As 1379 Supp 1-1997 Specification And Supply Of Concrete Sup 3i5c2j

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AS 1379 Supp 1-1997 Specification and supply of concrete - Commentary (Supplement to AS 1379-1997)

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AS 1379 Supp1—1997

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AS 1379 Supplement 1—1997 Specification and supply of concrete—Commentary (Supplement 1 to AS 1379—1997)

This Australian Standard was prepared by Committee BD/49, Manufacture of Concrete. It was approved on behalf of the Council of Standards Australia on 22 August 1997 and published on 5 October 1997.

The following interests are represented on Committee BD/49: Ash Development Association of Australia AUSTROADS Australasian Slag Association Australian Premixed Concrete Association Cement & Concrete Association of Australia Housing Industry Association Licensed to E.S.SURESH on 04 Jun 2002. Single licence only. Storage, distribution or use on network prohibited.

Master Builders Australia The Association of Consulting Engineers, Australia University of Newcastle

Review of Australian Standards. To keep abreast of progress in industry, Australian Standards are subject to periodic review and are kept up to date by the issue of amendments or new editions as necessary. It is important therefore that Standards s ensure that they are in possession of the latest edition, and any amendments thereto. Full details of all Australian Standards and related publications will be found in the Standards Australia Catalogue of Publications; this information is supplemented each month by the magazine ‘The Australian Standard’, which subscribing receive, and which gives details of new publications, new editions and amendments, and of withdrawn Standards. Suggestions for improvements to Australian Standards, addressed to the head office of Standards Australia, are welcomed. Notification of any inaccuracy or ambiguity found in an Australian Standard should be made without delay in order that the matter may be investigated and appropriate action taken.

This Standard was issued in draft form for comment as DR 96017.

AS 1379 Supp1—1997

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AS 1379 Supplement 1—1997 Specification and supply of concrete—Commentary (Supplement 1 to AS 1379—1997)

First published as AS 1379 Supplement 1 — 1997. Incorporating: Amdt 1—2000

PUBLISHED BY STANDARDS AUSTRALIA (STANDARDS ASSOCIATION OF AUSTRALIA) 1 THE CRESCENT, HOMEBUSH, NSW 2140 ISBN 0 7337 1469 2

AS 1379 Supp1 — 1997

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PREFACE This Commentary (AS 1379—Supplement 1) was prepared by Standards Australia Committee BD/49 on the Manufacture of Concrete. While it is intended for reading in conjunction with AS 1379, Specification and supply of concrete, it does not form an integral part of that Standard. Objective

The objective of this Commentary is—

(a)

to provide background reference material to the Clauses in the Standard;

(b)

to indicate the origin of particular requirements;

(c)

to indicate departures from previous practice; and

(d)

to explain the application of certain Clauses.

The clause numbers and titles used in the Commentary are the same as those in AS 1379 except that they are prefixed by the letter C. To avoid possible confusion between this Commentary and the Standard, clauses are cross-referenced within the text, while Commentary clauses are referred to as Paragraph C ... in accordance with Standards Australia policy.

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Gaps in the numerical sequence of Paragraphs in the Commentary indicate that the committee considered that commentary on these Clauses was not needed.

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AS 1379 Supp1 — 1997

CONTENTS Page

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SECTION C1 SCOPE AND GENERAL C1.1 SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C1.3 OTHER MATERIALS, PLANT OR METHODS C1.4 DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . C1.6 SPECIFICATION OF CONCRETE . . . . . . . . . C1.7 METHODS OF ORDERING . . . . . . . . . . . . . . C1.8 BASIS OF SUPPLY . . . . . . . . . . . . . . . . . . . .

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4 4 4 5 8a . 12

SECTION C2 CONCRETE MATERIALS AND CONSTITUENT LIMITATIONS C2.2 CEMENT CONSTITUENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C2.3 AGGREGATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C2.4 MIXING WATER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C2.6 BULK STORAGE OF MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . C2.7 LIMITATIONS ON CHEMICAL CONTENT OF CONCRETE . . . . . . .

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SECTION C3 PLANT AND EQUIPMENT C3.1 BINS AND SILOS . . . . . . . . . . . . . C3.2 WEIGHING EQUIPMENT . . . . . . . C3.3 LIQUID DISPENSING EQUIPMENT C3.4 MIXERS . . . . . . . . . . . . . . . . . . . .

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SECTION C5 SAMPLING AND TESTING OF CONCRETE C5.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C5.2 SLUMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C5.3 STRENGTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . C5.5 CHLORIDE AND SULFATE CONTENT . . . . . . . . C5.6 DRYING SHRINKAGE . . . . . . . . . . . . . . . . . . . .

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SECTION C6 SAMPLING, TESTING AND ASSESSMENT FOR COMPLIANCE OF CONCRETE SPECIFIED BY COMPRESSIVE STRENGTH INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C6.1 GENERAL REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C6.2 SAMPLING AND TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C6.3 PRODUCTION ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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SECTION C4 PRODUCTION AND DELIVERY C4.1 GENERAL . . . . . . . . . . . . . . . . . . . . C4.2 BATCH PRODUCTION . . . . . . . . . . . C4.3 CONTINUOUS PRODUCTION . . . . . . C4.4 DELIVERY . . . . . . . . . . . . . . . . . . . .

