SIST-TP CEN/TR 17086:2020

Further guidance on the application of EN 13791:2019 and background to the provisions

This Technical Report 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 2017 revision. The annex comprises worked examples of the procedures given in EN 13791.

Weiterführende Anleitung zur Anwendung der EN 13791:2019 und Hintergrund 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

• Status: Published
• Public Enquiry End Date: 19-Mar-2017
• Publication Date: 11-Nov-2020
• ICS: 91.080.40 - Concrete structures
• ICS: 91.100.30 - Concrete and concrete products
• Technical Committee: BBB - Concrete, reinforced concrete and prestressed concrete
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 21-Oct-2020
• Due Date: 26-Dec-2020
• Completion Date: 12-Nov-2020
• Ref Project: CEN/TR 17086:2020 - Further guidance on the application of EN 13791:2019 and background to the provisions

Standards Content (Sample)

SLOVENSKI STANDARD
SIST-TP CEN/TR 17086:2020
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
SIST-TP CEN/TR 17086:2020 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST-TP CEN/TR 17086:2020

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SIST-TP CEN/TR 17086:2020


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.

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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
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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

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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.

4

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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.
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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
2
coefficient of determination
R
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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
0
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
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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

8

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— 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
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(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 fac
...

SLOVENSKI STANDARD
kSIST-TP FprCEN/TR 17086:2017
01-april-2017
1DGDOMQMDQDYRGLOD]DXSRUDER(1LQR]DGMHGRORþLO
Further guidance on the application of EN 13791:2017 and background to the provisions
Weiterführende Anleitung zur Anwendung der EN 13791:2017 und Hintergrund zu den
Regelungen
Ta slovenski standard je istoveten z: FprCEN/TR 17086
ICS:
91.080.40 Betonske konstrukcije Concrete structures
91.100.30 Beton in betonski izdelki Concrete and concrete
products
kSIST-TP FprCEN/TR 17086:2017 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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kSIST-TP FprCEN/TR 17086:2017


FINAL DRAFT
TECHNICAL REPORT
FprCEN/TR 17086
RAPPORT TECHNIQUE

TECHNISCHER BERICHT

February 2017
ICS 91.080.40
English Version

Further guidance on the application of EN 13791:2017 and
background to the provisions
 Weiterführende Anleitung zur Anwendung der EN
13791:2017 und Hintergrund zu den Regelungen


This draft Technical Report is submitted to CEN members for Vote. 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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.

Warning : This document is not a Technical Report. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a Technical Report.


EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. FprCEN/TR 17086:2017 E
worldwide for CEN national Members.

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FprCEN/TR 17086:2017 (E)
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 . 13
5.5 Effects of curing. 14
5.6 Effects of vibration . 14
5.7 Effects of excess entrapped air . 14
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 . 15
6.5 Length/diameter ratio . 15
6.6 Flatness of end surfaces . 15
6.7 Capping of end surfaces . 16
6.8 Effect of drilling . 16
6.9 Reinforcement . 16
7 Scope (EN 13791:2017, Clause 1) . 16
8 Terms and definitions, symbols and abbreviations (EN 13791:2017, Clause 3) . 17
9 Investigation objective and test parameters (EN 13791:2017, Clause 4) . 17
10 Test regions and test locations (EN 13791:2017, Clause 5) . 17
11 Core testing and the determination of the in-situ compressive strength
(EN 13791:2017, Clause 6) . 18
12 Initial evaluation of the data set (EN 13791:2017, Clause 7) . 19
13 Estimation of compressive strength for structural assessment of an existing
structure (EN 13791:2017, Clause 8) . 19
13.1 Based on core test data only (EN 13791:2017, Clause 8.1) . 19
13.2 Based on a combination of indirect test data and core test data (EN 13791:2017,
Clause 8.2) . 23
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13.3 Use of indirect testing with selected core testing (EN 13791:2017, Clause 8.3) . 27
14 Assessment of compressive strength class of recently supplied concrete using in-situ
testing (EN 13791:2017, Clause 9) . 27
14.1 General (EN 13791:2017, Clause 9.1) . 27
14.2 Use of core test data (EN 13791:2017, Clause 9.2) . 28
14.3 Indirect testing plus selected core testing (EN 13791:2017, Clause 9.3) . 28
14.4 Screening test using general relationship with rebound number (EN 13791:2017,
Clause 9.4) . 29
14.5 Screening test using an indirect test — specific concrete cylinder/cube relationship
(EN 13791:2017, Clause 9.5 and Annex A) . 31
14.6 Use of comparative testing . 31
Annex A (informative) Examples of the calculations . 34
A.1 Example A: Calculating the rebound number . 34
A.2 Example B: Calculating the in-situ strength from core test data . 36
A.2.1 Example B.1 . 36
A.2.2 Example B.2 . 36
A.3 Example C: Assessing the data for a test region to check whether it contains two or
more compressive strength classes . 36
A.4 Example D: Check for statistical outliers . 39
A.5 Example E: Calculation of characteristic in-situ compressive strength from core test
data . 41
A.6 Example F: Establishing a correlation between an indirect test and in-situ
compressive strength . 42
A.7 Example G: 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 . 45
A.8 Example H: Estimating the characteristic in-situ compressive strength using indirect
testing and three cores taken from the weaker area. 48
A.8.1 Example H.1 . 48
A.8.2 Example H.2 . 48
A.9 Example I: Screening test using a general relationship . 50
A.10 Example J: Screening test using a rebound hammer that has been calibrated against
test specimens made from the same concrete . 52
A.11 Example K: Assessment of compressive strength class of concrete as placed using
indirect testing and selected core test data . 56
A.12 Example L: Assessment of compressive strength class of recently supplied concrete
using core test data . 57
Bibliography . 59

