BS 8297 2017
BS 8297 2017
BS 8297 2017
8297:2017
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ICS 91.060.10
Contents Page
Foreword iii
Introduction 1
1 Scope 2
2 Normative references 2
3 Terms and definitions 5
4 Materials and components 7
4.1 Customer requirements 7
4.2 Specifications for materials and components 8
4.3 Steel 9
Table 1 — Recommended grades of austenitic or Duplex (Austenitic/ferritic) stainless steel used
for fixings 9
Table 2 — Recommended grades of fasteners 10
4.4 Materials for jointing and pointing 11
4.5 Flashings, weatherings and cavity trays 11
4.6 Coating agents 11
5 Design of cladding units 12
5.1 General 12
5.2 Structural design 12
5.3 Thickness of concrete cover to reinforcement 13
5.4 Fire-resisting wall units 14
5.5 Support conditions and bending moments 14
Figure 1 — Assumptions for design of units and corbels/brackets 16
5.6 Sizes of units including thickness 17
5.7 Movement and tolerances 17
5.8 Dimensional stability 18
5.9 Accommodation of dimensional changes 18
Table 3 — Coefficients of thermal expansion of buildings materials 19
Table 4 — Extreme temperatures of UK structures 19
Table 5 — Rate of shrinkage of concrete (as a percentage of its potential) 21
5.10 Other factors affecting design 21
5.11 Passive fire protection to resist the spread of fire 23
5.12 Signs and attachments to cladding units 23
5.13 Thermal insulation 24
5.14 Acoustic properties 24
6 Position and detail of joints 24
6.1 General 24
6.2 Movement joints 25
6.3 Compression joints 25
6.4 Sealed joints 25
6.5 Open drained joints 26
Figure 2 — Examples of jointing details 27
Figure 3 — Open drained joint with plain baffle 28
6.6 Impregnated foam sealing strips and sealing strips 28
7 Support and attachment of units to the structure 29
7.1 Support 29
7.2 Methods of attachment 29
7.3 Design of fixings 29
Figure 4 — Typical restraint fixing to concrete structure — vertical section 30
Summary of pages
This document comprises a front cover, and inside front cover, pages i to iv, pages 1 to 55, an inside back cover and
a back cover.
Foreword
Publishing information
This British Standard is published by BSI Standards Limited, under licence from The British
Standards Institution, and came into effect on 31 October 2017. It was prepared by Technical
Committee B/524, Precast concrete products. A list of organizations represented on this committee
can be obtained on request to its secretary.
Supersession
This British Standard supersedes BS 8297:2000, which is withdrawn.
Presentational conventions
The provisions of this standard are presented in roman (i.e. upright) type. Its recommendations are
expressed in sentences in which the principal auxiliary verb is “should”.
Commentary, explanation and general informative material is presented in smaller italic type, and does
not constitute a normative element.
The word “should” is used to express recommendations of this standard. The word “may” is used in
the text to express permissibility, e.g. as an alternative to the primary recommendation of the clause.
The word “can” is used to express possibility, e.g. a consequence of an action or an event.
Notes and commentaries are provided throughout the text of this standard. Notes give references
and additional information that are important but do not form part of the recommendations.
Commentaries give background information.
Introduction
This British Standard has been developed to give recommendations and good practice for specifiers
and manufacturers during the design, manufacture, transport and installation of architectural precast
concrete units.
Framed structures are often enclosed by precast concrete panels.These in turn frequently serve an
architectural role in providing the external appearance of the building. In such instances, the panels
are generically referred to as cladding. As well as providing the external envelope of a building,
similar precast concrete units might also be used for other purposes, such as decorative columns,
either loadbearing or not, balconies, and other substantial elements. Where units provide the
weathertight external envelope of a building, water and airtightness is ensured by appropriate design
and treatment of the joints between the units.
Units are required to carry their own weight and also any directly or indirectly imposed loadings.
Units are also required to resist wind loading, provide weather protection, acoustic and thermal
performance. They might also need to provide fire resistance and be resistant to accidental damage,
e.g. vehicle impact and internal or external explosion. Their construction needs to allow for the
dimensional tolerances of construction and movement of the building structure during its designed
lifetime. Where units are loadbearing, they need to be able to transmit loads as part of the structure.
Strength, durability and versatility are the inherent characteristics of precast concrete. This type
of precast concrete is almost inevitably bespoke, by virtue of its shape and size, colour and texture,
finishes or facings, to achieve the specified aesthetic requirements of individual building projects. The
manufacturing process is non-repetitive and might not lend itself to automation. The weight and size
of units can also be critical. Involving the manufacturer at the design stage can therefore often lead to
more efficient production and optimization of the fixing system.
1 Scope
This British Standard gives recommendations and guidance for the design, manufacture, transport
and installation of architectural precast concrete units in the form of:
a) units supported by and fixed to a structural frame or wall to perform a cladding role;
NOTE These might be part of the external envelope or separate elements, such as columns and balconies.
b) units which may neither be associated with, or form part of, a building;
c) units used as permanent formwork in part or in whole, but limited to architectural elements; and
d) the architectural function of a sandwich panel (see 3.7.3).
It includes recommendations on the measures which are to be taken to provide for permanent and
temporary movements and tolerances of the structure, to enable the cladding to perform its function
satisfactorily. It gives the minimum standards needed and the materials and methods of fixings most
frequently used. It applies to new buildings but many provisions might be applicable to alterations or
refurbishment of existing buildings.
Guidance is given on the quality of the finished product and verification of performance. The design
recommendations given in this British Standard are based on limit state design principles.
This British Standard is intended to be used in conjunction with BS EN 13369:2013 and BS EN 13670,
but provides comprehensive guidance specifically in relation to architectural precast concrete
cladding and therefore takes precedence in the items addressed.
