CEN/TR 17086:2020
(Main)Further guidance on the application of EN 13791:2019 and background to the provisions
Further guidance on the application of EN 13791:2019 and background to the provisions
This document explains the reasoning behind the requirements and procedures given in EN 13791 [1] and why some concepts and procedures given in EN 13791:2007 [2] were not adopted in the 2019 revision. The annex comprises worked examples of the procedures given in EN 13791:2019.
Weiterführende Anleitung zur Anwendung von EN 13791:2019 und Hintergründe zu den Regelungen
In diesem Dokument werden die Gründe für die in EN 13791 [1] enthaltenen Anforderungen und Verfahrensweisen dargelegt, und es wird erläutert, warum einige der in EN 13791:2007 [2] enthaltenen Konzepte und Verfahrensweisen nicht in die überarbeitete Fassung von 2019 übernommen wurden. Der Anhang enthält ausgearbeitete Beispiele für die in EN 13791:2019 enthaltenen Verfahrensweisen.
Guide pour l’application de la norme EN 13791:2019 et contexte des spécifications
Nadaljnja navodila za uporabo EN 13791:2019 in ozadje določil
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
01-december-2020
Nadaljnja navodila za uporabo EN 13791:2019 in ozadje določil
Further guidance on the application of EN 13791:2019 and background to the provisions
Weiterführende Anleitung zur Anwendung der EN 13791:2019 und Hintergrund zu den
Regelungen
Guide pour l’application de la norme EN 13791:2019 et contexte des spécifications
Ta slovenski standard je istoveten z: CEN/TR 17086:2020
ICS:
91.080.40 Betonske konstrukcije Concrete structures
91.100.30 Beton in betonski izdelki Concrete and concrete
products
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
CEN/TR 17086
TECHNICAL REPORT
RAPPORT TECHNIQUE
October 2020
TECHNISCHER BERICHT
ICS 91.080.40
English Version
Further guidance on the application of EN 13791:2019 and
background to the provisions
Guide pour l'application de la norme EN 13791:2019 et Weiterführende Anleitung zur Anwendung der EN
contexte des spécifications 13791:2019 und Hintergrund zu den Regelungen
This Technical Report was approved by CEN on 4 October 2020. It has been drawn up by the Technical Committee CEN/TC 104.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2020 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 17086:2020 E
worldwide for CEN national Members.
Contents Page
European foreword . 4
Introduction . 5
1 Scope . 6
2 Symbols and abbreviated terms . 6
3 General principles adopted for the revision . 7
4 In situ compressive strength and other concrete properties assumed in the EN 1992-
1-1 design process . 8
4.1 General . 8
4.2 Concrete compressive strength based on test specimens . 9
4.3 Concrete compressive strength based on the strength of cores from the structure . 11
5 Differences between test specimens and concrete in the structure . 11
5.1 Introduction . 11
5.2 Reference test specimen . 12
5.3 Effects of the moisture condition on in situ specimens . 13
5.4 Effect of maturity on concrete strength . 14
5.5 Effects of curing. 14
5.6 Effects of vibration . 15
5.7 Effects of excess entrapped air . 15
6 Testing variables that influence core strength . 15
6.1 Introduction . 15
6.2 Direction relative to the casting . 15
6.3 Imperfections . 15
6.4 Diameter of core . 16
6.5 Length/diameter ratio . 16
6.6 Flatness of end surfaces . 16
6.7 Capping of end surfaces . 16
6.8 Effect of drilling . 16
6.9 Reinforcement . 16
7 Scope in EN 13791:2019, Clause 1 . 17
8 Terms and definitions, symbols and abbreviations in EN 13791:2019, Clause 3 . 17
9 Investigation objective and test parameters in EN 13791:2019, Clause 4 . 18
10 Test regions and test locations in EN 13791:2019, Clause 5 . 18
11 Core testing and the determination of the in situ compressive strength in
EN 13791:2019, Clause 6 . 18
12 Initial evaluation of the data set in EN 13791:2019, Clause 7 . 19
13 Estimation of compressive strength for structural assessment of an existing
structure in EN 13791:2019, Clause 8 . 20
13.1 Based on core test data only (see EN 13791:2019, 8.1) . 20
13.2 Based on a combination of indirect test data and core test data (see EN 13791:2019,