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APPENDIX CB GUIDE TO THE SPECIFICATION OF SPECIAL-CLASS CONCRETE 31

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STANDARDS AUSTRALIA Australian Standard Specification and supply of concrete — Commentary (Supplement 1 to AS 1379 — 1997) S E C T I O N

C 1

S C O P E

A N D

G E N E R A L

C1.1 SCOPE The Standard was prepared for use in the specification and supply of all concrete, whether or not it is addressed in the scope and application of AS 3600, Concrete structures. It is not intended to apply to mortars or grouts. Requirements for mortars for masonry construction are given in AS 3700 and the methods for sampling and testing mortars in AS 2701.

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Requirements for grouts to be used for the grouting of prestressing tendons in ducts, are given in AS 3600. The supply of concrete by the premix concrete industry has been expressly but not solely contemplated in the preparation of the document. It is not intended that this Standard should take precedence over existing Australian Standards for the manufacture of specific concrete products. The publication of this edition of the Standard is intended to complete the independence of this Standard from AS 3600, Concrete Structures. Due to the lack of synchronization in publishing dates, for many years concrete as a material relied on both Standards to one degree or another. With the relatively recent revision of AS 3600 and the current edition of AS 1379, it is intended that AS 1379 is now a ‘stand-alone’ document for the specification and supply of concrete. C1.3 OTHER MATERIALS, PLANT OR METHODS Where materials, plant or methods not complying with this Standard are proposed and the intended use of the concrete is subject to the control of a building or other regulatory authority, approval for the use of those other materials, plant, or methods will need to be obtained from the authority. C1.4

DEFINITIONS

For the purpose of this Standard, the definitions below apply.

C1.4.4 Cement —the meaning of the term cement has been extended beyond its traditional meaning of portland cement to include supplementary cementitious materials. For the purpose of this Standard cement can be portland or blended cements as defined in AS 3972, or a combination of these and fly-ash, silica fume or ground granulated iron blast furnace slag. C1.4.7 Customer — the ‘’ and its derivatives have been replaced by ‘Customer’ for consistency with ISO 8402 terminology. C1.4.11 Project assessment—Section 6 requires suppliers to continuously monitor the strength of the concrete supplied by ‘Production assessment.’ Project assessment is an additional assessment that may be specified, and attracts an additional cost.

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AS 1379 Supp1 — 1997

Currently AS 3600 requires project assessment to be specified for all special-class concrete. Project assessment may also be specified for normal-class concrete. The customer should not assume the supplier is aware of any liability to conduct project assessment. It is the responsibility of the customer to commission an appropriate testing body to carry out project assessment. The supplier may be an available choice for this purpose. The customer may commission the supplier to carry out the specified project testing, making appropriate arrangements for the recovery of the costs involved. Project assessment is discussed further in the section of this Commentary dealing with Section 6. C1.4.16 Supply The ‘Manufacture’ and its derivatives have been replaced by ‘Supply’ for consistency with ISO 8402 terminology. C1.4.17 Total free water—The only water considered to be available for hydration of cementitious materials is the ‘total free water’, being ‘added water’ and the ‘surface moisture’ of the aggregates.

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In this context, water separately added in the batching process, as distinct from water introduced as moisture content in the aggregates, is referred to as ‘added water’. The other component of total free water, the ‘surface moisture’ content of aggregates, is defined as the total moisture content less the ‘absorbed moisture’ content. ‘Absorbed moisture’ is that contained within capillary fissures in the aggregate particles. It is considered to remain within the aggregates and not to be available to enter into chemical combination with the cementitious materials. When aggregates are in a condition where the only water they contain is that which can fully occupy the capillary fissures within the aggregate particles, they are said to be in a ‘saturated surface dry’ condition. To bring aggregates to this condition, they are slowly dried from a higher moisture content until the surface water has evaporated. The ‘saturated surface dry’ moisture content can then be determined. Any moisture the aggregates contain in excess of the absorbed moisture is referred to as ‘surface moisture’. C1.4.18 Water-cement ratio (w/c)— Water for this purpose is defined as the total free water discussed at Paragraph C1.4.17 above, and cement as discussed in Paragraph C1.4.4. C1.6

SPECIFICATION OF CONCRETE

C1.6.1 General ‘Normal-class concrete’ is presented as the specification method likely to suit the majority of applications. Its properties are limited, albeit in reasonably wide ranges, in respect of a number of key parameters such as strength grade, density, aggregate size, slump, and chemical composition. The quality compliance provisions of Section 6, originally promulgated in AS 3600 in 1988, anticipated that the majority of specifications would call up normal-class concrete. The entire concept of production assessment is based on this assumption. In turn the customer s level of protection also improves. ‘Special-class concrete’ provides the specifier with the opportunity to specify parameters or values which are not permitted in normal-class concrete. C1.6.2 Standard strength grades The majority of structural concrete lies in the range of strength grades from 20 to 50 MPa. For such concrete, the committees responsible for AS 3600 and this Standard recommend the selection of one of the values 20, 25, 32, 40, or 50 MPa. The advantages of standardizing strength grades lies in the consequent increase in the volume of statistical information available for each mix and in avoiding a variety of unnecessary mix designs.

