BSI Standards Publication: Assessment of In-Situ Compressive Strength in Structures and Precast Concrete Components [PDF]

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BS EN 13791:2019

BSI Standards Publication

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Assessment of in-situ compressive strength in structures and precast concrete components

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BS EN 13791:2019

BRITISH STANDARD

National foreword This British Standard is the UK implementation of EN 13791:2019. It supersedes BS EN 13791:2007 and BS 6089:2010, which are withdrawn. The background to EN 13791:2019, as well as further guidance and worked examples, is given in CEN/TR 17086 (publication anticipated in 2020).

All the relevant content of BS 6089:2010 is now covered by BS EN 13791:2019, including its national annex; PD CEN/TR 17086 (publication anticipated in 2020); and BS EN 12504‑1:2019, including its national annex. This British Standard should be used in conjunction with BS EN 125041:2019 and its national annex.

A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. © The British Standards Institution 2020 Published by BSI Standards Limited 2020 ISBN 978 0 580 96061 1 ICS 91.080.40

Compliance with a British Standard cannot confer immunity from legal obligations. This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 January 2020. Amendments/corrigenda issued since publication Date

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The UK participation in its preparation was entrusted to Technical Committee B/517/1, Concrete production and testing.

EN 13791

EUROPEAN STANDARD

BS EN 13791:2019

NORME EUROPÉENNE EUROPÄISCHE NORM

August 2019

ICS 91.080.40

Supersedes EN 13791:2007

English Version

Assessment of in-situ compressive strength in structures and precast concrete components Évaluation de la résistance à la compression sur site des structures et des éléments préfabriqués en béton

Bewertung der Druckfestigkeit von Beton in Bauwerken oder in Bauwerksteilen

This European Standard was approved by CEN on 7 July 2019. CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member. This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions. 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 © 2019 CEN

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Ref. No. EN 13791:2019 E

BS EN 13791:2019

EN 13791:2018 (E)

Contents

Page

European foreword ...................................................................................................................................................... 3 1

Scope.................................................................................................................................................................... 7

2

Normative references.................................................................................................................................... 7

3 3.1 3.2

Terms, definitions, symbols and abbreviations .................................................................................. 8 Terms and definitions ................................................................................................................................... 8 Symbols and abbreviations ....................................................................................................................... 10

4

Investigation objective and test parameters ...................................................................................... 12

5 5.1 5.2

Test regions, test locations and number of tests ............................................................................... 15 Test regions..................................................................................................................................................... 15 Test locations ................................................................................................................................................. 15

6

Core testing and the determination of the in situ compressive strength ................................. 17

7 7.1 7.2

Initial evaluation of the data set .............................................................................................................. 18 Evaluation of the test region to determine if it represents a single concrete strength class18 Assessment of individual test results within a test region ............................................................ 19

8 8.1 8.2 8.3

Estimation of compressive strength for structural assessment of an existing structure ... 21 Based only on core test data ..................................................................................................................... 21 Based on a combination of indirect test data and core test data ................................................. 22 Use of indirect testing with at least three core test data ................................................................ 24

9 9.1 9.2 9.3 9.4 9.5

Assessment of compressive strength class of concrete in case of doubt .................................. 25 General.............................................................................................................................................................. 25 Use of core test data ..................................................................................................................................... 26 Indirect testing plus selected core test data ....................................................................................... 27 Screening test using a general or specific relationship with an indirect test procedure ... 28 Procedure where the producer has declared non-conformity of compressive strength ... 29

Annex A (informative) Guidance on undertaking an investigation ......................................................... 30 Annex B (informative) Example of a generic relationship between rebound number and compressive strength class ....................................................................................................................... 38 Bibliography ................................................................................................................................................................. 41

2

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Introduction .................................................................................................................................................................... 4

BS EN 13791:2019 EN 13791:2019 (E)

European foreword This document (EN 13791:2019) has been prepared by Technical Committee CEN/TC 104 “Concrete and related products”, the secretariat of which is held by SN.

This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by February 2020, and conflicting national standards shall be withdrawn at the latest by February 2020.

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 supersedes EN 13791:2007.

The main changes compared to EN 13791:2007 are:

a) the standard is fully revised but for continuity the methodological approaches and scope is retained as well as much of the previous layout;

b) the primary focus is on the determination of the characteristic in situ compressive strength for application with EN 1990 and EN 1992-1-1;

c) more comprehensive guidance is provided on applying the procedures, particularly with respect to defining a test result, a measurement, volume of concrete, test location, small test region and test region; d) requirements to set out the purpose of the investigation, procedures to be adopted, test methods, test locations and test regions to be defined prior to commencing the testing, are included;

e) Clause 8, "Estimation of compressive strength for structural assessment of an existing structure", covers the previous requirements for assessment of characteristic in situ compressive strength by either testing cores or indirect methods; f)

Clause 9, "Assessment of compressive strength class of concrete in case of doubt", covers previous requirements for the assessment where conformity of concrete based on standard tests is in doubt;

g) approaches A and B in EN 13791:2007 are no longer valid; h) EN 13791 is aligned with the requirements of EN 206.

According to the CEN-CENELEC Internal Regulations, the national standards organisations of the following countries are bound to implement this European Standard: 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 the United Kingdom.

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BS EN 13791:2019

EN 13791:2018 (E)

Introduction (1) This document covers two applications of in situ strength assessments. These are:

— to estimate in situ characteristic compressive strength of a test region and/or in situ strength at specific locations;

— assessment of compressive strength class of concrete supplied to a structure under construction where there is doubt about the compressive strength based on results of standard tests or doubt about the quality of execution.

(2) Both applications have a number of common steps as shown in Table 1, but the assessment methods differ. The reason for this difference is that with the estimation of the in situ strength (Clause 8) there is no presumption as to what this should be and the uncertainty associated with the number of data are taken into account when estimating the value. The in situ strength determined in accordance with Clause 8 is a value based on testing a finished structure or element, as referred to by EN 1992-1-1:2004, A.2.3. NOTE Information may be available on the original quality of the supplied concrete, but the in situ strength may have changed over time.

(3) Most of the procedures in Clause 9 apply where there is verification that the concrete supplied is in accordance with the producer's declaration of performance for compressive strength but test results from samples taken on site indicate non-conformity, and where this difference cannot be resolved by other means. As the procedures given in CEN standards for the verification of the declaration of performance are regarded as being reliable, the assumption is that the concrete conforms to the specified characteristic strength and the applied statistical tests check the validity of this hypothesis.

Where a Clause 9 assessment indicates non-conformity of compressive strength then the 9.5 procedure should be adopted by the producer and other involved parties. (4) The Clause 8 and Clause 9 procedures have different approaches that may lead to significantly different outcomes.

(5) Unless indicated otherwise, the provisions given in this document apply to concrete structures made from normal-weight, lightweight or heavyweight concrete. (6) This document only covers the use of a single relationship between an indirect test method (UPV or rebound hammer) and compressive strength. The combined use of both UPV and rebound hammer techniques with core strength is a useful technique, but the procedures are not detailed in this document.

(7) This document was developed with the expectation that it will be used with EN 1992-1-1. If it is used in conjunction with other design standards, some of the factors may need modification. In addition, this document uses the EN 1992-1-1:2004, 3.1.6, recommended value of 1,0 for the factor αcc and EN 1992-1-1:2004, A.2.3, recommended value of 0,85 for the factor η. Where national provisions adopt different values for these coefficients then adjustments to the appropriate formula within this Standard may be required.

(8) Techniques outside the range of those specified in this document may be given in provisions valid in the place of use. For example, these include: — combining two indirect test methods with core testing;

— use of cores of diameter less than 50 mm; — use of pull-out testing; 4

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BS EN 13791:2019 EN 13791:2019 (E)

— a screening test conforming to the principles specified in 9.4;

— in the Clause 8 procedures, provisions for less than 8 cores without indirect testing; — assessing the strength gradient across a section after a fire;

— in the Clause 9 procedures, comparing an element where the concrete quality is in doubt with a similar element containing conforming concrete.

In addition, provisions valid in the place of use may give requirements for other aspects not specified in this document. For example, these include: — relationship between 2:1 and 1:1 core compressive strengths if a value other than 0,82 is justified on the basis of test data for the local materials; — relationship between in situ compressive strength and core length to diameter ratio for values other than 2:1 or 1:1;

— relationship between in situ compressive strength for lightweight concretes and core length to diameter ratio; — adjustment to core strength for cores containing transverse reinforcement;

— relationship between core strength and the strength of a cast cylinder of equal diameter and length; — factors when the assessment is other than with EN 1992-1-1 or EN 1990;

— factor η given in A.2.3 of EN 1992-1-1:2004 where the national provisions use a value different to the recommended value of 0,85;

— in 8.3 different criteria for structural assessment;

— in 9.2 and 9.3 different criteria where the criteria for compressive strength in EN 206:2013+A1:2016, B.3.1, were not used for the assessment of a number of loads delivered to a construction site; --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

— guidance on appropriate actions where the producer of the concrete has declared non-conformity or where the concrete has been proven to be non-conforming. (9) Guidance on undertaking an investigation is given in Annex A.

(10) Further guidance and background information on this revision of EN 13791 and worked examples of the calculations are given in CEN/TR 17086 [1].

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EN 13791:2018 (E)

Table 1 — Guidance on relevant clauses Action

Clause

Objective of the investigation

Clause 4, A.1

Selection of test methods

A.3, A.4

Selection of assessment method:

A.2

for determination of in situ strength based on: — core test data;

8.1

— core and indirect testing.

8.3

— indirect testing calibrated against test specimens;

8.2

or, for assessment of compressive strength where production control data show conformity and identity testing data indicate non-conformity based on: — core test data;

9.2

— screening test.

9.4

— indirect testing and selected core testing;

9.3

Selection of test regions and test locations

Determination of in situ strength from core test data

9.5

5.1, 5.2, A.4 Clause 6

Evaluation of data set to see if it comprises a single concrete Evaluation of data set to see if it includes outliers Assessment and use of the data

6

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7.1 7.2

A.4, A.5, A.6

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Procedure where the producer has declared non-conformity of compressive strength

BS EN 13791:2019 EN 13791:2019 (E)

1 Scope (1) This document:

— gives methods and procedures for the estimation of the in situ compressive strength and characteristic in situ compressive strength of concrete in structures and precast concrete components using direct methods (core testing) and indirect methods, e.g. ultra-sonic pulse velocity, rebound number;

NOTE To align with the design standard EN 1992-1-1, where the compressive strength is based on 2:1 cylinders, the in situ compressive strength is based in 2:1 cores of diameter ≥ 75 mm.

— provides principles and guidance for establishing the relationships between test results from indirect test methods and the in situ compressive strength;

— provides procedures and guidance for assessing the conformity with the compressive strength class of concrete supplied to structures under construction where standard tests indicate doubt or where the quality of execution is in doubt.

(2) This document provides requirements for determining the in situ strength at test locations and the characteristic strength of test regions, but how this information is to be applied needs to be considered in the light of the specific situation and engineering judgement applied to the specific case.

(3) This document does not include the assessment of the quality of concrete for properties other than compressive strength, e.g. durability-related properties. (4) This document is not for the assessment of conformity of concrete compressive strength in accordance with EN 206 or EN 13369, except as indicated in EN 206:2013+A1:2016, 5.5.1.2 or 8.4.