8.2) . 25
13.3 Use of indirect testing with selected core testing (see EN 13791:2019, 8.3). 30
14 Assessment of compressive strength class of supplied concrete in case of doubt in EN
13791:2019, Clause 9 . 30
14.1 General in EN 13791:2019, 9.1 . 30
14.2 Use of core test data (see EN 13791:2019, 9.2) . 31
14.3 Indirect testing plus selected core testing (see EN 13791:2019, 9.3) . 32
14.4 Screening test using general or specific relationship with an indirect test procedure
(see EN 13791:2019, 9.4) . 32
14.5 Procedure where the producer has declared non-conformity of compressive
strength in EN 13791:2019, 9.5 . 36
14.6 Use of comparative testing . 36
Annex A (informative) Examples of the calculations . 39
A.1 Example A1: Calculating the rebound number . 39
A.2 Example A2: Calculating the in situ strength from core test data . 41
A.2.1 Example A2.1 . 41
A.2.2 Example A2.2 . 41
A.3 Example A3: Assessing the data for a test region to check whether it contains two or
more compressive strength classes . 42
A.4 Example A4: Check for statistical outliers . 45
A.5 Example A5: Calculation of characteristic in situ compressive strength from core test
data . 47
A.6 Example A6: Establishing a correlation between an indirect test and in situ
compressive strength . 48
A.7 Example A7: Using combined indirect testing and core testing to estimate the
characteristic in situ compressive strength and the compressive strength at a
location where only an indirect test result is available . 52
A.8 Example A8: Estimating the characteristic in situ compressive strength using
indirect testing and three cores taken from the weaker area . 55
A.8.1 Example A8.1 . 55
A.8.2 Example A8.2 . 55
A.9 Example A9: Screening test using a generic relationship . 56
A.10 Example A10: Screening test using a rebound hammer that has been calibrated
against test specimens made from the same concrete . 59
A.11 Example A11: Assessment of compressive strength class of concrete as placed using
indirect testing and selected core test data . 63
A.12 Example A12: Assessment of compressive strength class of recently supplied
concrete using core test data only . 64
Bibliography . 66
European foreword
This document (CEN/TR 17086:2020) has been prepared by Technical Committee CEN/TC 104
“Concrete and related products”, the secretariat of which is held by Standards Norway.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document should be read in conjunction with EN 13791:2019.
Introduction
(1) To achieve a balanced standard, CEN/TC 104/SC 1/TG 11 comprises experts with different
backgrounds and affiliations. The membership of TG 11 is given in Table 1.
Table 1 — Membership of the European Technical Standard Committee,
CEN/TC 104/SC 1/TG 11, responsible for the revision of EN 13791
Member Affiliation
Professor Tom Harrison Convenor
Dr Chris Clear Secretary
Vesa Anttila Rudus, Finland
Prof. Wolfgang Breit (papers only) Technische Universität Kaiserslautern, Germany
Dr Neil Crook The Concrete Society, UK
Ir. F.B.J. (Jan) Gijsbers CEN/TC250/SC2
Bruno Godart IFSTTAR, France
Dr. Arlindo Gonçalves Laboratório Nacional de Engenharia Civil, Portugal
Christian Herbst JAUSLIN + STEBLER INGENIEURE AG, Switzerland
Rosario Martínez Lebrusant Jefe del Área de Certificación y Hormigones, Spain
Dorthe Mathiesen (papers only) Danish Technological Institute, Denmark
David Revuelta Instituto Eduardo Torroja, Spain
Dr.-Ing. Björn Siebert followed by
Deutscher Beton- und Bautechnik-Verein E.V.
Dr Enrico Schwabach
Swedish Cement and Concrete Research Institute,
Prof. Johan Silfwerbrand
Sweden
Ceyda Sülün followed by Francesco Biasioli ERMCO
José Barros Viegas (papers only) BIBM
Dr.-Ing. Ulrich Wöhnl German expert and member of former TG11
Christos A Zeris (papers only) National Technical University of Athens, Greece
(2) In addition, guidance on rebound hammer and pulse velocity testing was provided by David Corbett
of Proceq, Switzerland and statistical help with combining core and indirect test results was provided by
André Monteiro of the Laboratório Nacional de Engenharia Civil, Portugal.