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The use of standard strength grades whenever possible will result in more test results being available for statistical analysis and will generate more reliable statistical parameters with which to assess quality. This will enhance the effectiveness of suppliers production assessment procedures, the principal tool for maintenance of the quality of concrete production. If special-class concrete is specified, it is preferable if the use of non-standard strength grades is avoided. If the strength grade is one of the standard grades, test data can sometimes be grouped with the data used for production assessment of normal-class concrete of the same strength grade even though concrete may be special-class. A technical distinction is introduced in this Clause with the use of the term ‘design’ characteristic strength. Characteristic strength is defined at Clause 1.4.5 as ‘that value of the material strength, as assessed by the standard test, which is exceeded by 95 percent of the material.’ The word ‘design’ is prefixed to it in this Clause to make clear that in this context the value referred to is the value upon which the designer relies. It is not to be confused with the value calculated from the test results for a particular grade which will be expected to exceed the ‘design’ value. C1.6.3

Normal-class concrete

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C1.6.3.1 General This Clause is an amalgamation of two clauses from the 1991 version of the Standard. It now contains all the surviving provisions from both clauses dealing with limitations on cement types and lightweight aggregates. Any restrictions on the use of fly-ash, ground slag or chemical ixture or a limitation of basic shrinkage strain after 56 days drying to less than 1000 × 10-6, would change the classification to special-class as follows: (a)

Mass per unit volume The range of from 2100 kg/m3 to 2800 kg/m3 was chosen to accommodate satisfactory dense aggregates commercially available. Lightweight concrete (< 2100 kg/m3) would necessarily be special-class concrete.

(b)

Chemical content Limiting the chloride ion content to 0.8 kg/m3 reflects a consensus of views as to a safe maximum to prevent corrosion of embedded ferrous metals and any other deleterious chemical reactions from chlorides. Similarly, the limit on sulfates of 50 g/kg of cement is consistent with all data on a safe maximum to ensure long-term durability.

(c)

Basic shrinkage strain The maximum basic shrinkage strain, after 56 days drying, of 1000 × 10-6 is a value to which suppliers of normal-class concrete in any area of Australia are committed. Two matters as follows are relevant when considering the appropriateness of this limit: (i)

Practicality In some areas the locally available aggregates and cement for the production of concrete will result in concrete with a basic shrinkage strain approaching this value. A lower maximum value for normal-class concrete would, in such areas, preclude the use of economical and otherwise satisfactory materials for use in normal-class concrete. Many areas have materials economically available which will ensure the basic shrinkage strain of concrete in those areas is considerably less than 1000 × 10-6. Designers should ascertain the shrinkage characteristics of commercially produced concrete in any area by inspecting the records of suppliers in that area. It is preferable that such enquiries be made before the specification of lower shrinkage strain limit is contemplated.

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(ii)

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Consistency A median value of 700 × 10-6 is used in AS 3600 for the basic shrinkage strain as an alternative to values determined from tests on the concrete proposed to be used or similar local concrete. The maximum to 1000 × 10-6 specified in AS 1379 includes allowance for the range of values each side of the median and for the testing precision in the determination of shrinkage. The AS 3600 provision predated the drafting of AS 1379, but was based on a similar body of experience of the values of shrinkage strain attained across the nation. Further discussion of basic shrinkage strain occurs in Clause C1.6.4.

(d)

Strength gain characteristics The needs for strength growth characteristics vary. Relatively high early strengths are needed in some circumstances, for example, when stripping time for suspended slabs is critical. A minimum mean 7-day strength is included in the Standard to inform customers what they may anticipate if normal-class concrete is specified.

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When higher early age strengths are needed special-class concrete should be specified. The previous edition of the Standard approached the minimum early strength issue by imposing limits, expressed in a formula, on the allowable proportion of supplementary cementitious materials to portland cement. With the changes to AS 3972 ‘Portland and blended cements’ which allows up to 5% of ‘mineral additions’ and the comparatively new use of silica fume, the formula approach would become cumbersome and it is considered that a performance rather than a prescriptive requirement is more consistent with current specification practice. None the less it must be emphasized, especially in the absence of an additional early strength specification parameter, that concrete takes some time to achieve its potential strength. The importance of extended continuous curing to achieve the full potential properties also needs to be emphasized in this context. C1.6.3.2 Basic parameters The six (6) basic parameters that need to be specified when ordering normal-class concrete are as follows: (a)

Standard strength grades. The significance of standard strength grades is discussed in Paragraph C1.6.2 above.

(b)

Slump.

(c)

Maximum aggregate size.

(d)

Method of placement.

(e)

Any requirement for project testing.

(f)

Level of air entrainment if required.

In the absence of specific advice, default values of maximum aggregate size and project testing have to be established. When a customer orders a specific value of slump, that slump becomes the supplier’s target. The tolerances in Table 6 are to provide for the operator’s inability to precisely assess slump in the production process. The specification of slump of normal-class concrete is commonly established by the designer in contractual documents, often without discussion with those responsible for placing the concrete or certain knowledge of the details of the method of placement.

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There is merit in the alternative of allowing the customer to specify the slump to the supplier, after considering the alternative methods of placement and finishing. The customer may need to strike a balance between the higher cost of purchasing concrete with a higher slump and the considerable cost associated with the difficulties of placing concrete at lower slump. For example, it is usually more expensive to pump concrete at an 80 mm slump than a 100 mm slump. Due to local aggregate characteristics, suppliers in some areas may not be able to meet the shrinkage requirements of this Standard at a specified slump of >80 mm. In this case the supplier cannot offer normal-class concrete of >80 mm slump, but only lower slump. It is the responsibility of the supplier to decide if a >80 mm slump normal-class concrete can be supplied. C1.6.4

Special-class concrete

Shrinkage strain Special-class concrete may be specified as having maximum values of basic shrinkage strain below or, indeed, above 1000 × 10−6. The specification of maximum basic shrinkage strains less than 1000 × 10−6 may carry with it some disadvantages. Depending on the material resources available in the region, it may be — (a) Licensed to E.S.SURESH on 04 Jun 2002. Single licence only. Storage, distribution or use on network prohibited.