(5) This document does not cover the procedures or criteria for the routine conformity control of precast concrete components using either direct or indirect measurements of the in situ strength.

2 Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 206:2013+A1:2016, Concrete — Specification, performance, production and conformity EN 1990:2002, Eurocode — Basis of structural design

EN 1992-1-1:2004, Eurocode 2: Design of concrete structures — Part 1-1: General rules and rules for buildings EN 12350-1, Testing fresh concrete — Part 1: Sampling

EN 12390-2, Testing hardened concrete — Part 2: Making and curing specimens for strength tests

EN 12390-3, Testing hardened concrete — Part 3: Compressive strength of test specimens

EN 12504-1, Testing concrete in structures — Part 1: Cored specimens — Taking, examining and testing in compression

EN 12504-2, Testing concrete in structures — Part 2: Non-destructive testing — Determination of rebound number --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

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EN 13791:2018 (E)

EN 12504-4, Testing concrete — Part 4: Determination of ultrasonic pulse velocity

EN 13369:2018, Common rules for precast concrete products EN 13670, Execution of concrete structures

3 Terms, definitions, symbols and abbreviations 3.1 Terms and definitions For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses: •

•

IEC Electropedia: available at http://www.electropedia.org/

ISO Online browsing platform: available at http://www.iso.org/obp

NOTE

Abbreviations related to expressions of compressive strength and their meaning are given in 3.2.

3.1.1 core length factor factor for converting the core test measurement or a core test result to the equivalent value of the same diameter core with a length that is twice its diameter

3.1.2 indirect test non-destructive test in accordance with either EN 12504-2 for rebound number or EN 12504-4 for ultrasonic pulse velocity (UPV) 3.1.3 load quantity of concrete transported in a vehicle comprising one or more batches

3.1.4 maturity function of age and temperature such that for a given concrete, any batch with the same maturity has the same compressive strength --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

Note 1 to entry: Maturity is often expressed as equivalent age in days at 20 °C. In accordance with EN 13670, maturity calculations shall be based on an appropriate maturity function, proven for the type of cement or combination of cement and addition in use.

3.1.5 rebound number median of at least nine valid rebound hammer readings taken at one test location after adjusting where necessary for the orientation of the rebound hammer Note 1 to entry:

Note 2 to entry:

The rebound number is expressed as a whole number.

The procedure for determining the rebound number is specified in EN 12504-2.

3.1.6 screening test indirect test procedure with a generic or specific relationship to compressive strength 8

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BS EN 13791:2019 EN 13791:2019 (E)

Note 1 to entry: strength class.

The established relationship may be used to indicate conformity to a specified compressive

3.1.7 small test region for structural assessment a small test region is one that is sufficiently small for the variations in the insitu compressive strength to be primarily due to the selected test locations and testing variability and not due to variations in the quality of the concrete supplied 3.1.8 test location limited area selected for measurements usually used to estimate one test result that is to be used in the assessment of in-situ compressive strength Note 1 to entry:

See Clause 6 (9) and 8.1 (2) for the exception.

Note 1 to entry:

A test region contains test locations.

Note 1 to entry:

A test result may comprise a single ≥ 75 mm diameter core or a single UPV measurement.

Note 1 to entry:

The procedure for determining the UPV is specified in EN 12504-4.

3.1.9 test region one or several similar structural elements or precast concrete components known or assumed to be made from concrete with the same constituents and the same compressive strength class or equivalent to the defined volume associated with identity testing for compressive strength 3.1.10 test result arithmetic mean of the measurements or in the case of a rebound number the median of the measurements taken at a test location 3.1.11 ultrasonic pulse velocity UPV speed at which an ultrasonic pulse passes through concrete

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3.2 Symbols and abbreviations CLF

core length factor

Gp

critical value according to Grubbs’ test

kn

characteristic fractile factor [SOURCE: EN 1990:2002]

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

s

se xi,cor x0

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sc

estimate of the overall standard deviation of in situ compressive strength NOTE 1 See Formula (6) for the calculation of s.

residual standard deviation, which is a measure of the spread of the core strength test data around the fitted regression curve NOTE 2 See Formula (8) for the calculation of sc.

standard deviation of all the estimated strength values, which is a measure of the spread of the estimated core strengths around its mean value NOTE 3 See Formula (7) for the calculation of se. indirect test value at test location "i" that is used for the correlation

indirect test value at test location "0" (where the in situ strength is required for structural assessment purposes)

mean of the m indirect test values used for the correlation NOTE 4 The abbreviations used for compressive strength are given in Table 2. x

10

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BS EN 13791:2019 EN 13791:2019 (E)

Table 2 — Abbreviations used for compressive strength Abbreviation

Description and explanation

fc or fc,cube

Compressive strength determined from samples of concrete taken in accordance with EN 12350-1, made into cylinder or cube specimens and cured in accordance with EN 12390-2 and tested in accordance with EN 12390-3.

fc,core fc,1:1core or fc,2:1core

Compressive strength of a core determined in accordance with EN 12504-1. NOTE

This is a generic abbreviation used to cover all length to diameter ratios.

Compressive strength of a core determined in accordance with EN 12504-1.

NOTE Where the length to diameter ratio of the core is 1:1 the abbreviation fc,1:1core is used and where the length to diameter ratio is 2:1, the abbreviation fc:2:1core is used.

fc,is

Compressive strength of a core taken at a test location within a structural element or precast concrete component expressed in terms of the strength of a 2:1 core of diameter ≥ 75 mm.

fck,is

Characteristic in situ compressive strength (expressed as the strength of a 2:1 core of diameter ≥ 75 mm), i.e. the in situ compressive strength below which 5 % of test results are expected to fall if all the volume of concrete under consideration had been cored and tested.

fc,is,est

NOTE 1 If more than one core is taken at a test location, the test result is the mean of the individual test measurements. NOTE 2 This value is based on the in situ moisture condition and it is not adjusted to a standard moisture condition.

NOTE 1 These values are not normalized to a standard moisture condition. NOTE 2 The in situ volume of concrete under consideration is unlikely to be the same volume used to determine the conformity of the fresh concrete in accordance with EN 206. It is generally a smaller volume.

Estimated in situ compressive strength at a specific test location.

fc,is,highest

Highest value of in situ compressive strength in a set of "n" test locations (expressed as the strength of a 2:1 core of diameter ≥ 75 mm).

fc,is,lowest

Lowest value of in situ compressive strength in the set of "n" test locations (expressed as the strength of a 2:1 core of diameter ≥ 75 mm).

fc,is,reg fck,spec fc,m(i)is

NOTE If more than one core is taken at a test location, the core test values for each test location are averaged and the "highest value" is the highest of these averaged measurements. NOTE If more than one core is taken at a test location, the core test values for each test location are averaged and the "lowest value" is the lowest of these averaged measurements.

Indirect test value converted to its equivalent in situ compressive strength using a regression equation. Minimum characteristic strength of 2:1 cylindrical test specimens associated with the specified compressive strength class. NOTE For example fck,spec is 30 MPa for compressive strength class C30/37. See EN 206 for all strength classes.

Mean in situ compressive strength of a set of "i" test locations (expressed as the strength of a 2:1 core of diameter ≥ 75 mm).

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4 Investigation objective and test parameters (1) Prior to commencing testing on site, the following shall be determined and documented: a) objective of the investigation;

b) standards, test methods and assessment techniques to be applied;

NOTE 1

See A.3 and test method standards for limitations on test methods.

c) test region(s) and test locations;

d) number of measurements per test location;

e) if cores are being taken, the diameter and length of the cores to be taken from the surface;

NOTE 2

f)

The specified diameter of the core refers to the finished core diameter and not the hole size.

where the cores are to be cut to obtain the trimmed length(s) for testing;

g) technique to be used to prepare the ends of the cores;

h) whether sampling and testing shall be undertaken by a laboratory that has accredited procedures according to ISO/IEC 17025 [3];

i)

j)

method of reinstatement after cores have been taken;

any deviations from the procedures specified in this document.

(2) Figure 1 and Figure 2 are flowcharts to help select the appropriate techniques and clauses. --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

(3) Guidance on undertaking an investigation is provided in Annex A.

12

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BS EN 13791:2019 EN 13791:2019 (E)

Figure 1 — Flowchart for the estimation of characteristic in situ compressive strength for the test region and the in situ compressive strength at specific locations

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EN 13791:2018 (E)

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Figure 2 — Flowchart for assessment of compressive strength class of supplied concrete in cases of doubt

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BS EN 13791:2019 EN 13791:2019 (E)

5 Test regions, test locations and number of tests 5.1 Test regions (1) The test regions shall be defined. They may comprise a series of similar elements, one large element or the defined volume associated with identity testing (on-site control) for compressive strength. Different concretes with regard to mix design shall have separate test regions. Where the concrete strengths are not known, engineering judgement shall be applied to group elements into test regions and the test results checked to see whether they comprise more than one concrete. NOTE 1

With existing structures it may not be known whether the concrete: —was produced on or off-site;

—was supplied as a designed or prescribed concrete;

—came from different sources, at different times;

—has undergone variations in curing due to variable exposure.

For these reasons the in situ concrete may fall across a range of compressive strength classes.

(2) Concretes from different batching plants may be placed in the same test region provided the same mix design and constituents are used, e.g. on a large site or ready-mixed concrete plant where there are two or more batching plants.

(3) Where the elements under investigation comprise precast concrete components and in situ concrete, the precast concrete components and the in situ concrete shall form different test regions.

(4) The concept of a small test region is used in this document. Such a small test region shall not include loads that are known or suspected as being significantly different to the other loads comprising this test region. NOTE 2

See definition in 3.1.7.

(5) For Clause 9 procedures, if the volume of concrete is not more than about 30 m3, supplied in a single day and there is no indication that one of the loads may be different to the others, it may be assumed that the supplied concrete does not vary significantly and the variation in test results is primarily due to location within the test region and test variability.

5.2 Test locations

(1) The number of test locations per test region is dependent on the volume of concrete involved, the purpose of the testing and the required confidence of the estimation. The number of test locations per test region shall be determined and specified.

(2) The selection of the test locations shall enable the objective of the investigation (see Clause 4) to be achieved. Each test location shall be determined and specified. The minimum number of test locations are specified in 8.1 and Clause 9. NOTE 1

Guidance on the assessment of existing structures is provided in EN 1998-3 [4].

(3) The number of individual test measurements to achieve a test result varies with the method of test, see Table 3.

(4) Where the objective of the investigation is to estimate the characteristic in situ compressive strength (fck,is), the test locations within the test region shall be selected to take account of the typical variations in strength within the elements, see A.4 for guidance on selecting test locations. --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

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(5) The site conditions to be considered shall include:

— general site location, and ease of transporting test equipment; — accessibility to suspect region onsite;

— safety of personnel onsite and of the general public.

(6) When selecting test locations avoid prestressing steel and ducts and try to avoid:

— cracked areas;

— highly stressed or critical sections; — reinforcement.

(7) The use of a covermeter or radar to help ensure the proposed locations are free of reinforcement or prestressing steel is recommended.

