# PD CEN TR 16369-2012

BSI Standards Publication Use of control charts in the production of concrete PD CEN/TR 16369:2012National foreword This Published Document is the UK implementation of CEN/TR 16369:2012. The UK participation in its preparation was entrusted by Technical Committee B/517, Concrete, to Subcommittee B/517/1, Concrete production and testing. 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 2012. Published by BSI Standards Limited 2012 ISBN 978 0 580 76828 6 ICS 03.120.30; 91.100.30 Compliance with a British Standard cannot confer immunity from legal obligations. This Published Document was published under the authority of the Standards Policy and Strategy Committee on 30 November 2012. Amendments issued since publication Date Text affected PUBLISHED DOCUMENT PD CEN/TR 16369:2012 TECHNICAL REPORT RAPPORT TECHNIQUE TECHNISCHER BERICHT CEN/TR 16369 October 2012 ICS 91.100.30; 03.120.30 English Version Use of control charts in the production of concrete Utilisation des cartes de contrôle pour la production du béton Anwendung von Qualitätsregelkarten bei der Herstellung von Beton This Technical Report was approved by CEN on 20 May 2012. It has been drawn up by the Technical Committee CEN/TC 104. CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United Kingdom. EUROPEAN COMMITTEE FOR STANDARDIZATION COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG Management Centre: Avenue Marnix 17, B-1000 Brussels © 2012 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. CEN/TR 16369:2012: E PD CEN/TR 16369:2012CEN/TR 16369:2012 (E) 2 Contents Page Foreword 4 Introduction .5 1 Scope 7 2 Symbols and abbreviations 7 3 Statistics for Concrete 8 3.1 Normal distribution of strength 8 3.2 Characteristic strength and target strength .8 3.3 Standard deviation 10 3.4 Setting the target strength . 13 4 Simple Data Charts . 14 5 Shewhart Charts . 15 5.1 Introduction . 15 5.2 Shewhart action criteria . 16 5.2.1 Points beyond UCL or LCL 16 5.2.2 Points beyond UWL or LWL 16 5.2.3 Patterns within control limits 16 5.3 Control of standard deviation 16 5.4 Example Shewhart chart 16 5.5 Modified application of Shewhart control chart 17 6 CUSUM . 19 6.1 Introduction . 19 6.2 Controlling mean strength . 22 6.3 Controlling standard deviation 22 6.4 Controlling correlation . 23 6.5 Design of V-mask 24 6.6 Action following change 24 7 Multivariable and Multigrade Analysis . 26 7.1 General . 26 7.2 Multivariable 26 7.3 Multigrade 27 8 Speeding the Response of the System 28 8.1 Early age testing . 28 8.2 Family of mixes concept 28 9 Guidance on Control Systems 30 9.1 Abnormal Results . 30 9.2 Handling mixes outside the concrete family . 30 9.3 Handling mixes not controlled by compressive strength requirements 31 9.4 Test rates . 32 9.5 Action following change 33 10 EN 206-1 Conformity Rules for Compressive Strength 33 10.1 Basic requirements for conformity of compressive strength 33 10.2 Assessment period . 34 10.3 Conformity rules for compressive strength . 34 10.4 Achieving an AOQL of 5 % with CUSUM 36 10.5 Non-conformity . 37 11 Implementing Control Systems . 38 12 CUSUM Example . 38 PD CEN/TR 16369:2012CEN/TR 16369:2012 (E) 3 12.1 Reference mix and concrete family . 38 12.2 Main relationship . 39 12.3 Applying adjustments . 40 12.4 CUSUM calculation 41 12.5 CUSUM action following change . 45 12.6 Further data and a change in standard deviation 47 Bibliography 51 PD CEN/TR 16369:2012CEN/TR 16369:2012 (E) 4 Foreword This document (CEN/TR 16369:2012) has been prepared by Technical Committee CEN/TC 104 “Concrete and related products”, the secretariat of which is held by DIN. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. PD CEN/TR 16369:2012CEN/TR 16369:2012 (E) 5 Introduction It is safe to assume that ever since manufacturing commenced, attempts have been made to control the process in order to improve quality and drive down costs. The application of statistical techniques to manufacturing was first developed by physicist Walter A. Shewhart of the Bell Telephone Laboratories in 1924. Shewhart continued to develop the idea and in 1931 he published a book on statistical quality control [1]. Shewhart recognised that within a manufacturing process there were not only natural variations inherent in the process, which affected quality but there were also variations that could not be explained. Shewhart recognised that it is possible to set limits on the natural variation of any process so that fluctuations within these limits could be explained by chance causes, but any variation outside of these limits, special variations, would represent a change in the underlying process. Shewhart’s concept of natural