(3) Contact and exchange of information was also maintained with RILEM Technical Committee
TC ISC 249, which works on onsite non-destructive assessment of concrete strength.
(4) Where a reference is cited to a paragraph without being preceded by a reference to a standard, e.g.
EN 13791:2019, Clause 6, the reference is to a paragraph in this document. For example ‘13.3 (2)’ means
paragraph (2) in 13.3 of this document.
1 Scope
This document explains the reasoning behind the requirements and procedures given in EN 13791 [1]
and why some concepts and procedures given in EN 13791:2007 [2] were not adopted in the 2019
revision. The annex comprises worked examples of the procedures given in EN 13791:2019.
2 Symbols and abbreviated terms
For the purposes of this document, the following symbols and abbreviated terms apply.
CLF core length factor
CoV coefficient of variation
f or f compressive strength of standard test specimens, 2:1 cylinder or cube
c c,cube
f or
core compressive strength associated with a length: diameter ratio of either 1:1 or 2:1
c,1:1core
f
c,2:1 core
f design compressive strength in the structure
cd
f minimum characteristic compressive strength of test specimens based on 2:1 cylinders
ck
f minimum characteristic compressive strength of test specimens based on cubes
ck, cube
f in situ compressive strength
c,is
f characteristic in situ compressive strength (expressed as the strength of a 2:1 core of
ck,is
diameter ≥ 75 mm)
f assumed characteristic compressive strength in the structure
ck,is,28
f assumed characteristic compressive strength in the structure after 28 days
ck,is, > 28
f specified minimum characteristic strength
ck,spec
specified minimum characteristic cube strength (Some CEN members specify cube
f
ck,spec,cube
strength)
f highest value of f for a set of ‘n’ results.
c,is,highest c,is
f lowest value of f for a set of ‘n’ results
c,is,lowest c,is
f
estimated in situ compressive strength at a specific test location
c,is,est
f indirect test value converted to its equivalent in situ compressive strength using a
c,is,reg
regression equation
f mean (average) concrete compressive strength of 2:1 test cylinders
c,m
f mean (average) value of a set of ‘n’ values of f
c,m(n)is c,is
k factor applied to the sample standard deviation
n
k reduction factor for α
t cc
m number of valid indirect test results in test region under investigation
n number of core test results
p number of parameters of the correlation curve
coefficient of determination
R
s estimate of the overall standard deviation of in situ compressive strength
s residual standard deviation, which is a measure of the spread of the core strength test
c
data around the fitted regression curve
s standard deviation of all the estimated strength values, which is a measure of the spread
e
of the estimated core strengths around its mean value
s sample standard deviation of reference element(s)
r
s sample standard deviation of element(s) under investigation
s
UPV ultrasonic pulse velocity
mean UPV/rebound number of the reference element
X
r
mean UPV/rebound number of the element under investigation
X
s
x indirect test value at test location ‘0’ (where the in situ strength is required for structural
assessment purposes)
x indirect test value at test location i that is used for the correlation
i,cor
x mean (average) of the m indirect test values used for the correlation
α coefficient taking account of long term effects on the concrete compressive strength
cc
γ
partial safety factor for concrete for persistent and transient design situations
c
3 General principles adopted for the revision
(1) The scope of the revision retains covering both the estimation of compressive strength for the
structural assessment of an existing structure (EN 13791:2019, Clause 8) and assessment of compressive
strength class of supplied concrete in case of doubt (EN 13791:2019, Clause 9). Presenting EN 13791 as
two parts was considered as it would emphasize the differences between the estimation of compressive
strength for a structural assessment and assessment of compressive strength class of supplied concrete
in case of doubt. It was decided to keep EN 13791:2019 as a single standard to avoid duplication of
requirements.
(2) EN 13791 was not drafted to cover exceptional situations. EN 13791 aims to cover the most common
situations.
(3) As the objective was to produce a technically sound European standard and not a collation of national
provisions, the requests to refer to provisions valid in the place of use were resisted. Nevertheless,
techniques not specified and topics not addressed by EN 13791:2019 may be detailed in national
provisions or left to the investigator involved.