This British Standard does not provide recommendations relevant to units incorporating glass
fibre reinforced concrete (GFRC/GRC), semi-dry or small wet-cast masonry units instead of larger
architectural wall cladding panels (see BS 1217), nor the design of the supporting structure to which
the units might be attached.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes provisions of this document. For dated references, only the edition cited
applies. For undated references, the latest edition of the referenced document (including any
amendments) applies.
Standards publications
BS 1881‑208, Testing concrete — Recommendations for the determination of the initial surface
absorption of concrete
BS 4449, Steel for the reinforcement of concrete — Weldable reinforcing steel — Bar, coil and decoiled
product — Specification
BS 4482, Steel wire for the reinforcement of concrete products — Specification
BS 4483, Steel fabric for the reinforcement of concrete — Specification
BS 5606, Guide to accuracy in building
BS 6093, Design of joints and jointing in building construction — Guide
BS 6100‑9, Building and civil engineering — Vocabulary — Work with concrete and plaster
BS 6180, Barriers in and about buildings – Code of practice
BS EN 10080, Steel for the reinforcement of concrete — Weldable reinforcing steel — General
BS EN 10088‑2, Stainless steels – Part 2: Technical delivery conditions for sheet/plate and strip of
corrosion resisting steels for general purposes
BS EN 10088‑3, Stainless steels – Part 3: Technical delivery conditions for semi‑finished products, bars,
rods, wire, sections and bright products of corrosion resisting steels for general purposes
BS EN 12155, Curtain walling — Watertightness — Laboratory test under static pressure
BS EN 12179, Curtain walling — Resistance to wind load — Test method
BS EN 12878:2014, Pigments for the colouring of building materials based on cement and/or lime —
Specifications and methods of test
BS EN 13050, Curtain walling — Watertightness — Laboratory test under dynamic condition of air
pressure and water spray
BS EN 13055, Lightweight aggregates
BS EN 14019, Curtain walling — Impact resistance — Performance requirements
BS EN 15651‑1, Sealants for non-structural use in joints in buildings and pedestrian walkways — Part
1: Sealants for façade elements
BS EN 15167‑1, Ground granulated blast furnace slag for use in concrete, mortar and grout — Part 1:
Definitions, specifications and conformity criteria
BS EN ISO 3506‑1, Mechanical properties of corrosion-resistant stainless steel fasteners – Part 1: Bolts,
screws and studs
BS EN ISO 3506‑2, Mechanical properties of corrosion-resistant stainless steel fasteners – Part 2: Nuts
BS EN ISO 6946, Building components and building elements — Thermal resistance and thermal
transmittance — Calculation method
BS EN ISO 11600:2003+A1:2011, Building construction – Jointing products — Classification and
requirements for sealants
BS EN ISO 13788, Hygrothermal performance of building components and building elements —
Internal surface temperature to avoid critical surface humidity and interstitial condensation —
Calculation methods
BS ISO 15510:2014, Stainless steels — Chemical composition
DD CEN/TS 772-22, Methods of test for masonry units – Determination of freeze/thaw resistance of clay
masonry units
PD CEN/TR 15739: 2008, Precast concrete products – Concrete finishes – Identification
NA to BS EN 1990:2002+A1: 2005, UK National Annex for Eurocode – Basis of structural design
Other publications
[N1] EAD 330232-00-0601, Mechanical fasteners for use in concrete
[N2] ETAG 0034, Guidelines for European Technical Approval of kits for external
wall claddings
[N3] CWCT Standard for systemised building envelopes (all parts)
[N4] CWCT Technical Note 75, Impact performance of building envelopes: guidance on
specification
[N5] CWCT Technical Note 76, Impact performance of building envelopes: method for impact
testing cladding panels
3.3 cladding
form of building covering that supports its own weight and resists any other forces that
may act upon it
3.9 Joints
3.9.1 movement joint
purpose-designed joint that accommodates multi-directional movement between adjacent units and
takes account of manufacturing and construction tolerances
NOTE Also commonly known as an "expansion joint".
3.10.3 baffle
flexible preformed linear section designed to be fitted in grooves between adjacent units to minimize
direct entry of rain into an open joint
4.2.2.1 Cement
Cement should conform to the relevant standards as follows.
a) Common cement to BS EN 197‑1:2011.
b) Sulfate‑resisting cement to BS EN 197‑1:2011, SR0 or BS EN 197‑1:2011, SR3.
Combinations of CEM I as given in BS 8500‑2:2015+A1, Annex A and BS EN 197‑1:2011 with either:
1) fly ash conforming to BS EN 450‑1:2012, Category A or B;
2) ground granulated blast furnace slag (GGBS) conforming to BS EN 15167‑1; or
3) limestone fines conforming to BS 7979 or BS 8500‑2:2015+A1.
Combinations manufactured in the concrete mixer from Portland cement and GGBS or PFA should
conform to the relevant standards.
4.2.2.2 Aggregates
Aggregates general suitability should conform to the following.
a) Normal-weight and heavyweight to BS EN 12620 and the requirements specified in
BS 8500‑2:2015+A1.
b) Coarse crushed concrete aggregate (CCA) to BS EN 12620 and the requirements specified in
BS 8500‑2:2015+A1.
c) Granulated, including pelletized, blastfurnace slag and air-colled blastfurnace slag to
BS EN 12620 and the requirements specified in BS 8500‑2:2015+A1.
d) Lightweight aggregates to BS EN 13055 and the requirements specified in BS 8500‑2:2015+A1.
e) Reclaimed aggregate to BS EN 206.
4.2.2.3 Admixtures
Admixtures should conform to BS EN 480‑1 and BS EN 934‑2.
4.2.2.4 Pigments
Where a specified coloured concrete requires a pigment, the pigment should conform
to BS EN 12878:2014. For reinforced concrete, the pigment should conform to
BS EN 12878:2014, Category B.
4.2.2.5 Water
Mixing water should conform to BS EN 1008.
