the main
contractor appoints a suitably experienced subcontractor who has a successful
track record of building good quality industrial slabs without defects (such as
excessive cracking, poor tolerances etc.). The quality of construction control
and workmanship is also quite variable between ‘specialist’ flooring
contractors.
Foundations
Standard details at the edge of the building are included in the SSN Typical
Details [13].
To minimise restraint to movement the slab should be independent of any
foundations, columns or walls which penetrate the slab.
Columns
Generally, foundations are piled or pads, with the superstructure fixed directly to
the foundations. The slab is commonly laid at a later date, preferably when the
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building envelope has been constructed to aid curing, It will run directly up to the
edge of each column, so will need an isolation joint around the columns.
Walls
Generally there are walls at the edge
of the building. The SSN Typical
Details show a thickening below the
wall line. As there is a change in
depth, it is normal to locate a joint
(typically an expansion joint) where
the slab ‘necks’ and without which
significant restraint is introduced.
A free movement joint to avoid
restraint between the slab and any
thickenings is therefore strongly
recommended.
Figure 5. Perimeter wall (Detail G004 from [13]).
Rather than building a chamfer, some contractors prefer to build a vertical face as
they find it easy to form (see standard detail G006 [13]). Discuss the detail with
the contractor where possible.
The depth below ground level needs to meet the requirements of the Building
Regulations in the UK. Refer to Part A: Minimum depth of strip foundations,
section 2E4 [15].
Where walls are located within the building, choose from the following
approaches:

provide a thickening with joints where the slab necks (see detail G011 [13])

build the wall on a separate foundation and cast the slab up to the wall (similar
detail to G008 [13]). This would be the preferred detail to avoid increased
restraint.

build the wall on the slab. Check the slab for a line load and inform the
geotechnical engineer so that long term settlement can be reviewed.
Construction
There are two main construction techniques for industrial slabs: large area pours
and long strip construction. The construction method and pour layout adopted will
determine the final joint layout.
Large area pours
This is generally the Contractors preferred method where mechanical laying
equipment is available and where MHE movements are not in defined corridors
between racking.
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Formed free-movement joints that open up to 20mm (it is unlikely that this much
movement will occur when intermediate joints are used) are provided at
perimeters (40m to 50m is the normal maximum dimension) with sawn restrainedmovement joints cut on a 6m grid in both directions.
It is now more common to omit the intermediate joints, where measures such as
fibres are used, and where knowledge of how the slab will be trafficked permits
this to be considered. In general, where fewer joints are used with wider joint
spacing, a higher level of workmanship, construction control and appropriate
detailing and mix design is needed to achieve satisfactory results.
A variation on large area pour is wide bay construction, which gives an improved
control on tolerance.
Long strip construction
This is general adopted where there are MHE movements in defined corridors, or
where mechanical laying equipment is not available.
Strips of ground bearing slab, each 4m to 6m wide are laid between formwork,
with restrained-movement joints at 6m centres along the length of the slab. Strips
can be laid alternately, with infill strips subsequently placed. A greater control of
tolerance is achievable with this form of construction.
6.2
Residential
Introduction
The design basis for residential slabs is much less codified than for industrial
slabs. Residential floors in general have small panels, light loads and in most
cases have finishes applied. Cracking, surface regularity, abrasion resistance and
strength are normally not significant issues. Unreinforced slabs are often used.
BS 8204 [1] [2] and the NHBC [3] are the principal design guides.
Slab Thickness
The BS 8204 and NHBC (chapter 5.1, ‘Substructure and ground bearing floors’)
guidance for slab thickness is that, “Ground bearing concrete floor slabs should
be not less than 100mm thick, including monolithic screed where appropriate”.
A 100mm slab is therefore the minimum code recommendation, and to avoid
curling risk etc. a greater slab thickness would generally be recommended.
100mm may be adequate in a private dwelling on good ground conditions with
careful detailing, however it would not be recommended for a multi-occupancy
residential building like a hotel, student accommodation, multi-storey flats, care
homes etc. (i.e. the type of residential projects which Arup are most likely to be
involved in).
In most cases, on reasonable ground conditions, a 150mm slab with one layer of
A142 fabric reinforcement mesh is likely to provide an adequate design. Often in
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lightly loaded slabs experience has shown that the reinforcement can be placed
centrally or in the top, to assist in distributing shrinkage cracking.
Sub-base
BS 8204 states that “the tolerance on a sub-base should be +0mm/-25mm”,
consistent with TR34 (see also section 5.4 of this Note).
Importantly, the NHBC adds that “ground bearing slabs are not acceptable where
fill exceeds 600mm in depth”. In this case, a suspended slab should be used.
Joints
A joint is generally detailed between the slab and the wall construction.
Generally, no internal joints are required. If they are, refer to BS 8204 for more
details.
6.3
Slabs for other “normal” non-residential uses
Introduction
Ground bearing slabs are often used in schools, universities, public facilities,
offices, canteens etc. Although loads are generally much lighter than in industrial
uses, a mesh design sizing based on TR34 [7], rather than BS 8204 [1], [2] is
recommended as it gives a rigorous check and calculation.
Generally, in these buildings, the slab:

will have an applied finish in most areas (possibly not in plant rooms)

will be insulated to meet building regulation requirements

may have internal line loads from internal walls

could have a requirement for heating elements built into the floor build up.
Table 3 presents a comparison of the characteristics of these ‘normal’ use slabs
with those of industrial slabs.
Table 3. Comparison with more heavily loaded industrial slabs.
Characteristic
Slab strength critical
Heavily loaded industrial slabs
Yes – high point loads and UDLs
Placement of mesh
Bottom (for bending strength)
Use of induced joints
Yes, regular, crack control
Typical surface
requirements
Impact & abrasion resistance, use
of sealants/toppings, surface
regularity and level,
reinforcement of joints
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Lightly loaded “normal” slabs
No (only to bridge soft or hard
spots)
Top (for crack control where
unjointed) or middle (in 150mm
slabs to allow saw cut joints
above)
Possibly not (if all aspects careful
detailed and top mesh used)
To receive a floorcovering
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It goes without saying that, where the technical requirements of a slab are closer
aligned to those of an industrial slab, then the design should follow that approach
– refer Section 6.1 of this Note.
In contrast, where the characteristics of the slab align closer with those of a
‘normal’ use slab, it is appropriate to design accordingly.
Jointed vs unjointed slabs
Where slabs are only subject to loads with a modest intensity and a uniform
distribution, they will be subject to much lower bending and shear stresses. In
these situations it is advantageous to reinforce the slab with a layer of mesh placed
close to the top surface.
With such detailing, and provided the following key points are satisfied, slabs can
potentially be designed with wider spaced movement joints; sometimes omitting
joints entirely.

The mesh must be positioned in the top of the slab and sized correctly (with
calculations) for shrinkage and thermally induced stresses, sufficient to control
crack widths and spacing. (Expect an A252 mesh or larger.)

Local features (e.g. re-entrant corners, manhole openings, necks in the slab
etc.) must be carefully addressed.

