USE OF CONCRETE FAMILY CONCEPT FOR CONFORMITY CONTROL OF READY MIXED CONCRETE

Transcription

1 USE OF CONCRETE FAMILY CONCEPT FOR CONFORMITY CONTROL OF READY MIXED CONCRETE Lu Jin Ping*, American Concrete Institute Singapore Chapter Soh Guan Hong, Admaterials Technologies Pte Ltd, Singapore Goh Lee Yong, Admaterials Technologies Pte Ltd, Singapore 35 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2010, Singapore Article Online Id: The online version of this article can be found at: Thisarticleisbroughttoyouwiththesupportof SingaporeConcreteInstitute AllRightsreservedforCIPremierPTELTD YouarenotAllowedtoredistributeorresalethearticleinanyformatwithoutwrittenapprovalof CIPremierPTELTD VisitOurWebsiteformoreinformation

2 35 th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2010, Singapore USE OF CONCRETE FAMILY CONCEPT FOR CONFORMITY CONTROL OF READY MIXED CONCRETE Lu Jin Ping*, American Concrete Institute Singapore Chapter Soh Guan Hong, Admaterials Technologies Pte Ltd, Singapore Goh Lee Yong, Admaterials Technologies Pte Ltd, Singapore Abstract On 4 Jan 2010, the Building and Construction Authority (BCA) issued a circular on REQUIREMENT FOR READY-MIXED CONCRETE (RMC) CERTIFICATION FOR STRUCTURAL WORKS. The Singapore Accreditation Council (SAC) has also launched a Certification Scheme for Ready-Mixed Concrete (RMC) which provides for third-party certification of RMC producers in accordance to SS EN and SS 544 standards. Under the new certification scheme, BCA will require concrete products for building and civil engineering structures to be certified, and supplied by RMC plants which are certified under the Singapore Accreditation Council (SAC) Certification Scheme for RMC. This will include on-site batching plants, as well as plants supplying concrete for structural precast elements. To allow RMC producers sufficient time to prepare and obtain the certification, there will be a grace period of 9 months, effective from 1 Jan 2010, before this requirement takes effect from 1 Oct In Singapore, concrete plants are required to supply a very wide range of concrete mixes, with different strengths, consistencies, admixtures, aggregate sizes, etc. As a result of these largely differentiated requests, a plant often does not produce enough of some particular concrete mixes to apply the conformity criteria of EN for individual concretes. However, the concrete family concept allows the producers to combine strength results of different mixes for the conformity assessment, and allows a more continuous control of the production process. In this paper the principle and concept of concrete family will be introduced. The application of the concrete family concept in conformity control of compressive strength will be discussed. Some examples of the application of concrete family concept will be presented. The assessment of conformity control for concrete family will be also discussed.

3 1. Introduction In January 2010, BCA issued a issued a circular on REQUIREMENT FOR READY-MIXED CONCRETE (RMC) CERTIFICATION FOR STRUCTURAL WORKS. Under the certification scheme, concrete products for building and civil engineering structures are to be certified by a SINGLAS accredited certification body. According to SS EN 206-1, in the assessment of conformity, concrete mixes may be evaluated as an individual concrete, or as a concrete family. 2. Concept of Concrete Family SS EN defines a concrete family as a group of concrete compositions for which a reliable relationship between relevant properties is established and documented. The relationships between members of the family may be established by initial testing, existing production data and/or theory. However, high strength concrete (>C55/67 or >LC55/60) is excluded from the concrete family concept. In addition, lightweight concrete cannot be grouped into the same family as normal weight concretes, but they can form another family on their own. In applying the concept of concrete families, a Reference Concrete is chosen. The Reference Concrete is usually the most commonly produced, or one from the mid-range of the concrete family. Concrete families may be formed by concretes with: a) Cement of one type, strength class and source; b) Demonstrably similar aggregates and Type 1 additions; c) Water reducing/plasticising admixtures or without; d) Full range of consistence classes; e) A few strength classes. It is also to note that concretes containing Type II additions (eg. Pozzolanic or latent hydraulic addition), high range water reducer or superplasticizer, retarder or air-entraining admixtures should be put into a separate family or assessed as individual concretes. An example of a concrete family is given in Table 1. In this example, all members of the concrete have cement of the same type (OPC), demonstrably similar aggregates, and this concrete family consists of concrete from Grade C12/15 to C45/55 with consistency classes S2 to S4. 1. Advantages of Applying the Concrete Family Concept In the assessment of conformity, at least 35 test results are required for initial production assessment. For common concrete grades such as C28/35 and C32/40, there should be no problems achieving the 35 test results for assessment. However, in Singapore, the concrete orders may be varied, in terms of strengths, consistencies, admixtures used, aggregate sizes etc. Hence, there may be difficulties to achieve sufficient test data for certain grades of concrete. As such, combining different concrete grades into a concrete family can result in a more effective production control. Furthermore, combining data into families can reduce the time taken to detect any significant changes in production quality. For example, if three concretes are tested daily and 15 test results are needed to detect significant changes, it would only take 5 days to collect the results and detect if there are any significant changes, when the concrete are grouped into a family. Whereas if they are individual concretes, it would take 15 days before any changes are detected. In addition, the concept is that if a constituent material were to change in a way that affected concrete strength, all those concretes made with that constituent material would be affected. This is logical and shows that a change that affects one concrete will also affect others. By pooling the information on the performance of different concrete, a change in performance can be detected more quickly and appropriate action can be taken.