and special variations is clearly relevant to the production of concrete at a ready-mixed plant or precast factory and the requirement to achieve a specified compressive strength. Natural variations exist in the process due to variation in the raw materials (aggregate grading, chemical composition, etc), batching accuracy, plant performance, sampling and testing, etc. Special causes of variation outside of the natural variations could be due to changed constituent materials being used, weigh-scales losing accuracy, a new batcher, problems with testing equipment, etc. Control charts have found widespread use in the concrete industry in both ready-mixed concrete and precast concrete sectors as a tool for quality control. Control charts can be applied to monitor a range of product characteristics (e.g. cube/cylinder strength, consistence, w/c ratio), constituent materials (aggregate grading, cement strengths, etc.) or production (batching accuracy). Their most common application of control charts is as a means of continuously assessing compressive strength results in order to: check whether target strengths are being achieved; measure the variations from target (all products vary); identify magnitude of any variation; objectively define action required (e.g. change w/c ratio) to get the process back on target; identify periods and concretes where the strength was less than specified so that investigations can be carried out and corrective action taken. The use of control charts should not be treated in isolation from the rest of production control. For example routine checking and maintenance of weigh equipment will minimise the risk of a weigh-scale failure. Control charts provide information about the process, but the interpretation of the information is not a mechanical process. All the information available to the concrete producer should be used to interpret the information and make informed decisions. Did a change in quality occur when a new batch of constituent was first used? Is all the family showing the same trend? Are other plants using similar materials showing a similar trend? Such information leads to the cause of the change in quality being identified and appropriate action being taken. For example a loss of accuracy in the weigh-scales should lead to repair, maintenance and re-calibration and not a change in mix proportions. Where a change in mix proportions is required, the use of control charts can lead to objectively defined changes in proportions. Effective control of concrete production is more easily achieved when there are good relationships with the constituent material suppliers, particularly the suppliers of cementitious materials. Early warning of a change in performance from the constituent material supplier should be part of the supply agreement, e.g. that stock PD CEN/TR 16369:2012CEN/TR 16369:2012 (E) 6 clinker is being used during the maintenance period, and on the basis of this warning, the producer will decide the appropriate action. Some producers use changes in cement chemistry to predict changes in concrete strength. Effective production control is about using all this information to produce concrete conforming to its specification. Effective production control, which includes the use of control charts, significantly reduces the risk of non- conformity benefiting both users and producers of concrete. There are drawbacks to the existing method of assessment of conformity of mean strength adopted in EN 206-1 [3] including not following the CEN Guidance on the evaluation of conformity [2]. It is believed that control charts (already widely used as a quality assurance tool in factory production control) would provide an alternative and better means of ensuring the characteristic strength is achieved and it is a method that follows the CEN Guidance. PD CEN/TR 16369:2012CEN/TR 16369:2012 (E) 7 1 Scope This Technical Report reviews various control systems that are currently used in the concrete industry and, by the use of examples, show how the principles are applied to control the production of concrete. This CEN/TR provides information and examples of the use of method C in Clause 8 of prEN 206:2012. 