MESTRADO INTEGRADO EM ENGENHARIA CIVIL 2008/2009 REABILITAGAO E REFORGO ESTRUTURAL Modulo 4 Avaliagao do Estado das Estruturas Inspecgdo e Ensaios Concrete Core Testing for Strength Concrete Society Technical Report No. 11, 1976Concrete Society Technical Report No. 11 Meee mie) mci Report of a Concrete Society Working Party a RecastMembers of the Working Party Corresponding members *J..D. Dewar (Convenor). 8Se(En9), CEng, MICE, MITO E. A. Burks, 8Se(Eng). CEng. FICE, FINE AMC Technical Services Limited Burks, Green and Pariners L. Collis, Bse. Fos K. Green, Bse, CEng FICE, MISmuete. IME Messrs Sandberg Burks, Green and Pariners J.D. Llewellin, 84, CEng, MICE D.C. Lewis, asciéng), CEng, MICE WE C French (Construction) Limited Property Services Agency “W. E. Murphy, BSc(Eng), CEng. MICE, FINE Cement and Concrete Association 7J.M, Plowman. 8Se. PhD. ACG. DIC, FICE, FIStuct ALAIo Consulting Engineer “PA. Warren, Bsc RMC Technical Services Limited “Drafting panel Acknowledgements ‘This work on cores was initiated by the Materials Technology Divisional Committee of The Concrete Society in 1873. A first draft of this report was presented to 2 Concrete Society meeting on the Future for the Core, held in London on 12 September 1973, The working party is grateful to the contributors to discussion at that meeting. to all those who sent in written comments: following advertisement and distribution of copies of the Graft during 1973 and 1974, and more recently to Mr ‘A. E. Brooks of the Cement and Concrete Association Concrete Society Technical Repor No. 11 First published May 1878 158N 07210 1088 x Price £5:00, Published by The Concrete Society Terminal House, Grosvenor Gardens, London SW1W O&J Designed and printed by the Cement and Concrete Association Wexham Springs, Slough SL3 6PL Funer copies may be obtained trom: Publications Sales Unit, Cement and Concrete Association Wexham Springs, Slough SL3 6PL. ‘quoting reterence number 51.071 ‘© The Concrete Society 1976 ‘Although The Concrete Society (limited by guarantee) does its best howsoever and from whatsoever cause ausing. is accepted inthis respect by the Society is servants or agentsConerete Society Technical Report No. 11, ~ Concratecore testing forstrength May 1976 ay = ‘Resort of a Concrete Society Working Party Summary Contents ‘This report provides recommended procedures for obtaining andthe page 2 1 Scope compressive testing of concrete cores and for the interoretation of ies the results. Evidence from practice and research is provided for the pee. formulae ang conversion factors recommend. 3. 2 Introduction 3. The reasoning behind the procedures 3 Non-destuctive test methods 3. Background to the two procedures © Conversion factors 4 Use of estimated values 5 BS 18811970 5 Other uses for cores 6 3 Procedures for obtaining and compressive ase testing of cores, and interpreting the results” se 73.1 Making the decision to dil cores i psn 1022 Planning and preliminary work Cecloucas TAM 16 3.3 Obtaining the cores 18 34 Laboratory work 20 38 Estimating Actual or Potential Strength 223.6 Interpretation of resuts 24 Appendix 1: Correcting Core Suength for the influence of Excess Voisege 28 Appendix 2: & method of estimating Excess Voidage by visual means 27 Appendix 3: The influence of curing history upon Core Stents 28 Appendix 4: Conecting Core Strength for the influence of included stee! 29 Appendix 5: & worked example using the recommended Procedures 32. 4 Other uses for cores 33. 5 Evidence from research and practice 33° Introduction 33. Relationship berween Core Strength and Actual Strength 35 Estimation of Actual Strengtn 37 Relationship between ‘Actual ané Potential Strength 38 Estimation of Potential Strength 38 Ditference between Actual and Potential Strenath= 39 Etect of age upon Core Strength 40 Varibiity of results 41 Further research 41 References “The index on page 6 constitutes the detailed contents of Part 3. IsTITUTO suPEnion TEcwIco iiaConcrete core testing for strength Part 1. Scope The sim of this report is to recommend procedures to be followed when arilling and testing cores to assess the strength of concrete in pavements, in situ structures and precast units. The procedures are designed, on evidence from practice and research. primarily for concretes made with Portland cements and natural aggregates and for cotes sampled, treated and tested to BS 1881 : Part 4 1970. “The conversion factors given in the report are considered 10 be applicable generally to concretes containing admix- tures, but should be used with more caution for concretes which: contain lightweight or artificial aggregates: contain cements other then Portland: have extreme values for mix proportions: are inadequetely compacted: have been subjected to unusual. variable or extreme con- ditions have deteriorated. These cautions apply more to estimations of Potenti Strength than to estimations of Actual Strength. Definitions ‘The quality of concrete, as assessed by making and testing cubes in accordance with BS 1881, is different from that, of concrete in an in situ element, @ pavement or @ precast unit, There are many reasons for this, among them being ‘that the methods of compacting the cubes and storing ‘them until they are tested differ from the treatment given to the remainder of the concrete. In this report, the following terms are used for compres- sive strength relating to ‘potential’ or ‘actual’ quality of concrete. All except No. 4, Core Strength, are cube strengths or equivalent cube strengths. 1. Stanaerd Cube Strength ‘The compressive strength of a cube sampled, moulded and tested as defined in BS 1881 : 1970, 2. Potential Strength The notional strength of concrete considered as the average Standarc Cube Strength at 28 days for @ single batch of concrete moulded wholly as standard cubes. 3. Estimated Potential Strength An estimate of Potential Strength from a limited number of standard cubes or cores. 4. Core Strength ‘The ‘measured compressive strength’ of a core as de: fined in BS 1881 : Part 4 : 1970, clause 3.3. 5. Actual Strength ‘The notional strength of concrete at a single location, considered as the strenoth of a cube of the concrete as it exists in the structure 6. Estimated Actual Strength An estimate of Actual Strength from the test of @ core drilled from the structure.Part 2. Introduction ‘The reasoning behind the procedures The reasons for drilling cores for strength tests are com- monly to assess one or a combination of the following (1) The auality of the concrete provided to the construc: tion {Potential Strength). (2) The quality of concrete in the construction (Actual Strength). (3) The load factor of a structure to carry (2) the actual loading system (b) the designed loading system: (c) a projected loading system for @ new use (4) Deterioration in the structure due to (2) overloading: (©) fatigue: (€) chemical reaction (@) fire or explosion: (@) weathering. For purpose (3), an estimate of the Actual Strength provides @ measure of strength of tne concrete at @ par- ticular lecation which can be applied in structural calcula- tions. For purpose (4), values of Actual Strengths from cores drilled from affected and unaffected locations may enable assessments to be made of the degree and extent of deterioration. For some of these purposes, the use of cores may not provide the most accurate means of assessing the quality Concerned, but may be the most economic and practical method available. A simplified approach to the order of tests for different purposes is shown in Table 1. It is obviously preferable to use a primary test, Le. one correlating most directly with the strength aspect under review, whenever economic and practicable. It is unfor- tunate that. although an ultimate load test of a structure provides an accurate measurement of its strength, it also removes the structure, and proves an uneconomic method. (On the other hand, the further removed the test method is ‘rom the property of interest, the less reliable the informa, tion will be, Thus, the standard cube, as a tertiary test, supplies relatively litle information about the behaviour of the structural element, Concrete cores, although a primary means for assessing concrete strength in the structure. are unfortunately rele gated to a secondary position for assessing the strength of the standard moulded cube. However. when standard cube test