4 Theoretically, applying the concrete family concept will lead to smaller producers and end-users risk compared to conformity control using individual concrete, due to the higher number of data available when using the concrete family concept. Pooling data normally leads to higher standard deviations when the spread in the standard deviations of the members is not too large. In applying the concrete family concept, the standard deviation,, is higher. Thus, this results in a larger margin and increased factor of safety in the assessment of conformity control. Table 1: Example of Concrete Family Source: Admaterials Technologies Pte Ltd Product Certification Department 2. Transposition Methods All members compressive strength results need to be transposed to that of the Reference Concrete. The three methods of transposing are as follows, with the first method (Method A) being the most common: Method A: Method B: Method C: Strength method based on a straight line relationship between strength and water/cement ratio Strength method based on a proportional effect w/c ratio method

5 The actual method of transposing is not critical, as they give similar results over a limited range of strength classes. In Method A, the difference between the specified characteristic strength and the actual strength of the tested concrete is determined. This strength difference is then applied to the characteristic strength of the Reference Concrete to obtain the equivalent strength. For example, a C20/25 concrete is tested and it gave a cylinder strength of 28 MPa. The difference between the characteristic strength (20 MPa) and the actual strength (28 MPa) is thus +8 MPa. Applying this difference to the characteristic strength of the Reference Concrete (C25/30), the equivalent strength is 33 MPa. More details of the transposition are shown in Table 2. Table 2: Transposition of data Original Data, MPa Transposed Data, MPa C20/25 C25/30 C30/37 C20/25 C25/30 C30/ Applying the Concrete Family Concept in Conformity Control In the assessment of conformity control, three criterion need to be satisfied. When a family member is tested, the original compressive strength result has to conform to Criterion 2 in Table 14 of SS EN The member s result will be transposed to equivalent values of the Reference Concrete and assessed for conformity (Criterion 1, Table 14 of SS EN 206-1). Transposition of data may be based on strength, w/c ratio, or cement content. In addition, it has also to be assessed that each individual member belongs to the family (Criterion 3, Table 15 of SS EN 206-1). In the case where a member fails to meet Criterion 3, it is removed from the family and assessed individually for conformity. See Tables 3 and 4 for the three criterions. In Singapore, the use of 100mm cubes for compressive strength test is recommended. Table 3: Criterion 1 and 2 for initial and continuous production according to SS EN 206-1: 2009 Production Number n of test results for compressive strength in the group Criterion 1 Criterion 2 Mean of n results Any individual test (f cm ) N/mm 2 result (f ci ) N/mm 2 Initial 3 f ck + 4 f ck - 4 Continuous 15 f ck f ck - 4 Table 4: Criterion 3, confirmation criterion for family members according to SS EN 206-1: 2009 Number n of test results for compressive Criterion 3 strength for a single concrete Mean of n results (f cm ) for a single family member N/mm 2 2 f ck f ck f ck f ck f ck Figure 1 shows the procedure to be adopted, when assessing concrete families as part of the whole conformity control process.

6 Figure 1: Flow chart for the assessment of membership and conformity of a concrete family At28daysiseachindividualtestresultsf ck 4 (Criterion2) Yes No Declarethebatchorloadas nonconforming Foreachfamilymembertested,checkateach assessmentperiodusingconfirmationcriterionif thememberbelongstothefamily (Criterion3) Yes Ateachassessmentperiod,isthemeanstrength ofallthetransposedresultsgreaterthanorequal tothecharacteristicstrengthofthereference Concreteplus1.48xfamilystandarddeviation (Criterion1) Yes No No Removethisconcretefromthe familyandassessasan individualconcrete Declarethefamilyasnon conformingoverthe assessmentperiod Declarethefamilyasconformingoverthe assessmentperiod 4. Example of Applying the Concrete Family Concept in Conformity Control a) Introduction This example illustrates the use of the concrete family system to control concretes over three strength classes containing aggregates of 20 mm maximum sizes and to detail some ways in which data can be transposed. The assessment period is for initial production. b) The family The following concretes are included within this particular example of a concrete family and within the production control system: - CEM I-42.5; - demonstrably similar aggregates, crushed Granite and natural sand; - strength classes in the range C16/20 to C40/50; - slump classes S2 to S4; - concretes with and without water reducing/plasticizing admixture. The criteria for demonstrably similar aggregates are: - same geological type; - same deposit/quarry; - similar grading and shape; - similar filler content; - similar methylene blue value for the limestone based sand. - c) Reference Concrete The Reference Concrete is strength class C32/40 which is the most produced concrete product. d) Assessment period