The slab is due to receive floor coverings that are not sensitive to some
localised irregular cracking.
Both jointed and unjointed solutions require engineering thought and
neither should be seen as an easier option. Either way, there are coordination
decisions: jointed with the architect; unjointed more so with the contractor.
A reduced quantity of movement joints (both induced and formed) can simplify
the slab detailing and construction considerably and can equate to reduced
construction costs.
However, it requires a good level of construction control (workmanship), and
recognition by the client and architect that the approach does present a higher risk
of some uncontrolled cracking, whilst having the advantage of avoiding some of
the detailing considerations associated with providing joints.
The flow chart shown in Figure 6 maps out the design process for adopting an
unjointed design approach.
It should be noted that to provide the minimum reinforcement to restrain initial
cracks against further opening (in accordance with Ciria C660 and EC3 Part 2) a
higher level of reinforcement would be needed than the usual single layer of
mesh.
The expression for the reinforcement area that would be required for this is:
As,min = kck Act fctm(t) / fky
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For a C32/40 ‘typical’ scenario:
fctm(30) = 1.81 N/mm2
fky = 500 N/mm2
kck = k = 1.0 (external restraint and thickness h < 300mm)
So for a 150mm C32/40 slab the minimum reinforcement required such that when
a crack occurs the reinforcement will not yield before another crack forms would
be:
As,min = 1.0 x 1.81 / 500 = 0.00362% x 150 x 1000 = 543 mm2/m
This is considerable higher than the reinforcement provided by a single layer of
mesh.
After the first crack has formed the stress required to yield the fabric further is
therefore less than the stress required to form a full through thickness crack
somewhere else.
With joints, the maximum shrinkage is limited to the shrinkage strain multiplied
by the panel length.
With no joints, the maximum shrinkage is the strain from the whole pour length
which might be 30-40m long, so a single crack could potentially be quite large,
and the effects of this should be considered. In practice this often doesn’t occur as
there is always some degree of restraint leading to localised stress increases and
additional cracks forming, rather than a single dominant crack. This is not readily
predictable however, as localise restraint will be very dependent on construction
as well as design aspects.
Figure 5. Typical detail for a 150mm slab with central mesh, using a jointed
approach.
If an unjointed approach is not welcomed, the alternative is to place the mesh
centrally in the slab (typically an A193 mesh central in a 150mm thick slab depth)
and to use formed and saw cut joints at 6m to 7.5m centres. This jointed approach
has a lower risk of uncontrolled cracks occurring, may be more ‘safe’ and is
suitable where brittle or bonded floor finishes are proposed. The disadvantages are
that there is a cost associated with forming the joints, positioning of the joints
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needs consideration, and detailing the finishes and internal walls to coordinate
with the joints becomes important.
Figure 6. Flow chart for designing lightly loaded internal floor slabs.
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Table 4 presents a comparison of the jointed and unjointed approaches. Further
commentary is given in section 6.3.5 of this Note.
Table 4. Comparison of jointed and unjointed slabs for ‘normal’ buildings.
Jointed ‘normal’ lightly
loaded slabs
Unjointed ‘normal’ lightly
loaded slabs
Typical slab thickness
used 1
150mm
150mm
Typical mesh size 2
A192
A252
Mesh placement
At mid depth or top of slab
Top of slab
(typically 25-40mm cover –
requires check to EC2 for level
of exposure etc. Greater cover
may be required for any cast-in
inserts etc. Greater cover may
increase risk of unplanned
cracking)
Joints and panel ratios
Construction pour joints
dowelled.
Saw cut joints on typically 6m x
6mx bays.
Panel ratios to be 1:1.5 and no
spacing greater than 7.5m
between joints.
None required other than
construction pour joints, but
extreme care in detailing to
avoid restraint.
Reinforcement at re-entrant
corners etc. is required.
Saw cut depth typically 50mm
(1/3 slab thickness)
Advantages
Less risk of uncontrolled
cracking
Cost and detailing issues of
joints minimised.
Suitable when finishes can
accommodate some cracking.
Disadvantages
Cost of joints.
Coordination of joints with
walls, finishes, edge details,
slab features, drainage etc.
required.
Risk of uncontrolled cracking
somewhat higher.
Unsuitable for bonded or brittle
finishes.
Particular experience / control
of workmanship and detailing
important.
Notes:
1. Thickness requires thought; selection to suit ground conditions etc.
2. Mesh size requires calculation based on thermal cracking, bridging of soft spots etc.
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Applied finishes
The type of surface finish the slab is to receive will normally be determined by the
architect or client to suit the intended use of the building. It is important that we
understand what the applied finishes will be when we are designing the slab and
making decisions about whether to include movement control joints, isolation
joints around columns and specifying the surface finish.
Some finishes are inherently more sensitive to movement and cracking of the slab
than others. Table 5 lists the sensitivities of common finishes. Unjointed slabs are
only advocated when used in conjunction with the ‘sensitive’ and ‘less sensitive’
finishes although, in all instance, the expectations should be communicated to the
client.
Table 5. Floor finishes.
    


Sensitivity to “rippling”
      
Exposed
Bonded screed
Sensitivity to surface irregularity
Raised access
floor
 
Tiles
(e.g. ceramic)

Timber
    
Vinyl
Sensitivity to (controlled) cracking
= very sensitive
= sensitive
= less sensitive
Carpet
Insulation &
screed
Key:


(no tick)


   