4.3 Steel
4.3.1 Carbon steel reinforcement
Steel reinforcement for concrete should conform to BS 4449, BS 4482 and BS 4483, as appropriate.
4.3.3.1 General
Austenitic and duplex (austenitic/ferritic) stainless steel should conform to the following standards:
a) stainless steel sheet strip and plate to BS EN 10088‑2; and
b) stainless steel rod and bar to BS EN 10088‑3.
Stainless steel plate for restraint and loadbearing fixings should conform to the grades and uses
given in Table 1.
Table 1 — Recommended grades of austenitic or Duplex (Austenitic/ferritic) stainless steel used for fixings
If visually exposed, or specified to a higher grade, plate and strip grade 1.4401 (316) to BS EN 10088
or Grade 1.4404 (316L) should be used if plate is thicker than 16 mm. For steel rod and bar
1.4401(316) should be used to prevent tarnishing of the surface.
As cast‑in fixings perform a critical role in the securing of the cladding to the structure, the quality
control process should include a 100% visual inspection of the fixing type, size and secure positioning
in accordance with the intended design detail.
The specified mechanical properties of the material used in the manufacture of cast-in sockets should
take account of the performance classification of the proposed fastener from BS EN ISO 3506‑1. The
minimum thread engagement of the fastener into the socket should be specified.
4.3.3.3 Fasteners
Bolts and nuts should be of austenitic or Duplex (Austenitic/ferritic) stainless steel conforming to
BS EN ISO 3506‑1. The grades of fasteners should be as given in Table 2.
Table 2 — Recommended grades of fasteners
the fixings should be A4, D4 or D6 and if externally exposed in chloride bearing environments and of
aethestic interest should be A8 or D8.
NOTE Where duplex (austenitic-ferritic) plate is not readily available it is acceptable to use austenitic stainless
steel plate with duplex fasteners ensuring that the minimum relative corrosion resistant properties are maintained,
e.g. D2 with A2, D4 and D6 with A4 and D8 with A8, etc.
4.3.3.4 Welding
Welding of austenitic stainless steel components should conform to the requirements of
BS EN 1011‑1 and BS EN 1011‑3, and should only be undertaken where the necessary facilities,
expert knowledge and skills are available.
Procedures for testing welds should be agreed at the design and specification stage.
If the fixing provides structural stability, the components should conform to BS EN 1090 (all parts).
4.4.2 Sealants
Sealants should conform to BS EN ISO 11600:2003+A1, Type F requirements and should be selected
using the guidance given in BS 6213 and BS 6093. Sealants should be tested for adhesion, staining
(see 5.10.3) and compatibility with materials that they are likely to come into contact with.
The design life of the sealant might not equal the design life of the cladding and therefore regular
inspections should be carried out.
Materials used should conform to BS EN 15651‑1.
4.4.5 Baffles
Baffles should be made of resilient polymer or rubber material.
Where coating agents are used they should be applied to surfaces as late as possible to allow natural
curing of the base concrete to occur. Wherever possible, they should not be applied to individual
components in the factory to avoid inadvertent treatment of bedding surfaces to the base concrete
which could affect mortar or sealant bonding.
In advance of selecting the coating agent, it should be tested with the manufacturer’s products for
compatibility and checked with the coating agent manufacturer’s recommendations.
NOTE 2 See BA 85/04 [1] for further reference.
NOTE 3 Coating agents might vary the appearance of the concrete and might require future maintenance.
The design of cladding units should be in accordance with NA to BS EN 1990:2002+A1. The basis of
the structural design table in NA to BS EN 1990:2002+A1, Table NA.2.1, is the design working life of
the precast cladding should be specified at 50 years (category 4) which aligns with the design values
within BS 8500.
For loadbearing elements, the design of concrete and reinforcement should be in accordance
with BS EN 1992‑1‑1:2004+A1, and for the design of steel elements should be in accordance with
BS EN 1993‑1‑1.
For the design of stainless steel items such as supports and restraints, reference should be made to
BS EN 1993‑1‑4.
NOTE 2 Users might also wish to refer to industry publications such as Design manual for structural stainless steel
(Euro Inox and Steel Construction Institute) [2].
There should be close cooperation between the designer of the precast elements, and the designer
of any supporting structure. In particular the designer of the structure should be made aware of the
magnitude and location of the loads likely to be imposed onto the structure by the precast.
Appropriate dynamic (snatch) factors should be included to take account of crane types and
speed of lifting.
The point of application of loads, which could be affected by shims etc., should be taken into account.
NOTE 2 Where plain CEM I cements are required for colour and consistency BS EN 13369:2013, Table A.2 may
take precedence.
NOTE 2 For the purposes of architectural concrete cladding, XS1 exposure conditions are defined as being less than
100 m horizontally from high tide.
NOTE 3 Where a precast concrete panel is faced with natural stone or similar materials having a thickness greater
than 25 mm, cover may be reduced by 10 mm, providing the cover from the rear of the facing material is not
less than 20 mm.
Where different mixes are to be used for facing and backing, the mix or mixes providing the structural
strength of the unit should satisfy the requirements of BS EN 1992‑1‑1:2004+A1.
NOTE Guidance on measures to minimize the risk of alkali‑silica reaction in concrete are given in Concrete
Society Report TR 30 [3].
For concrete corbels, the local bending moments produced by the eccentricity of the vertical
reactions being transmitted to individual corbels or brackets, may be assumed to be distributed
over an effective width equal to the width of the corbel, plus twice the effective structural thickness.
See Figure 1b).
For bolted‑on metal brackets, the effective width should be taken as twice the effective thickness plus
the width between centres of bolts attaching one bracket (if two or more bolts spaced horizontally
are used). See Figure 1d).
If units are top hung, the reinforcement should be designed to resist the tensile force from their own
weight as well as the bending moments from any eccentricity created by the bearing detail. Account
should be taken of the concentration of such tensile forces at points of attachment to the structure.