results are not available or their validity is doubted. the core test may assist in finding answers to the two questions: (a) is the structural element of adequate strength? (b) was concrete complying with the specification sup- plied to the construction? and may allow some distinctions to be made between effects of external conditions, site workmanship and the concrete supplied to the construction. Non-destructive test methods ‘Two methods in general use. ultrasonic and rebound- hammer testing, can be useful in supplementing core testing by (1) locating areas of potential weakness or variability: (2) enabling areas between core lacations to be surveyed. thus reducing the number of cores required for large areas or volumes of concrete. ‘The use of either method to assess strength, without speci- fic correlation with relevant cube or core strengths, is deprecated, It can be seen from Table 1 that they are. at best. secondary or tertiary methods since they do not measure strength directly. For assessment of deterioration, non-destructive tests can be used for detailed surveys or for continuous moni- toring In addition, the covermeter is perticularly useful for avoiding steel when coring. Gamma radiography. although ot in general use, owing to the special safety requirements necessary, may be used to locate honeycombed areas and reinforcement at greater depths. Background to the two procedure: Two procedures are provided in Part 3, one for estimating Actual Strength and one for Potential Strength. The appro- priate locations for coring a structure will usually differ for the two purposes. For example, for Actual Strength it may Table 1 Simplified order of test methods for different aspects of strength. ‘Aspect of strength Test method Utimat Non-déstructive load test Load test Core test UUtimate strength of structural Primary Secondary Secondary Tertiary Tertiary member Loading capacity fof structural Primary Primary Secondary Tertiary Tertioy ‘member Concrete svength in struct Secondary Secondary Primary Secondary Secondary ‘Standard moulded Concrete cube Tertiary Tertiary Secondary Primary Tertiany strength9¢ relevant to core at a location of poorly cured or com- pacted concrete, whereas for Potential Stength it is agsential to core from concrete representative of a batch. Although the separate procedures will produce the most “eliable information for each purpose. this does not imply hat it will always be necessary to cut cores for both pur- noses. For example, if Potemtial Strengths estimated by the ‘ecommended procedure pass the compliance require- ‘nents of the specification, it would usually be unnecessary (0 take further cores to check Actual Strength. Similarly. if Actual Strengths are acceptable to the engineer, it may be seemed unnecessary to pursue Potential Strength, A single core provides a single estimate of Actual or Potential Strength only in the immediate vicinity of the sore. A number of cores ate needed to provide a reliable average value for assessing Potential or Actual Strength. Obviously. @ large number of cores taken for any situa- (on will increase accuracy but in many instances. when Jelays are not critical, when there is no criticality of the structural element or when division of responsibility is not necessary, @ small number may be accepted. If doubt per- sists, additional cores can be cut and tested within @ few ays. The recommendations are based on the concept of com- tomise between reasonable accuracy and cost. for @ wide spectrum of circumstances where the:cost implications of acceptance of low strength or rejection of satisfactory strength in @ structural element may be several orders of “nagnitude greater than the cost of core testing, Conversion factors The Core Strength and an estimate of Actual Strength at he age of extracting a core can be obtained with reason- able accuracy in many instances. Translation of 2 value Forward or backward in time reduces the accuracy, because the Core Strength-may very with age owing to cement characteristics, maturity, moisture history, internal move- ments, stressing due to load and any aggressive environ- nent, such that cores A, B, C and D in Figure 1 may have different strengths which beer no simple relation to each other. Potential Strength estimated back in time from core 4 will be more reliable than from core D and may be similar to Standard Cube Strength. provided allowance can 3e made for the factors which have affected the concrete rn the structure differently from the sme concrete in @ standard cube. To enable the most accurate values to be calculated for Actual and Potential Strengths, it is necessary to have sither (a) 2 range of conversion factors from which a selection can be made of the most appropriate ones for the particular circumstance: or (b) average values for conversion factors which will suit most circumstances, In some cases, where a significant influence can be ‘eadily quantified, e.g. the length/diameter ratio for a core, 1 is possible to provide @ range of conversion factors. In ‘ther cases, where measurements are likely to be unavail- able or difficult to assess, e.g. moisture history of concrete, “nethod (b) has to be used. Both methods have been used ‘or the procedures given in Part 3. and all factors are based on the comprehensive analysis of date from practice and “esearch provided in Part 5. twill be noted that the process of conversion in Part 3 sme Figure: tlustration of the relationship in time between Standard Cube Strength, Core Strengths, Actual Strengine ang Potential ‘Strengths ate single location in e structure differs from that given in BS 1881 by removing the un- necessary transition stage of first converting the Core Strength to a cylinder strength before conversion to a cube strength. The recommended procedure also allows for the influence of reinforcement. drilling damage. direction of drilling, curing and the age of testing in addition to in- fluences of size and shape. Recommendations for drilling locations and sizes of cores are made to minimize the effects of variation due to some of these factors. Use of estimated values Figure 2 illustrates the procedures for a case where it has been necessary to cut cores in different locations to assess both structural integrity and compliance of the concrete with the specification, where CP 110 provides the criteri for judgement. Estimated Actual Strengths can be compared, as an average of individually, with structural criteria and to ‘assess effect of environment or workmanship. The average Estimated Potential Strength for the batch can be com- pared with specification compliance criteria for Standard Cube Strength, ‘A worked example. demonstrating the use of the pro- cedures in a practical situation, is provided in Appendix 5 to Part 3, ‘Typically. Estimated Actual Strength is numerically less than the ‘estimated cube strength’ calculated to 8S 1881 1970 trom the Core Strength whereas the Estimated Poten- tial Strength is higher,ee a we Pt 2 580 « ounce etey a ees [eee Figure 2: Mlusttation of the ox ‘where CP 110 provides the cn a for judgement. BS 1881 :1970 BS 1881 : 1970 provides a basis for core testing from the stage of receiving a core at the laboratory up to the dete mination of the Core Strength. However. it does not pro- vide sufficient guidance on the location and extraction of cores or 2 sound basis for conversion to Actual or to Potential Strength. ‘The term ‘estimated cube strength’ in BS 1881 : 1970 is considered to be misleading and has been taken erron- eously in practice to imply equivalence with the strength of a cube sampled, made. cured and tested to Parts 1, 3 and 4 of the Standard for moulded specimens, It is recommended that measurement of concrete den- sity and the restriction of the ratio of length to diameter of ‘dures for assessing Actual and Potential Strengths a core after capping to 1-1-2 should be made mandatory requirements for core testing in the next revision of BS 1881. and that sawing of plane ends before capping should be included as a recommended practice. Specific guidance. as supplied now for cube testing. is required in relation to mode of core failure in the strengthrtest. Other uses for cores Although this reportis concerned specifically with concrete core testing for strength, there are many other uses for cores. A list of such uses is provided in Part 