(b)

necessary to import distant materials, thus increasing the cost of concrete; and/or unnecessarily deplete scarce resources.

Consultation with experienced suppliers in the region is advisable to determine what shrinkage may be expected with normally used materials, and the cost, if any, associated with specifying a limit less than 1000 × 10−6. In the event that more restrictive values for basic shrinkage strain are specified, the specification of a median value is recommended. A limit on the median is more capable of rational assessment than a limit on the maximum. In view of the inherent variability of the sampling and testing procedures, unusually high values may randomly occur. Where maximum value is specified, irrational assessment may occur. Proposed conventions for use in deciding prefixes to succinctly identify mixes Identifying prefixes for special-class concrete should follow the following conventions: (i)

Mix codes such as S25 may be used to identify a 25 MPa strength grade specialclass concrete, distinguishing it from N25, being a 25MPa strength grade normalclass concrete.

(ii)

Codes SF ‘x’ and ST ‘y’ may be used to identify special-class concrete with a design characteristic strength in flexure of ‘x’ MPa and indirect tension ‘y’ MPa, respectively.

(iii) Appropriate alphanumerical codes agreed between the supplier and the may be used to identify concrete specified by properties other than strength; for example, KC280 may be used to identify a kerb and channel mix with 280 kg cement/m3. High strength/performance concrete Where concrete strength grades greater than 50 MPa, or other high performance parameters, are proposed, it is recommended that the specification require the supplier to submit for approval to the customer or his agent a quality plan specific to the project. The plan should confirm the supplier’s ability to supply and deliver concrete conforming to the requirements of the specification. The plan should also be implemented for the duration of the project.

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Specifications of slump as a maximum value in lieu of the default mean value The practice of specifying slump in accordance with the normal-class provisions is encouraged. The specification of maximum slump or any non-standard method of slump specification is discouraged to avoid confusion.

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C1.7 METHODS OF ORDERING The purpose of this Clause is to establish standard guidelines for ordering concrete produced in accordance with this Standard, so that when placing and accepting an order, there is a clear understanding between the purchaser and the supplier, with regard to the expectations of the former and the responsibilities of the latter.

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The Standard covers the specifications, manufacture (and delivery where applicable) of plastic concrete containing certain basic ingredients and having specified plastic state properties. Further, the purchaser should have confidence that the concrete supplied will achieve specified hardened state properties at the nominated time when handled, cured and tested in accordance with the relevant procedures.

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Whether or not the concrete placed into the structure achieves the same hardened state properties as the test specimens is affected by the handling, placing and compacting techniques employed, the methods and duration of curing used and the method, sequence and timing of any formwork stripping. As these factors are outside the control of the supplier, assessment of compliance of concrete on the basis of testing of the concrete in the structure is outside the scope of this Standard. In fact assessment for compliance based on testing the concrete in a structure is not covered by any Standard, however AS 3600 does give a method of estimating the strength of concrete in a structure. In the absence of test results on samples made from the fresh concrete, testing of concrete in the structure can be undertaken, but only to provide guidance as to whether the concrete produced by the supplier would have achieved the required hardened state properties. In pursuance of the principles established when AS 1379 was first published, suppliers will continue to classify orders as ‘performance’ or ‘prescription’ according to whether the supplier accepts, or declines responsibility for selecting and proportioning the mix ingredients to meet specified or ordered performance parameters. However, there is now an additional requirement for specifying (and ordering) concrete as ‘normal-class’ or ‘special-class’, as distinguished by their respective specifications in Clause 1.6.3 and Clause 1.6.4 of the Standard. It is intended that normal-class concrete should provide a ‘standardized’ range of concretes which are suitable for the majority of applications in domestic, commercial, industrial and institutional buildings. There will of course be other areas, such as particular civil engineering structures, where normal-class concrete would also be suitable. The principal criterion for deciding whether normal-class concrete is appropriate for a particular application, is to determine whether the limited number of parameters permitted to be specified for that class (see Clause 1.6.3) is sufficient to ensure that concrete with the desired properties can be provided. If it is considered to be insufficient, then specialclass concrete is required and the different or additional parameters needed to ensure the desired results are to be specified (Clause 1.6.4) and noted in the order (Clause 1.7). Apart from being an essential requirement for ordering, it is in the customer’s interest to determine beforehand which class of concrete is the most appropriate for the project, as there will usually be a cost difference between the two classes. If there is a ‘structural specification’ for supply of the concrete, the decision as to which class is appropriate will usually have been made by the specifier well before an order is placed. If there is no specification, or the specification is not clear on which class is appropriate, it will be essential for the customer and the supplier to reach an agreement on the appropriate class, before any order is placed. Having established which class of concrete is required, the following ordering procedures of (a) or (b) should be followed as appropriate: (a)

Ordering normal-class concrete Where normal-class concrete has been specified, or it has been agreed between the customer and the supplier that normal-class concrete is appropriate for the intended use, the order should contain only the following information, as appropriate: (i)

The quantity of concrete required and sufficient information to ensure that the receives the concrete ordered.

(ii)

The standard compressive strength grade, designated in accordance with Clause 1.6.3.2(a). COPYRIGHT

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(iii)

A standard slump selected in accordance with Clause 1.6.3.2(b), at the point of acceptance.

(iv)

The maximum nominal size of aggregate selected in accordance with Clause 1.6.3.2(c).

(v)

The level of air-entrainment, if any in accordance with Clause 1.6.3.2(f).