Account should be taken of the support conditions during lifting and handling, where they differ
from those applying when lifting into final position (e.g. lifting at discrete points; with final support
on a continuous nib). When designing elements for use during lifting, dynamic load factors should be
applied, appropriate to the type of crane.
NOTE 2 Orientation and re-orientation during handling and geomtetry of the panel are critical factors. These are
in addition to load factors relating to static design, as per the recommendations given in PD CEN/TR 15728.
The line of action of the load (V) should be the most severe case possible, such as the edge of the
corbel, the bottom of any chamfer, or the outer edge of discrete shims and permissible deviations.
The vertical reinforcement in the panel should take account of local increases in tension and bending
at supports.
If substitution of previously specified cast‑in anchors by post‑drilled fixings is considered, the edge
distance parameters should be carefully checked in accordance with the manufacturer’s product data.
Sharp arrises and thin projections should be avoided, particularly in large units as they are
easily chipped in handling. Chamfered or radiused arrises resist damage and mask irregularities
of alignment.
5.6.2 Thickness
The thickness of units should be designed to resist the loads to be imposed upon them. The minimum
thickness should have regard for:
a) stuctural design, temporary and permanent;
b) facing or finish;
c) environmental exposure;
d) concrete mix, reinforcement requirements, cover;
e) architectural geometry; and
f) joint details.
NOTE A panel may be a consistent thickness or be designed with stiffening ribs to its perimter and/or
its mid span.
The thickness of the unit and any stiffening ribs should be adequate to allow the provision of support
and restraint fixings as well as handling fixings (see Figure 4 and Figure 5). In particular, holes for
dowels, bolts and lifting points should be encompassed by reinforcement to resist rupture, and the
reinforcement should have its appropriate cover (see 5.3).
The position of the panel joint should be carefully considered in relation to its supporting/restraining
structure, where possible avoiding locations of the structure which are susceptible to greater levels of
movement (deflection).
NOTE Further guidance on dimensional deviations is given in BS 5606, BS 6093 and BS 6954. BS 6954‑2
approaches tolerances on a statistical basis.
If facing and backing materials are different, differential movement should be taken into account to
reduce bowing.
NOTE 2 Units might also be susceptible to bowing caused by moisture or thermal gradients. These might be cyclic.
Since a significant part of the total drying shrinkage takes place during the first month after casting,
concrete units should be left as long as possible before installation. It is inadvisable, however, to
install matured units to a structure still subject to substantial shrinkage and/or creep (see 5.9.5).
Cladding units should be stored so as to allow movement and prevent sagging.
The size, thickness and profile of the units should be taken into account when considering
dimensional changes.
NOTE 2 More detailed guidance can be found in Design for movement in buildings [4] and Estimation of thermal
and moisture movements and stress [5].
d) the temperature of the various components when the cladding was fixed.
NOTE 3 For buildings in the United Kingdom with modern standards of thermal insulation and air conditioning,
the temperatures tabulated in Table 4 can be used as a guide to the extremes likely to be experienced.
Condition Temperature
°C
Winter Summer
a) Ambient temperature (in the shade) -10 25
b) External Cladding – light colour ‑20 50
Cladding – dark colour ‑20 65
c) Free‑standing Concrete – light colour ‑20 45
structures or fully Concrete – dark colour ‑20 60
exposed structural Metal – light colour ‑25 50
members Metal – dark colour ‑25 65
d) Internal Normal use 10 30
Empty – out of use -5 35
on how much of the total load has been applied to the foundations at the time of cladding fixing and on the nature
of the soil, for example clays consolidate at a much slower rate than sands after each load increment.
5.9.4.1 General
The three aspects of elastic deformation that should be taken into account are column shortening,
beam deflection and wind sway.
NOTE The significance of column shortening depends on the height and type of construction and on the sequence
of construction.
For dry‑clad beams, greater movements can be anticipated and should be allowed for.
NOTE 2 For the purpose of calculating deflections, Young's modulus for steel can be taken as E = 2.1 × 105 N/
mm2 and for any concrete casing a value of E = 2.1 × 104 N/mm2 may be used, together with the moment of inertia of
the uncracked rectangular section.
The structural engineer should advise the anticipated movements of the structure that affect
the cladding.
NOTE 2 The designed deflection limit of precast cladding is usually L/500 as determined by BS EN 1992 for
beams and slabs.
NOTE 3 Elastic shortening of concrete columns is usually considered with creep (see 5.9.5).
Crack widths should be limited to 0.3 mm for stone, tile or brick faced elements and 0.15 mm for all
other concrete finishes, see 8.1 to 8.5.
Calculations for deflections of long slender beams and cantilevers in particular should take into
account the effects of shrinkage and creep as well as of elastic strain.
The structural engineer should advise the anticipated movements of the structure that would affect
the cladding.
Notwithstanding BS EN 1992‑1‑1, under normal climatic conditions in the United Kingdom the rate of
shrinkage (expressed as a percentage of its potential) should be assumed to be as shown in Table 5.
Table 5 — Rate of shrinkage of concrete (as a percentage of its potential)
Effective Period
thicknessA) of
element
14 days 28 days 3 months 1 year
100 15% 25% 35% 70%
150 10% 15% 30% 50%
300 <10% 10% 20% 40%
The effective thickness is the ratio of twice the volume of the concrete divided by the exposed surface
A)
area.
NOTE 2 In addition, concrete structures exposed to outdoor climate may exhibit seasonal, cyclic movement
of ± 0.4 times its long term drying shrinkage. However, cladding the concrete frame can significantly reduce its
seasonal movement.
NOTE 3 Creep in concrete causes continued deformation without additional load.The magnitude of the creep is
dependent on the stress in the concrete, the ambient relative humidity, and the age of the concrete when loaded.