(8) Where indirect test methods are to be applied, see the relevant test method standard for performing the test and the relevant section of this document for guidance on the minimum number of test locations per test region. Table 3 — Types of test and their relationship between test locations and regions

Test

Location

Region

Compressive strength from cores (EN 12504-1)

A test result may be the strength of a single core, the mean core strength if more than one core is taken at the test location, e.g. when a long core is divided into two or more shorter cores. See also Clause 6 for requirements for cores with diameters less than 75 mm.

Rebound numbera (EN 12504-2)

The test result in accordance with EN 12504-2 is the rebound number and it comprises the median of a minimum of 9 valid readings at a test location.

The minimum number of valid test results for estimating the characteristic in situ compressive strength of a test region is eight provided the core diameter ≥ 75 mm, see 8.1 (2), where it is recommended to core at least ten test locations, to allow for possible outliers. For a small test region a lower number of valid test results may be permitted, see 8.1 (6). The minimum number of valid test results from ≥ 75 mm diameter cores for use in combination with indirect testing is three, see 8.3, where at least four test locations should be cored to allow for a possible outlier.

UPVa (EN 12504-4)

A test result may be a single measurement of the ultrasonic pulse velocity measured directly or indirectly, through a section of concrete, or the mean ultrasonic pulse velocity if more than one measurement is taken at the test location.

A regularly spaced rebound hammer survey will show variations in concrete surface hardness over the structure and identify parts of the test region where cores should be taken or further investigations undertaken. A regularly spaced UPV survey will show variations in concrete density over the structure and identify parts of the test region where cores should be taken or further investigations undertaken.

a Neither the rebound number nor ultrasonic pulse velocity are direct measurements of compressive strength, but when specifically calibrated against the concrete used in the structure they may be used to estimate the in situ compressive strength, see Clause 8.

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NOTE 2 If the plot of frequency against test value is a symmetrical bell shape around the mean value, the distribution of test results may be taken as being Gaussian. If there are low peaks in the distribution, these indicate test locations that might need further investigation.

6 Core testing and the determination of the in situ compressive strength

(1) For the Clause 9 procedures, core testing shall not be undertaken on cores with a maturity less than that used as the basis for conformity testing, e.g. 28 days at 20 °C.

(2) Core testing shall be carried out in accordance with EN 12504-1 where specimens are stored in sealed containers, apart from when they are either trimmed to length or the ends are capped ready for testing. (3) The densities of the cores should be determined in accordance with EN 12390-7 and reported. NOTE 1

The density of the core is helpful when interpreting core measurements.

(4) Cores with a trimmed length : diameter ratio of 2:1 or 1:1 and a diameter ≥ 75 mm shall be specified except where it is not practical. If due to reinforcement detailing it is not practical to use ≥ 75 mm diameter cores, core diameters not less than 50 mm shall be specified. NOTE 2 No requirements are specified for converting cores other than 2:1 and 1:1 into an in situ compressive strength (fc,is).

NOTE 3

Further guidance on selecting core sizes is given in A.4.

(5) Cores should be free from reinforcement. Where a core contains reinforcement that is arranged perpendicular to the direction of loading, this shall be recorded and evaluated separately. (6) Any core that contains reinforcement in the direction of coring or close to the direction of coring shall be rejected immediately and a further core taken from the same test location.

(7) For determining the in situ strength, the core test result is converted to the equivalent value of a 2:1 core using the core length factor (CLF). For normal-weight and heavyweight concrete the factor for converting a 1:1 core to a 2:1 core is 0,82 unless a different value is given in the provisions valid in the place of use or a different value has been justified by testing. For other length to diameter ratios, the CLF shall be given in provisions valid in the place of use. For lightweight concretes the CLF shall be given in the provisions valid in the place of use or justified by testing. (8) The requirements to determine the in situ compressive strength at a test location are given in Table 4.

NOTE 4 The aggregate size has a significant influence on the measured strength when the core diameter divided by the upper aggregate size is less than about 3. NOTE 5 The direction of coring is normally expressed as either vertical or horizontal to the element as cast. This standard, or its predecessor EN 13791:2007, does not differentiate between either direction of coring.

(9) Where cores equal to or greater than 50 mm diameter and less than 75 mm diameter are being taken for the purposes of determining the mean strength and there is no interest in obtaining an estimate of the compressive strength at each test location, a single core may be taken at each test location (see 8.1 for the minimum number of test locations). NOTE 6 The strength of smaller cores have a higher variability and therefore the minimum number of cores has been increased to give the same confidence in the test result. There is evidence that with 20 mm upper aggregate size, 100 mm diameter 2:1 cores are approximately 7 % stronger than 50 mm diameter cores (see EN 12504-1), but there was insufficient evidence to quantify the difference for 1:1 cores and so it is not taken into account.

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NOTE 7 To have the same confidence in the test result at a specific test location as that given by an ≥ 75 mm core, the mean of a number of small diameter cores (see Table 4) is needed; however, the same confidence in the mean strength to a test region may be achieved by increasing the number of test locations and taking a single small diameter core at each test location (see 8.1)

Table 4 — Requirements to achieve a test result for a test location

Requirement

Length : diameter ratio Minimum number of core compressive strength values to achieve a test result at a test location In situ compressive strength at test location (fc,is)

Core diameter

Core diameter

50 mma

≥ 75 mm

Nominal: 1:1b Permitted range: 0,90:1 to 1,10:1

Nominal: 2:1 Permitted range: 1,95:1 to 2,05:1

Nominal: 1:1 Permitted range: 0,90:1 to 1,10:1

3

1

1

CLFc (mean of fc,1:1core values)

mean of fc,2:1core valuesd

CLFc (mean of fc,1:1core values)d

a For diameter above 50 mm and less than 75 mm, the minimum number of core compressive strength values should be interpolated and specified. b No provisions are given for 2:1 cores with a diameter of 50 mm. c

See Clause 6 (7) for the value of the CLF.

d

If a single core has been taken, fc,is = fc,2:1core or CLF x fc,1:1core.

7 Initial evaluation of the data set

7.1 Evaluation of the test region to determine if it represents a single concrete strength class (1) Where it is not known that the proposed test region contains a single compressive strength (more likely in the case when Clause 8 procedures are to be applied), all available information on the production control and site records should be used to determine the test regions and locations that need specific investigation. Although it is reasonable to assume that the concrete within a single element comprises one compressive strength class, there are rare exceptions where the assumption is not valid, e.g. when a load destined for another element was discharged into the element under investigation; however, such a load might also be identified as a high or low outlier, see 7.2. Visually inspect the location and strength data to check if there are any anomalous test results that may indicate the test region contains two or more compressive strengths. While careful selection of the test regions will minimize the risk of including two compressive strengths in a test region, it does not exclude the possibility. NOTE 1 For example, the test region may have been based on all the columns within a building. If the data appears to be from two populations, "similarly located elements" would be all columns of a similar size on one or more floors (the strength of the concrete in the columns may have been lower in the upper floors or some columns may have been cast with higher strength concrete in order to take care of temporary cold weather).

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NOTE 2 The location of the core (top, middle or bottom of an element) will have an impact on strength, but if the recommendations in A.4 are followed, these variations will be reduced. Generally it is safe to assume that the concrete in any single element comprises concrete from a single strength class.

(2) If there is evidence from the test results that the test region may contain two compressive strengths, either: — split the data set into two test regions, but note the minimum requirements for a test region or

— split the data into two sets and determine if the mean strengths are different using, for example, a t-test.

NOTE 3 As Clause 8 covers the determination of a characteristic in situ compressive strength and this is based on the mean strength and standard deviation, a test that determines if the mean strengths are significantly different is the appropriate approach. NOTE 4 There is a natural strength variation dependent on casting height due to compacting procedure and efficiency.

(3) If a t-test is being used to determine if the mean strengths are different or one group has a higher value than the other group, the variances shall be pooled.

(4) If the mean strengths are shown to be significantly different the data shall be split into two test regions; if the mean strengths are not significantly different, the data set shall be regarded as being a single test region. (5) This check is not required for investigations under the Clause 9 procedures.

7.2 Assessment of individual test results within a test region

(1) If a data set appears to contain one or more test results that are unusually low or high, these test results should be checked to determine if they are statistical outliers. NOTE 1

See A.6 for guidance on handling outliers.

(2) A set of indirect test results may also contain outliers, which may indicate a need for further investigation at this test location, e.g. a core test. (3) By assessing, for example, the difference between the lowest or highest test result and the mean of all the test results, it is possible to determine if the lowest or highest result is a statistical outlier. The action to take if one or more results are statistical outliers is a matter of engineering judgement. (4) Any established method for assessing statistical outliers is permitted. NOTE 2

See, for example, ISO 5725 [5] and ASTM E178 [6].

(5) The Grubb test may be used to determine statistical outliers provided the data are distributed normally. The highest test value of n consecutive test values should be considered an outlier when f c,  is,  highest − f c, m s

( n )  is

(1)

> Gp

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(6) Critical values (Gp) for testing for outliers are given in Table 5, which are based on a significance level of 1 %. NOTE 3

Other significance levels may be adopted for establishing the Gp values.

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Table 5 — Critical values (Gp) for testing for outliers

Number of test values 4

1,496

7

2,139

5 6 8 9

10 11 12 13 14 15 16 17 18 19

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20 25 30 35 40 50 60 70 80 90

100 120 140 160 180 200 250

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Gp

1,764 1,973 2,274 2,387 2,482 2,564 2,636 2,699 2,755 2,806 2,852 2,894 2,932 2,968 3,001 3,135

3,236 3,316 3,381 3,482 3,560 3,621 3,673 3,716 3,754 3,817 3,867 3,910 3,946 3,978 4,042

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(7) The lowest test value of n consecutive test values should be considered an outlier when f c, m

( n )  is

− f c,  is,   lowest   s

> Gp

(2)

(8) In the case of possible outliers at both extremes, this technique should be first applied to the value that deviates most from the mean. This technique may be applied twice to a set of data for a test region. Before the test is repeated, the first outlier shall be excluded from the calculation of the mean and standard deviation. Each outlier shall be documented and evaluated individually. If more than two test results are outliers, this may be an indication that the test region comprises at least two concretes and this possibility should be examined. Some of the other techniques may permit more than two outliers, but the possibility that the test region comprises more than one compressive strength class should also be considered.

8 Estimation of compressive strength for structural assessment of an existing structure 8.1 Based only on core test data

(1) The in situ compressive strength values (fc,is) are checked to ensure that all values are valid. All valid test results are used to estimate the mean in situ compressive strength (fc,m(n)is) and the sample standard deviation s of the test region in the structure under investigation.

(2) Except for small test regions, the estimation of the characteristic in situ strength shall be based upon a minimum of:

— eight valid test results of in situ compressive strength based on ≥ 75 mm diameter cores in accordance with Table 4 or — twelve valid values of in situ compressive strength each based on a single 50 mm diameter cores from concrete with a upper aggregate size ≤ 16 mm. (3) When applying Formula (3), the sample standard deviation shall be the calculated sample standard deviation s, or the value that provides a coefficient of variation of 8 %, whichever is the greater. (4) The characteristic in situ compressive strength (fck,is) is estimated from the lower of: = f ck , is    f c, m n is − k n s

( )

where

kn is taken from Table 6, or, = f ck , is  f c, is, lowest + M

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NOTE 1 A minimum value of the coefficient of variation is a safeguard against the use of an unrealistic low value where core test results are abnormally close.

(3)

(4)

where

the value of M is based on the value of fc,is,lowest and taken from Table 7.