4. Some of these other uses may aid interpretation of core strengths. It is possible that detailed guidance on them may be merited in the futurePart 3. Procedures for obtaining and compressive testing of cores, and interpreting the results Index pape 7 10 10 10 10 10 10 u 2 2 1“ 15 18 16 16 16 16 16 16 v 7 7 7 18 18 8 18 18 9 19 6 20 20 20 20 2 2 2 2 2 2 24 28 a 8 28 3.1 Making the decision to drill cores 31.1 Gene 3122 Proceaure 3.2.3 Definition 3112.2 Geneva 31.2.3 Preliminaries 31.24 Restictione 3.1.2.5 Precision of Estimetee Actua ng Potent! Strengthe 2.2 Planning and preliminary work 32.1 Gene 32.2 Procesue 32.2.1 General 3.2.2.2. Necessity fr the west and its aims 3.2.2.3 Determination of genera riling arexand location of, teintorcement 32.2.4 Number of cores 3225 Location 32.26 Sae of cores 32.2.7 Ancilary date 32.2.8 Testing laboratory 32.2.9 Required stengih levels 32.2.10 Supervision 3.3 Obtaining the cores 3.3.1 General 332 Procedure 3.32.1 Driling and extraction 3322 Examination 3.323 Core isenification 3.3.2.4 Diiling report 3.3.2.8 Designation of test length and despatch 10 the laboratory, 3.4 Laboratory work 3.4.1 General 3.4.2 Procedure 3.4.2.1 Trimming to rest length 34.2.2 Examination 34.2.3 Capping 34.2.4 Density cetermination 3.4.2.8 Testing the core 3.5 Estimating Actual or Potential Strength 38.1 General 35.2 Proceaure 3.5.2.1. Estimation of Actual and Potential Swengtns 35.2.2 Correction for the presence of stel in the test core 3.5.2.3 Significance of Estimated Actual end Potential Strengths 3.6 Interpretation of results 36.1 General 362 Procedure 3.6.2.1 Actual and Potential Strengths Appendix Conecting Core Strength for the influence of Excess Voidage ‘Appendix? A method of estimating Excess Voidage by visual means Appendix 3 The influence of curing history upon Core Strength Appendix 4 Comecting Core Strength for the influence of included stee! Appendix A worked example using the recommended Procedures, cube Strength. Thus Actual Strength and Potenti cube strengths. To aid users of the Procedures. key aspects have been boxed or printea in bolder type. Clauses relating to Actual Strength only and to Potential Strength only are shown respectively on the left- and right-hand sides of a page and have the prefixes A/ and P/ respectively. All strengths in Part 3 are stiengths of their equivalent, except for the measured Core ‘Strength are equivalent3.1 Making the.decision to dri 3.1.1 General Before deciding to drill cores for compressive testing, itis essential that full tion be given to the necessity for the test. its aims and the value Of the results which will be obtained. The consideration will generally hinge fon whether it is required to establish the serviceability of a structural element trom an assessment of the strength of the concrete in it (Actual Strength), or to estimate the strength of the concrete provided for the ‘manufacture of the element (Potential Strength) and normally expressed as the average strength of a number of cubes sampled, cast and tested in accordance with BS 1881 Either situation may arise when a cube test result is deemed not to comply with a given specified value. CP 110 advises on the necessity of checking the validity of the result as the first action. Three possible situa~ tions then arise. (1) Where, on investigation. all the parties having # direct professional or ‘commercial interest in the matter (TM Parties) are agreed that the test result is valid, further testing is not normally justified and subsequent action is best based on a consideration of the magnitude of the result. (2) Where the Parties are agreed that some testing deficiency or deficien- cies have been present, the test result should be rejected and further consideration of the true potential quality of that batch of concrete is, not normally justified. If, however. the location of the concrete is judged critical by the engineer responsible for the performance of the structute, he may deem it necessary to obtain a valid assessment of the ‘Actual or Potential Strength and will be obliged to resort to the inter- ‘pretation of the results of some test or tests of the quality of the concrete in the work. The conclusions drawn in such cases should not. how- ‘ever, be regarded as proof of non-compliance on the part of the concrete producer. (3) Where the Parties disagree regarding the validity of the cube test ‘result, a proposal to make a second estimate of the Potential Strength of the concrete may be seen as an acceptable form of arbitration. In this situation, it is essential that the methods of test and interpretation of the results be agreed by the Parties before proceeding, otherwise the disagreement may become extended to the second estimate of Poten- tial Strength and little has been gained. In many cases the method used for estimating the Actual Strength or Potential Strength will be the core test. This part of the report recommends Procedures for the taking and testing of cores, and interpretation of the results such that the Actual Strength or the Potential Strength may be obtained with agreement, item by item, between the Parties. It is seldom possible to obtain valid and fully appropriate estimates of both Actual and Potential Strengths from the same cores. Hence, the ‘special conditions appropriate to the two approaches should be appraised by using the Procedure given in 3.1.2 before taking further action. ACTUAL STRENGTH (Clauses A/...) 3.1.2 Procedure A/3.1.2.1. Definition Estimated Actual Strength is defined as the strength of concrete sampled from an element and tested in accord- ‘ance with this Procedure such that the result, expressed as {an equivalent cube strength, is an estimate of the concrete strength as it exists at the sampling location, without correction for the effect of curing history, age or degree of ‘compaction. POTENTIAL STRENGTH (Clauses P/. P/3.1.2.1 Definition Estimated Potential Strength is defined as the,strength of concrete sampled from an element and tested in accord- ance with this Procedure such that the result is an estimate of the strength of the concrete provided for the manufac ture of the element expressed as the 28 day BS 1881 cube strength, allowance being made for differences in curing history. age and degree of compaction between core and BS 1881 cube.A/3.1.22 General ‘The core test is used for the estimation of Actual Strength when the values obtained will enable the serviceability of the element sampled to be assessed and where other forms of test. which may be more convenient, comprehen- sive. faster, cheaper or less damaging to the appearance of perlormance. are deemed unacceptable for reasons of inaccuracy. Reasons for requiring an estimate of Actual Strength may range from fire or other damage, to failure to test soncrete in @ critical location or the use of an unknown or suspect grade of concrete. Resolution of a dispute regard- ing the validity of cube test results, however, requires the estimation of Potential Strength. ‘The Actual Strength depends upon the quality of the concrete provided for the manufacture of the element plus the quality of workmanship and subsequent history. Interpretation of the estimate. regarding serviceability of the element containing the concrete, will depend upon the design philosophy involved. A/3.1.2.3 Preliminaries H workmanship or subsequent history are suspect or if standard cube test results are not available, core testing aan Actual Strength of the concrete is appro- However, when the Standard Cube Strength is Known for the batch or may be reesonably inferred, and i the workmanship and subsequent history are deemed acceptable, the element may be essessed by the following recommended procedure; on the basis of this assessment itmay be deemed unnecessary to proceed with the estima: tion of Actual Strength from cores. (1) Estimate the probable location of the suspect concrete in the element by observation, from records or by the use of non-destructive tests (e.g. ultrasonic or re- bound-hammer tests). and ascertain that, with the ‘exception of cube strength. the concrete is otherwise acceptable regarding compaction. finish, etc. (2) Examine the design of the element to determine the part of the suspect concrete that will be"most highly stressed in service. (3) Calculate. according to the design method applicable to the element, the ratio (A): Compressive strength required at most highly stressed location in suspect concrete Compressive strength required at most highly stressed location in element as @ whole or Design compressive stress (maximum for section) at ‘most highly stressed location in suspect concrere Design compressive stress (maximum for section) at ‘most highly stressed location in element as @ whole 4) Calculate the ratio (8): Stendard Cube Strength of suspect concrete Minimum cube strength permissible by design method for the most highly stressed location in element (ess than or equal to the specified cube strength) i) If (8) is greaterthan (A). the element may be regarded as having adequate strength (subject to comparable P/31.22 General ‘The core test is used for the estimation of Potenti ‘Strength when the values obtained may assist in the reso- lution of a dispute (usually resulting from disagreement over the validity of test cubes) regarding the quality of concrete used in the manufacture of an element, where other forms of test, which may be more convenient, com- prehensive. faster. cheaper or less damaging to the apps ance or performance. are deemed unacceptable for reasons of inaccuracy. ‘The precision of an estimate of Potential Strength is such that it should not be regarded as an alternative to @ valid BS 1881 cube test ‘The Potential Strength is a measure of the quality of the concrete provided for the manufacture of the element but is not influenced by the quality of workmanship or the subsequent history. Interpretation of the estimate, regarding specification ‘compliance, will depend upon the wording of the speci- {ication involves. P/3.1.2.3 Preliminar H concrete of suspected sub-specification quality exists in the element (by virtue of sub-specification but disputed cube test results), the Potential Strength of the batch may be judged from core tests. In addition. because of the possibility of intermixing of batches of fresh concre uring placing. assessment of Potential Strength may ‘appropriate for batches closely associated with the suspect batch.checks on bond strength etc.) and proceeding with the estimation of Actual Strength fram core tests may be judged unnecessary. A/3.1.2.4 Restrictions The estimation of Actual Strength from cores can be under taken at any age, irrespective of the workmanship applied to the concrete (compaction. curing) and after chemical cor physical deterioration; and for concrete made from any type of cement and aggregates. P/3.1.2.4 Restrictions The estimation of Potential Strength is restricted 10 con- crete made from Portland cement and dense (normal- weight) aggregates. The concrete may not be younger than 28 days when tested, whilst interpretation becomes progressively less precise as the age increases beyond 28 days. Restrictions are imposed by the Procedure on the loca- tions within elements which may be drilled to provide test cores, and in some circumstances these may preclude the determination of Potential Strength from core tests, Occasionally cores complying with the restrictions imposed by these Procedures will permit the estimation of both Potential Strength and the Actual Strength required for assessing element serviceability It may be appropriate to sample concrete containing a localized or un- typical defect. In this event, the Actual Strength estimated should not be interpreted as typical for the element or for a batch. It would be most inappropriate to use such a sample for Potential Strength. ‘The estimation of Potential Strength from core test results must take into account factors likely to make the strength ‘of the concrete in the core (Actual Strength) differ trom that in the standard BS 1881 cube. These factors include the influence of voidage and curing differences, and both ‘ate functions of the nature of the concrete concerned. Curing differences (e.g. on-site curing compared with water curing) will in particular. produce a strength differ- ence (between Actual and Potential Strengths) which will depend upon (a) the particular curing history of the concrete in the core and the age attest: and (©) the strength-gain rate. which is a function of the chemical and physical properties of the cement and aggregates. and the composition of the cement. The strength-gain rates of Portland cements are more variable at early ages than they are after 28 days. In pat- ticular. the gain in strength after 28 days of the site-cured concrete in cores may be very small for concretes made with Portland cement and dense aggregates. and hence tne ratio of Actual Strength (at ages > 28 days) 10 Poten- tial Strength (28 day BS 1881 Standard Cube Strength) is nearly constant for a given curing history of the concrete in the core. Research data are available which allow an estimate of the ratio to be used for Portland cements, dense aggregates. curing histories typical of construction in the UK and ages after 28 days; this Procedure for the estimation of Potential Strength is restricted to such situa~ tions This Procedure cannot at present be used for concretes containing non-Portland and pozzolanic-mixture types of cement. because the ratio of Actual Strength : Potential Strength for these is not at present well enough known Similarly. the influence of lightweight aggregates upon the ratio is not known adequately for lightweight concrete to be covered by this Procedure. although there is some reason to believe that the ratio Actual Strength : Potential Strength will increase with increasing aggregate absorp tivity.A/2.1.2.5 Precision of Estimated Actual Strength ‘The core testis intrinsically more vs P/3.1.2.5 Precision of Estimated Potential Strength ble than the cube test, well cut and capped cores and well made cubes having typical coefficients of variation due to testing alone of 6 and 3% respectively. Hence. the average estimate of the Actual Strength at 3 given sampling location Gerived from n cores is likely to lie (with 95% confidence) within + 12%//n only of the tue value of the concrete contained in the core samples, it must therefore be appreciated that, when a comparison of the average estimate and a structurally required value is, made, 2 tisk of the comparison being inconclusive will exist which can only be reduced by increasing n and resolved by the application of engineering judgement. In addition, the factors necessarily incorporated in an ‘estimation of Potential Strength from core test results are such that litle improvement in the reliability of the estimate is likely to be obtained with any increase in the number of cores beyond four per batch of suspect concrete. Fewer than four cores per batch of suspect concrete a not permitted under this Procedure but even with four or more, the average estimate of the Potential Strength ob- tained is likely to lie at best within + 15% only of the rue value for the batch, for sampling and testing reasons alone. In cases where the curing history of the element sampled is abnormal and not allowed for, the average estimate of Potential Strength can be further at error. It must hence be appreciated that when 2 comparison of the average estimate and @ specified value is made, 2 risk of the comparison being inconclusive will exist which cannot be reduced and can only be resolved by agreement. 