(vi)

The intended method of placement (Clause 1.6.3.2(d)).

(vii)

Whether project assessment is to be carried out by the supplier.

Clearly Items (ii) to (v) above are performance requirements. Furthermore, apart from the limitations imposed by Section 2 of the Standard, there is no restriction on the supplier regarding the selection or proportioning of the mix ingredients. An order for normal-class concrete is therefore a performance order. It follows that in accepting an order for normal-class concrete, the supplier also accepts responsibility for supplying concrete which will have both the specified plastic-state properties, and the potential to attain the specified hardened-state properties, and for ensuring that due has been taken of the other information contained in the order.

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(b)

Ordering special-class concrete Clause 1.6.4 of the Standard indicates that any concrete which is not normal-class is classified as special-class. Clause 1.7 of the Standard requires that if the concrete is special-class, the class is to be further qualified as ‘performance’ or ‘prescription’. At this point, it is important to that, like any other wholesaler or retailer, it is the supplier’s prerogative to accept or decline an order for a product. Hence, whether an order for special-class concrete is accepted as a ‘performance’ order or a ‘prescription’ order will depend entirely on agreement between the supplier and the customer. An order for special-class concrete may be accepted as either a performance order, or a prescription order. If the order is in the form given in Paragraph C1.7b(i) below, and it is agreed between the parties to the order that the supplier accepts responsibility for proportioning the concrete ingredients so that the specified, or otherwise agreed, properties or characteristics of the plastic and hardened concrete can be achieved, then the order will be accepted as a ‘performance order’. If this does not apply, it is understood that the supplier does not agree to accept responsibility for achieving some or all of the specified, or otherwise agreed, properties or characteristics of the plastic and hardened concrete. The order is then classed as a ‘prescription order’ and the information listed in Paragraph C1.7b(ii) below will be required. However, the supplier is still responsible for carrying out all other applicable requirements of this Standard. Appendix B may be used as a guide to the ordering of special-class concrete, subject to the limitations given above. (i)

Special-class performance concrete A special-class performance order should contain only the following information as appropriate: (A)

The quantity of concrete required, and sufficient information to ensure that the receives the concrete ordered.

(B)

The relevant ‘Special-Class’ designation, Clause 1.6.4, followed by the ‘Performance’.

(C)

The strength grade, if applicable.

(D)

The slump, quoted in multiples of 10 mm, and the point of acceptance.

(E)

The maximum nominal size of aggregate, selected from the standard sizes specified in AS 2758.1.

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(F)

The level of air entrainment, if any.

(G)

The intended method of placement.

(H)

Whether project assessment is to be carried out by the supplier.

(I)

Any other construction, performance or quality criteria that may influence the selection of materials or proportioning of the ingredients. These criteria may include a minimum cement content or a maximum water-cement ratio, or both, but if other material proportions are specified, the order will be classed as a prescription order and the information given in Paragraph C1.7b(ii) will be required.

It can be seen that this is almost identical with an order for normal-class concrete, except that the properties specified under Items (C), (D) and (E) do not have to be the standardized normal-class values. The other important exception is the information to be provided under Item (I). In the long run, this will determine whether the supplier accepts the order on a ‘performance’ basis. Information that would need to be included under Item (I) would include the following: (1) Any early-age strength limitation. (2) Any shrinkage strain limitation (see Clause 5.6.2). Licensed to E.S.SURESH on 04 Jun 2002. Single licence only. Storage, distribution or use on network prohibited.

(3) The inclusion of additives such as fibres or colouring pigments. (4) A requirement for colour control of the hardened concrete. As indicated in Item (I) a requirement for specific material proportions will generally preclude acceptance on a performance basis. (ii)

Special-class prescription concrete A special-class prescription order should contain at least the following information: (A)

The quantity of concrete required and sufficient information to ensure that the receives the concrete so ordered.

(B)

The relevant ‘Special-Class’ designation, in Clause 1.6.4, followed by the word ‘Prescription’.

(C)

The particle density, maximum nominal size and grading of the coarse aggregate, within the ranges provided for in AS 2758.1, or alternatively, the source of aggregate supply.

(D)

The particle density and grading of the fine aggregates, within the ranges provided for in AS 2758.1, or alternatively, the source of aggregate supply.

(E)

The type of portland or blended cement, selected from AS 3972.

(F)

The limitations, if any, on the use of other cement constituents.

(G)

The limitations, if any, on the type and proportions of ixtures that may be used.

(H)

The proportions of aggregates and cement, by mass except as otherwise permitted by Clause 4.1.2.

(I)

Either the maximum water-cement ratio, based on saturated surface-dry aggregates and total water content; or the required slump at the point of acceptance.

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Persons requiring special-class prescription concrete are reminded that concrete properties are very sensitive to variations in the properties of the ingredients contained in the mix. Because concrete properties can be achieved with particular proportions in one area, there is no guarantee that these properties can be achieved in an area with different sources of materials. It is therefore incumbent on the specifier to ascertain whether all prescribed requirements can be satisfied with the materials available to suppliers in the region in which the concrete is to be supplied. C1.8

BASIS OF SUPPLY

C1.8.2 Volume of plastic concrete This Clause addresses what the industry commonly refers to as ‘yield’, i.e. the actual volume of concrete supplied, as against the volume ordered and charged. There is an unavoidable variation in the volume of concrete produced from any set of batch weights arising from batching deviations within the allowed tolerances, variations in the moisture content of aggregates, density of aggregates and to a lesser degree other factors such as slump variations and temperature.