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NOTE 2

Table 6 — kn values for use in Formula (3)

n

8

kn

2,00

10

12

16

20

30

1,92

1,87

1,81

1,76

1,73

Formula (3) and Table 6 align with EN 1990:2002+A1:2005, Annex D.

∞

1,64

Table 7 — Value of margin M to be applied in Formula (4) Value of fc,is,lowest MPa

Margin MPa

≥ 16 < 20

3

≥ 20

4

≥ 12 < 16

2

< 12

1

(5) It is permitted to use the log normal form of Formula (3). In this case, the formulae given in EN 1990:2002, D.7.2 shall be applied using the unknown coefficient of variation option. The sample standard deviation of the natural logarithms of the strength values shall be the respective calculated value or the value that provides a coefficient of variation of 8 %, whichever is the greater. (6) The minimum number of cores and the assessment criteria for a small test region may be specified in provisions valid in the place of use or the procedure given in (7) may be adopted.

(7) For a small test region comprising one to three elements and a total volume not exceeding approximately 10 m3, at least three cores ≥ 75 mm in diameter shall be taken including at least one core from every element in the test region and calculate the in situ compressive strength (fc,is). If the core locations represent concrete that will remain in the structure, take the lowest value of three or more cores (provided the spread of test results is not more than 15 % of the mean value) as being the in situ compressive strength (fck,is) for structural assessment purposes.

If the spread of results is more than 15 % of the mean, this is an indication that more information about the test region should be sought.

8.2 Based on a combination of indirect test data and core test data

8.2.1 Use of indirect testing that has been specifically calibrated against core data taken from the structure under consideration

NOTE 1 It is often possible to develop a good correlation with 8 pairs of test results. The recommendation to take 10 pairs of test results is to allow for possible outliers and to ensure that the correlation is based on at least 8 pairs of test results.

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(1) Ideally, the indirect testing survey should be undertaken prior to coring. The data from this survey should be used to select the positions for coring. Cores shall be taken at locations where indirect test values are available. At least 10 pairs of test results should be obtained and the core test locations should cover all the extent of the indirect test values, including the extremes, if structurally safe to core at these extremes locations. The core test results shall be converted to values of in situ compressive strength (fc,is) and the indirect test results shall be plotted as the x-axis against the in situ compressive strength values (y-axis). The best-fit linear regression through these points shall be determined and judged if it is reasonable for the evaluated concrete (type, age, concrete).

BS EN 13791:2019 EN 13791:2019 (E)

(2) Whenever practical, sufficient core data should be obtained to establish a specific correlation between the indirect test and the in situ compressive strength. If there are less than 8 pairs of test results, the procedure given in 8.3 should be followed.

NOTE 2 As the range of strength values is likely to be limited, fitting a linear regression is usually adequate. The equations in this clause are directly applicable to linear equations. NOTE 3

No guidance is provided on what is an adequate correlation.

The data set shall be assessed for atypical values.

NOTE 4 The analysis of standardized residuals is a good procedure for detecting atypical values. A standardized or studentized residual is the quotient resulting from the division of a residual by an estimate of its standard deviation.

(3) Using the established linear regression equation, all valid indirect test values are converted to their equivalent regression equation values (fc,is,reg) even at test locations where there are actual core test results. While determining the regression equation values, the regression equation shall not be extrapolated by more than 4 MPa at both ends of the proven relationship. NOTE 5 If the actual core test results are included in the calculations in 8.2.2 and 8.2.3, an error is introduced due to double-counting the variability.

(4) These regression equation values shall be used to estimate the characteristic in situ compressive strength (see 8.2.2) and may be used to the estimation of the in situ compressive strength at specific locations (see 8.2.3). 8.2.2 Estimation of the characteristic in situ compressive strength for a test region (1) The mean in situ compressive strength is estimated as: f c, m

= ∑ ( f c, is, reg ) /  m ( m ) is   

(5)

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(2) The overall standard deviation of the in situ compressive strength of the test region is determined from: = s  s c2   +  s e2

(6)

The value of se is given by: se =  

∑

m

  − f c, m m is  f c, is, reg     ( ) i =1   m−1

The value of sc is given by:

∑ i =1 ( f c,is − f c,is,reg ) n

sc =

n−2

2

(7)

2

(8)

NOTE Formulae (8) and (9) are only valid where the correlation has two parameters. For example is of the form of (y = a + b · x), or (ln y = a + b · ln x). If this is not the case, the term (n − 2) is replaced with (n − p), where p is the number of parameters in the formula.

(3) The value of sc shall be the calculated value or 2,0 MPa, whichever is the greater.

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(4) The effective number of degree of freedom associated with the overall standard deviation s is calculated from: neff =

where n m

s 2 + s 2  e   c s c4

n−2

+

2

(9)

s e4

m−1

is the number of pairs of test results used for establishing the correlation curve and is the number of estimated strength values.

(5) Formulae (3) and (4) are then used to estimate the characteristic in situ compressive strength except that in Table 6 n is replaced with (neff + 1) rounded to the nearest integer. In the application of Formula (4), fc,is,lowest is the lower of the lowest estimated or lowest measured core strengths 8.2.3 Estimation of the in situ compressive strength at a specific location --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

(1) It is not safe to use the mean relationship to estimate the in situ compressive strength at a specific location as there is a 50 % probability that the actual strength is less than the estimated strength. Consequently the in situ compressive strength at a specific location shall be determined as the value of the indirect test result converted to the value on the lower limit curve of the prediction interval for a significance level α of 5 % (one tailed test). (2) For structural assessment purposes, the estimated in situ compressive strength at a specific test location ( f c, is,est ) is calculated using the following formula: f c, is= , est

f c, is, reg

1 − t 0,05,  n − 2    ( ) sc 1 + n +

( x0 − x )

2

∑ i =1 ( x i,cor − x ) n

2

(10)

where 0,05 in t(0,05,n-2) is the alpha value for a one-tailed test with (n − 2) degrees of freedom.

NOTE 1

Formula (10): This strength is not the characteristic compressive strength of the test region.

(3) Formula (10) is only valid for linear correlations. Where linearity is achieved after transformation of variables, the transformed variables shall be used in the calculations.

(4) If there is a ≥ 75 mm diameter core test result at the specific test location, this value shall be used and not the estimated value. NOTE 2 A single 50 mm diameter core is insufficient to give confidence in the in situ compressive strength at a test location.

8.3 Use of indirect testing with at least three core test data

(1) This technique may be applied to a test region comprising not more than 30 m3 of concrete to estimate the in situ compressive strength using indirect methods without calibration where there is no issues over the compressive strength of the supplied concrete.

NOTE 1 See Clause 9 for the situation where there are issues related to the compressive strength of concrete supplied.

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(2) Using ultrasonic pulse velocity testing or rebound hammer testing, survey the test region to determine variability and identify those locations of lower compressive strength. Take at least three ≥ 75 mm diameter or an equivalent number of smaller diameter cores (see Table 4) from the area around the location(s) with the lowest indirect test result and calculate the in situ compressive strength (fc,is). If the core locations represent concrete that will remain in the structure, take the mean value of three or more cores (provided the spread of test results is not more than 15 % of the mean value) as being the in situ compressive strength (fck,is) for structural assessment purposes (see NOTE 3). NOTE 2

See A.3 for limitations on the use of the rebound hammer.

NOTE 3 National provisions may specify, or the engineer involved may select, different criteria for structural assessment purposes, see A.2 (5).

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(3) Where the spread of the test results is higher than 15 % of the mean value, if an investigation provides a justified reason for rejecting one of the core test results, the in situ compressive strength (fck,is) shall be taken as being the mean of the remaining valid values.

9 Assessment of compressive strength class of concrete in case of doubt 9.1 General

(1) Doubt on the achievement of the specified compressive strength class of concrete in structures under construction might arise from various sources. Doubt about the in situ quality may arise from doubts about the quality of the concrete supplied to the site, problems during the execution of the works or after some exceptional event on site. The term "doubt" includes, but is not limited to, the following:

— insufficient compressive strength of samples taken for production control leading to a declaration of non-conformity; — insufficient compressive strength of samples taken for identity testing; — problems during execution of the works.

(2) The criteria in 9.2 and 9.3 are based on and applicable where the criteria for compressive strength in EN 206:2013+A1:2016, Annex B, B.3.1 were used for the assessment of a number of loads delivered to a construction site.

(3) The concrete under investigation shall be split into test regions, for example these test regions might align with the lots used for identity testing. The test region should not exceed more than about 180 m3.

(4) If the procedures in this clause are satisfied, the defined test region shall be accepted as having conformed to the specified compressive strength class. From this it may be concluded that the concrete delivered to site, any adjustments to the concrete on site and any deviation on the execution with respect to placing, compacting and curing as required by EN 13670 or EN 13369, as appropriate, were not significant with respect to compressive strength.

All parties involved should be involved in the decision on the required procedures to assess the compressive strength class of concrete as placed and to minimize cost it may be preferable to consider options in order of least resource, i.e.: — screening test (see 9.4);

— indirect testing plus selected core test data (see 9.3);

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The screening tests in 9.4 provide a fast and non-damaging to the structure method of assessment that may confirm the concrete in the structure came from a conforming population. Failure to satisfy these conservative requirements is not proof of non-conformity but it means that direct testing by coring is needed to resolve the issue. When drilled cores show densities that are clearly lower than standard test specimens, the reason(s) should be clarified, see A.5.

NOTE 1 Failure to satisfy the criteria indicates that the concrete may not have achieved its specified compressive strength class, problems occurred during transport of concrete to the site, any adjustments to the concrete made by the user might have been significant, the execution in placing, compacting and curing the concrete did not conform to EN 13670 or EN 13369, as appropriate, due allowance was not made for the impact of the early-age temperature cycle, or any combination of these factors. The producer and user may need to identify which factors are significant, but this involves taking into account any changes to the concrete supplied by the producer, the voidage and reinforcement in the cores and the maturity of the core at testing. Guidance on quantifying these factors is not provided in this European Standard.

(5) Where the in situ compressive strength test results do not verify the criteria of this clause, the structural adequacy of the works and, if relevant, the implications for durability should be checked. (6) If cores are to be tested, the user and producer shall agree on the core diameter and length to diameter ratio and the laboratory to undertake the testing.

(7) If conformity to the specified compressive strength class had been based on cube testing, the assessment of the compressive strength class as placed may be based on the minimum characteristic cube strength associated with the specified compressive strength class.

(8) The null- hypothesis in the procedures in 9.2 to 9.4 is that the concrete conformed to the specified compressive strength class and the procedures assess whether this is, or is not, a valid hypothesis. (9) Where the producer has declared non-conformity, the procedures in 9.5 apply.

NOTE 2 In this situation it is not technically correct to assume that the concrete conformed to its strength specification and using a statistical test to check if this assumption is correct.

9.2 Use of core test data

(1) Each test region shall be split into volumes of approximately 30 m3. Where there is less than 30 m3 it may be treated as a single volume provided the concrete was supplied on a single day and there was no information that one of the loads may be different to the others. The minimum number of test locations for each volume is specified in Table 8. Using the procedures specified in Clause 6, cores are taken at each test location to obtain a test result for each test location. If both of the criteria given in Table 8 are satisfied, the conformity of compressive strength may be accepted for the test region under investigation. NOTE 1

Thirty cubic metres aligns with three 10 m3 truckloads or four 7,5 m3 truckloads.

