STANDARDS

Why the BS 8500 durability

tables are the way they are
The British Standard for concrete durability recommendations are set out in a format partly standardised at
European level by EN 206(1) but extended to include cover where corrosion of reinforcement is considered. The
recommendations were developed from a combination of UK experience and some consideration of durability
modelling. As set out in BS 8500-1(2) Tables A.4 and A.5, they are for intended working lives of up to 50 and up to
100 years respectively. Chris A Clear – of the British Ready-Mixed Concrete Association and chair of BSI Technical
Committees: B/517 Concrete, B/517/1 Concrete production and testing and B/517/1/20 Concrete – Specification
and Production – sets out some of the background, history and incorporated relationships to these tables.

T
he BS 8500-1 guidance is deemed CEM III/A concretes. It is important to align the BS 8500-1 recommendations with
to be sufficient for the majority note that the current BS 6349-1-4 and BS those of BS 6439-1-4, as trying to define the
of civil engineering and building 8500-1 requirements are the same in terms required quality of concrete for a particular
works but for particular applications of maximum w/c ratio and nominal cover, exposure is difficult where there are different
reference to alternate Standards or specialist and almost the same in terms of minimum interpretations of the exposure classes
literature may be considered desirable. cement content and strength class. It is also themselves.
For agricultural buildings BS 5502-21(3) interesting to note the small increase in cover For BS 8500, XC3 moderate humidity
and BS 5502-22(4) should be referred to and for CEM III/A concrete from 50 to 60mm, and XC4 cyclic wet and dry are amalgamated
for maritime works then BS 6349-1-4(5) from 1972 to 2015. For CEM I concrete, into class XC3/XC4, as it is regarded as
may be considered most appropriate. As the increase is significantly greater – from impractical to distinguish between them for
the maritime Standard contains durability 50 to 90mm – with the large recent increase design. There is some logic to considering
recommendations for seawater exposure to nominal cover from 60 to 90mm with carbonation and chloride ingress mechanisms
then engineers have questioned why these publication of the most recent editions of BS separately but in real terms a reinforced
are not the same as those contained within 6349-1-4 and BS 8500-1. It can be argued concrete element exposed to corrosion
BS 8500-1. In terms of water:cement (w/c) that what is currently understood by XS3 is induced by chlorides XD1, XS1, XD3 and
ratio, the recommendations are aligned not the same as the terms severe, very severe XD4 will always be subject to moderate
but the BSI committee responsible for BS and extreme, as used by previous Standards, humidity or cyclic wet and dry and so must
8500-1 decided to retain a standard set of and for this reason the current exposure also be exposed to XC3/4 carbonation as well.
relationships between maximum w/c ratio, classes are reviewed in the next section. The BS 8500-1 recommendations take this
minimum cement or combination content, into account within its Tables A.4 and A.5.
and strength class, and this meant that Exposure classes For exposure class XC3/4 in combination
complete alignment with BS 6349-1-4 was The current exposure classes were first with XD3, XS1 or XS3 at any cover, then
not possible. aligned with Eurocodes with the first a minimum strength class of C25/30 is set,
publication of BS 8500-1 in 2002. This with a minimum cement or combination
Brief history introduced the EN 206 exposure classes content of 320kg/m3 at a maximum w/c ratio
Standards committees generally adopt with environments classified by their of 0.55 irrespective of cement type.
the policy of only modifying guidance deterioration mechanism with respect
where there is both a need and technical to concrete or reinforced concrete. Table Highest minimum cement content
justification to do so. For this reason the 2 (page 51) is a summary for corrosion The maximum strength of the aggregate
history of the durability recommendations induced by carbonation classes XC3, XC4 largely controls the maximum strength that
inevitably influenced how they are today. As as well as corrosion induced by chlorides can be achieved by the concrete from which
a historical comparison, the various British from seawater for XS1, XS2 and XS3. The it is made. Where the cement or combination
Standard recommendations for the durability table includes abstracts from informative contains proportions of additions, such as
of reinforced concrete exposed to what is examples applicable in the UK from BS fly ash in excess of around 20% or GGBS
now called XS3 for a design working life of 8500-1 and descriptions from the maritime in excess of around 50%, then this also
not less than 50 years, are summarised here Standard BS 6349-1-4. This comparison is influences the 28-day ceiling strength. This
in Table 1 (page 50), for both CEM I and indicative of the difficulty in trying to fully may be because it may take two months or