It can be assumed that about 40%, 60% and 80% of the final creep develops during the first month, 6 months
and 30 months under load respectively, when concrete is exposed to conditions of constant relative humidity.
Further guidance is given in BS EN 1991‑1‑1.
NOTE 4 As an example, over the long term, a "normal" 12-storey concrete framed building might shorten by 5 mm
to 6 mm per storey due to elastic and creep and drying shrinkage. Depending on circumstances, shortenings per
storey might vary.
NOTE 5 For further guidance on shrinkage and creep refer to the Properties of concrete for use in Eurocode 2 [7].
5.10.3 Staining
To avoid the possibility of the face of the cladding becoming stained by the run-off of metals from
adjacent cladding finishes, non‑staining materials such as stainless steel should be used. The
provision of lightning conductors should be detailed to obviate staining or damage to the cladding.
NOTE Some natural stones can migrate organic material onto the finished face of the stone during weathering
and drying. This is normally only a temporary phenomenon, but can take several weathering cycles to naturally
diperse. BS 8221‑1 advises that the brown stain usually fades.
Certain timbers and timber based products contain water soluble compounds and some exterior
wood finishes are susceptible to chalking, which can cause discolouration. Advice on the selection of
timber species and finishes should be sought from the relevant trade associations.
Staining can occur with some sealants and reference should be made to the manufacturer stating the
nature of the unit and its facing. Care should also be taken with the application of sealant primers,
some of which can discolour with time if misplaced on the face of the building. Where adjacent
trades apply seals in contact with the precast, testing for compatibility should be carried out before
application.
Water should not be able to run-off limestone, reconstructed stone or concrete directly onto any
other porous material. The introduction of projecting window sills/copings aids this process.
Finishes can vary considerably in their weathering characteristics and designers should be aware of
the likely changes in colour and texture of either in the design of the cladding or in the selection of the
mix for the units. Similar consideration should be applied to the colour of the jointing materials.
In a face sealed cladding system the design should ensure that water/air is prevented from entering
the building (see 6.4). A condensation risk assessment should be carried out to ensure that no
condensation build-up occurs within the system. When the precast concrete panels are being used as
a rainscreen, it should be anticipated that rain penetration can occur within the cavity between the
cladding and the inner leaf or backing wall. Such cavities should therefore have adequate provision
for drainage and for damp‑proofing.
In either situation the interfaces with adjacent trades (e.g. windows, openings, and backing wall)
should be established early and design consideration given to the systems of others, to ensure
localized interfaces do not create a risk of condensation or water penetration.
To prevent trapping water within the unit (see 4.6), the use of applied weatherproof compounds
should not be used.
NOTE 2 Whilst uneven weathering of cladding is unavoidable, the pattern can be controlled by the design of
projections and true or false jointing details.
5.10.5.1 Metal
Sheet metal for flashings and weatherings should be selected after evaluation of the conditions of use,
exposure and chemical action due to contact with other materials. Aluminium, zinc and their alloys
should not make direct contact with other metals due to the risk of bi-metallic corrosion when in the
presence of moisture. Aluminium should not be incorporated within or attached to uncured concrete
NOTE See CWCT Technical Note 24 [8].
Aluminium should also be separated from cured damp concrete, with dense PVC, EPDM, Zinc
chromate coating, or bituminous paint.
Firestopping should meet the same performance requirements of the compartment floor/wall. Cavity
barriers should be composed of non‑combustible materials having at least 30 min fire integrity and
15 min fire insulation, with adequate provision for drainage and ventilation including cavities trays,
etc. They should both be adequately fixed and supported to ensure that they remain effective for the
life of the cladding, in accordance with the manufacturer’s recommendations. (See also 7.3.6.2.)
NOTE 2 Requirements concerning cavity barriers are contained in part B of the Building Regulations [9] and
BS 9991 for residential and BS 9999 for general building types.
d) fixings are designed and applied to the manufacturer’s recomended edge, spacing and depth
dimensions; and
e) any voids formed are correctly sealed after the attachment has been applied.
NOTE The early design consideration of attachments can allow the casting-in of anchors within the panel for
fixing of the attachement.
NOTE 2 Guidance on the acoustic properties of concrete panels are given in CIRIA Special Report 87 [10] and
Architectural precast concrete, section 5.5 [11].
As the acoustic assessment of components and their interconnection with other building elements is
an extremely complicated matter, the principal designer should seek specialist consultant advice for
acoustic building assessments.
Joints should be designed with appropriate widths and materials, if any, to suit the individual project,
in accordance with BS 6093.
The joints between adjacent buildings, structure or panels are either movement or compression
joints.They should both allow the relative movement between the structure and the cladding to take
place without overstressing the cladding units and fixings.
NOTE 2 The type of joint and any sealant is determined by the type, size, thickness of the cladding units and the
degree of movement anticipated.
Design of joints should be simple, to ease manufacture and installation. They should be capable of
accommodating the accumulated tolerances of both the frame and units, whilst maintaining the joint
width within their working range.
NOTE 3 The surface finish (concrete, brick or stone) and panel design at the interface with the joint can also effect
the detail of the seal, both visually and technically.
If a movement joint (commonly known as an expansion joint) has been designed within the
claddings supporting structure, it is essential the expected movement through the building structure
should also be taken through the cladding. The amount of movement to be accommodated at these
points can be appreciable and should be advised by the structural engineer at the earliest possible
design stage.
The installed width of the joint should always be greater than the calculated movement. If the joint
is to be sealed the width of the joint should be such that the strain capacity of the sealant is not
exceeded (see also 6.4).
A vertical movement joint should be provided between all adjacent panels. Particular attention
should be given to the movement expected at parpets and copings, especially in a stacked panel
system, as the accumulation of thermal movement in the panels is applied at this relatively
vunerable interface.
Movement joints may be designed as a sealed or open joint. In either case it is essential that any
temporary works item and construction debris should be removed from the joint.