3.2 Planning and preliminary work 3.2.1 General The basis for the decision 10 cores for the estimation of Potential or Actual Strength should be communicated to the appropriate interested erties (i.e. those having professional or commercial interest in the con- crete), and a planning meeting convened. preferably on site. In some cases, the complexity of the drilling locations may indicate that it is desirable to have a representative of the dling contractor at the meeting, 3.2.2 Procedure 3.2.2.1 General Planning and pretimir ry work should cover the following points. (a) The necessity for the test and its aims. (b) Evidence of the location of the suspect concrete from site records or non-destructive test survey results. (c) Proposed drilling locations, number and size of test cores. (4) Ancillary work, e.g. density tests and curing history. (©) Strength levels required by the specification (Potential Strength) or design (Actual Strength) and action to be taken if the esti- mates obtained from cores are clearly greater, less or inconclusive. (1). Responsibilities of individuals regarding execution of the work Detailed recommendations relevant to points (a) to (f) are given below, and it is emphasized that the successful application of this Procedure in a core testing situation depends upon the comprehensiveness of the planning ‘meeting and the degree of agreement reached between the interested parties before further action is taken, 13.2.2.2 Necessity for the test and its aims full discussion of Actual Strength is given in section 1.2, but certain points should be appreciated, |) Actual Steength is the strength of the concrete as it ‘ists in the element at the time of drilling, 2) Actual Strength is the end result of the quality of the snerete used, the workmanship applied to it and all other ° P/3.2.2.2 Necessity for the test and its aims A full discussion of Potential Strength is given in section 3.1.2, but certain points should be appreciated. (2) Potential Strength is the strength of the concrete as itwould have been at 28 days after being made into cubes, cured and tested in accordance with BS 1881 (2) Potential Strength relates to the quality of the con-historical or environmental factors up to the time of xiling (3) An estimate of Actual Strength obtained from core results is of limited accuracy relative to the true Actual Strength of the concrete in the cores. (4) An estimate of Actual Strength can be applied to a given design method to appraise the actual serviceability of the element sampled, without introducing the concept of a safety factor for strength (see 8S Code of Practice CP 110) with virtually no restrictions on the nature of the concrete involved. 3.2.2.3 Determination of general dri reinforcement crete used. corrections-being rade to allow for the effect (on core strength of tre workmanship applied to the con- ‘crete and other historical and environmental factors up to the time of drilling (3) An estimate of Potential Strength obtained from core results is of very limited accuracy relative to the Standard Cube Strength of the batch of concrete sampled. Types of concrete which may be sampled are also restricted (4) An estimate of Potential Strength from cores can be compared with cube strengths required by a specification and, by introducing a safety factor for strength (see BS Code of Practice CP 110), can be used to assess the potential serviceability of the element sampled by assum- ing normal workmanship, curing etc. for the element. (8) An estimate of Potential Strength trom cores may help to resolve a dispute over the validity of suspect cube test results. ling area and location of ‘The location of the suspect concrete in the element should be determined by visual inspection or from records. If necessary, a non-destructive test should be used to determined its boundary with other concret ‘The probable location of steel within the expected depth of the core ‘sampling should also be determined by using a covermeter (preferably) or from site records, and its position relative to the expected drilling surface marked on the element, Asset of cores for the estimation of Potential Strength must relate to a single suspect batch of concrete if comparison with disputed cube test results for that batch is to be made. The boundary of the suspect concrete should therefore be checked to confirm that the volume of concrete contained is not greater than the size of the suspect batch. The section of core to be tested should not include the top 20% (to a limit of 300 mm) of the lift concerned. The top 60 mm should not be included in any case. In rare cases. these requirements may preclude the estimation of Potential Strength from cores. Where this upper concrete is present in the suspect batch, its location should be clearly marked. Note. Most concrete will, when placed, display a degree of seci- mentation (gravity drauving dene materials to the bottom of the lift land displacing the lighter air and water upwards). Hence, a lite aie dnd rather more water ate likely to be trapped inthe concrete towards the 10p of the lift, leading to reduction in strength. Research shows tat. generally. the concrete weakened by sedi mentation will be restricted to the upper 20% of the lit depth in shallow lite (< 15 m), and to the upper 300 mm in deep lifts (215m. ‘The Procedure requires. for estimation of Potential Strength, that the concrete in the tett cores should not diffe in composition from ‘that which would nave been sampled for making cubes to 8S 1881. 0 that concrete from the upper 20% of the lft (or 300 mm for lifts ‘deeper than 1-5 m) is regarded as unrepresentative of the concrete supplied to the construction and not appropriate for coring. Research also shows that the sttength of the top 50 mm layer of conerete in & lift i often significantly less than that of the mass of Concrete Below owing to the rapid dying it may receive oF the vag aries of the weather and of site curing procedures. Since the correc tions for the influence of curing upon the ratio Actual Strength Potential Strength made later in thie Procedure are only accectably accurate for the mass of concrete below the surface layer, the depth of concrete which. owing to both curing and sedimentation problems, may not be included in test cores for the estimation of Potential ‘Strength is: the upper 20% of the lift (minimum thickness 50 mm, ‘maximum thickness 300 mm), uA/3.2.2.4 Number of cores The number of cores required to provide an estimate of Actual Strength at e single sampling location is one. How- ‘ever, the Actual Strength estimated from @ single core is likely to lie (with 86% confidence) within 12% only of the true Actual Strength of the concrete in that core, I the number of cores taken from the sampling location is increased, the reliability of the average estimate of Actual Strength improves as follows, Number of 95% confidence limits on cores mean estimate of Actual (a) Strength 1 £12% 4 = 6% 8 = 4% 16 3% tt may be of assistance, when considering the number of cores 10 be drilled. 0 bear in mind that. whilst it might be quite proper to deem a small sectioned column struc- turally inadequate from the result of a single core test, on the basis that one core represents @ good sample of the quemtity of concrete which could lead to feilure, it would rot be proper $0 to do if the single core had been taken from a large element. Hence, itis recommended that the number of cores (n) tilled should reflect the volume of concrete truly liable to, render the element structurally inadequste, end thet the subsequent interpretation of the estimates of Actual Sttength should accommodate a tolerance of =12%/v/non their mean. Where the individual ultimately responsible for ‘interpretation finds the potential region of ‘net proven’ of +125//n'for the initially proposed value of 7 toc large, 17 should be increased to an acceptable value. A/3.2.2.5 Location Cores may be drilled from any location within the suspect concrete, according to the purpose of the investigation, and preferably clear of reinforcement. in most cases, how- ever, the serviceability of the element sampled can best be uudged by drilling where the ratio Required compressive strength or design compressive stress ‘Actual Strength of suspect concrete ‘5 expected to be highest. ‘The Actual Strength is usually lowest in the top 20% {50 mm minimum, 300 mm maximum) of a lift, However, he applied compressive stress in service will usually be i) @ maximum at mid-span in beams and slabs and near he extreme fibres of the compression zone (topside of imple beems and slabs) and ii] near unitorm with height and across the cross-section 1 walls and short columns. ence, cores from beams and slabs will generally be taken ‘om that part of the suspect concrete nearest to mid-span nd drilled from the compression (usually top) face: vhereas those from walls and columns will generally be aken from the top of the suspect concrete in the lift and ray be drilled from any surface. 