NOTE 2 Table 8 and Table 10: The value of 0,85 is the recommended value of conversion factor ƞ given in EN 1992-1-1:2004, A.2.3.

26

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Table 8 — Criteria for assessment based on core test data Number of approximate 30 m3 volumes in test region

Minimum number of test locations for each volumea

Mean of core test results for the test regionb

Lowest test resultb,c

1d

3

—

≥ 0,85 (fck,spec − M)

5 to 6

2

≥ 0,85 (fck,spec + 2)

≥ 0,85 (fck,spec − M)

2 to 4 a

b

2

≥ 0,85 (fck,spec + 1)

≥ 0,85 (fck,spec − M)

See Clause 6 for the minimum number of cores to obtain a test result for each test location.

The core strength may be expressed as fc,1:1core or fc,2:1core depending upon the selected value of fck,spec.

c Where M = 4 MPa for compressive strength class C20/25 or higher. For C16/20, C12/15 and C8/10 the margin M shall be reduced to 3, 2, and 1 respectively. d Provided it is treated as a single volume, see 9.2 (1).

9.3 Indirect testing plus selected core test data

(1) The concrete under investigation shall be divided into test regions not exceeding approximately 180 m3.

(2) At least the number of test locations as given in Table 9 shall be tested by the selected indirect test method. Whenever practical to do so, take at least one indirect test measurement at locations within test regions for every delivery NOTE For the Clause 9 procedures, it is reasonable to assume that the carbonation depth has not exceeded 5 mm and therefore the use of a rebound hammer is an acceptable method.

(3) At each test location a rebound test in accordance with EN 12504-2 or a UPV measurement in accordance with EN 12504-4 shall be undertaken. The apparatus, the test procedure and the expression of test results shall be in accordance with EN 12504-2 or EN 12504-4 as appropriate.

(4) At the test locations specified in Table 10 a core test result in accordance with Clause 6 shall be obtained. Table 9 — Minimum number of test locations for indirect test measurements for the test region

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Number of approximate 30 m3 volumes in test regiona

Minimum number of indirect test locations

1b

9

2 to 4 5 to 6

12 20

a If the volume comprises a large area, the indirect testing should be increased so that it is representative of the variations within the test region. b Provided it is treated as a single volume, see 9.2 (1).

(5) If both of the criteria given in Table 10 are satisfied, the conformity of compressive strength may be accepted for the test region under investigation.

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EN 13791:2018 (E)

Table 10 — Locations for selected coring and assessment criteria

Number of approximate 30 m3 volumes in test region

1d

2 to 4 5 to 6 a

b

Minimum number of test locations for coringa

One core at each of the two lowest indirect test values for the test region

One core at lowest indirect test value for the test region and one core at each of the 2 test locations closest to the median rebound number or the mean UPV for the test region

Mean of core test results at the locations closest to the median rebound number or the mean UPV for the test regionb

Lowest test resultb,c

—

≥ 0,85 (fck,spec − M)

≥ 0,85 (fck,spec + 2)

≥ 0,85 (fck,spec − M)

≥ 0,85 (fck,spec + 1)

≥ 0,85 (fck,spec − M)

See Clause 6 for the minimum number of cores to obtain a test result for each test location.

The core strength may be expressed as fc,1:1core or fc,2:1core depending upon the selected value of fck,spec. c Where M = 4 MPa for compressive strength class C20/25 or higher. For C16/20, C12/15 and C8/10 the margin M shall be reduced to 3, 2, and 1 respectively. d Provided it is treated as a single volume, see 9.2 (1).

9.4 Screening test using a general or specific relationship with an indirect test procedure

(1) The screening-test may be used to estimate the uniformity of the concrete composition in the test region to e.g. determine variability, identify those locations of lower compressive strength, and to estimate whether the specified compressive strength class has been achieved. (2) A generic or a specific relationship between concrete strength and a rebound number or pulse velocity shall be established by procedures given in provisions valid in the place of use or a generic relationship may be provided in the national provisions.

NOTE 1

An example of an established generic relationship is given in Annex B.

NOTE 2

See A.3 and test standards for guidance on limitations of indirect test methods.

(3) This procedure shall be limited to where the indirect test method is appropriate. (4) The same type and model of rebound hammer/UPV equipment that was used to establish the relationship shall be used to test the structure.

(5) While the results of this procedure may be accepted as an indication that the concrete conformed to its specified compressive strength class, failure to meet the criteria is not proof that the concrete did not meet the specified compressive strength class. If the concrete fails to meet these criteria, one of the procedures given in 9.2 or 9.3 shall be applied.

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(6) The rebound test is primarily used to estimate the uniformity of the concrete composition. As proof of load bearing capacity, a compressive strength class according to EN 206 may be assigned to in situ concrete using relationships given in the provisions valid in the place of use or indicating in the national provisions that the relationship given in Annex B may be applied.

9.5 Procedure where the producer has declared non-conformity of compressive strength

(1) In the case of precast concrete components, the manufacturer shall follow the procedure specified in EN 13369:2018, B.5. In addition the manufacturer shall identify the cause(s) of the non-conformity and take action to reduce the risk of further non-conformities.

(2) In the case of ready-mixed or site-mixed concrete, where the producer has declared non-conformity, the producer shall provide the following information to the involved parties, or where the information is not available/unknown identify that this is the situation: — identification of the concrete that was non-conforming;

— estimated characteristic compressive strength of the supplied concrete immediately prior to placing in the structure; — data on which this estimated characteristic compressive strength was based;

— reasoning leading to this estimated strength;

— the causes of this non-conformity;

— proposed actions to reduce the risk of further non-conformity.

NOTE Provisions valid in the place of use may provide further guidance on the appropriate actions to take in this situation.

(3) Where relevant, the implications for durability should also be considered, but this topic is not within the scope of EN 13791.

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Annex A (informative)

Guidance on undertaking an investigation A.1 Information required from the tests (1) Knowledge of the in situ compressive strength of concrete in a structural member as part of an assessment of a structure might be required for one or more of the following reasons: a) structural assessment:

— prior to refurbishment or new use;

— prolonged service life;

— after deterioration of concrete due to: — overloading; — fatigue;

— chemical action;

— fire;

— explosion;

— weathering including freeze–thaw action;

— to ascertain whether the in situ strength of concrete is acceptable for: — the designed loading system; — the actual loading system;

— a projected loading system for a new use;

b) assessment of a test region where there is verification that the concrete supplied is in accordance with the declared compressive strength but test results from samples taken on site indicate nonconformity resulting from, for example: — nonconformity or suspected questionable compressive strength of concrete supplied;

— air content in excess of the maximum permitted;

— water added on site under the instructions of the user without control testing being undertaken;

c) assessment where the producer has declared non-conformity;

d) problems during execution involved in placing, compacting or curing of concrete. 30

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EN 13791:2018 (E)

BS EN 13791:2019 EN 13791:2019 (E)

(2) In planning an investigation, it is important to discriminate between a structural assessment of an existing structure using the Clause 8 procedures and an assessment of compressive strength class of concrete in case of doubt using the Clause 9 procedures. (3) For structural assessment, it may be that the strength of the concrete is known or at least known in some areas, and the purpose of the survey is to determine the strength in areas not known or of interest due to damage, refurbishment or change of use.

(4) Where the concrete strength supplied to the structure is unknown, e.g. there are no as-built records; the estimation of characteristic in situ compressive strength shall be conservative. The less data available the more conservative the estimated characteristic strength should be to compensate for the uncertainty. There are no definitive rules for the application of the test results. For example, the engineer may decide to replace concrete in the structure identified as giving low test results and hence be justified in excluding these test results from the estimation of characteristic in situ compressive strength.

(5) If information is available about an existing structure, it should be used to help define the test regions and indicate what strength classes should be expected after taking account of strength development over time. With existing structures and no reliable construction data, it may be appropriate to check any estimate of the compressive strength with an estimate of the compressive strength from past use. (6) Even where the strength of the supplied concrete is known, it may be helpful to assess the actual strength developed over time.

(7) Where there is verification that the concrete supplied is in accordance with the declared compressive strength but test results from samples taken on site indicate non-conformity, the validity of whether the concrete in the structure came from a population meeting the specified compressive strength class is being tested. As with identity testing to EN 206, the presumption is that the concrete came from a conforming population and the assessment based on in situ testing is performed to determine if this is true or false. (8) Any structural investigation should be carefully planned and executed to ensure that the information obtained is sufficient to provide an adequately reliable assessment of concrete strength in a structure. The detailed test programme will depend upon the reason for the investigation and whether:

a) an estimate of the characteristic in situ compressive strength of concrete in a structural member is required; b) an estimate of the in situ compressive strength at specific locations within an element is required;

c) the investigation is required to determine the strength of concrete on the immediate surface, near to the surface, or at a greater depth (see A.3); d) additional information is required, e.g. uniformity and density of concrete.

A.2 Method of assessment of in situ strength

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(1) For statistical reasons a minimum number of test results may be required to achieve the required level of confidence, but the number of tests shall take account of practical issues such as access, impact on the structure, cost, and the effectiveness of re-instatement at the test locations.

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(2) When the purpose of the investigation is to determine the characteristic in situ compressive strength and not the compressive strength class of the concrete in case of doubt, the options are: — core testing (see 8.1);

— indirect methods with calibration against core data (see 8.2);

— indirect methods with selected coring (see 8.3).

(3) If indirect testing is planned, check limitations of the indirect test method to ensure that there would be a reliable relationship between the indirect test method and coring, see A.3 for guidance.

(4) With all these options it is necessary to:

— select and specify the test region(s) (see 5.1);

— select and specify the test locations (see 5.2) and the number of tests per test location (see 5.2);

— select and specify the core diameter and length : diameter ratio (either 2 : 1 or 1 : 1);

— specify the method of end preparation of the cores (see EN 12504-1);

— specify the method of re-instatement at the locations where cores are to be taken.

(5) The guidance supplied in this document needs to be considered in the light of the specific situation and engineering judgement applied to the specific case. Whether the value of characteristic in situ strength so estimated should be used in a structural assessment will depend upon the particular circumstances. If the calculated value is based on a large number of core or indirect test data, it is an appropriate value for structural calculations. However as the number of test data decreases, the probability that, with an unknown structure, the structure contains (unknown) weaker areas increases.

(6) While coring gives the most reliable measure of in situ compressive strength at a test location, coring is expensive and the holes where the cores were extracted need re-instatement. Coring on its own gives limited information about a structure. Consequently, the trend in practice for older structures is to use indirect testing to obtain a detailed assessment of the uniformity of the concrete in the structure and then use coring to establish a specific relationship between the indirect test measurements and in situ compressive strength.

A.3 Selection of test method

(1) The confidence with which it is possible to assess in situ strength of concrete will increase with the number of assessments made. In the case of some tests (e.g. ultrasonic pulse velocity, rebound number) little extra cost is incurred by obtaining a large number of test data. In other cases (e.g. core data) the cost of each test is appreciable. The decision on the number and type of tests to be made will, therefore, be based upon an assessment of the cost of obtaining an estimated in situ strength of adequate reliability. Core testing provides the most direct measure of in situ strength.