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In order to achieve a minimum 98% yield, the mean value of the volumes yielded from the batch weights for one cubic metre obviously has to exceed 98% and the volume of concrete supplied over a long period will be at least equal to the cumulative volume shown on the identification certificate. The determination of volume of a batch of concrete involves measuring the weight of the batch. This can be done by accumulating the batch weights of the ingredients used in the production of the batch. If water is batched by a volumetric metre, a conversion to mass will be necessary. Several factors can cause the in situ measured volume to differ from the volume of plastic concrete as determined above. These factors include handling and compaction, the effects of hardening, temperature changes, formwork deflection and spillage. A quite small unintentional increment in the thickness in a slab amounts to a significant percentage increase in the volume of concrete used while proper compaction can reduce the delivered volume by between 3% to 5%. C1.8.3 Identification certificate A ‘delivery docket’ or similar document issued by the manufacturer and containing at least the information specified in this Clause is considered to be an ‘identification certificate’. Where the customer is also the supplier, the records required by Clause 4.1.6 may be considered to be an identification certificate. The intent of recording water additions is to provide an audit trail to facilitate the analysis of a nonconforming batch. For all the reasons discussed elsewhere it cannot be used as a primary control of total water content at the time of delivery.

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S E C T I O N C 2 C O N C R E T E M A T E R I A L S A N D C O N S T I T U E N T L I M I T A T I O N S C2.2 CEMENT CONSTITUENTS Silica fume is included in this edition of the Standard. Its use has been well documented in recent years and the requirements for its properties are specified in AS 3582.3.

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The use of supplementary cementitious materials generally has progressively increased over the last 20 years. There are good reasons to this growth as follows: (a)

Improvements in certain properties of both plastic and hardened concrete over those achievable with portland cement alone.

(b)

Economy, since the supplementary cementitious materials are industrial by-products which are often cheaper than cement.

(c)

Energy conservation, reducing the demand for the energy-intensive manufacture of cement.

(d)

Reduced carbon dioxide emissions to the atmosphere due to reduced portland cement manufacture.

(e)

Resource conservation, reducing the demand for the raw materials for cement manufacture.

(f)

Reduction in the pressure on landfill for disposal of the otherwise waste materials.

(g)

There is usually the potential for enhanced strength gain after 28 days.

Good curing practice is essential to develop the strength potential of any concrete, and this is especially so when supplementary cementitious materials are used. Generally concrete made using mixtures of portland cement and supplementary cementitious materials attains early strength more slowly than that made with portland cement alone. C2.3 AGGREGATES Because the Australian Standard for aggregate, AS 2758.1, includes a number of alternative options, it cannot be used on its own as a specification for contract purposes. The options selected as appropriate to the intended use or performance will therefore need to be separately specified. Usually it is the supplier s responsibility to specify aggregate quality. C2.4 MIXING WATER The range of impurities to be monitored has been reduced by removing limits for chlorides, sulfates, sulfides and sodium equivalent. The rationale for the change is that limits are elsewhere specified for chloride and sulfate ions in the hardened concrete which embrace impurities introduced from all sources including water. Test methods have been updated to include where appropriate, methods not previously available. C2.6

BULK STORAGE OF MATERIALS

C2.6.1 Cement constituents The requirement to use bagged cement in the same chronological order as it was received has been eliminated. The requirement was absolute, and as such could not be achieved in practice. It was considered the commitment to achieve the specified quality requirements, particularly strength, would preclude any need to expressly specify what is commonly known and observed as good practice in the use of bagged cement.

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C2.7 LIMITATIONS ON CHEMICAL CONTENT OF CONCRETE Some small amounts of chlorides can be introduced from water, cement, aggregates and ixtures. The limit of 0.8 kg/m3 of concrete for chlorides was selected after a review of extensive literature on the subject, especially from North America and Europe where the problem of corrosion of reinforcement had reached alarming proportions, largely due to the use of deicing salts. Experience indicates that sulfate levels above 5% by weight of cement may impair durability. The method of sampling for testing the chloride or sulfate contents is specifically intended to exclude any contaminants introduced after the concrete has been discharged from the mixer.

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If further contamination during or after placing is suspected, further samples of in situ concrete should be tested. The issue of compliance of such samples lies beyond the scope of this Standard.

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E Q U I P M E N T

BINS AND SILOS

C3.1.1 General Requirements have been reworded to include expressions such as ‘as far as practicable’, as it was impracticable to comply with the previous wording expressed in absolute . Bins and silos in most cases should carry prominent labelling of their contents to prevent the accidental addition of material belonging to other storage. C3.2

WEIGHING EQUIPMENT

C3.2.2 Accuracy The current clause allows calibration to be done by an ‘accredited’ rather than an ‘independent’ organization, reflecting the fact that many suppliers have competent in-house calibration facilities. C3.3

LIQUID DISPENSING EQUIPMENT

Licensed to E.S.SURESH on 04 Jun 2002. Single licence only. Storage, distribution or use on network prohibited.