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 STANDARDS

Table 1 – Summary of reinforced concreteA) durability requirements for exposure class XS3B) for an intended working life of not
less than 50 years
Code of Practice, Concrete made with: Concrete made with:
British Standard Portland cement, CEM I Blast-furnace cement, CEM III/AC)
mccD), mwcE) fck,cube cnomF) mccD), mwcE) fck,cube cnomF)
kg/m3 MPa mm kg/m3 MPa mm
CP 110-1: 1972 330 0.45 50 50 330 0.45 50 50
BS 6349-1: 1984 400 0.42 40 50 400 0.42 40 50
BS 8110-1: 1985 400 0.45 50 50 400 0.45 50 50
BS 8110-1: 1997 400 0.45 50 50 400 0.45 50 50
BS 6349-1: 2000 400 0.40 50 60 360 0.50 37 50
BS 8500-1: 2002 360 0.40 50 60 360 0.40 45 50
BS 8500-1: 2006 380 0.40 50 60 380 0.40 45 50
BS 6349-1-4: 2013 360 0.35 50 90 380 0.35 50 60
BS 8500-1: 2015 380 0.35 55 90 380 0.35 50 60
A) Maximum aggregate size 20mm
B) Assuming XS3 is equivalent to exposure classes previously described as severe, very severe or extreme
C) Assuming a nominal GGBS content of 40%, representative of UK Portland blast-furnace cements 1925–1990
D) Minimum cement or combination content
E) Maximum free water:cement ratio
F) Nominal cover assuming required minimum cover plus an allowance of 10mm.

more for these cements to react as much around the UK where local natural aggregates around 30% fly ash or up to around 50%
under standard curing as a Portland cement for concrete are used. GGBS, then the rate of carbonation just
(CEM I) does in 28 days. Figure 1 shows On this basis, a relationship is incorporated depends on strength class.
a set of idealised relationships between into BS 8500-1 within a combination of the
28-day strength and cement or combination carbonation tables and a table linking the Chloride ingress
content. Without testing, it is not possible maximum w/c ratio to a minimum cement Resistance to chloride ingress is mainly
to assess what the ceiling strength will be, content, for a range of maximum aggregate dependent upon the cement type and the
but it is evident that the ceiling strength sizes. For designated GEN and RC concretes, w/c ratio, with aggregate quality being a
is approached where the cement or and XC exposures classes to BS 8500-1 Tables secondary factor. The 2002 version of BS8
combination content is around 380kg/m3. A.4 and A.5, the incorporated relationships 500-1 introduced the concept that where
The highest minimum cement content in are summarised in Table 3 (page 52). higher proportions of fly ash or GGBS are
all the BS 8500-1 durability tables is set at The ‘associated strength classes’ within incorporated as part of the cement in concrete
380kg/m3. Table 3 are largely indicative and the then a lower strength class is to be expected.
intention is to simply indicate a strength class In the realisation that reinforced concrete
Compressive strength class that a designer could be reasonably confident exposed to chloride ingress is particularly
For concrete design, the designer selects a of achieving at the maximum w/c ratio and vulnerable to corrosion, there is an increase
compressive strength class and for the most minimum cement content. Where designated in the standard minimum cement contents
economic design it is ideal if the strength concretes are specified or where a strength at each w/c from 0.35 to 0.60 by 20kg/m3
class is just achieved at the maximum w/c and class is specified then the strength class is relative to the recommendation for other
minimum cement content specified. a requirement and the producer may need exposure classes. A maximum w/c ratio for
Due to the inherent differences in to use a lower w/c ratio and higher cement concrete exposed to chloride ingress was set
performance of the range of cements, content than that indicated to achieve the at 0.60, but the 0.60 maximum only applied
additions and use of chemical admixtures required strength class. to the XD1 exposure class where carbonation
across the UK, it will never be possible to For XC exposure classes, the BS 8500-1 is considered the dominate deterioration
define these relationships. durability requirements are based on strength mechanism. These relationships that set the
Notwithstanding this, there is some class, where the higher the strength the pattern for the current BS 8500-1 Tables A.4
confidence that from a maximum w/c ratio higher the resistance to carbonation. This and A.5 are summarised in Table 4 (page 52).
and minimum cement content it is possible comes from the established BRE view that The strength classes for XS and XD
to identify an indicative strength class that for CEM I concretes, or concretes containing exposures were developed on an indicative
should be achievable in most locations combinations of CEM I with either up to basis although BS 8500-1 does not describe