Sensitivity to moisture from slab drying
Notes:
1) The above assumes ‘normal’ usage. Particular requirements of the building may alter the
suggested sensitivity level (e.g. indoor sports courts)
2) The criteria for defining the surface regularity does not normally need to be as stringent as
for an industrial floor slab where the leaning of forklifts and high bay raking needs to be
limited.
Further guidance on potential requirements for exposed concrete floors (such as
abrasion resistance, slip resistance, colour and appearance etc.) can be found in
[12].
Where a direct finish such as carpet, vinyl or timber is to be applied, the slab will
need to dry to an equilibrium level before the finish is laid. This can take 6 months
or even a year. An alternative, if such a long period cannot be accommodated in
the programme, is to apply a thin (3-5mm) epoxy or latex levelling compound to
the slab surface. This will trap moisture within the slab, preventing it from
evaporating upwards and causing delamination, and comes with the added benefit
of levelling and smoothing the slab surface. This technique may not always be
possible, for example where a bonded screed is to be applied to the slab, so it is
recommended that it is discussed with the project team at an early stage. The
decision whether to adopt a levelling compound would typically be made by the
contractor following discussion with the floor finish supplier.
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Slab surface finish
The type of surface finish required for the cast slab will be dictated by the type of
finishes to be applied. It should be specified by the architect with input, as
required, on compatibility with the structure provided by the engineer.
Where floor coverings are to be applied, it is normally sufficient to refer to the
National Structural Concrete Specification (NSCS) [16]. Where tighter control on
tolerances is required, characteristics such as surface regularity and flatness can be
defined using the definitions given in TR34 [7], however these are normally
reserved for warehouses, industrial slabs or special applications and are likely to
require a specialist subcontractor if they are to be achieved successfully.
The descriptions from the NSCS are reproduced in Appendix A of this Note for
ease of reference. More details on the tolerances for flatness, level etc. are
available in the NSCS section 10.9.
Note that we are sometimes asked by contractors or architects to specify the
method for forming the surface finish. The NSCS is not prescriptive, and it should
be up to the constructor to use their own knowledge and experience to determine
the appropriate means to meeting the specified requirements. An experienced
contractor or sub-contractor should be aware of the issues surrounding different
finishing techniques, and how best to achieve the categories identified by the
NSCS. We should be very careful about specifying a particular process because, if
it is not carried out properly and problems occur, the lines of responsibility will
have become blurred.
One particular issue to be aware of is the occurrence of ‘reinforcement rippling’
which gives a quilted appearance to the surface of the slab. The rippling affect is
often more pronounced on suspended slabs which are typically thicker and more
densely reinforced than ground bearing slabs, however occurrences are also
relatively common on thinner slabs where a top mesh is used. There is a Concrete
Society advisory note [17] which provides further background, and suggests that
the only way to avoid the issue is to carry out a power trowelled or floated finish
operation.
Contraction joints
Where required, the layout of contraction joints is important, since joints or cracks
in the slab will generally be repeated in the finishes.
Agreeing the location of any contraction joints (which may be planned
construction pour joints or saw cut joints) with the architect is therefore necessary.
The exclusion, or reduction of joints will be likely to be well received by the
architect, since their detailing and coordination will be simplified by not having to
coordinate joint locations with internal subdividing partitions (which are much
more prevalent in institutional buildings than they are in industrial buildings, but
rarely on a 6m x 6m grid).
However, even where an unjointed slab can be justified, it may also be prudent to
include isolation joints around structural columns – see section 6.3.6.
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Joints along block work wall lines are also recommended. Where provided, these
should be aligned to one side of block work wall lines (where they will be under
the skirting), not bridged by the block walls.
Under heavier walls, where these sit on the slab, slab thickenings or a separate
strip foundation may be required.
Where joints are used (typically at 6m to 7m centres), panels should not exceed
1.5:1 ratio of sides. If they do, a crack across the middle of the panel is very
likely.
Isolation joints around Columns etc.
If movement control joints are required, isolation joints around columns will be
necessary.
However, if movement control joints in the slab are not included it may still be
beneficial to include isolation joints around columns. The decision on whether to
include them will need to take into account a number of considerations, including:



the type of superstructure (joints are easier to form around concrete columns
than steel)
buildability of the details
other issues such as detailing of the DPM.
Four common scenarios are shown in Table 6. Detailing of the DPM is not shown
and needs to be discussed with the architect at an early stage as it could affect the
practicality of forming effective isolation joints.
For situations 1, 3 & 4 in the Table it is recommended that additional
reinforcement is included to prevent cracks propagating from the corners of the
columns. It is recommended that these bars are orthogonal with and nestled within
the mesh, rather than being placed diagonally. While it is common to see details
drawn with bars at 45 degrees to the corner, this will mean that the additional bars
are located within the third and fourth layer and are much less effective at
intercepting and stopping cracks.
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Table 6. Isolation joint details.
Detail
1. Concrete columns & foundation set down.
Any type of floor finish.
Comments
 Inclusion of a soft joint between slab
and column faces is recommended.
 Foundations are often set down to
make construction of drainage easier,
and/or stop “hard spots”.
 It will be common for the column to
be poured up to kicker level before
the slab is formed. This is the
structurally preferable sequence as
the number of different pours is
minimised.
 Orthogonal crack control bars in the
slab are used
2.

Steel/concrete column & foundation set up.
Any type of floor finish.

3.
Steel column, with raised access floor



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Inclusion of a soft joint between slab
and column is of little or no benefit
(no vertical movement possible, and
horizontal movement can only take
place if the slab and base are
structurally separate with a slip
membrane such as two layers of
polythene between them.
Crack control bars are not usually
required unless the foundation size is
small.
Concrete surround to steel column
below slab level.
Inclusion of a soft joint between
concrete surround to column and slab
is unlikely to have any adverse
structural effects, so is recommended.
Such a soft joint should also be easy
to construct.
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Detail
4. Steel column with thin finishes
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Comments
 The benefits of including a joint need
to be considered on a project by
project basis, and discussed openly
with the rest of the project team,
weighing up against practical aspects
(e.g. the position of the DPM,
buildability, etc).
o Benefit of a soft joint will be
limited to allowing horizontal
movement. Differential vertical
movement will be prevented by
the column encasement.
o The detailing of the DPM may
make the inclusion of a soft joint
impractical.
o Whilst it is fiddly to apply the
compressible board around the
profile of the column, the
alternative is to have it
unsupported of infill in between
the column flanges prior to
casting the slab.
Slab thickness
For lightly loaded applications the thickness of the slab is rarely governed by
strength, unless the ground conditions are particularly poor or the slab is
particularly thin.
As previously discussed, almost all available methods for designing ground
bearing slabs are intended for industrial type slabs subject to point loads and
heavy UDLs. The choice of slab thickness for most lightly loaded applications
tends to be made based on engineering judgement and past experience.
150mm thick slabs are typical, as for slabs any thinner than this, there is a risk due
to subgrade tolerances of thin sections (125mm or less) that would potentially be
subject to excess cracking or curling due to shrinkage.
Amongst the available published guidance, Curtins et al [8] has attempted to apply
some science to sizing the thickness by dividing ground conditions into four
categories of decreasing stiffness and increasing variability. These categories are
used to size a notional ‘soft spot’ over which the slab needs to be capable of
spanning, as shown in Figure 7.
Whilst the selection of category for soil classification will be open to a degree of
interpretation, and the sizes of soft spots suggested could be seen as being
somewhat arbitrary, a comparison of this method against the designs for past
projects does show a good degree of correlation.
The Curtins method has not been widely tested in Arup, however it does appear to
offer a good alternative to relying solely on past experience and engineering
judgement.
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Figure 7. Critical ‘soft spot’ dimension for slab design.
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Other issues
7.1
Perimeter cladding support
Table 7 shows the three different types of edge detail that are commonly used for
supporting perimeter cladding.
Table 7. Slab edge details.
Details
A) Suspended “boot” beam spanning
between pads/pile caps
B) Ground bearing “boot”
Comments
Reasons that this detail might be adopted:
 Brittle cladding
 Marginal ground conditions
 Where different settlement is predicted
between pads/pile caps and the slab edge
beam, e.g.:
o different loading conditions
o founding in different strata
Reasons that this detail might be adopted:
 Lightweight or non-brittle cladding e.g.
metal cladding, ’Sto’ render
 Good ground and shallow depth
 Similar settlement expectations between
frame (pads/pile caps) and cladding
(“boot”)
There are some buildability considerations: tall
bar spacers are required for the different layers
of mesh; it maybe better to schedule U / L bars.
C) Separate strip or trench footing
Reasons that this detail might be adopted:
 Reasonable ground at relatively shallow
depth
 Suited to load bearing perimeter walls
rather than framed construction
 If slab is going to be constructed after
the envelope
This detail is more common for domestic scale
buildings
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Under-slab insulation
Insulation may be required below ground bearing slabs.
The strength and compressibility of any insulation should be reviewed and it
should be of suitable grade and stiffness to provide the necessary structural
support. Note that data provided in manufacturer’s literature may not include
factors of safety, giving a false sense of the grade of insulation to be used. An