(4) Requirements have been placed in the EN 13791:2019 normative text and guidance in its Annex A
and this document.
(5) Statistical principles are applied and this has consequences when there are small sets of data. For all
other things being equal, a small set of data will lead to a lower estimate of the characteristic in situ
compressive strength when applying the EN 13791:2019, Clause 8 procedures. On the other hand, in the
EN 13791:2019, Clause 9 procedures, the smaller data set, the lower is the risk of rejecting concrete.
(6) Uncertainty of measurement is not taken into account but there are recommendations as to the
minimum number of test results to help ensure the estimates are reliable. This means that with respect
to uncertainty of measurement, the producer and user risks are the same.
(7) EN 13791 [1] is drafted to be compatible with EN 1990 [3], EN 1992-1-1 [4] and EN 206 [5]. The
recommended value of 0,85 for the factor η in A.2.3(1) of EN 1992-1-1:2004 [4] has been applied and if
national provisions use a different factor, the national annex to EN 13791 would need to provide the
appropriate value. Where EN 13791 is used with design standards other than EN 1992-1-1 then some
factors may need to be reviewed or adjusted, but this is outside the scope of the revision.
(8) As the EN 1992-1-1 is based on 2:1 cylinder strengths, the in situ compressive strength in EN 13791
is expressed as the strength of a 2:1 core.
(9) For structural assessment, the output of EN 13791:2007 [2] was the estimated compressive strength
class of the concrete prior to placing in the structure. At the request of the structural engineers, the
approach was changed to estimating either the characteristic in situ compressive strength for the test
region or the in situ compressive strength at a specific location.
(10) When estimating the in situ compressive strength for the structural assessment of an existing
structure (EN 13791:2019, Clause 8 procedures), the strength is estimated purely from the data analysis
with no presumption as to the concrete strength.
(11) When assessing the compressive strength class of supplied concrete in case of doubt (EN
13791:2019, Clause 9 procedures), it is assumed that the concrete conformed to its specification with
respect to compressive strength and the truth of this assumption is tested. For statistical analysis, this
assumption is known as the null hypothesis. This is the same philosophy as used in EN 206 [5] for
conformity and identity testing and in EN 13791:2007 [2].
(12) The criteria in EN 13791:2019, 9.2 and 9.3 are based on the identity testing criteria for compressive
strength given in EN 206:2013+A1:2016, Annex B, B.3.1.
(13) It is possible that an EN 13791:2019, Clause 8 calculation from core results may indicate that the
estimated in situ strength is insufficient, whilst an EN 13791:2019, Clause 9 analysis may indicate that
the concrete placed conformed to the specified strength class.
NOTE For example, EN 13791:2019, 9.2 would accept a small element with a mean of three cores giving an in
situ compressive strength below the 0,85f provided every core is not less than 0,85(f ‒ 4) and in this
ck, spec ck, spec
situation a structural analysis is not needed. Nevertheless, if the same three core test results were used in the
EN 13791:2019, 8.1(7) procedure, the lowest core test result would be taken as the characteristic in situ
compressive strength and this value used in a structural analysis based on EN 1990.
(14) When interpreting the data, engineering judgement will be required. For example, EN 13791:2019
now includes procedures for identifying statistical outliers, but whether any outliers are included in the
estimation of the characteristic in situ compressive strength is left to engineering judgement.
4 In situ compressive strength and other concrete properties assumed in the
EN 1992-1-1 design process
4.1 General
(1) Before describing the background to the provisions in EN 13791:2019, this section sets out the
assumptions related to the in situ concrete compressive strength and other concrete properties in the
1)
EN 1992 series structural design process. The EN 1992 series of standards is commonly known as
Eurocode 2.
(2) For structural design, various concrete strength and deformation properties (mechanical properties)
are defined in EN 1992-1-1, namely:
1) The standards in the EN 1992-series are:
EN 1992-1-1, Eurocode 2: Design of concrete structures — Part 1-1: General rules and rules for buildings
EN 1992-1-2, Eurocode 2: Design of concrete structures — Part 1-2: General rules — Structural fire design
— compressive strength;
— tensile strength;
— splitting tensile strength;
— flexural tensile strength;
— modulus of elasticity;
— Poisson’s ratio;
— coefficient of thermal expansion;
— creep coefficient;
— drying shrinkage strain and autogenous shrinkage strain;
— stress-strain relationship.