2 Symbols and abbreviations AOQ Average outgoing quality AOQL Average outgoing quality limit C mraConstant giving the cement content increase required to produce a 1N/mm 2increase in strength dc Change in cement content Dl Decision interval G Gradient f ciIndividual test result for compressive strength of concrete f ckSpecified characteristic compressive strength f cmMean compressive strength of concrete k Statistical constant L lLower limit LCL Lower control limit LWL Lower warning limit n Number of samples q nStatistical constant that depends upon n and the selected AOQL s Sample standard deviation UCL Upper control limit UWL Upper warning limit x iTest result NOTE According to EN 206-1 [3], a test result may be the mean value of two or more specimens taken from one sample and tested at one age. x Mean value of ’n’ test results σ Estimate for the standard deviation of a population PD CEN/TR 16369:2012CEN/TR 16369:2012 (E) 8 3 Statistics for Concrete 3.1 Normal distribution of strength Compressive strength test results tend to follow a normal distribution as illustrated in Figure 1. A normal distribution is defined by two parameters, the mean value of the distribution and the standard deviation (σ ), which is the measure of the spread of results around the mean value. A low standard deviation means that most strength results will be close to the mean value; a high standard deviation means that the strength of significant proportions of the results will be well below (and above) the mean value. The area under the normal distribution between two values of ‘x’ represents the probability that a result will fall within this range of values. The term ‘tail’ is used to mean the area under the normal distribution between a value, e.g. a compressive strength, and where the frequency is effectively zero. For strength it is the lower tail, i.e. low strength results, that is important but for other properties, e.g. consistence, both the lower and upper tails are important. Key X cube strength, N/mm² Y frequency 1 target mean strength 2 specified Characteristic strength, f ck3 minimum strength (f ck– 4) 4 tail Figure 1 — Illustration of concrete strength distribution At the extremes of the strength range for a given set of constituent materials, the assumption of a normally distributed set of data may not be valid. It is not possible to have strengths less than zero and most concretes have a ceiling strength beyond which they cannot go. In these situations the data set is skewed. However as low strengths are of concern to specifiers, an assumption of normally distributed data does not lead to problems in practice. 3.2 Characteristic strength and target strength EN 206-1 [3] specifies the characteristic compressive strength of concrete in terms of a standard cylinder test or a standard cube test carried out at 28 days. The characteristic strength is defined in EN 206-1 [3] as the “value of strength below which 5 % of the population of all possible strength determinations of the volume of concrete under consideration, are expected to fall”. Put simply this means that if every single batch was tested, 5 % of the results would fall within the lower ‘tail’ of the normal distribution that starts 1,64σ below the actual mean strength. However the actual mean strength will not be known until the concrete has been PD CEN/TR 16369:2012CEN/TR 16369:2012 (E) 9 produced and tested and therefore the target mean strength (TMS) is usually set at some higher value to ensure the concrete achieves at least the specified characteristic strength. The target mean strength is given in Equation (1): TMS = f ck+ k × σ (1) where TMS = target mean strength f ck= characteristic compressive strength σ = estimate for standard deviation of population k = statistical constant k × σ = the margin The fixed point in the distribution is the specified characteristic strength and as the margin increases and/or the standard deviation increases, the target mean strength increases, see the following Example. EXAMPLE The target mean strength for a specified characteristic strength of C25/30 is given in Table 1. A standard deviation (σ ) of 3 N/mm 2is typical of a concrete with low variability and a value of 6 N/mm 2represents high variability. Table 1 — Target mean strength for specified characteristic strength of 30 N/mm 2(cube) Margin Area in lower tail (i.e. percentage below characteristic strength) Target mean strength (cube), N/mm 2σ σ σ σ = 3 N/mm 2σ σ σ σ = 6 N/mm 21,64σ 5 % 35 40 1,96σ 2,5 % 36 42 2,00σ 2,28 % 36 42 2,33σ 1,0 % 37 44 3,0σ 0,13 % 39 48 The numbers in this table have been rounded. A concrete strength below the characteristic strength is not a failure as statistically 5 % of the results are expected and accepted as to fall below this value. However for structural safety reasons, a batch with a concrete strength significantly below the characteristic strength is excluded, even though it forms part of the expected population. Consequently EN 206-1 [3] specifies a minimum strength requirement for individual results (f ci ) of (f ck– 4). Any batch below this strength is a non-conforming batch. The risk of non-confo