insulation layer may restrict the methods used to lay a slab, and this should be
reviewed at an early stage.
7.3
Underfloor heating
It is strongly recommended that heating systems are not laid in thin or lightly
reinforced ground bearing floor slabs. This is because the cyclical expansion and
contraction caused by the heating & cooling of the system is likely to cause
additional cracking of the slab. This risk is much higher in ground bearing slabs
than in more heavily reinforced suspended slabs.
It is suggested that the heating is instead laid in a screed above the slab (often with
insulation below the screed, above the slab), and this screed is specified by the
specialist supplier of the heating.
On a few occasions, where we have worked for a contractor on design and build
projects, the contractor has taken the decision to go against our advice and lay the
underfloor heating in the slab. On each occasion the driver has been cost (since
the omission of the screed results in cost and programme saving). In such cases it
is important to place our concerns in writing to the contractor/client and clearly
outline the risks of going against our advice.
7.4
Ground gases and radon
Where the ground floor slab is required to form part of a barrier against ground
gas (i.e. CO2, methane) and/or radon, reference should be made to the relevant
guidance, such as BS 8485 [18] and BRE Guide 211 [19].
It is important that the ground characteristics are defined at an early stage since
the level of protection required could influence the thickness of the slab and level
of reinforcement included.
A combination of membrane(s) and below slab ventilation systems may be used,
in conjunction with a degree of inherent resistance from the structure.
Special consideration may also need to be given to the detailing of service
penetrations and joints in the slab.
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7.5
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Mesh detailing: flying ends
It is often beneficial to specify fabric mesh with flying ends to reduce the numbers
of layers at joints (especially in corners) to a minimum. A comparison of lapped
mesh, with flying ends with conventional sheets is shown in Figure 8.
Figure 8. Lapped mesh, with (left) and without (right) flying ends.
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References
[1]
BRITISH STANDARDS INSTITUTE. BS 8204-1:2003 +A1:2009.
Screeds, bases and in situ floorings —Part 1: Concrete bases
and cementitious levelling screeds to receive floorings —
Code of practice. 2009.
[2]
BRITISH STANDARDS INSTITUTE. BS 8204-2:2003 +A2:2011.
Screeds, bases and in situ floorings —Part 2: Concrete
wearing surfaces — Code of practice. 2011.
[3]
NATIONAL HOUSE BUILDING COUNCIL. NHBC Standards 2014.
Part 5 – Substructure, ground floors, drainage and basements.
2014.
[4]
ARUP. IGN01. Pavement Design for Lightly Trafficked Roads and
Parking Areas, Civil Engineering and Utilities Network,
Jonathan Millard, August 2016.
[5]
ARUP. SGN 4.5. Ground-Bearing Floor Slabs, (later renamed 1997
SGN 10), November 1983.
[6]
GARBER, G. Design and Construction of Concrete Floors.
2nd Edition. June 2006.
[7]
CONCRETE SOCIETY. Technical Report 34. Concrete industrial
ground floors — A guide to design and construction.
4th Edition, October 2013.
[8]
CURTIN, W. G. et al, Structural Foundation Designers’ Manual,
2nd Edition. 2006. Available from Arup library as an e-book:
http://site.ebrary.com/lib/arupuk/detail.action?docID=102366
55
[9]
HIGHWAYS AGENCY. Manual of Contract Documents for
Highways Works. Volume 1, Specification for Highway
Works, Series 800, Road pavements – Unbound materials.
2009.
[10]
BRITISH STANDARDS INSTITUTE. BS EN 13285:2010. Unbound
mixtures – Specifications. 2010.
[11]
BRITISH STANDARDS INSTITUTE. BS EN 13242:2013.
Aggregates for unbound and hydraulically bound materials
for use in civil engineering work and road construction, 2013.
[12]
CHANDLER, J.W.E. Design of Floors on Ground. Technical
Report 550, MPA Cement, 1982.
[13]
ARUP, SSN UK Structural Typical Details, April 2016.
http://networks.intranet.arup.com/structural/tools/typical_deta
ils_drawings/uk/structural_typical_details/structural_typical_
details_home.cfm
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[14]
BRITISH STANDARDS INSTITUTE. BS 8500-1:2015 +A1:2016.
Concrete. Complementary British Standard to BS EN 206.
Method of specifying and guidance for the specifier. 2015.
[15]
HM GOVERNMENT. The Building Regulations 2010. Approved
Document A – Structure. 2013.
[16]
CEMENT AND CONCRETE INDUSTRY PUBLICATION.
CCIP-050. National Structural Concrete Specification for
Building Construction, 4th Edition; 2010.
[17]
CONCRETE SOCIETY. CAS06. Concrete Advice Note 6 –
Reinforcement ripple. 2003.
[18]
BRITISH STANDARDS INSTITUTE. BS 8485:2015. Code of
practice for the design of protective measures for methane
and carbon dioxide ground gases for new buildings. 2015.
[19]
BRE. BR 211. Radon: Guidance on protective measures for new
buildings. 2015.
Additional sources of guidance
[20]
ISTRUCTE. Technical Guidance Note 30 Level 1. Ground-bearing
slabs: The Structural Engineer, August 2013.
[21]
DEACON, C. Concrete Ground Floors: Their design, construction
and finish, 3rd Edition; Cement & Concrete Association,
1986.
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Appendix A
Supporting guidance
Structural Guidance Note
A1
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General Notes and Specifications
The following notes provide a summary of some of the key issues that need to be
covered on ground bearing slab design documents (drawings and specifications)
for both jointed and jointless construction, to ensure that the issues around early
age thermal cracking and joint layouts are adequately covered. These notes do not
attempt to comprehensively cover every aspects of ground slab specification such
as the mix design and other material requirements, etc. While it may be felt
appropriate to include some of this information in general notes or specifications,
it is more likely to be properly read and adhered to if it is included on the ground
slab drawings.
Red text is guidance to the reader, to be removed.
Purple text is highlighted as most likely to need to be edited to be project specific.
General
[Some typical notes – these will not cover all situations, and the notes will need
review in each instance to suit the specific project requirements]

