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

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
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
21-Oct-2020
Due Date
26-Dec-2020
Completion Date
12-Nov-2020

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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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CEN/TR 17086:2020 (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 ............................................................................................... 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.
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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
c,2:1 core
f design compressive strength in the structure

f minimum characteristic compressive strength of test specimens based on 2:1 cylinders

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
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
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
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
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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

data around the fitted regression curve

s standard deviation of all the estimated strength values, which is a measure of the spread

of the estimated core strengths around its mean value
s sample standard deviation of reference element(s)
s sample standard deviation of element(s) under investigation
UPV ultrasonic pulse velocity
mean UPV/rebound number of the reference element
mean UPV/rebound number of the element under investigation

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

partial safety factor for concrete for persistent and transient design situations

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

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

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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

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

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

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

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

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;

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

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);

is the partial safety factor for concrete, with a recommended value of 1,5 for persistent and transient

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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kSIST-TP FprCEN/TR 17086:2017
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
c,2:1 core
f design compressive strength in the structure
f characteristic compressive strength of test specimens based on 2:1 cylinders
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
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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R coefficient of determination
s sample standard deviation
s standard error of the correlation
s standard deviation of all the estimated values of in-situ strength
s sample standard deviation of reference element(s)
s sample standard deviation of element(s) under investigation
UPV ultrasonic pulse velocity
mean UPV/rebound number of the reference element
mean UPV/rebound number of the element under investigation

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 of the m indirect test values used for the correlation

α coefficient taking account of long term effects on the concrete compressive strength

γ partial safety factor for concrete for persistent and transient design situations

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

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

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

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

strength concrete at 28 days is assumed as (Table 3.1 of EN 1992-1-1:2004)
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

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

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;

k = 0,85 when the strength is determined after 28 days (3.1.2(4) of EN 1992-1-1:2004).

α is the coefficient taking account of long term effects on the concrete compressive strength. This

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

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 %)
...

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