(2) Ultrasonic pulse velocity and rebound hammer tests do not measure the strength of concrete but some other property (pulse-transit time in the case of UPV and surface hardness in the case of rebound hammer) that has a concrete-specific relationship to compressive strength. If the relationship between ultrasonic pulse velocity or rebound hammer and compressive strength is established for a particular concrete as described in this document, a safe and reliable relationship may be determined. The relationship between rebound number and strength is different if the concrete is carbonated. It is best to avoid using the rebound number to assess strength on carbonated concrete, but it is still useful in determining the locations to core. --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

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(3) The relative merits and limitations of tests for various depths from the surface are summarized in Table A.1. There are other tests, e.g. gamma radiography and radar, not listed in Table A.1, but the main purpose of using those tests is something other than the determination of compressive strength. (4) Due to the uncertainty associated with using a limited number of test data, it is recommended that any investigation (other than a small test region) where the number of cores is less than 10 be supported by additional indirect test data, e.g. rebound number. (5) The effect of damage to the structure caused by the testing needs to be taken into account and where damage will occur the method of reinstatement needs to be specified. (6) The choice of test methods should include consideration of:

— general site location and ease of transporting test equipment;

— likelihood of obtaining relevant test results due to geometry, structure, reinforcement, and relative humidity; — accessibility to test region;

— access to electric power, water, compressed air; — safety of personnel onsite and general public; — climatic conditions;

— availability of suitably trained and qualified personnel;

— delays in construction or operation whilst testing is conducted and decisions are made;

— damage to the structure caused by the testing; — delays in completion and handover; — cost.

Table A.1 — Relative merits and limitations of various tests for measuring in situ compressive strength Region tested

In depth Near to surface

Immediate surface

NOTE

a

Test

Standard

Accuracy of strength estimate

Speed of test

Ease of test

Economy of test

Lack of damage to structure

**

**

*

*

Core

EN 12504-1

****

Pull-out

EN 12504-3

** b

Ultrasonic pulse velocity Rebound number

EN 12504-4

EN 12504-2

More asterisks indicate a better performance.

** a

***

***

***

****

** a

****

****

****

***

**

**

**

*

Only if calibrated for the particular concrete under investigation.

b Pull-out testing is more commonly used to determine the strength of young concrete and for this use the accuracy of the strength estimation is higher than given in this table. A lower accuracy is given for the applications covered in Clause 8 as carbonation and incipient delamination may affect the accuracy. --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

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(7) The rebound hammer gives a measure of surface strength and not an assessment of the concrete quality throughout the section. The use of the rebound hammer is not appropriate in certain conditions including: — carbonation depths greater than 5 mm;

— where controlled permeability formwork or surface hardeners have been used;

— fire damaged concrete;

— concrete surfaces the surface has been lost due to chemical or freeze‐thaw action.

(8) Pulse velocity measurements give an assessment throughout the section depth, but the test result is influenced by the moisture content of the concrete and other factors, see EN 12504-4:2004, Annex B. The direct transmission method is more accurate than the indirect transmission method.

(9) The accuracy of estimates of in situ strength obtained from indirect non-destructive tests depends upon the reliability of the correlation between test method and core strength. This document provides methods for obtaining reliable safe relationships. Two procedures for using combined techniques are described below.

— Use of a comprehensive survey with indirect testing, e.g. rebound hammer, with sufficient core testing to establish the relationship between the indirect method and in situ compressive strength for the concrete under investigation. Then all the test data are converted into their in situ compressive strengths. These data are then used to determine the characteristic in situ compressive strength and areas that need more detailed consideration. — Use of an indirect method to locate the lower compressive strength in a test region not exceeding 30 m3 from which to obtain a few cores (see 8.3). In this procedure, there are insufficient core data to establish the relationship between the indirect method and core strength. (10) The reference method is core testing or direct testing of certain precast products.

A.4 Additional guidance for assessment based on core test data (1) Core testing is undertaken in accordance with EN 12504-1.

(2) If the engineer plans to assess the adequacy of the structure using European standards such as EN 1992-1-1, the core tests should ideally be carried out on cores with a length : diameter ratio of 2:1. It is not always practical to take 2:1 cores and therefore the option of using 1:1 cores is also provided but these test results are converted to an equivalent 300 mm by 150 mm diameter core using the relationship given in Clause 6. Clause 6 notes that the aggregate size has a significant influence on the measured strength when the core diameter divided by the upper aggregate size is less than about 3. Ideally the diameter of the cores should be in the range 75 mm to 100 mm. It is strongly advised that the core locations should be such that the cores contain no reinforcement. This had led to using cores with smaller diameters (in the range of 50 mm to 70 mm), but such measurements are more variable [7]. To overcome this variability, this document requires more smaller-diameter cores at each test location with the "test result" being the mean of these cores or more test locations with a single core (see Clause 6 and 8.1).

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(3) The confidence given to the calculated in situ compressive strength when testing identical concrete is estimated from the repeatability/√n. For 100 mm diameter cores with ends prepared by grinding, there is a 95 % probability that the true mean value is within ±14 %/√n of the calculated value [8]. Therefore, it is recommended that a minimum group of four cores should be taken to represent a small test region. If more than four cores are taken from a small test region, the confidence in the mean in situ compressive strength will increase. More cores are needed from larger test regions as there may be variability in the placed concrete as well as variability in testing and location. (4) End preparation by grinding is the most precise but other methods have similar precision for normal strength concretes if they are undertaken by laboratories experience in the capping method [8]. End preparation by grinding is recommended for estimated compressive strengths higher than 50 MPa.

(5) The diameter, length to diameter ratio (2:1 or 1:1) of the cores, where the core taken from the structure is to be sawn to get the test specimen(s) and end preparation method should be specified to the coring/testing company. The diameter is the diameter of the core, not the hole. A small tolerance on the finished length is permitted (see Clause 6), e.g. a 2:1 100 mm diameter core would have a finished length in the range 190 mm to 210 mm. A core with a length : diameter ratio within the tolerances given in Clause 6 is accepted without adjustment for length. If these recommendations are not followed, the individual core test results may need correcting for:

— length : diameter ratio to convert the core compressive strength (fc,core) to in situ compressive strengths (fc,is);

— reinforcement transverse to the direction of loading.

(6) Clause 6 requires a core containing longitudinal or near longitudinal reinforcement to be rejected on visual examination and a replacement core taken.

(7) Both the core compressive strength (fc,core) and the in situ compressive strength (fc,is) should be reported. To be able to assess the structure correctly, the engineer should specify, where appropriate, the reporting of the following additional (additional to the core test results) information from core testing: — density of the core;

— excess voidage;

— any other observations that may be relevant, e.g. cracks or cold joint in core.

(8) The test locations for cores should be such that after cutting the core to length, the core does not contain: — concrete from within 30 mm to any surface;

— concrete within 50 mm or 20 % of the top of the lift, whichever is the higher amount, in sections where the depth of lift is not more than 1,5 m; — concrete from the top 300 mm of the lift, where the depth of lift is 1,5 m or more.

(9) These recommendations are to help ensure that the test results are representative of the bulk of the concrete in the structure.

A.5 Assessment of compressive strength class of concrete in cases of doubt

(1) The procedure to follow will depend upon the cause of the doubt. Where the producer has declared non-conformity, a statistical test based on the assumption that the concrete conformed to its specified compressive strength class is not valid and the procedures in 9.5 should be followed. --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

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(2) Where there was a problem with the execution with respect to placing (including delayed placing), compaction or curing or an exceptional event on site and the concrete has been accepted as conforming to its specification, it may still be appropriate to use the 9.2, 9.3 or 9.4 procedures to determine if the concrete strength is adequate.

(3) Where there is a potential nonconformity of concrete, guidance valid in the place of use, e.g. CIRIA C519: Action in the case of nonconformity of concrete structures [9], should be followed. The initial investigation should include examination of production records and testing procedures. An initial step will be check that the concrete test procedures used to establish conformity and those casting doubt on the conformity were undertaken in accordance with the relevant standards. If the initial investigation identifies the need for in situ testing to resolve a dispute over the compressive strength of the concrete supplied to the site, the options are: — screening test (see 9.4);

— indirect testing followed by selected coring of the weakest concrete (see 9.3);

— core testing (see 9.2).

(4) For these assessments, the null hypothesis is that the concrete came from a conforming population and the fewer the data that are available the less evidence there is to reject the hypothesis and the concrete.

(5) The procedures in Clause 9 assess the compressive strength of the in situ concrete and if the result is positive, the concrete supplied to the structure is accepted as having conformed to the specified strength class, any adjustments to the concrete delivered to site were not significant and that the level of care in placing, compacting and curing the concrete conformed to EN 13670 or EN 13369, as appropriate.

(6) Where the additional testing options 9.4, 9.3, and 9.2, or combinations thereof, indicate that the conformity of concrete to the specified strength class is not proven a verification of the structural adequacy taking into account the estimated strength may need to be undertaken in accordance with provisions valid in the place of use.

(7) Where the concrete failed to meet the Clause 9 criteria, it may be necessary to establish the cause of the low strength using, for example, a procedure to determine the strength this concrete would have achieved if it had been made into test specimens. Nevertheless, with a marginal failure and the uncertainties associated with estimating this strength, it will be difficult to prove that the concrete did, or did not, conform to its specification.

(8) Estimating the strength the concrete would have achieved if it had been made into test specimens rather than placed and then cored is a complex process with a high level of uncertainty. The reasons for this are that the core strengths shall be adjusted for at least the volume of entrapped air above that found in test specimens, curing and maturity to estimate the strength that this concrete would have given if it had been made into test specimens. The maturity of concrete depends upon its temperature and curing history, which is not normally known, and maturity is a function of cement type and content, addition type and content, admixture type, section thickness, formwork type, placing temperature and ambient temperature. The strength–maturity relationship depends upon the type and source of cement. The source of cement is more significant when taking cores at early ages (before a maturity equivalent to 28 days at 20 °C has been achieved). However, from a structural viewpoint, it is preventing inadequate strength in the structure that matters, regardless of whether this was due to erroneous constituent selection, poor concrete, poor execution on site or a combination of these factors.

(9) A further complication arises where the section is large and the concrete has undergone the typical temperature rise and fall, as the strength of such concrete may be up to 20 % less than standard cured specimens [10]. --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

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(10) While a complete analysis of potential strength is difficult and the outcome uncertain, there are aspects of this analysis that are reliable and could be taken into account when assessing the cause(s) of the low strength.

(11) If water has been added to the concrete under the instructions of the client, according to EN 206, the volume of added water should have been recorded on the delivery ticket and the producer should be able to provide evidence of the impact of this added water on the compressive strength.

NOTE Added water may also have an impact on durability but this aspect of performance is not covered in this document.

(12) Using the same sources of constituents and the same mix proportions, it might be appropriate to undertake a test programme to establish the relationships between: a) variations in water content and compressive strength; b) variations in air content and compressive strength;

c) variations in level of compaction and compressive strength.

Using this information in conjunction with the core test data and the other information may help identify the cause(s) of the low strength. --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

A.6 Acceptance of test data

(1) Before any programme is commenced, it is desirable that there is complete agreement between the interested parties on the validity of the proposed testing procedure, the criteria for acceptance, and the appointment of a person and/or laboratory to: — take responsibility for the testing;

— interpret the test results.

(2) It is strongly recommended that testing be undertaken by a laboratory that is accredited for the whole of the test procedure, not just the testing. This will minimize the risk of a dispute over the quality of the test data.