NOTE 2 The vertical load can be transferred, between panels, using permanent steel shims or temporary shims and
a wet applied mortar bed.
Compression joints may be designed as a sealed or open joint. If sealed the position of the shims or
mortar bed should be designed to sit behind the seal. If open jointed the visual impact of the shims
should be considered.
6.4.2 Sealant
NOTE 1 General guidance on the choice of sealants is given in BS 6213, BS EN ISO 11600:2003+A1 and CWCT
Technical Note No. 19 [12] and CIRIA Report R178, Sealant joints in external facades [13]. However as the subject is
complex it is advisable to seek guidance from the sealant manufacturer at an early stage. See also 4.4.2.
Some sealants can cause staining with particular substrates and specifiers should refer to the sealant
manufacturer to confirm the suitability of the selected sealant. On precast concrete cladding staining
is unlikely to occur if a sealant is used in conjunction with a recommended primer. Porous facings
(stone, terracotta, etc.) are more prone to staining and should be tested for compatibilty before use.
Sealants should be applied in a joint with a minimum joint depth of 10 mm (see Figure 2a).
NOTE 2 It is not considered practical to apply sealants satisfactorily in joints less than 6 mm wide.
A primer should be applied on concrete surfaces for adhesion and to guard against the possibility
of staining.
NOTE 3 Where double seals are provided the joint width would typically be no less than 12 mm, excluding
tolerances (see Figure 2b).
The cavity between the double seal should be drained at the lowest point along any one continuous
length of seal, unless sealing between isolated elements with no risk of water tracking into
other elements.
NOTE 4 It might be possible to post apply a sand/grit to the face of the uncured sealant, to acheive a textured
finish. However, the consistency and longevity of the sand/grit cannot be assured. Consult the sealant manufacturer
for advice before application.
In vertical joints, a baffle (see 4.4.5) to limit the amount of water reaching the barrier should be
placed not less than 50 mm back from the external face in grooves cast in the edges of the units
(see Figure 3). A strip of the material is the most useful form of baffle, but other shapes might also
be satisfactory. Baffles are expected to have a long life but wherever possible the joints should be
designed so that the baffle can be replaced. They should also be designed to resist vandalism.
c) Double seal with porous facing material d) Double face seal with a third rear seal
e) Joggle joint
NOTE Joint width sizes designed in accordance with BS 6093.
The joint width should be calculated in accordance with BS 6093 and should not normally be
less than 12 mm.
The edges of the units should have a vertical groove to receive the baffle and may have other grooves
to assist in preventing rain, driven across the face of the unit and into the joint, passing to the back
of the joint. The baffle should be brought to the face of the unit at intervals wherever the joint is
continuous down the face of the building, so as to shed the water. Alternatively, a flashing should
be incorporated. Figure 3 shows a method of making the junction of a vertical drained joint with a
horizontal joint.
Horizontal joints should be designed so that their faces are protected by the lower edge of the unit
(above the joint) projecting downwards for a distance of at least 50 mm measured from the top of the
upstand of the unit below the joint. A flashing should be provided at the intersection of vertical and
horizontal joints, and it should provide continuity of the 50 mm overlap at the junction.
structure of the building and the manufacture of the precast units. In most cases, for averaged‑sized
panels, four restraint fixings should be provided, notionally at the corners of the unit.
Figure 4 — Typical restraint fixing to concrete structure — vertical section
7.3.4.1 General
The design of fixings should be in accordance with BS EN 1993‑1‑1 and BS EN 1993‑1‑1‑4.
Technical literature or design software from the anchor manufacturer should be checked to
determine if the performance quoted is for “cracked” or “non‑cracked” concrete and anchors are
selected accordingly.
NOTE 2 Guidance is given in [N1] EAD 330232-00-0601 on the testing of fixings in cracked and
non‑cracked concrete.
For significant duration of exposure to fire, e.g. in excess of 30 min, special measures should be taken.
These include:
a) specifying anchors with performance certification appropriate to the conditions of use;
b) increasing the embedment depth of resin bonded anchors to match the durability of the bond to
that of the nut/stud connection at the surface; and
c) applying a fire protection coating to the fixing and surrounding area, to a minimum radius
equivalent to twice the embedment depth.
NOTE 2 Advice on the selection and application of sprayed mineral coatings is given in BS 8202‑1.
NOTE 3 The yield strength and Youngs modulus of most steels are reduced to about half at 500 °C.
When the cladding panels, and its fixings are required to offer fire protection, the panel to panel joints
should also be assessed.
NOTE 4 Guidance on the performance of fixings in fire is given in CFA Guidance Note, Fixings and fire [15].
7.3.7 Anchors
Clear guidance should be given to installers, on action to be taken if reinforcement is struck during
drilling. This should include consultation with the responsible engineer, especially where cutting
through bars or repositioning anchors is required. To facilitate the latter, additional or elongated
holes might be required in bracketry. Repositioned anchors should be set at a distance from the
aborted hole equivalent to the depth of the aborted hole.
NOTE 2 As this usually equates to cover-on reinforcement, this dimension can be used when designing additional or
elongated holes in brackets.
Aborted holes should be filled with an approved product of equal or greater strength than the
parent material.
NOTE 2 Exposed concrete finishes produced with special cements might have an initial colour that varies from
their final appearance following atmospheric exposure.
Where stainless steel dowel pins are used for attaching stone (see Figure 6) the following
should apply.
a) Fixings should be made with dowels not less than 4.7 mm in diameter inclined at
approximately 45° to 60° to the back of the stone.
b) Approximately 50% of the dowels should be reversed in direction.
c) Each dowel should be fitted with a flexible grommet, with a wall thickness not less than 3 mm, to
allow some differential movement.
d) The depth of penetration of the dowel into the stone should be two‑thirds of the thickness of the
stone and the depth of the hole should be no greater than the dowel +2 mm
e) The embedded length of the dowel into the concrete should not be less than 60 mm.
f) Small, narrow or irregular shaped stones should be supported by a sufficient number of fixings.
g) The bottom edges of stones which do not have a physical support or another stone immediately
below them should have fixings not more than 100 mm above the edge and also within the
reinforcement cage.
h) A bond breaker should be provided between the rear of the stone and the backing concrete, to
allow for differential movement.
i) The number of pins required should be demonstrated by a test applying a factor of safety of 8.