(3) The properties listed in 4.1(2) are assumed to be related to the compressive strength of concrete
except for Poisson’s ratio and the coefficient of thermal expansion. The appropriate relationships are
given in EN 1992-1-1 [4] for normal weight aggregate concrete and for lightweight aggregate concrete.
Additional properties of concrete, which are relevant for structural fire design, are given in EN 1992-1-2.
(4) As in EN 13791:2019, distinction is made in this section between two situations, namely the situation
in which the concrete compressive strength in the structure is based on test specimens (see 4.2) and the
situation in which the concrete compressive strength in the structure is based on cores extracted from
the structure (see 4.3). Normally the first situation applies to new structures whereas the second
situation applies to existing structures for which a structural assessment is required.
(5) The standards in the EN 1992 series are intended to be used for the structural design of buildings and
civil engineering works in concrete (EN 1992-1-1:2004, 1.1.1), i.e. for new structures. For the structural
assessment of existing buildings and civil engineering works in concrete, additional rules are being
2)
developed by the European Concrete Design Committee . These additional rules will become available
as part of the second generation of Eurocodes, which are expected to be published around 2023. The
information given in 4.3 is based on current draft proposals and consequently may be subject to change
before publication.
4.2 Concrete compressive strength based on test specimens
(1) The concrete compressive strength in the structure is related to the compressive strength of test
specimens, namely the characteristic (5 %) 2:1 cylinder strength (f ) or the characteristic (5 %) cube
ck
strength (f ) (EN 1992-1-1:2004, 3.1.2(1)P).
ck, cube
(2) The 2:1 cylinder strength is assumed to be 0,82 times the cube strength. The factor 0,82 is the average
value of the ratio between the 2:1 cylinder strength and the cube strength for the range of concrete
strength classes, C12/15 to C90/105, covered by EN 1992-1-1:2004, Table 3.1 (see 5.2).
2) CEN/TC 250/SC 2
EN 1992-2, Eurocode 2: Design of concrete structures — Part 2: Concrete bridges — design and detailing rules
EN 1992-3, Eurocode 2: Design of concrete structures — Part 3: Liquid retaining and containment structures
(3) According to EN 1992-1-1, the variation of the concrete compressive strength in the structure is given
as a lognormal distribution. The average concrete compressive strength f for normal and high strength
cm
concrete at 28 days is assumed as (EN 1992-1-1:2004, Table 3.1) given in Formula (1).
f = f + 8 (values in MPa) (1)
cm ck
(4) The characteristic (5 %) concrete compressive strength in the structure at 28 days (f ) is
ck,is,28
assumed to be 85 % of the corresponding characteristic (5 %) strength (f ) of 2:1 cylinder test specimen
ck
at 28 days, see Formula (2):
f = 0,85 × f (2)
ck,is,28 ck
NOTE The factor 0,85 is the recommended value of the conversion factor η in A.2.3(1) of EN 1992-1–1:2004.
(5) After 28 days a strength increase of 18 % (1/0,85) is assumed. Formula (3) takes this strength gain
into account:
f = (1/0,85) × 0,85 × f = f (3)
ck,is,>28 ck ck
(6) The value of the design concrete compressive strength in the structure f is defined in (3.1.2(4) and
cd
3.1.6(1)P of EN 1992-1-1:2004) and reproduced as Formula (4):
f = k α f /γ (4)
cd t cc ck c
where
k is a reduction factor for α with:
t cc
k = 1,0 when the strength is determined at 28 days;
t
k = 0,85 when the strength is determined after 28 days (3.1.2(4) of EN 1992-1–1:2004).
t
is the coefficient taking account of long term effects on the concrete compressive strength. This
α
cc
coefficient is also known as the Rüsch factor for reduced strength under sustained load. The
recommended value of α is 1,0 (3.1.6(1)P of EN 1992-1–1:2004);
cc
is the partial safety factor for concrete, with a recommended value of 1,5 for persistent and transient
γ
c
design situations (2.4.2.4(1) of EN 1992-1–1:2004).