50 concrete www.concrete.org.uk JǬǥǜ ǻǹǺǿ

 STANDARDS

Table 2 – EN 206 exposure classes XC3, XC4 and XS

EN 206 class designation EN 206 description of BS 8500-1: 2015 Table A.1 – abstracts from BS 6349-1-4: 2013 Table 1 and 2
the environment informative examples applicable in the UK exposure descriptions
XC3 Moderate humidity External reinforced and prestressed concrete –
surfaces sheltered from, or exposed to, direct
Corrosion induced Cyclic wet and dry –
rain…
by carbonation XC4 …Reinforced or prestressed concrete surfaces
protected by waterproofing
Exposed to airborne External reinforced and prestressed concrete Airborne salt environment – exposed
XS1 salt but not in direct surfaces in coastal areas to airborne salt but not in contact with
contact with seawater seawater or splash
Permanently Reinforced and prestressed concrete surfaces Submerged environment – permanently
Corrosion induced XS2 submerged completely submerged or remaining saturated, submerged
by chlorides from eg, concrete below mid-tide level
seawater
XS2/XS3 – – Frequently wetted lower tidal, backfilled
Tidal, splash and spray Reinforced and prestressed concrete surfaces in Infrequently wetted upper tidal, splash/
XS3 zones the upper tidal zones and the splash and spray spray, ‘dry’ internal faces of submerged
zones, including exposed soffits above seawater structures

Figure 1: An idealised relationship

pattern between 28-day strength
and cement or combination
content.

recognised within the BS 8500-1 Tables A.4

and A.5 where for air-entrained concrete
exposed to freezing and thawing and ingress
of chlorides the maximum strength class may
be reduced to C28/35.

Future developments
Knights(6) provides some background to
the revision of the BS 6349-1-4 durability
tables, where modelling was carried out
based on that outlined by Bamforth(7). In
terms of the requirements for w/c ratio, BS
8500-1 has incorporated the BS 6439-1-4
recommendations for resisting corrosion
them as such, except for unreinforced exposure is recommended for precast pre- induced by chlorides from seawater and
concrete in seawater. tensioned concrete. there is a wish that the requirements will
be further aligned. It is accepted that for
Lowest w/c values Freeze/thaw specialist maritime structures then there will
Due to the most recent advances in Each 1% of entrained air is known to reduce be scope for more detailed guidance than that
admixture technology it is possible to produce 28-day strength by around 5% but strength provided in the general concrete Standard.
and supply concrete at a maximum w/c ratio is not as significant as entrained air for It is also accepted that durability
of 0.35, but as BS 8500 points out, it will determining the durability against freeze/ modelling, particularly with validation by
not always be possible throughout the UK. thaw conditions. For concrete to resist field testing and with development and
Precast pre-tensioned concrete units made freezing and thawing, a minimum of 4% adoption of appropriate test methods, should
to a strength class of C40/50 or higher with entrained air is recommended. So for air- be the basis for the most sustainable concrete
cements containing less than 25% fly ash entrained concrete it is not generally practical durability design. Until modelling or other
or 46% GGBS have been found to perform to compensate for the potential 20% or performance-based methods are developed
well. On this basis, for an intended working more strength reduction by using additional for use in general construction, it is likely that
life of at least 100 years, a minimum cover of cement to reduce the w/c ratio where the the deemed to satisfy limit value approach as
35mm for XS1 exposure and 60mm for XS3 strength class is C28/35 or higher. This is set out in the current BS 8500-1 will continue

