FprCEN/TR 17086

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

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

This draft Technical Report is submitted to CEN members for Vote. It has been drawn up by the Technical Committee CEN/TC

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,

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are

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

© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. FprCEN/TR 17086:2017 E

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

(EN 13791:2017, Clause 6) ....................................................................................................................... 18

12 Initial evaluation of the data set (EN 13791:2017, Clause 7) ....................................................... 19

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

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

compressive strength .................................................................................................................................. 42

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

This document (FprCEN/TR 17086:2017) has been prepared by Technical Committee CEN/TC 104

(1) To achieve a balanced standard, CEN/TC104/SC1/TG11 comprises experts with different

Table 1 — Membership of the European Technical Standard Committee, CEN/TC104/SC1/TG11,

Prof. Wolfgang Breit (papers only) Technische Universität Kaiserslautern, Germany

(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

(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

(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

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.

For the purposes of this document, the following symbols and abbreviated terms apply.

f indirect test value converted to its equivalent in-situ compressive strength using a

x indirect test value at test location ‘0’ (where the in-situ strength is required for structural

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

(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

(2) EN 13791 was not drafted to cover exceptional situations. EN 13791 aims to cover the most

(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

(4) Requirements have been placed in the EN 13791 normative text and guidance in its Annex B and

(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

(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

(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

(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

(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

(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

(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

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

(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.

(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

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

(2) For structural design, various concrete strength and deformation properties (mechanical

(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

(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

(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

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

(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

(4) The characteristic (5 %) concrete compressive strength in the structure at 28 days (f ) is

assumed to be 85 % of the corresponding characteristic (5 %) strength (f ) of 2:1 cylinder test

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.

(6) The value of the design concrete compressive strength in the structure f is defined as (3.1.2(4) and

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

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

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

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