(3) Where a core test result is shown to be a statistical outlier (either suspicious or rejected), the reason should be determined. When on re-examination it is concluded that the outlier was not a valid test result, the core test result should be rejected and not used in the assessment of the strength in a structure or precast concrete component. When the core test result is valid and represents a local defect, e.g. an area that is not properly compacted, the action to be taken shall be determined. For example, the local area might need removal and replacement. When the outlier represents a local defect that is being remedied, the core test result should not be included in any calculation of characteristic in situ compressive strength. There are situations where an outlier needs to be taken into account when assessing structural adequacy, e.g. where the weak area is not being removed and replaced.

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Annex B (informative)

Example of a generic relationship between rebound number and compressive strength class

(1) The following example is taken from the procedure given in the German National Annex to EN 13791:2006.

(2) The apparatus, the test procedure and the expression of test results shall be in accordance with EN 12504-2. At least 9 test locations within the test region shall be selected and at each test location a rebound test in accordance with EN 12504-2 shall be undertaken.

(3) Testing shall be undertaken by a person who has been adequately trained in the use of rebound hammers.

(4) The rebound numbers for all the test locations in the test region are used to determine the median of the rebound number for the test region (second column in Table B.1 or Table B.2).

(5) Provided all of the following conditions are satisfied: — the concrete is normal-weight concrete;

— controlled permeability formwork or surface hardeners were not used;

— a Type N rebound hammer having an impact energy of 2,207 Nm was used for measuring the rebound number based on the rebound distance (R) or by energy or velocity measurements (Q); — the carbonation depth does not exceed 5 mm;

— the rebound numbers meet both the criteria in column 1 and column 2 of Table B.1 (rebound distance) or both the criteria in column 1 and column 2 of Table B.2 (energy or velocity differential);

— the associated compressive strength class (column 3) may be assumed. NOTE 5 mm.

For the Clause 9 procedures, it is reasonable to assume that the carbonation depth has not exceeded

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BS EN 13791:2019 EN 13791:2019 (E)

Table B.1 — Rebound number based on the rebound distance (type R) and associated EN 206 compressive strength classes for normal-weight concrete Lowest rebound number from all test locations in the test region

Median of the rebound numbers for the test region

EN 206 compressive strength classa

≥ 26

≥ 30

C8/10

≥ 32

≥ 35

≥ 30 ≥ 35 ≥ 37 ≥ 40 ≥ 44 ≥ 46 ≥ 48 --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

≥ 50 ≥ 53 ≥ 57 ≥ 62 a

≥ 66

At a confidence level of the 10th percentile.

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≥ 33

C12/15

≥ 38

C20/25

≥ 40 ≥ 43 ≥ 47 ≥ 49 ≥ 51 ≥ 53 ≥ 57 ≥ 60 ≥ 65 ≥ 69

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C16/20 C25/30 C30/37 C35/45 C40/50 C45/55 C50/60 C55/67 C60/75 C70/85 C80/95

39

BS EN 13791:2019

EN 13791:2018 (E)

Table B.2 — Rebound number based on the energy or velocity differential, (type Q) and associated EN 206 compressive strength classes for normal-weight concrete Lowest rebound number from all test locations in the test region

Median of the rebound numbers for the test region

EN 206 compressive strength class a

≥ 25

≥ 34

C8/10

≥ 36

≥ 45

≥ 29 ≥ 42 ≥ 46 ≥ 51 ≥ 56 ≥ 58 ≥ 60 ≥ 62 ≥ 64 ≥ 66 ≥ 69 ≥ 71

a

At a confidence level of the 10th percentile.

≥ 40

C12/15

≥ 49

C20/25

≥ 52 ≥ 56 ≥ 60 ≥ 62 ≥ 64 ≥ 66 ≥ 68 ≥ 71 ≥ 73 ≥ 75

C16/20 C25/30 C30/37 C35/45 C40/50 C45/55 C50/60 C55/67 C60/75 C70/85 C80/95

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BS EN 13791:2019 EN 13791:2019 (E)

Bibliography

[2]

[3]

[4] [5] [6]

[7] [8] [9] [10]

1

CEN/TR 17086 1, Further guidance on the application of EN 13791 and background to the provisions EN 12390-7, Testing hardened concrete - Part 7: Density of hardened concrete

ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories

EN 1998-3, Eurocode 8: Design of structures for earthquake resistance - Part 3: Assessment and retrofitting of buildings INTERNATIONAL STANDARDS ORGANISATION, Accuracy (trueness and precision) of measurement methods and results, ISO 5725

AMERICAN SOCIETY FOR TESTING MATERIALS, Standard practice for dealing with outlying observations, ASTM E178

EUROPEAN STANDARDIZATION COMMITTEE, Inter-laboratory comparisons in support of CEN/ISO standards called up in EN 206, Concrete. Final report on experiment C, June 1997

THE CONCRETE SOCIETY, In situ concrete strength. An investigation into the relationship between core strength and standard cube strength. The Concrete Society, Camberley, UK, 2004 AINSWORTH P.R., HOPKINS C.J. Action in the case of nonconformity of concrete structures. C519. CIRIA, 2000. ISBN 978 0 86017 519 3

HARRISON T. A., Concrete properties: Setting and hardening, Advanced Concrete Technology — Concrete Properties, 2003, p4/11

Under preparation. Stage at the time of publication: FprCEN/TR 17086:2017.

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[1]

41

BS EN 13791:2019

National Annex NA (informative)

Guidance and complementary provisions

NA.1 Introduction NA.1.1 General --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---

(1) This national annex sets out additional guidance and complementary provisions for the use of BS EN 13791:2019 in the UK to ensure that technically sound and established processes continue to be covered by the standard, even though consensus could not be achieved at the European level.

NA.1.2 National provisions

(1) BS EN 13791:2019 permits national provisions in a number of situations. Table NA.1 is the UK committee's position in respect of those permissions. Table NA.1 — UK committee's position on permissions

Clause

Permission

UK committee's position

(paragraph) Introduction (6)

Combining two indirect test methods Guidance on this approach is not provided. with core testing

Introduction (8)

Use of cores of diameter less than 50 mm

Introduction (7)

Introduction (8)

Introduction (8)

Different factors for αcc and η

Use of pull-out testing

A screening test conforming to the principles specified in 9.4

Introduction (8)

Assessing the strength gradient across a section after a fire

Introduction (8)

Relationship between in situ com‑ pressive strength for core length to diameter ratio other than 2:1 and 1:1

Introduction (8)

42

Relationship between 2:1 and 1:1 core compressive strength if a value other than 0.82 is justified on the basis of test data for the local materials

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BS EN 1992‑1‑1:2004+A1:2014 recommends for αcc: 0.85 for compression in bending or axial loading; and 1.00 for all other phenomena. BS EN 1992‑1‑1:2004+A1:2014 permits η × γc to be reduced to 1.3 for direct testing of compressive strength on the structure. Not recommended. Consequently, guidance is not provided. Guidance is not provided.

Given the wide range of aggregates in use in the UK, a general relationship is not considered to be appropriate. The procedure given in Annex B is not recommended for use in the UK. A specific screening test developed for the concrete under consideration is an acceptable approach and a recommended first step if testing the structure is necessary. A proce‑ dure for developing such a relationship is given in PD CEN/TR 17086[NA.1]. Guidance is not provided.

Given the range of constituents used in the UK, there is no evidence to permit a general relaxation. A high‑ er factor is permitted if justified by test data for the concrete being investigated. The need for other length to diameter ratios is ex‑ ceptional and no guidance is provided.

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BS EN 13791:2019 Table NA.1 (continued) Clause

Permission

UK committee's position

(paragraph) Introduction (8) Introduction (8)

Introduction (8) Introduction (8) Introduction (8)

Introduction (8)

NA.6.3 specifies a value of 0.91 if no other value has Relationship between in situ com‑ been established by testing for converting 1:1 cores pressive strength for lightweight concretes and core length to diameter to in situ compressive strength. ratio Adjustment to core strength for cores Guidance is given in BS EN 12504‑1:2019, National containing transverse reinforcement Annex NA. Relationship between core strength and the strength of a cast cylinder of equal diameter and length Factors when the assess‑ ment is other than with BS EN 1992‑1‑1:2004+A1:2014 or BS EN 1990:2002+A1:2005

In the UK, BS EN 1992‑1‑1:2004+A1:2014 and BS EN 1990:2002+A1:2005 are applicable. No guid‑ ance is provided for other standards.

Factor ƞ given in The UK adopts the recommended value and there‑ BS EN 1992‑1‑1:2004+A1:2014, A.2.3 fore no different value is needed. where the national provisions use a value different from the recommend‑ ed value of 0.85 See NA.5. Guidance on appropriate actions where the producer of the concrete has declared nonconformity or where the concrete has been proven to be nonconforming

8.1 (6), (7)

Provisions for fewer than eight cores without indirect testing

8.3 (2)

Different criteria for structural as‑ sessment

9.4 (1)

Comparing an element where the concrete quality is in doubt with a similar element containing conform‑ ing concrete

9.2 and 9.3

Although it is recognized that there may be a differ‑ ence, no relationship is provided.

Different assessment criteria where the criteria for compressive strength in BS EN 206:2013+A1:2016, B.3.1 were not used for the assessment of a number of loads delivered to a con‑ struction site

The procedure given in 8.1 (7) is an acceptable procedure. No additional requirements are specified and the parties involved are free to select a proce‑ dure that fits the specific situation. The UK accepts the given criteria.

The UK uses the criteria given in BS EN 206:2013+A1:2016, B.3.1 and consequently different criteria are not required. A procedure is set out in NA.5.5.

NA.2 Normative references [Cl 2 of BS EN 13791:2019] BS EN 206:2013+A1:2016, Concrete — Specification, performance, production and conformity BS EN 1990:2002+A1:2005, Eurocode — Basis of structural design

BS EN 1992‑1‑1:2004+A1:2014, Eurocode 2: Design of concrete structures — Part 1-1: General rules and rules for buildings

BS EN 12504‑1:2019, Testing concrete in structures — Part 1: Cored specimens — Taking, examining and testing in compression

BS 8500‑2:2015+A2:2019, Concrete  — Complementary British Standard to BS  EN  206  — Part  2: Specification for constituent materials and concrete --`,,``,,,,,`,`,,```,`,`,`-`-``,```,,,`---



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BS EN 13791:2019

NA.3 Terms and definitions, symbols and abbreviations [Cl 3 of BS EN 13791:2019] NA.3.1 Terms and definitions [Cl 3.1 of BS EN 13791:2019] (1) The terms and definitions given in 3.1 apply together with the following additions.

(2) The core strength given in Table 2 (fc,1:1core or fc,2:1core) should not be significantly affected by the presence of transverse reinforcement. See BS EN 12504‑1:2019, National Annex NA. NA.3.1.1 excess voidage

voidage in excess of that present in test specimens that have undergone standard compaction

NA.3.2 Symbols and abbreviations [Cl 3.2 of BS EN 13791:2019]

(1) The symbols and abbreviations given in 3.2 apply together with the symbols in Table NA.2. Table NA.2 — Symbols

Symbol

Explanation

Kv

correction factor for excess voidage

X̅ r

mean ultrasonic pulse velocity (UPV)/rebound number of the reference test region

Sr

Ss

X̅ s

sample standard deviation of reference test region

sample standard deviation of the test region under investigation

mean UPV/rebound number of the test region under investigation

NA.4 Estimation of compressive strength for structural assessment of an existing structure — based only on core test data [Cl 8.1 of BS EN 13791:2019]

(2) BS EN 1992‑1‑1:2004+A1:2014 requires 2:1 cores for structural assessment, and the use of such specimens does not require adjustment for establishing the in situ compressive strength. The use of 2:1 cores is not always practical or appropriate if structural assessment is based on other design codes.