2 P/3.2.2.4 Number of cores The minimum number of cores required by the Procedure to provide an estimate of the Potential Strength of » batch of suspect concrete is four. These cores are required to be of uniformly and well compacted concrete and, where possible, free of embedded reinforcement. I @ poorly compacted layer of concrete or steel is found to be present in core when itis extracted from an element. it will often be possible to trim the core length to remove ‘these and stillretain an adequate test length. In other cases. it may be necessaty 10 make an on-the-spot decision to Grill one or more extra cores. In exceptional cases, the location of steel in the element may make it impossible to obtain steel-iree test cores. Where avoidance of steel does Prove impossible, it must be understood thet the subse- ‘quent interpretation of the test results is less reliable. ‘The mean estimate of Potential Strength obtained from four or more individual core test results (even with well ‘compacted, uniform and steel-free cores) cannot be re- {garded as a reliable statement of the true 28 day BS 1881 cube strength to better than + 15%. Hence, @ potential region of ‘not proven’ of = 15% must be anticipated when. the subsequent comparison of the Estimated Potential Strength with some specified or required value is made, ‘even after exercising great care in sampling the cores and allowing for exceptional curing and compaction or the presence of steel P/3.22.8 Location Cores drilled tor the estimation of Potential Strength should be taken such that each represents an approx imately equal amount of the suspect concrete. However, in addition to the obvious need to avoid badly compacted concrete and reinforcement, this Procedure requires that the test length of core should not contain concrete from ‘the top 20% (50 mm minimum, 300 mm maximum) of the lift which the suspect batch partly or wholly comprised, Note. Where Potential Strengths to be estimated, the coring loce- jons and the length of core subjectec to comprestion testing must be Carefully chosen s0 thatthe concrete sampled has been aflected es litle as possible by the effects of sedimentation during placing and uncerigin curing, Secimentation occurs with most types of concrete ang placing techniques, whatever the depth of the lift may be (ie. slabs as well as columns). such that an excess of water and air mi be trapped in the upper layers ofthe concrete rendering it untypical of that used to make the element and therefore unsuitable for coring for the estimation of Potential Strength, Hence. avoidance of this upper layer of ‘unrepresenta- tive’ (unacceptable for assessment of Potential Strength) concrete will generally dictate the drilling location and Girection as follows, (2) Wells, columns and deep beems will normally be Grilled horizontally below the unrepresentative upper layerWhere the element is of such slender proportions that the removal of test cores could lead to doubts regarding future serviceability (even after making good). cores should be drilled from what is judged to be the nearest suspect concrete in an acceptably non-critical location. Occasionally, the concrete may show. under visual or non-destructive examination. a region of apparently lower than-average quality large enough to influence the ser- of the lift-or element cores-have to be drilled vertically from the top of the element, they should be of such length that the required test length (not less than one diameter) can be obtainec after removal of the unrepresentative con- crete. (b) Shallow beams and slabs will normally be drilled ver- tically downwards through both unrepresentative and representative concrete, only the latter being used for the viceability of the whole element. In such cases. it may be appropriate to take cares from this concrete for the estima- tion of Actual Strength. Where. however, a localized or untypical defect is present in the drilled core, either by Geliberate selection of the dtilling location at an observed defect or by chance, it will be generally inappropriate to infer the serviceability of the element from any estimate of ‘Actual Strength obtained from that core. test length of core. Where feasible, however. grilling up- wards may facilitate the avoidance of reinforcement and reduce the extent of the drilling required to obtain suitable test cores. It should be noted that, in rare cases involving small batches or very large and deep lifts. the suspect concrete may lie totally within the unrepresentative upper layer of the lift. In such cases, Potential Strength cannot be d mined satisfactorily and, should cores be taken from unre- resentative concrete, the estimated Potential Strength yielded by application of the formulae and factors given in this Procedure may be depressed (relative 10 the true Potential Strength) by 30% or more. (ce) Pavements, which ate invariably drilled downwards from the surface. may be constructed in one layer or two, In the former case, the top 60 mm, or 20% of the slab depth if this is greater, should be regarded as unrepresentative concrete and should not be included in the test length. Ifa slab has been laid in two courses. the upper will normally be about 60 mm in thickness and it will not be possible to obtain a core (even of unrepresentative con- crete) from this course in accordance with this Procedure. ‘The upper course should, however. have been compacted while:the lower course was stil plastic: if this was done, the slab can be considered as being of one ‘lift’ so that little, if any. of the lower course need be considered to be of unrepresentative concrete. Pavements are usually compacted by the application of ‘a vibrating beam to the upper surface. The effectiveness of the vibration tends to diminish with depth. Particular attention should, therefore. be paid to the examination of the cores extracted and their voidage before proceeding to estimate Potential Strength. ‘The face of the element from which cores will be drilled is usually fixed by the above and by practical considerations. However. it may be noted thet tilling is usually easier and cheapest in the vertically downward direction and hardest and dearest in the vertically upward direction. In some special cases, it must be anticipated that aesthetic considerations regarding the appearance of the element after coring may influence the selection of sampling locations. Also. whilst cores should never be cut from locations such that the coring itself renders the element unserviceable, ‘there will invariably be a need to ‘make good! after coring. Note. Whilst making-good is not a topic which has been investigated in depth in the production ofthis document. itis suggested that restitution should be by ether (1) ramming a dry (low-sheinkage) concrete of suitable potential strength: (2) pouting Portiand cement grout or epoxy resin into the dry hole and inserting a cast Cylinder (100 mm and 180 mm standard test cylinders should fit nominal 100 mm and 150 mm holes witha slight clearance) or an untested core of suiteble length anc quality lof concrete, and using @ pumping’ oF ‘screwing’ action to bed the cylinder in the grout ‘ot resin and force it out through the narrow annular gap. Where the maintenance of appearance is ctitcal, consideration might be given to sawing off and returning the outermost part of the core To its original position in the riled hole, leaving only the internal portion and the annular gap to be fled with fresh materia. 133.226 Size of cores Cores of both 100 and 150 mm nominal diameter are permitted under these Procedures, provided the nominal maximum aggregete size does not exceed 25 mm and 40 mm respectively. Whenever possible, however, 150 mm. diameter cores should be drilled, as less variability due to dtilling and more iable results are obtained, with the following exceptions: (2) when the reinforcement is congested and 100 mm diameter cores are therefore less likely 10 contain pieces ot steel: () when sampling deeperthan 150 (b) when use of @ 160 mm drill bit mm from the surface isnot de- and rejection of unrepresent- sired (e.9. when the compres tive concrete will not permit the sion concrete ina shallow beam production.of @ test core of for slab is less than 150 mm adequate length. thick). Exception (b) above is based on the fact that test cores may not, under this. Procedure, have a length less than one diameter nor greater than two diameters, when capped for test. In general, however, short cores are preferred 10 long cores (length/diameter, d = 1-0 to 1-2), Note. This preference is based on 3 consideration of several factors. (9) Short cores have a geometry closer to that of cubes than long cores. Consequently, ‘compressive testing machines which perform satistectoniy when testing cubes should aio perform satisfactorily when testing short cores, Long