Refer to the earthworks specification / drawing(s) for requirements for
excavation, fill and for slab subbase materials and compaction.
[For smaller projects, earthworks specification and subbase requirements may
be given on the drawings]

The compacted subbase is to be blinded with fine material to fill surface voids
and the finished surface is to be rolled to closure. A surface layer of fines or
sand is not to be used as this may not provide a suitably stable platform to
support the membrane and reinforcement chairs etc. during construction.

The surface tolerance of the subbase is to be +0mm, -10mm.
[TR 34 recommends 25mm maximum downward tolerance on subbase level,
however tighter tolerances are achievable and can be specified]

The slab is to be laid on one layer of minimum 1200 gauge BBA polythene
with 150mm lapped, taped and sealed joints (polythene membrane providing
the function of acting as a slip membrane). Waterproofing (DPM)
requirements to be confirmed by the architect (the minimum slip membrane
polythene may be satisfactory to provide the requirement, subject to the
architect specifying that this is the case).

For details of the required extent and specification of below slab insulation
refer to Architects details.
[Project Specific. Note only required where there is below-slab insulation.
The assumed design stiffness of subgrade needs to take account of insulation]
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Ground bearing slabs: good practice guide

The finish tolerance is to be FM2 [Project specific] in accordance with
concrete society technical report TR 34 (4th Edition).

Concrete Grade C32/40 / RC40 / etc [Project specific]

Maximum aggregate size to be 20mm

Minimum cover to all reinforcement 35mm / 40mm / etc [Project specific]

Provide pairs of diagonal bars in ground slab at all re-entrant corners / box
outs in slab, as shown on Arup substructure/reinforcement drawings.

The contractor is to consider his pour sequence, bay size and construction joint
locations to minimise slab shrinkage, and submit proposals to Arup prior to
construction.

The position of construction joints proposed by the Contractor shall be such as
to avoid distress or damage to the Works particularly from thermal movement
or shrinkage effects and are to be agreed with the Architect to avoid sensitive
areas.