NOTE 1 It is assumed that the increase in the compressive strength after 28 days is offset by the reduction of the
compressive strength due to long term effects (Rüsch factor). This implies in fact an assumed value of 0,85 for α .
cc
NOTE 2 The value of 0,85 (Formula (2)) is included in the partial safety factor for concrete.
NOTE 3 It is assumed that all variations related to execution (placing, compaction and curing of concrete) are
covered by the partial safety factor for concrete provided execution is in accordance with the requirements of
EN 13670 [6].
(7) When the strength is determined on the basis of the characteristic (5 %) strength of 2:1 test cylinders
at 28 days f , the design value of the concrete compressive strength in the structure is calculated using
ck
Formula (5):
f = k α f /γ = 1,0 × 1,0 × f /γ = f /γ (5)
cd t cc ck c ck c ck c
(8) When the strength is determined on the basis of the characteristic (5 %) strength of 2:1 test cylinders
after 28 days f , the design value of the compressive strength in the structure is calculated using
ck, > 28
Formula (6):
f = k α f /γ = 0,85 × 1,0 × f /γ = 0,85 × f /γ (6)
, ,
cd t cc ck >28 c ck >28 c ck,>28 c
4.3 Concrete compressive strength based on the strength of cores from the structure
(1) Structural assessment of existing concrete structures may be based on the strength of cores, which
are extracted from the structure.
(2) The design value of the concrete compressive strength in the structure is derived from the
characteristic (5 %) value of the concrete compressive strength, i.e. the value which has an exceedance
probability of 95 %.
(3) It is assumed that the characteristic (5 %) concrete compressive strength in the structure equals the
characteristic (5 %) compressive strength (f ) of 2:1 cores extracted from the structure.
ck,is
(4) When the strength is estimated on the basis of the 2:1 core strength (f the design value of the
ck,is),
concrete compressive strength in the structure is calculated using Formula (7):
f = k α f /γ = 0,85 × 1,0 × f /γ = 0,85 × f /γ (7)
cd t cc ck,is c ck,is c ck,is c
(5) Using Formula (7) the value of the partial safety factor γ for concrete may be reduced to a
c
recommended value of 1,3 (A.2.3 in EN 1992-1-1:2004). This is to allow for the reduction in uncertainties
as the compressive strength is derived from the structure directly.
5 Differences between test specimens and concrete in the structure
5.1 Introduction
(1) There are a number of reasons why the strength in situ is different to that assessed on test specimens.
Depending upon the purpose of the investigation, these differences may need to be taken into account
somewhere in the assessment.
(2) All execution that is in accordance with the requirements of EN 13670 [6] is intended to be covered
by the partial safety factor for materials. For concrete this factor is 1,5 and it covers statistical variation
in produced concrete, conversion to in situ strength and statistical variation of in situ strength within the
limits set by execution in accordance with EN 13670. The portion of this factor allocated to the conversion
to in situ strength is 1,2 (i.e. the inverse of 0,85). Indications are that this portion is in the range from 0,75
to 0,95. Tests from offshore concrete thick slip-formed walls with much re-vibration gave values above
1,0, while members with less re-vibration like domes are high but below 1,0. Little is actually known on
the statistical distribution of in situ strength and what fractile is < 0,85f , but it is probably larger than
ck
5 % in many cases; however, experience indicate that a partial factor for concrete of 1,5 is still adequate.
(3) When the in situ strength is measured by cores, the portions of the partial factor taken into account
are:
— the conversion to in situ strength;
— most of the statistical variation in produced concrete;
— some of the statistical variation in in situ strength.
Nevertheless, the procedures in EN 13791:2019, Clause 9 only take the portion of the partial factor
allocated to conversion to in situ strength into account. Given the uncertainties associated with the
allocation of the portions, the structural designers were adamant that no further allowance should be
made when applying the EN 13791:2019, Clause 9 procedures.
(4) Many of the following influences on in situ compressive strength are only relevant when assessing
responsibility for non-conformity under EN 13791:2019, Clause 9 procedures, i.e. maturity, curing
compaction.
(5) Clause 5 and Clause 6 contain what was originally in EN 13791:2007, Annex A plus additional
information.