In the absence of test data and where stones are provided in accordance with BS 8298‑3:2010,
Annex A, not less than 11 stainless steel pins per m2 should be provided.
Some handmade bricks are likely to prove unsuitable due to their inconsistent shape, dimensions or
properties, and early reference should be made to the cladding manufacturer to discuss feasibility.
Testing of brick, tile and slip faced units for durability and adhesion should be in accordance with
[N2] ETAG 0034 (heat-rain conditioning) and DD CEN/TS 772‑22 (freeze/thaw conditioning). After
both conditions have been applied there should be no detectable deterioration (spalling or cracking).
Following durability conditioning a sample of bricks should be tested for bond within the concrete
panel as described in BS EN 1015‑12. Bond strength tests on individual bricks should achieve no less
than the designed wind pressure on cladding ×10 (factor of safety) in tension at the point of contact
between brick and concrete.
NOTE 2 See BS EN 1991‑1‑4 for wind pressures to be applied.
Inspection of panels in the manufacturer’s factory should take into account that the products will be
in varying states of maturity, reactive to moisture conditions and curing.
NOTE 2 The aspects affecting final maturity of finishes are:
a) the nature of the materials used and the movement of free minerals embedded within
the material;
b) storage and weathering in the factory; and
c) environmental conditions.
Inspection should be in accordance with PD CEN/TR 15739:2008 which defines:
a) 5 m distance for areas intended to be seen at close range (entrance hall, corridor); and
b) 10 m distance for faces intended to be seen from any other range.
9 Manufacture
9.1 General
Competent persons should be employed to supervise all stages in the production and erection of the
units. All tests on materials, the making and testing of cubes and the maintenance and calibration of
all mixing and measuring plant should be carried out under direct supervision, operating within a
recognized quality plan including a project specific manufacturing inspection and test plan.
Where oversize units are not acceptable, the designer should make the necessary allowances in the
dimensioning of the unit as given in Table 6.
Table 6 — Permissible deviations in the manufacture of cladding units
Table 6 (continued)
Target size Permissible deviation
g) Anchors and inserts 1) Isolated insert or group of ± 6 mm
inserts
2) Individual insert relative to ± 3 mm
others within a group
3) Non‑structural cast‑in items Twice the above
9.5 Marking
Each unit should be clearly marked with an identification symbol either on the unit or by securely
fixing a durable label to it, identifying the:
a) project name/reference;
b) panels' unique reference; and
c) panels' weight.
10.4 Storage
Storage of units should be arranged so that delivery in accurate sequence for site fixing is possible.
During storage of units, protection should be provided against staining, particularly from corroding
metal, and accidental damage.
transportation or fixing. All temporary supports should be adequately fixed. Wedges, shims, spacers
and other items not designed for permanent inclusion should be removed and the voids made good.
NOTE Further information is given in BS 8000‑0 and BS 8000‑2.2.
Safe and adequate access should be provided to and about the working area. Lifting areas should be
selected so that the units clear all permanent and temporary structures during the lift.
NOTE 2 Attention is drawn to the Factories Act 1961 [18] and regulations governing the use of lifting and transport
equipment when testing erection gear.
Temporary supports, jointing and bedding materials, fixings and tools should be pre‑positioned at
the fixing point. All temporary wedges, shims, spacers and other items not designed for permanent
inclusion in the structure should be removed and the spaces made good as necessary.
11.3.4 Fixings
NOTE Guidance on site based methods of test are given in CFA Guidance note, Procedure for site testing
construction fixings [19], which includes advice on test equipment, number of tests, assessment of results etc.
NOTE Recommendations on the use of laboratory based test procedures are given in BS 5080‑1.
Small units such as copings which are manhandled may be stood on pads of mortar and tapped
home. Cladding units requiring mechanical handling/lifting should have support, using one of the
following methods.
a) Permanent support points, using packing shims, without the use of cementitious materials.
These are positioned under the unit support corbels or at two defined locations under
a continuous nib.The size, positions and material of such shims should be specified on
the drawings.
b) Where continuous support bedding of cementitious materials is a design requirement, units
should be initially supported at specified points by levelling shims or screws that should
subsequently be relaxed, or by plastic shims which shed the load gradually onto the bed joint
filler. Upon completion of alignment, the bed joint should be either grouted or dry‑packed with
cementitious materials. The composition of such grouts/dry packing should be specified.
Grid reference lines for actual joint centres should be established for a row along an entire elevation
of panels. During initial erection of panels, these should be accurately centred within the grid spacing
to be occupied. Panels should be positively fixed to the structure to an accurate line and level.
NOTE 2 Special circumstances could arise when it might be necessary to leave the first row of panels not finally
bedded. This is to allow for subsequent fine adjustments to align vertical and horizontal offsets in joint widths,
minimization of offsets arising from bow or twist, etc., after subsequent rows of panels are erected above.
The finished work should have a satisfactory appearance, being square, regular, true to line, level and
plane, with a satisfactory fit at all junctions.
The following permissible deviations from theoretical centre lines should be used to achieve the
satisfactory appearance of the cladding, uniformity of width and consistency of joints between panels:
a) The average width of an individual joint between panels compared with nominal design width of
joint should not vary by more than ± 6 mm, otherwise unacceptable variations to straightness of
line of joint vertically, floor‑to‑floor, and horizontally might occur.
b) Panel edges at a joint out of parallel should not taper by more than 5 mm in overall height of
joint between panels.
c) Difference in alignment of a panel edge, from one panel to another, should not exceed 6 mm.