www.concrete.org.uk JǬǥǜ ǻǹǺǿ concrete 51

 STANDARDS

Table 3 – RelationshipsA) between w/c ratio, aggregate size and associated strength classes for designated GEN and RC concretes, and XC exposure
classes to BS 8500-1
w/c ratio Minimum cement content for maximum aggregate size of: Designated concretes Associated strength classes
40mm 20mm 14mm 10mm
— — 120 — — GEN0 C6/8
— — 180 — — GEN1 C8/10
— — 200 — — GEN2 C12/15
— — 220 — — GEN3 C16/20
0.70 240 240 260 280 RC20/25 C20/25
0.65 240 260 280 300 RC25/30 C25/30
0.60 360 280 300 340 RC28/35 C28/35
0.55 280 300 320 340 RC30/37, RC32/40 C30/37, C32/40
0.50 300 320 340 360 RC35/45 C35/45
0.45 320 340 360 360 RC40/50 C40/50
0.40 360 380 380 380 — —
0.35 380 380 380 380 — —
A) Relationships where there is no exposure for corrosion induced by chlorides (XS or XD exposure classes). This includes the relationships incorporated within
BS 8500-1 Tables A.4 and A.5 corrosion induced by carbonation (XC exposure classes), BS 8500-1 Tables A.7 and A.15

Table 4 – BS 8500-1 Tables A.4 and A.5 relationships between w/c ratio, minimum cement content and 28-day strength for XD2, XD3, XS1, XS2 and
XS3 exposure
w/c ratio Min. cement Cement type
content CEM I,IIA, IIB-S, CEM I-SR0, IIB-V, IIIA IIB-V ≥25% fly ash, IIIA IVB-V, IIIB
CEM I-SR3 ≥46% GGBS

0.55 320 C28/35 C25/30 C25/30 C20/25

0.50 340 C32/40 C28/35 C28/35 C25/30
0.45 360 C35/45 C32/40 C32/40 C28/35
0.40 380 C40/50 C35/45 C35/45 C32/40
0.35 380 C45/55 C40/50 C40/50 C35/45
A) Strength values included BS 8500-1: 2015+A1: 2016 Tables A.4 and A.5

to have a part to play for the design of durable Method of specifying and guidance for the specifier. BSI, 5. BRITISH STANDARDS INSTITUTION, BS 6349-1-4.
reinforced concrete. ■ London, 2015+A1:2016. Maritime works. Part 1-4 – General. Code of practice for
3. BRITISH STANDARDS INSTITUTION, BS 5502-21. materials. BSI, London, 2013.
References Buildings and structures for agriculture. Part 21 – Code 6. KNIGHTS, J. Revised durability tables for reinforced
1. BRITISH STANDARDS INSTITUTION, BS EN 206. of practice for selection and use of construction concrete in the maritime environment. Concrete, Vol. 47,
Concrete – Specification, performance, production and materials. BSI, London, 1990. No.10, December/January 2013/2014, pp.40–42.
conformity. BSI, London, 2013, incorporating 4. BRITISH STANDARDS INSTITUTION, BS 5502-22. 7. BAMFORTH, P.B. Enhancing reinforced concrete durability.
corrigendum May 2014. Buildings and structures for agriculture. Part 22 – Code Guidance on selecting measures for minimizing the risk of
2. BRITISH STANDARDS INSTITUTION, BS 8500. Concrete. of practice for design, construction and loading. BSI, corrosion of reinforcement in concrete. Technical Report
Complementary British Standard to BS EN 206. Part 1 – London, 2003+A1:2013. 61, The Concrete Society, Camberley, 2004.

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