C3.3.2 Accuracy of metering Water is usually added in two or more increments, because the variability of moisture contents in aggregates storage precludes the exact prior determination of the quantity of water necessary to produce a specified slump. The first and principal addition of water is that added concurrently with the cement and aggregates using weigh batching or rugged high capacity volumetric meters usually protected by filters. This is regulated to produce a slump not exceeding the value specified. In all cases an accuracy of at least ±2% can be relied upon from such equipment. The next addition is an adjustment decided upon after the mixing has proceeded sufficiently for an experienced operator to assess the consistency of the batch and estimate the further addition, if any, necessary to produce the specified slump. In larger pre-mixed concrete plants this may be done through a different meter to that used for the first addition, in order to make the loading station available to the next mobile mixer and maintain production. A ‘slump stand’, being a freestanding standpipe with a suitable platform for the operator to view the batch is a common installation used for this purpose. The measuring device may be less accurate than that used for the first addition, but should be sufficiently accurate to ensure that the total of the two increments is recorded to an accuracy of at least ±2%. The second addition would usually be less than 10% of the first. Again in a premix concrete operation, water may be added after the concrete leaves the plant, and if so an estimate of the quantity added by the operator may be the only means of assessing the quantity added due to the unreliability of truck-mounted water meters. C3.4

MIXERS

C3.4.1(a) Performance The concept of testing for uniformity of mixing has been substantially rationalized in this revision. The primary thrust now is to validate the capacity for each different model of mixer in use to mix uniformly. Once a particular model of mixer has been proven to mix concrete satisfactorily, it can safely be assumed all such mixers of that model will continue to do so unless they are worn to a degree that will preclude their continued performance or hardened concrete has been allowed to build up in them. Full uniformity tests are time consuming and expensive. To routinely do them when there is no reason to expect that the mixer is not mixing efficiently has no value. The thrust of the current clauses is — (a)

to prove new mixer types by exhaustive testing of prototypes;

(b)

to validate existing mixer models by exhaustive testing of one of the series;

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(c)

to regularly inspect for wear and cleanliness in place of wasteful repetitive uniformity testing at arbitrary intervals;

(d)

to effect necessary repairs and cleaning promptly when the need is perceived; and

(e)

to re-assess mixers by test after any major repair (e.g. design alteration).

The last item is in recognition that repair can involve the complete rebuilding of the mixing vessel with its baffles and fins. A major repair is seen to be any which may have the potential to reduce the mixer s uniformity of mixing. This would include a drum rebuild under less stringent control than would be found at the original supplier’s factory, but would not include the complete replacement of a drum with a new unit manufactured by the original supplier. An alternative and less rigorous uniformity test is called for after minor repairs, and in cases where inspection raises doubt about the mixer’s capacity to mix uniformly. This involves all but the comparison of coarse aggregate content and the mass per unit volume of the air free mortar.

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The importance of good practice in charging the mixer correctly must be recognized as a vital prerequisite for uniformity of mixing. Large mixers such as mobile mixers used in the premix industry may have difficulty mixing uniformly if care is not exercised in the sequence of loading materials. A reasonably uniform blending of the solid batch materials should be achieved as they enter the mixer. If poor uniformity is apparent without any clear reason indicating the condition of the mixer, loading practice should be examined.

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D E L I V E R Y

GENERAL

C4.1.2 Method of measuring quantities of ingredients This edition amendment removes some unintentional ambiguity present in the 1991 edition. The words ‘proportioned by mass’ have been replaced with ‘measured by mass’ and the words ‘directly or indirectly’ have been deleted. Volume batching of solid ingredients is now only permissible for concrete with a characteristic strength of 15 MPa or less. The use of timed flow of material through a gate opening intermittently calibrated by weight is not considered to constitute measurement by mass. C4.2 BATCH PRODUCTION C4.2.1 Tolerances on batch ingredients

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C4.2.1.1 Ingredients other than water Whatever tolerances apply, the supplier is also constrained by the quality and yield requirements unequivocally expressed in Clause 1.8. Indeed, given a philosophy of performance rather than prescriptive specifications, the need for many of the prescriptive clauses still remaining in this 1997 Standard could be debated. Regardless of this argument, the barriers to compliance with the tolerances in the 1991 edition were identified and this edition addresses two issues which arose. These are as follows: (a)

Multiple cementitious materials and aggregates The 1991 edition was less than clear as to whether the batching tolerances were applicable to each separate aggregate size and cementitious material type of the total quantities. This edition addresses this issue. It expresses explicit limits for the total of all cementitious materials and for each individual cementitious material. It requires compliance for the total mass of all aggregates, the total mass of fine aggregate and the total mass of coarse aggregate. It does not call up limits on separate components, if any, of fine or coarse aggregates.

(b)

The quantification of tolerances The provisions of the 1991 edition were unattainable with current plant. Two issues are relevant, material in free fall and variation in moisture contents of aggregates, as follows: (i)

Material in free fall The flow of material into a weigh hopper does not stop the instant a gate or valve is closed. Some material will be falling between the gate and the surface of the material already in the weigh hopper, referred to as material in free fall. The amount of material in free fall has to be anticipated by both manual and computer controlled batching operations, interrupting the feed of material before the intended mass has reached the weigh hopper and ed on the scale or digital readout. The allowance for free falling material is intuitive for a manual operation, and preset for automated plants. The free falling mass for any one material varies with the distance from the gate or valve to the surface of the material already in the weigh hopper, the moisture content of aggregates, particularly of fine aggregates, while humidity and fineness will affect the amount of cementitious materials in free fall. The efficiency of automated plants in anticipating the mass in free fall has not been found to be better than that of an experienced manual operator, although the human variability may be greater. COPYRIGHT

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Variation in moisture content of aggregates The weight of each aggregate to be batched has to be adjusted to allow for its moisture content. No sufficiently fast and accurate means of measuring moisture content of aggregates has been developed to achieve this.