The positions and details of all construction joints not shown on the drawings
are to be agreed with XXX [Project specific] before work commences.

Concrete finishes shall be compatible with the Architect’s finishes
specification and details.

Widespread cracking or cracks exceeding 0.5mm width will not be accepted,
and will require repair using a suitable injection crack repair method, which
will be subject to approval.
[Consider carefully if this is required, subject to finishes etc.]
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Ground bearing slabs: good practice guide
Jointed Ground Bearing Slabs
[Additional notes specifically for projects with jointed slabs]

The final selection of pour sizes and sequence, construction joint locations and
spacing’s of saw cuts is to be selected by the Contractor so as to minimise slab
shrinkage by minimising restraint to movement. The final mix design shall
minimise drying shrinkage. Construction joints are to be agreed with the
architect to avoid sensitive areas / finishes. Proposals are to be submitted to
Arup at least three weeks prior to construction for comment.
Specify
mix
designation
Final mix design
certification to meet
Arup Specification
Indicative
joint layout
Final joint
layout
Arup
Responsible
Approval
Responsible
Comment
Contractor
-
Responsible
-
Responsible
Architect
-
Approval
Comment
Approval

The spacing and arrangement of sawn and formed joints should follow the
indicative arrangement on the drawings. There is a risk of cracking in all
concrete floors. The risk increases with the size of bays and distance between
stress relief joints, Joints should generally be at less than 6m centres. Panel
aspect ratios should not be greater than 1:1.5 following saw cutting.

Sawn joints shall be 3-4mm wide and cut as soon as practicable after placing
the concrete (nominally 24 hours or less after placing). Appropriate experience
on the part of the ground works contractor is required as saw cutting too early
damages the surface, and if cutting too late, cracks may already have formed
and therefore would be likely to continue to propagate in uncontrolled manner.

Any cracks that develop should be monitored and, where appropriate
(typically where >0.5mm wide), will be required to be repaired by a technique
such as resin injection. The repair of concrete structures, both materials and
methods, is covered by EN 1504. Further guidance is given in Concrete
Society Technical Report 69. Repair proposals are to be submitted to Arup
prior to carrying out any repair.
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Structural Guidance Note
2018 SGN 01
Ground bearing slabs: good practice guide
Jointless Ground Bearing Slabs
[Additional notes specifically for projects with jointless slabs]

The design of structural concrete to Eurocode 2 will normally control and
distribute the cracking of concrete and limit such cracks, which may be
random, to about 0.3mm width. However, the factors that influence the
formation, size and distribution of cracks are many and varied. They include
the structural design, but also the site controlled activities such as the concrete
mix design, placing and curing of concrete, time of striking and loading the
slabs, pour size and slab restraints, temperature and humidity etc. Depending
on the variables noted above, it is possible that, in some areas, the cracks in
the floor slabs may be wider than 0.3mm and are likely to be worse and more
random at ground floor level as ground bearing slabs are only nominally
reinforced. The cracks will tend to form at the time of casting but may not be
noticeable until the concrete dries out over time.

The architect should consider the appropriateness of their proposed finishes
and check that no specified finishes will be particularly sensitive to such
cracking. It is assumed a thin (say 5mm) epoxy levelling screed will be
applied to the top of all slabs before flooring finishes are laid.

Underfloor heating pipes must not be cast into ground bearing slabs.

The ground bearing slab is mesh reinforced without contraction joints. The
Contractor is to consider his pour sequence, bay size and construction joint
locations to minimise slab shrinkage, and submit proposals to Arup prior to
construction.

Construction joint locations are also to be agreed with the Architect to avoid
sensitive areas. The architect’s attention is drawn to the likelihood of random
hairline cracks (typically less than 0.5mm in width) forming in the slab and
the need to ensure that any applied finishes are compatible with this.

For applied finishes a generous period of time (as recommended by
manufacturers) is recommended in the programme after casting slabs and prior
to beginning finishes to minimise the amount of shrinkage still to occur after
application of finishes, and avoid locking excessive moisture into the slab. For
such areas of more sensitive finish, in accordance with good practice the
subcontractor will need to take due account by using flexible adhesives,
flexible grout, and including movement joints in the finishes at regular
intervals, all in accordance with the architectural specification and
manufacturers recommendations.
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NSCS
The following is an extract from NSCS [16]:
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