5.2 Reference test specimen
(1) EN 13791:2019 is based on in situ strength being expressed as the strength of 2:1 cores. Cube
strengths are approximately 20 % higher than the strength of 2:1 cylinders due to the lateral restraint
from the test machine platens. This difference is taken into account in EN 206 by having different
minimum characteristic strength requirements for 150 mm cubes and 150 mm diameter by 300 mm
cylinders. The default core length factor (CLF) of 0,82 given in EN 13791:2019, Clause 6 for normal-
weight and heavyweight concretes is the average ratio between these different measures of compressive
strength. The use of a different CLF is permitted where justified by testing. The CLF is used to convert 1:1
cores to the equivalent strength of 2:1 cores.
(2) The ratio between 2:1 cylinder and cube strength in the EN 206 compressive strength classes is given
in Table 2. Some of the differences are due to rounding errors, but the higher factor for the higher strength
classes is a reflection of the evidence that as the compressive strength class increases, the factor
increases.
(3) No CLF is given in EN 13791 for lightweight concretes. If the same approach as used for normal-
weight concrete were to be applied, the factor would be 0,91 based on Table 13 of EN 206:2013+A1:2016
[5], but EN 206 permits other relationships if they are established and documented.
(4) ASTM [7] use for normal weight concrete a ratio of 0,87 to convert 1:1 cores to 2:1 cores and some
literature [8] indicates that the 0,82 factor is conservative particularly for high strength concrete. The
evidence is mixed and further research is being encouraged to provide definitive guidance. The default
relationship should be reviewed at the next revision of EN 13791.
(5) While the difference between a CLF of 0,82 and, for example, 0,87 is small (see Example A2), it may
be the difference between acceptability and rejection. For this reason EN 13791:2019 permits other CLFs
to be used if proven by testing.
Table 2 — Ratio between 2:1 cylinder strength and cube strength for the EN 206 compressive
strength classes for normal-weight and heavyweight concrete
EN 206 compressive 2:1 cylinder strength Cube strength
Ratio
strength class MPa MPa
C8/10 8 10 0,80
C12/15 12 15 0,80
C16/20 16 20 0,80
C20/25 20 25 0,80
C25/30 25 30 0,83
C30/37 30 37 0,81
C35/45 35 45 0,78
C40/50 40 50 0,80
C45/55 45 55 0,82
C50/60 50 60 0,83
C55/67 55 67 0,82
C60/75 60 75 0,80
C70/85 70 85 0,82
C80/95 80 95 0,84
C90/105 90 105 0,86
C100/115 100 115 0,87
Average ratio 0,82
(6) A number of the key reasons for differences between test specimens and 2:1 cores taken from the
structure are given in the following sub-sections and in Clause 6.
5.3 Effects of the moisture condition on in situ specimens
(1) The moisture condition of the core will influence the measured strength. A dry specimen will have a
higher core strength than a wet core all other things being equal.
NOTE According to EN 13791:2007, A.2.1 [2], if cores are tested wet, the strength is reduced by 8 % to 12 %
compared to testing in a dry condition. An implication of this fact is that if a core is tested wet the measured strength
could be enhanced by at least 8 % when calculating the dry in situ strength. 'Wet' is defined as soaked underwater
at (20 ± 2) °C for at least 48 h before testing.
(2) EN 13791 requires the core to be tested at a moisture condition similar to the in situ moisture
condition. This is an appropriate moisture condition when determining the in situ characteristic strength
in accordance with EN 13791:2019, Clause 8.
(3) In elements that function in a dry or semi-dry condition, the compressive strength in the structure is
thus enhanced over that of standard test specimens; but in other circumstances, for elements that
function in a wet condition, e.g. foundations, the in situ strength is not enhanced in the same way.
(4) As test specimens are cured in the wet condition, the option of testing cores for the EN 13791:2019,
Clause 9 procedures was reviewed. The EN 1992-1-1 [4] design process has a factor of 0,85 to account
for differences between 2:1 cylinders and the concrete in the structure, i.e. the concrete in the structure
may be up to a factor 0,85 less than that of test specimens. The 0,85 includes various elements but each
element is not allocated a specific portion of the factor. Most of the difference between test specimens
and the structure are negative, i.e. the in situ strength is lower than that of test specimens; however, as
most in situ concrete has a moisture content less than saturated and this increases strength, difference in
moisture content is a positive factor. By changing the curing to wet curing, the negative portions are
retained and the positive portion is removed. This was not regarded as a technically sound approach. As
explained in 5.1(3), core testing takes account of more than what is allocated to the 0,85 factor and
consequently there is some additional margin.