cores, on the other hand. may be susceptible toe latent, ang otherwise undetected, fault in the machine (possibly resulting from piaten rotation uncer load) end fii unéer 8 combination of Compression and bending, with reduction in the subsequent estimate of Potential Swrengtn, (2) Long cores. particularly it exacted from slender sectioned elements, may vary in voidage along their length, Hence, whilst thet failure lose. ang the subsequent estimate of Potential Strength which may be mage, will be symptomatic of the section of core having the grestest voidage. any allowance made for the etiect of voids willbe based on the average (over-all) voidage of the core anc. as # conse- quence, he estimate of Potential Strength will be depresses, (3) The relation beween cube and core strength is clesrly dependent upon the dite {erences in geometry of the two specimens and this fact i Vaken into account in the formulae given in clause 3.5.2.1. However, és is almost always the case when two itferent test variables are being compared. their elation to each othe is not unique land may be influenced 10 some degree by many factors other than (in this cate) ‘geometry slone. For this reason, it ie argued that the use of snort cores (having [Geomenry close to that of cubes) presents a correlation Detween the core and cube ‘eat least influenced by considerstions of macrnne sensitivity (refered ton 1 above), ‘aggregate type and size, cement type, workability level ofthe concrete when placed, Poissons ratio ang platen restraint. etc. (4) From elements of small section. short cores are mare readily obtsines than long one, (6) The lower driling coste associsted with short cores are evidently desirable and ameliorate the unfortunate requirement in some cases for rejection of unepreser tative concrete from the test length. (8) The weakening of he element, damage to reinforcement. and extent of msking good are all minimized, noice Dia. Test Possible probleme Choice Dia Test Possible problems length length (mm) (ene) (mm) (mm) 160«180, May include stee! bars intest First 150150 ‘May include steel bars in est length. length. May sample concrete to 2 greater depth than desired [Not permitted tor sampling 180-300 (as length increases). 180 300 slabs thine than 200 mm. 700 100 ‘Not permitted if nomi 700~—«100 ‘Not permitted # nominal aggregate size exceeds 28 mm. aggregate size exceeds 25 mm, May sample concrete at lesser Cr for sampling slabs thinner eptn than oesites (it snort than 180 mm." cores used), May produce less reliable * 100200 results Lest 100 700 “in all cases. the test length of core must nt include unrepresemtative concrete. Avoidance of this concrete when diiling into the sides oF sotfits of deep elements is generally easy. but when drilling down- \wards the depth of coring must allow for the subsequent rejection ofLnrepresentative concrete-tom-thedest length. Thus, if sia i to be dlled from the upper surface, ts depth will dictate the length cote dalled anc tested Slab Core_———Lengthof core eeph ia, ejected tested (em) (mm) (mmm) (oom) 750 60 180-240 a0 100 60 100-200 150 50 150-200 280 100 50 100-200 150 50 150 ae 100 50 100-150, 180 100 180 50 100 ‘When ariling less than the full thickness of an element, iti recom ‘mended te dil beyond the required distance to ensure that adequate length is recovered on extraction. P/3.2.2.7 Ancillary data ‘The basic method of estimating Potential Strength given by this Procedure assumes that suspect concrete is well ‘compacted and of normal curing history. It may be neces- sary to make special adjustments to Estimated Potential Strengths, however, for compaction or for curing if either has been abnormal. The following ancillary deta should therefore be obtained in anticipation of need. (a) Potent! Density of concrete ‘The Potential Density of concrete (0, is the 28 day cube density, determined by displacement, which would have been obtained from well compacted cubes made and cured, in accordance with BS 1881. Where the 28 day densities of cubes taken from the suspect concrete are not in dispute, their average should be used as Dp, Where valid cube densities for the suspect concrete are not available. however. D, should be estimated from the mean density of 28 day cubes taken from the same mix (assuming these are valid). adjusted for any agreed def ciencies in the suspect concrete. As a working rule it may be assured that, with the exception of concrete so grossly rmisbatched as to have been not recognizable as the speci- fied grade or otherwise acceptable concrete (i.e. concrete which should not have been placed at all), agreed evidence of excessive workability or moderate misbatching can be regarded as reducing cube density by 1% (or 2% for both). In cases where even the mean density of 28 day cubes for the mix is not available (perhaps because the volumes of ‘cubes have not been determined by water displacement). tse may be made of the fact that the mean cube density for the mix should approximately equal the plastic density (where known) + the cement content of the mix (kg/m?) divided by 20. (b) Curing history ‘The curing history of the suspect concrete should be deter- mined from site and meteorological records with regard to: (1) ambient conditions when placed: (2) protection and curing provided: (3) age at removal of formwork: (4) level of temperature and humidity over period between lacing and coring, 183.2.28 Testing laboratory ‘The testing laboratory selected for the preparstion and testing of cores should be appraised and agreed as acceptable to the interested parties regarding its capability for conducting the necessary examination, density tests. capping and compressive testing to the requirements of this Pro- cedure. Particular attention should be paid to recent verifications of the performance of the compressive testing machine, 3.2.2.9 Required strength levels The drilling and testing of cores should not proceed until an understanding has been reached regarding the levels required for estimates of Actual or Potential Strength, special attention being paid to corrections for the pre- sence of steel or excessive voids in the cores. and abnormal curing: (see section 3.5.2 and Appendixes to 4). A course of action in the event of an estimate being inconclusive should also be agreed. 3.2.2.10 Supervision In addition 10 the provision of drilling equipment and services. the cutting of cores requires supervision by a responsible person acceptable to the interested parties. The drilling supervisor should be capable of using his judgement to see thet unnecessary damage to the structure is avoided and ensuring that each core is properly drilled, extracted and labelled. He should elso provide lisison between the various interested parties, the drilling contractor and ‘the testing laboratory. ‘The brief to the drilling supervisor should cover the following: (1) The exact location of the suspect concrete and any part of it deemed unsuitable for coring (i.e. unrepresentative concrete), and the probable location of steel likely to lie within the drilling depth. (2) The diameter and number of test cores required and the corres- Ponding drilling points on the surface of the suspect concrete. (3) The depth of concrete to be extracted from each location, the part of that depth which is to be the test length and the procedure to be adopted if a core breaks off at less than the target length. (4) Procedure to be adopted if inspection of the core reveals features which may invalidate the result or affect its interpretation, e.g. under-compaction; steel; cracks. (5) Instructions to be given to the testing laboratory. 3.3 Obtaining the cores 3.3.1 General Obtaining the cores involves the drilling, extraction, examination and identi- fication of the cores, the recording of relevant data, designation of test lengths and despatch to the laboratory. 3.3.2 Procedure 3.3.2.1 Drilling and Extraction Test cores should be drilled by 2 skilled operator using well maintained equipment complying with the dimensional requirements of BS 4019 : Part 2. Diamond-impregnated, water-cooled bits may be driven by electric or air-driven motors, but it should be noted that considerations of electrical safety will normally preclude the use of electric motors when drilling upwards. 