7 The assessment period is 3 months. This example is based on actual production data taken during the first three months of e) Initial testing and relationships Based on an exponential equation, the relationship between the strength and w/c ratio is established from laboratory trials and theory. For this plant the relationship was 150mm cube strength. f cyl = 135 / exp (2,50 w/c ratio) [1] The target mean strength for a concrete is obtained from target mean strength = specified f ck + K where = standard deviation obtained from previous production data K = a constant greater than1,65. The value of this constant reflects the production capabilities of the plant and the degree of commercial risk the producer is willing to assume. In reality producers do not work on the basis of 5% of the actual test results being below the characteristic value. In this case the value of K was 1,95. Using the target mean strength, the producer applies Equation 1 to obtain the w/c ratio. The free water content of the concrete is obtained from the following equation that again has been obtained by a combination of laboratory trials and theory water content, w = (0,65S/M 0,33 ) - (0,8M/log 10 (1 + S/5)) [2] where w = water content in l/m 3 ; M = nominal maximum aggregate size in mm; S = average slump for the class in mm. The average slumps used are 70mm for S2, 125mm for S3 and 190mm for S4. Equation 2 is used to obtain the water content and from the water content and the w/c ratio, the cement content is calculated. f) Relationships between members of the family and the Reference Concrete In this example, the strength method based on a straight line relationship between strength and w/c ratio is used. g) Strength method based on a straight-line relationship between strength and w/c ratio In order to make the assumed straight-line relationship between strength and w/c ratio reasonable, the strength range has to be limited. The difference in strength between the specified characteristic strength and the actual strength of the tested concrete is determined. This strength difference is then applied to the characteristic strength of the Reference Concrete to obtain the equivalent strength. For example, A C15/20 concrete is tested and it gave a 150mm cube test result of 31,7 MPa. The difference between the characteristic cube strength (20) and the actual strength is +11,7 MPa. This is applied to the characteristic cylinder strength of the Reference Concrete (40) to give an equivalent strength of 51,7 MPa.

8 S/n h) Conformity control Each individual original result is checked against the criterion 2 in of EN i.e. f ck - 4, and all 42 results passed. The Reference Concrete containing all the transposed data was assessed against the criterion 1 in of EN for initial production: the mean strength of 3 non-overlapping consecutive test results f cm Mean strength f ck + 4 As shown in table 5, this criterion is satisfied. Grade Actual Strength Table 5, Conformity Criterion 1 and 2 Criterion 2 Converted to Mean strength (3 Actual strength Reference Concrete non-overlapping) > fck-4 (G40) 1 G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED G PASSED 57.3 Criterion 1 fcm > PASSED 52.4 PASSED 49.8 PASSED 54.1 PASSED 50.8 PASSED 55.6 PASSED 57.7 PASSED 54.8 PASSED 63.4 PASSED 56.2 PASSED 49.3 PASSED 55.1 PASSED 53.6 PASSED 57.3 PASSED

9 The assessment of relationships is given in Table 6. An inspection of the data given in Table 6 shows that the confirmation tests (Criterion 3) are passed, and consequently all the concretes are retained within the family. Strength Class, 150mm cube Table 6: Assessment of test data for conformity control Consistence Class (Aggregate Size) No of Data Mean Strength Conformity Control (Criterion 3) G30 All Pass S S G35 All Pass S S C40 All Pass S S S C45 All Pass S S n = Conclusion In the assessment of conformity control in accordance to SS EN 206-1, it is useful and advantageous to group different concretes into concrete families. In Singapore, where concrete products are varied, it is an efficient way of collecting enough test data for conformity assessment, and it is advantageous to both producers and end users in terms of detecting changes in concrete quality. Examples have been given to demonstrate the application of the concrete family concept, and the assessment for conformity in a concrete family. References [1] SS EN 206-1: 2009: Specification for Concrete Part 1: Specification, performance, production and conformity, SS EN 206-1: 2009 [2] CR 13901: The Use of the concept of concrete families for the production and conformity control of concrete, CEN Technical Report, 2000 [3] Harrison, T.A: Guidance on the application of the EN conformity rules, Quarry Products Association, 2001.