The offset in planes formed between vertical faces of one panel to another should not exceed 6 mm.
Bowed panels (within allowable manufacturing tolerances) should be arranged so that offset
between adjacent panels caused by bowing does not exceed offset tolerances.
The widths of joints should be such as to ensure that joints perform as intended and conform with the
recommendations of the joint sealant or gasket manufacturer.
Particular attention should be given to permanently exposed surfaces, especially arrises and
decorative features. The protection may be by timber strips, hessian or polyethylene, but should not
be such as would damage, mark or otherwise disfigure the units.
NOTE 2 Where external tubular scaffolding and boards are provided, on certain designs of cladding (e.g. columns
and spandrel panels) it is feasible to arrange to provide thick plastic sheeting as protection against rain, snow and
frost. It might be necessary to avoid contact with the units. Such protection might have to be removed when other
trades commence their work.
Timber battens protecting cills or other arrises should remain in position as long as possible.
Suitable packing should be used to ensure that scaffolding does not damage erected units.
NOTE 3 Particular problems of protection also exist where no external tubular scaffolding is to be used for erection
of storey height panels, and special arrangements might be necessary to provide access for protection work.
Unless precautions are taken for winter working, the use of mortar should be avoided when there is a
risk of frost damage. Joints should be protected against frost.
A 5% hydrochloric acid solution is usually satisfactory but trials should be carried out to determine
the minimum acid strength capable of removing the deposits or stains. Protection of adjacent
materials should be reviewed prior to washing off lime bloom.
11.9.3.1 General
Any large cementitious deposits should be removed, as far as possible, with wooden or plastic
implements to avoid damage to the surface.
All pointing should be checked and throats and weep holes should be thoroughly cleaned.
The cementitious surface of as‑cast or steel‑floated finishes is likely to be visibly affected by acids.
Cleaning methods should be chosen to minimize the effect on existing surfaces and fine textures.
Concrete and rendered finishes contain cement and aggregate. Since aggregates can vary from hard
to soft and from acid‑soluble to acid‑insoluble, and cements are acid‑soluble, the proposed cleaning
method should be trialled on a discrete area.
11.9.3.5 Brickwork
The cleaning of newly erected brickwork and terracotta faced cladding units should be in accordance
with BS 8221‑1.
movement as elements on the building. The structure supporting the test prototype should be similar
in stiffness and reaction to that supporting the cladding.
The scope of the test prototype(s) should be representative of typical design scenerious applicable to
the final construction and should be defined within the performance specification.
Joint details and connections should replicate those used in the construction. The prototype(s) should
also incorporate windows, curtain walling or other interfaces, where appropriate.
Unless interfaces can be designed out they should be tested. Concentrating efforts on construction
interfaces during design development and testing phases should reduce the number of problems
experienced during the construction phase.
When installing and dismantling the test prototype, any variations to the agreed details should be
recorded on a set of the test prototype assembly drawings. The extent of water penetration into the
system should also be recorded.
12.2.4.1 General
There should be two wind resistance tests:
a) a serviceability test; and
b) a safety test.
c) long‑term drying;
e) corrosion of reinforcement;
NOTE 2 Further guidance on the identification and treatment of cracking can be found in Concrete Society
Technical Reports TR22 Non‑structural cracks in concrete [22]; and TR44 The relevance of cracking in concrete to
corrosion in reinforcement [23].
If appropriate cracks should receive immediate remedial treatment to preserve long term durability.
13.3 Joints
The width of joints should be checked visually to ensure that they have not exceeded the design and
installtion tolerances or deviated from the alternative dimensions contained within the operations
and mantenance manual. Particular attention should be paid to compression joints and designated
movement joints.
Baffles in open drained joints should be checked for damage, slippage and alignment.
Annex A (normative)
Checklists for the exchange of information
A.1 General
Before tenders are invited, a cladding specialist should be consulted to discuss practical time scales,
availability and suitability of materials, etc.
NOTE 1 This is especially important for contracts with large quantities of special aggregates or finishes.
NOTE 2 The information required in order to schedule the work satisfactorily, obtain an accurate price, minimize
supply and production difficulties, and ensure safe and practical site fixing, is given in A.2, A.3 and A.4.
Bibliography
Standards publications
For dated references, only the edition cited applies. For undated references, the latest edition of the
referenced document (including any amendments) applies.
BS 4255‑1, Rubber used in preformed gaskets for weather exclusion from buildings –
Part 1: Specification for non‑cellular gaskets
BS 5080‑1, Structural fixings in concrete and masonry – Part 1: Method of test for tensile loading
BS 6954‑2, Tolerances for building – Part 2: Recommendations for statistical basis for predicting fit
between components having a normal distribution of sizes
BS 8000‑0, Workmanship on construction sites — Introduction and general principles
BS 8000‑2.2, Workmanship on building sites – Part 2: Code of practice for concrete work –
Section 2.2: Sitework with in situ and precast concrete
BS 8202‑1, Coatings for fire protection of building elements – Part 1: Code of practice for the selection
and installation of sprayed mineral coatings
BS 9991, Fire safety in the design, management and use of residential buildings — Code of practice
BS 9999, Fire safety in the design, management and use of buildings — Code of practice
BS EN 998‑2, Specification for mortar for masonry — Masonry mortar
BS EN 1992‑4, Eurocode 2 — Design of concrete structures — Design of fastenings for use in concrete
BS EN 10027, Designation systems for steel – Part 2: Steel numbers
BS EN 13116, Curtain walling — Resistance to wind load — Performance requirements
BS EN 13369:2013, Common rules for precast concrete products
BS EN 13670, Execution of concrete structures
BS EN 14992, Precast concrete products — Wall elements
PD CEN/TR 15728, Design and use of inserts for lifting and handling of precast concrete elements
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