(5) By having a difference between the EN 13791:2019, Clause 8 and Clause 9 curing conditions for cores,
problems are created if the same set of cores are used both for an assessment under EN 13791:2019,
Clause 9 and an estimation of in situ compressive strength under EN 13791:2019, Clause 8. This might be
the case if the concrete was shown to be non-conforming and it was necessary to check whether the
structure is still sufficiently safe at a lower concrete strength. For this structural check an unknown
moisture correction would have to be introduced depending upon the degree of saturation in the in situ
concrete. Given these difficulties, the required curing of cores is the same for the EN 13791:2019, Clause 8
and Clause 9 procedures.
5.4 Effect of maturity on concrete strength
(1) The rate of strength gain both pre and post-28 days is dependent upon the cement type, addition type,
admixtures and the temperature of the concrete.
(2) Provided there is sufficient water for hydration, concrete will continue to develop strength with time.
(3) Design is usually based on the characteristic strength of laboratory samples of concrete moist-cured
for 28 days at 20 °C. When comparing the strength of test specimens with the in situ strength this should
be done at equal maturity, not equal age. Maturity is a function of both time and concrete temperature
and there are various maturity functions that are widely used [9].
(4) The partial factor for materials does not include differences in maturity; however, the design may
include an ageing factor to take account of strength development over time. A proven increase in strength
with time may be decisive in deciding whether a structure has an acceptable strength.
(5) When estimating in situ compressive strength under EN 13791:2019, Clause 8, maturity is usually not
a critical issue as the structure is likely to be several years old. When interpreting an assessment under
the EN 13791:2019, Clause 9 procedures, the maturity of the concrete is something that should be
considered.
(6) EN 13791:2019, Clause 6 requires for the Clause 9 procedures that core testing is not be undertaken
on cores with a maturity less than that used as the basis for conformity testing, e.g. 28 days at 20 °C.
5.5 Effects of curing
(1) The strength of a core will be influenced by the curing history of the structure and the age of the
concrete when the core is taken.
(2) The effects of curing are complex and include:
— if the temperature of the concrete is low for the first few hours, the ultimate strength is increased
compared with standard curing;
— if the temperature of the concrete is high for the first few hours, the ultimate strength is decreased
compared with standard curing;
— as the peak temperature of the concrete increases due to the release of the heat of hydration or due
to accelerated curing, the early strength is increased but the long term strength is reduced [9];
— maturity equations are valid for normal conditions and they may not reflect the effects listed above.
(3) All the above assumes that there is sufficient water in the concrete as to not impede hydration. This
assumption may not be valid for slabs that are not adequately cured or for concrete with a very low water
content. There is no simple factor for taking the effects of curing into account. But it is clear that if the
concrete is placed on a hot day, placed in a large section or cured by accelerated methods, the effect of
curing on in situ strength may be significant. The effects of early temperature history on the eventual
concrete strength will depend on cement type, section size, formwork type and placing temperature. For
these reasons it is not easy to estimate the combined effects but it is known that hydrating concrete
reaching temperature in excess of 70 °C will have lower ultimate strength than concrete cured at lower
temperatures and may undergo deleterious reactions such as delayed ettringite formation.
(4) The partial safety factor used in the design process takes account of some effects of curing history.
5.6 Effects of vibration
(1) Internal water and air movements caused by the process of vibrating the concrete will cause
variations in the strength. The reduction in strength at the top of a vertical element is a well-recognized
phenomenon.
(2) Self-compacting concrete is claimed to eliminate these variations, but such a claim is only valid if the
concrete is not prone to static segregation.
(3) The partial safety factor used in the design process takes account of the effects of vibration.
5.7 Effects of excess entrapped air
(1) Increased voidage decreases the strength. Approximately 1 % voidage decreases the strength by 5 %
to 8 %.
(2) The entrapped air in the structure is likely to be higher than that in test specimens and as a
consequence the in
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