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European foreword
This document (FprCEN/TR 17086:2017) has been prepared by Technical Committee CEN/TC 104
“Concrete and related products”, the secretariat of which is held by DIN.
This document is currently submitted to the Vote on TR.
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Introduction
(1) To achieve a balanced standard, CEN/TC104/SC1/TG11 comprises experts with different
backgrounds and affiliations. The membership of TG11 is given in Table 1.
Table 1 — Membership of the European Technical Standard Committee, CEN/TC104/SC1/TG11,
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 Deutscher Beton- und Bautechnik-Verein E.V.
Swedish Cement and Concrete Research Institute,
Prof. Johan Silfwerbrand
Sweden
Ceyda Sülün 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) Unless stated otherwise, all references to EN 13791 refer to the 2017 revision, prEN 13791:2017
[1].
(5) Where a reference is cited to a paragraph without being preceded by a reference to a standard, e.g.
EN 13791:2017, Clause 6, the reference is to a paragraph in this Technical Report. For example ‘13.3
(2)’ means paragraph (2) in subclause 13.3 of this Technical Report.
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1 Scope
This Technical Report 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 2017 revision. The annex comprises worked examples of the procedures given in EN 13791.
2 Symbols and abbreviated terms
For the purposes of this document, the following symbols and abbreviated terms apply.
f or f compressive strength of standard test specimens
c c,cube
f or core compressive strength
c,1:1core
f
c,2:1 core
f design compressive strength in the structure
cd
f characteristic compressive strength of test specimens based on 2:1 cylinders
ck
f characteristic compressive strength of test specimens based on cubes
ck, cube
f in-situ compressive strength
c,is
f characteristic in-situ compressive strength
c,is,ck
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 characteristic strength
ck,spec
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 average concrete compressive strength of 2:1 test cylinders
c,m
f mean 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
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2
R coefficient of determination
s sample standard deviation
s standard error of the correlation
c
s standard deviation of all the estimated values of in-situ strength
e
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
0
assessment purposes)
x indirect test value at test location i that is used for the correlation
i,cor
x mean 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 13791:2017, Clause 8) and to assess whether the
recently supplied concrete conformed to the specified compressive strength class (EN 13791:2017,
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 the assessment of the
compressive strength class of recently supplied concrete. It was decided to keep EN 13791 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 may be detailed in
national provisions or left to the investigator involved.
(4) Requirements have been placed in the EN 13791 normative text and guidance in its Annex B and
this Technical Report.
(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. On the other hand, in the EN 13791:2017, Clause 9 procedures, the smaller data
set, the lower is the estimated risk of rejecting concrete.
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(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 conversion factor η in A.2.3 (1) of EN 1992-1-1 [4] has been used
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 placed 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 13791:2017, 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 recently supplied concrete using in-situ testing
(EN 13791:2017, 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) It is possible that an EN 13791:2017, Clause 8 calculation from core results may indicate that the
estimated in-situ strength is insufficient, whilst an EN 13791:2017, Clause 9 analysis may indicate that
the concrete placed conformed to the specified strength class.
NOTE For example, EN 13791:2017, Clause 9 would accept a small element with an average of three cores
giving an in-situ compressive strength below the 0,85fck, spec, but not less than 0,85(fck, spec ‒ 4) and in this situation
a structural analysis is not needed. Nevertheless, if the same three core test results were used in the
EN 13791:2017, Clause 8 procedure, the average would be taken as the characteristic in-situ compressive strength
and this value used in a structural analysis based on EN 1990. In, albeit rare situations, this estimated
characteristic in-situ strength may not be adequate from a structural viewpoint.
(13) When interpreting the data, engineering judgement will be required. For example, EN 13791 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, 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.


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
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(2) For structural design, various concrete strength and deformation properties (mechanical
properties) are defined in EN 1992-1-1, namely:
— 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, 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 specimen (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 (1.1.1 of EN 1992-1-1:2004), i.e. for new structures. For the
structural assessment of existing buildings and civil engineering works in concrete, additional rules are
2)
being 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
2020. 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 ) (3.1.2(1)P of EN 1992-1-1:2004).
ck, cube


2) CEN/TC250/SC2
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
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(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 Table 3.1 of EN 1992-1-1:2004 (see 5.2).
(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
cm
strength concrete at 28 days is assumed as (Table 3.1 of EN 1992-1-1:2004)
2
f = f + 8 (values in N/mm ) (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
ck
specimen at 28 days:
f = 0,85 × f (2)
ck,is,28 ck
NOTE 1 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, thus:
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 as (3.1.2(4) and
cd
3.1.6(1)P of EN 1992-1-1:2004):
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
α is the coefficient taking account of long term effects on the concrete compressive strength. This
cc
coefficient is also known as the Rüsch factor for reduced strength under sustained load. The
recommended value of αcc is 1,0 (3.1.6 (1) P of EN 1992-1-1:2004);
γ is the partial safety factor for concrete, with a recommended value of 1,5 for persistent and
C
transient design situations (2.4.2.4(1) of EN 1992-1-1:2004).
NOTE 2 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
αcc.
NOTE 3 The value of 0,85 (Formula (2)) is included in the partial safety factor for concrete.
NOTE 4 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 %)
...