NA.5 Assessment of compressive strength class of concrete in case of doubt [Cl 9 of BS EN 13791:2019] NA.5.1 Introduction

(1) NA.5.2 provides further guidance on the assessment of compressive strength class of recently supplied concrete using in situ testing where nonconformity has been declared in accordance with 9.5.

(2) NA.5.3 sets out a comparative testing procedure for assessing placed concrete where there is no doubt concerning its compressive strength class with placed concrete where there is doubt. The in situ testing is carried out using either ultrasonic pulse velocity or rebound number.

44

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(1) In general, the procedure given in 8.1 should be used. Nevertheless, it is recognized that if the test region is small, it may be appropriate to estimate the in situ strength with fewer than eight cores. The risks associated with such a procedure, as described in PD CEN/TR 17086[NA.1] should be taken into consideration when planning to use a low number of core test results without an indirect test survey.

BS EN 13791:2019 NA.5.2 Concrete declared as nonconforming NA.5.2.1 General (1) Concrete might be declared as nonconforming for a number of reasons, such as:

• failure of an individual batch to satisfy the BS EN 206:2013+A1:2016 minimum compressive strength criterion;

• failure to satisfy the BS EN 206:2013+A1:2016 mean compressive strength criterion for an assessment period; and •

failure by the contractor to satisfy the Engineer's specification — for example when a purchaser has instructed the producer to change the specification by adding water at site but the Engineer requires conformity to the original specification.

NOTE This situation is different from that of investigating an unknown structure, because in this case the producer is likely to supply additional data on the concrete supplied as part of the analysis of the nonconformity and its consequences.

(2) As the concrete producer has declared nonconformity in accordance with 9.5, there should not be a need to establish responsibility. An exception is where the producer has accepted that the concrete is nonconforming on the basis of being instructed to add additional water on site by the purchaser and the purchaser is claiming that the concrete would have been nonconforming even if they had not changed the specification. (3) The options available to the Engineer are described in NA.5.2.2, NA.5.2.3 and NA.5.2.4. NA.5.2.2 Accept producer's estimated characteristic compressive strength

(1) In many cases of nonconformity the producer is able to estimate the actual characteristic strength of the concrete supplied from the test data, batch records and knowledge of the cause of the nonconformity. Provided that the uncertainty associated with such an estimate is taken into account, the Engineer may use such data for checks on the structure without having to undertake in situ testing. (1) Even when the concrete is nonconforming or part of a nonconforming population, the structure might be adequately strong. A structural assessment is needed to determine whether this is the case. NA.5.2.4 Determine the characteristic in situ compressive strength

(1) The characteristic in situ compressive strength may be determined using any of the procedures specified in Clause 8 or using the producer's estimate of characteristic in situ compressive strength and checking this estimate with the Clause 9 procedures.

NA.5.3 Comparative testing

(1) In addition to the three methods given in BS EN 13791:2019, Clause 9 for estimating compressive strength in case of doubt, comparative testing may be used in some circumstances. For example, when the recently supplied concrete under investigation is in one or more of a series of elements where other elements have been accepted — that is the other elements have been made with conforming concrete and the level of care in placing, compacting and curing the concrete conforms to BS EN 13670:2009 or BS EN 13369:2018 as appropriate — the approach is to compare the concrete in the elements under investigation (test region under investigation) with the concrete in elements that have been accepted (the reference test region). This may be done using ultrasonic pulse velocity or rebound number. It is not a suitable procedure to use with cores because if cores are taken, there is no need to compare them with a reference test region. 

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45

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NA.5.2.3 Determine whether a batch or a limited number of batches satisfies the minimum compressive strength criterion

BS EN 13791:2019 (2) The null hypothesis1) is that the mean strengths are the same. The fewer the data, the lower the chance of showing that the hypothesis is not correct. For this reason it is recommended that indirect test values are taken at not less than 20 test locations in the test region under investigation. The same number of indirect test values should also be taken from equivalent test locations in the reference test region.

(3) By applying the accepted statistical principles of hypothesis testing, this technique may be applied to any combination of data sets, but this general approach is not detailed in this national annex.

(4) Agree a reference test region that is similar to the test region under investigation. Select a reference test region that has a similar maturity to the test region under investigation or select a more mature element where the difference in maturity will have a minor effect. Select a set of 20 test locations that are the same in the test region under investigation and the reference test region (to minimize differences due to location in the element). At the 20 test locations in the reference test region measure the ultrasonic pulse velocity or the rebound number. Calculate the mean value (X̅ r) and the sample standard deviation (Sr). Repeat with the test region under investigation and in this case the mean value is denoted X̅ s and the standard deviation as Ss. NOTE 1 The rebound number at each test location is the median value of at least nine valid readings and is expressed as a whole number. The median values for each test location are used to calculate the mean value of the medians for each region, X̅ r and X̅ s, which is then used for the statistical comparison.

(5) Calculate



(NA.1)

If the numerical value is not more than 2.024 and not less than -2.024, then there is no significant difference, at the 0.05 two-tailed significance level, between the concrete under investigation and the reference concrete.

NOTE 2 The range from +2.024 to -2.024 is for a two tailed t-test at a significance level of 0.05 for 38 degrees of freedom (i.e. n = 40 results). The limits are denoted t0.05, 38.

(6) With this procedure the higher the significance level, the lower the numerical difference between the mean strengths to be regarded as significant, but it would be unusual to use a significance level higher than 0.05. As reducing the number of pairs of results from 20 will increase the numerical difference between the mean strengths for it to be regarded as significant, this should be avoided wherever practical. NOTE 3

See PD CEN/TR 17086[NA.1].

NA.6 Additional guidance for assessment based on core test data [Cl A.4 of BS EN 13791:2019] NA.6.1 Length to diameter ratio (1) Where practical, either 2:1 or 1:1 cores should be taken in accordance with BS EN 12504‑1:2019 and BS EN 13791:2019. If the limitations for core locations given in A.4 (8) are applied, an adjustment for the direction of drilling is not appropriate. NOTE BS EN 12504‑1:2019, National Annex NA does not include a formula for converting other length to diameter ratios into in situ compressive strength.

1) This is the hypothesis that there is no significant difference between specified populations, any observed difference being due to sampling or experimental error.

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BS EN 13791:2019 NA.6.2 Adjustment for transverse reinforcement (1) Cores containing reinforcing bars should be avoided wherever possible by the selection of an appropriate core diameter and the use of cover meters when deciding where to cut the cores. Provided the amount and position of any transverse reinforcement in the core comply with the requirements given in BS 12504‑1:2019, National Annex NA, NA.3, an adjustment for the presence of reinforcement is not appropriate. For other cases the impact of reinforcement on core strength is variable and the measured value obtained is unlikely to represent the strength of the concrete. These results should be discarded.

NA.6.3 Adjustments to core strength of lightweight concrete

(1) The factor for converting 1:1 cores of lightweight concrete to in situ compressive strength should be taken as being 0.91 except where a different value has been proven by testing. NOTE The factor 0.91 is the average of the ratio of cube to cylinder strength given in BS EN 206:2013+A1:2016, Table 13.

NA.7 Additional guidance for the assessment of compressive strength in cases of doubt [Cl A.5 of BS EN 13791:2019] NA.7.1 General

(1) Cores drilled for the estimation of characteristic compressive strength should not be taken from areas of poorly compacted concrete. The test locations should be such that the test length of the core conforms to the recommendations given in A.4 (8). Whenever practical, the tested cores should have a length to diameter ratio of 1:1 (i.e. a length to diameter ratio in the range 0.90:1 to 1.10:1, see Clause 6), as these proportions represent the commonly used cube for conformity purposes in the UK. (2) This publication does not take into account differences in compressive strength based on core diameter, and it is established that, on average, smaller diameter cores are weaker than larger diameter cores. Consequently, where practical, the core diameter should be the same as the size of the specimens used for conformity (e.g. 100 mm diameter core if 100 mm cubes were used for conformity). (3) When reporting core test data for a Clause 9 procedure, an estimate of the voidage should be provided. See BS EN 12504‑1:2019, National Annex NA for guidance on reporting this value. However, the core strength in the test report should not include any adjustment for voidage.

NA.7.2 Adjustment for excess voidage

(1) After a nonconformity of recently supplied concrete has been reported, it may be appropriate to establish to what extent poor compaction may have contributed to this nonconformity (see general guidance in A.5). (2) Where the excess voidage exceeds 2.5%, the estimated fully compacted in situ cube strength and any subsequent estimate of the characteristic compressive strength of the supplied concrete should not be regarded as being reliable. However, excess voidage above 2.5% is an indication that the concrete has not been compacted properly. (3) Air entraining admixtures entrain air and many other admixtures entrain a small percentage of air. The air content of test specimens that have undergone standard compaction should be established to enable excess voidage to be estimated.

(4) There are two uncertainties associated with the effect of excess voidage. The first is the estimate of excess voidage itself, and it is often regarded as being subjective if it is done by comparison with

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47

BS EN 13791:2019 the reference samples and not by measurement. This uncertainty associated with a comparison can be minimized by having more than one determination of the excess voidage and taking the average value.

NOTE The method for estimating excess voidage given in BS EN 12504‑1:2019, National Annex NA, NA.4.2 has limitations. The comparison with reference photographs is subjective and although attempts are being made to develop a method to scan and record the areas of voids, such a method is not yet fully developed [NA.2].

(5) The other uncertainty is associated with the relationship between excess voidage and strength reduction. The values given in Table NA.3 are average values. The uncertainty associated with this relationship has not been established. (6) An estimate of in situ strength assuming fully compacted concrete may be calculated from:

where the excess voidage correction factor, Kv, is given in Table NA.3.

(NA.2)

Estimated excess voidage

Correction factor to fully compacted in situ strength (Kv)

0.0

1.00

1.5

1.09

0.5 1.0

2.0

2.5A)

1.03

1.06 1.12 1.15

Where the excess voidage exceeds 2.5% it is unlikely that any estimate of the fully compacted in situ cube strength using an assumed voidage correction factor is reliable.  A) 

(7) Variations in compaction will not be the only reason there are differences in compressive strength of test specimens and in situ concrete. Notwithstanding this caveat, applying a strength correction for excess voidage may give a useful indication of strength if the concrete had been fully compacted.

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Table NA.3 — Correction factor for excess voidage, Kv

BS EN 13791:2019

Bibliography [NA.1] PD CEN/TR 17086,2) Further guidance on the application of EN 13791 and background to the provisions

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[NA.2] TRUE, G.F. and SEARLE, D. Digital imaging and analysis — cores aggregate particles and flat surfaces. Concrete, June 2012. pp. 16-18.

2) Under preparation. Stage of CEN's Technical Report at the time of publication: FprCEN/TR 17086 (publication anticipated in 2020).



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