7 Seige ‘The drill should be kept rigidly positioned during coring. by bracing or kentledge, otherwise badly ridged or curved cores may be obtained with possible reduction in measured strength.Care must be taken to ensure that a suitable and uniform pressure is applied to the drill bit such that the optimum drilling rate for the concrete is, achieved. Too little pressure will prevent the diamond cutting action, where- 85 100 great a pressure will cause excessive diamond wear, Before any core is broken aut. itis necessary to ensure that the depth drilled is not less than that planned. If, unexpectedly. steel has been encountered (evidenced by change in drilling noise or speed. or colour of cooling effluent), the supervisor should make use of his brief to decide whether this is acceptable, whether to drill further in order to obtain a steel-free length. or whether to drill a replacement core, Core removal is usually achieved satisfactorily by insertion of a cold cchisel down the side of the core to cause breaking off at or close to the bottom of the drilled length. followed by extraction using the drill or tongs. ‘The supervisor should satisfy himself that the drilling methods being used are not causing signficant distortion or damage to the cores. 3.3.2.2 Examination (On extraction, each core should be examined by the supervisor to ensure that the required test length can be obtained (preferably steel-free: and, in the case of cores for Potential Strength determination. not containing poorly compacted concrete). If this is not the case, extra cores should be drilled from locations adjacent to those of the rejected cores. ‘The rejected cores may be required to be retained for further examination. 3.3.2.3 Core identification Each core should be given a distinct, unique and indelible code number which is marked on the cut surface within the expected test length and ‘cross-referenced on a simple sketch of the element drilled. Marks should also be made on the core to indicate distances in millimetres from the drilling surface so that the exact location in the element from which the test core came can be confirmed even when the ends have been trimmed. Dnting ace 180° ‘Sige view of ailed care, before tmimming. showing merks at 50 mm ‘spacing measured from oniling surface. -280- ‘nd alse code number for aentfication placed within langtn tnatis expected fo be used as the fast length. 3.3.2.4 Drilling report ‘The supervisor should prepare, while the drilling work proceeds, a simple record of any observations likely to have a bearing on the validity of inter- pretation of the core test results. This record, together with the dri locations and core identification reference numbers, constitutes the drilling report. 3.3.2.5 Designation of test length and despatch to the laboratory The supervisor should ensure that the cores are despatched to the testing laboratory without damage together with instructions as to which part of each (relative to the drilling depth marks) is to be capped, examined, photographed and tested. In designating the test length of the as-drilled core, the supervisor should ‘ensure compliance with the intentions of the interested parties and with the following rules, where possible (rule 1 being more important than rule 2 te). 7(1) The test lengin should be, after trimming and capping. not less than one not more than two Giameters, and preferably not more than 1-2 diameters (2) - (2) Sections showing poor eom- paction or other defects should not be included in the test length (3) = (3) Unrepresentative concrete (upper 20%. 50 mm min- imum ot 300 mm maximum) of the lift depth should not be includedin the testlength. (4) Sections containing multiple or large pieces of steel should not be included in the test length. (5) Sections containing one small bar should not be included in the rest length 3.4 Laboratory work 3.4.1 General Laboratory work covers the trimming, capping and testing of each core, together with an examination (including assessment of voidege) and den- sity determination. 3.4.2 Procedure 3.4.2.1 Trimming to test length ‘The laboratory should trim each core (see clause 3.1.5 of BS 1881 : Pant 4) in accordance with instructions given by the drilling supervisor, ensuring ‘that the trimmed length is not less than 95% of its diameter nor likely to exceed twice the diameter when capped. Trimming with a masonry or diamon¢ saw is preferred, but careful hang trimming may be acceptable. 3.4.22 Examination ‘The laborstory should examine and photograph (see also Appendix 2) each test core in accordance with instructions given by the drilling supervisor. Notes will normally be required of the following. (2) Any lack of homogeneity of the concrete within or between cores. (b) The extent of voids visible on the drilled surface, particularly those which form honeycombing. (©) The position of cracks, driling damage or steel, defects and in- clusions should be marked on the core, and sketch records made of their locations and size relative to the over-all geometry of the core. (2) The approximate size of the aggregate and its type (bearing in mind that cut aggregate usually looks small and, in the case of ‘gravels, more variable in type than might have been expected). (e) Whether the aggregates seem continuously or gap graded, and any distinctive features of the fine aggregate. Notes BS 1881 - Part 4, clause 3.1.3, gives @ method of classifying the extent of voids on the basis of their number and size cistibution. This method does not, however, enable 2 sirect estimate of the percentage of voids to be made. Appendix ? 10 this Procedure gives ‘method of estimating voisage which may well, with experience in its use, fin favour ‘bs 2 quantittive version of the BS 1881 classification and prove preferable 10 the Setermination of vidage from density tests. ~ Comments regarding the apparent cement content of the concrete an its orig water contemt should only be made by an experienced concrete technologist and should always be viewed with cautionExamination and photographing under diffrent moisture conditions can assist in highiighting specitic features. eg. 8 wat eurace if preferable for viewing the 0-95) significantly different from the others and should be rejec- ted if there is evidence that the core concerned was ab- normal in respect of location in the lift, steel content, voidage. lack of homogeneity. cracks or drilling damage when compared with the other cores, If ris greater than the value given by: 7 1 4 43 5 32 6 28 7 26 8 25 the lowest result should be rejected irrespective of othe considerations (p > 0°975). ‘The estimate of Potential Strength required for further interpretation (see section 3.6) is the mean of the 0 individual estimates or the 7 ~ 1 individual estimates remaining after rejection of the lowest. Note. The above procedure for rejecting an abnormally low result i necessitated by the fact that meny unaccounted for phenomens oF elects may occasionsly produce an individual low and spurious sult. “The occurrence of an abnormally high spurious result i¢ much rarer. Where, however. there is 900d reason to doubt the validity of {an individual high result. the procedure given above may be used (substituting ‘highest’ for ‘lowest’) to cecide whether or not to eject it Itis impossible for these Procedures to make comprehensive and exhaustive recommendations regar ing the interpretation of estimates of Actual or Potential Strength because of the wide range of situations and specifica- tions which may be concerned. A number of points are given. however. which in many cases may provide @ basic Procedure for resolving an investigation. A worked example, demonstrating the use of the procedures in a practical situation, is provided in Appendix § (pages 29 to 31). 6.2 Procedure /3.6.2.1 Actual Strength he individual or mean estimates of Actual Strength may 2 used in a number of ways depending upon the purpose ‘the investigation. Certain points should be borne-in ind, however. ) Because this Procedure requires the test core to be vaked in water prior to test, @ reduction (of up to about 2 P/3.6.2.1 Potential Strength ‘The mean Estimated Potential Strength yielded by the procedure given in clause P/3.5.2.3 can be used either for ‘comparison with disputed cube results or directly with the specification for the concrete. (and hence to assess the potential serviceability of the element). (1) Where comparison with the specification is the prim=