By compounding the moisture content inaccuracy (a percentage of the mass weighed) with mass in free fall, (a constant for each weighing) a range was generated within which the designed mass of material could be relied upon to fall. Adverse combinations of tolerances for aggregates and cementitious materials, i.e. overbatching aggregates and underbatching cement, were then examined. A tolerance of +0.2 to -0.4 in the overall aggregate to the cementitious materials ratio was used to arrive at the proposed figures. To achieve this it was necessary to ‘skew’ the tolerances, allowing a greater negative tolerance in measuring aggregates and a greater positive tolerance when measuring cement. Suppliers have a further restraint imposed from Clause 1.8.2 which specifies minimum ‘yield’, i.e. the assurance that the volume of concrete supplied is substantially that stated on the identification certificate (or delivery docket). Underbatching of both aggregates and cement could result in breach of this Clause. Management of yield properly belongs in the supplier’s quality management system. C4.2.1.2 Licensed to E.S.SURESH on 04 Jun 2002. Single licence only. Storage, distribution or use on network prohibited.

(a)

Water

Two methods of control currently in use are as follows:

By control of slump The importance of water control in the production of quality concrete is well and widely understood. The fact that the predominant method of controlling water has been by achieving the desired slump and NOT by measuring the water is not so well or widely understood. This reality is due to the lack of any sufficiently responsive and accurate means of measuring the moisture content of aggregates, discussed in Paragraph C4.2.1.1(b)(ii) above in relation to the batching of materials other than water where it creates a similar problem. The control of slump is an indirect control of water, and in practice is more reliable for making adjustments from batch-to-batch throughout a day than even the best available measurements of water content. When accompanied by periodic checks that the total batch water is within the tolerances set for the mix design, it has proved for many years adequate to control the strength of concrete. The tolerances specified in Clause 5.2.3 are intended to allow for human error in the assessment of slump. The supplier should aim to achieve a mean slump that is close to the specified slump. The supplier should not aim to achieve slump results higher than the specified slump.

(b)

By control of water-cement ratio To meet a specification for water-cement ratio, or total water content, it becomes necessary to measure and record the total water content. Repetitive sampling and measurement is necessary to detect variations within the stored material. This involves reliance on automatic devices of limited accuracy or the attendance of additional personnel at additional cost. It may also be necessary to restrict production rates in order to make the measurements.

It should be said that the value of the enterprise is questionable because of the limitations to the frequency of sampling to detect variations in the moisture contents of material even within a batch and the variation possible in the actual measurement, automated or manual. Allowing for these variations in measurement accuracy and imperfect sampling, in the best case measurements of moisture content cannot be relied upon to be more than ±1.0% in fine aggregate and ±0.5% in coarse aggregates. The effect in a cubic metre of concrete of these errors may be as much as 15 to 20L, or up to 10% of the total free water.

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Control of water by control of slump would usually result in less variation in total water content than attempting to monitor added water. Any procedure involving control of water content by measurement also has to be augmented by procedures to ensure the slump is within the tolerances in Table 6. The specification of the tolerance appropriate for water-cement ratio is dominated by the achievable accuracy in the control of total water. The quantity of water per unit volume of concrete is relatively constant over a wide range of mix designs. These considerations gives rise to the logic that any tolerance for water-cement ratio is best expressed as a percentage of the target water-cement ratio, and not as a constant. As discussed earlier the water content per unit volume of concrete is estimated to be subject to a variation of 10%. This possible variation determines the selection of a tolerance of ±10% of the target value of the water-cement ratio. This neglects the compounding effect of variations in the targeted cement content which would call for a greater tolerance.

Licensed to E.S.SURESH on 04 Jun 2002. Single licence only. Storage, distribution or use on network prohibited.

Examples of the absolute value of the tolerance are — Specified value of w/c ratio

Tolerance

0.60

±0.06

0.35

±0.035

A difference in philosophy of the specification of water-cement ratio exists. The supply industry prefers that the water-cement ratio be specified as a median value. Some public authorities prefer to specify a maximum value. By way of compromise both practices were itted. If the water-cement ratio is not specified to be a maximum value complying ratios will range 10% either side of the specified value. Alternatively if a maximum water-cement ratio is specified, complying ratios will range from the specified value to 20% less than the specified value. C4.2.2

Batch mixing

C4.2.2.1 General The 1991 edition required mixers to be completely discharged before being loaded with a new batch. Although probably not the intention when drafted, this could be construed to mean a complete washing out of the drum to remove even the mortar coating remaining after a batch is discharged. Wording has been amended to remove this possibility. The coating of mortar remaining after all concrete has been discharged is arguably best left in the mixer, as after discharge of the next batch a similar coating will remain, which will be taken from the ingredients of the next load if the mixer is completely free of all material prior to loading. Special needs to completely empty and clean mixers occur in some circumstances such as when fibre or colour are used intermittently in a mixer. Aside from the debate, if any, on the mortar coating remaining in the mixer, if all mixers were to be washed out completely prior to loading a new batch, existing waste disposal units at all plants in Australia (with very few if any exceptions) would be grossly overloaded, resulting in illegal discharges into drainage systems. Installing substantially larger units is precluded by site area restraints in most plants and in any case their economic viability is severely prejudiced by the capital and operating costs of substantially larger disposal facilities. The usage of water would also increase dramatically. If trucks are washed out after every load, and without the recycling of water, the total use of water for washing and for inclusion in the batch is approximately three times that contained in the batch. COPYRIGHT

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In some cases, even returned concrete can be allowed to remain in the mixer safely. Suppliers’ quality assurance systems are designed to control the judgement and procedures used. These are company specific, and would prove cumbersome to include in this Standard. In the last analysis the supplier is constrained by the performance requirements of all the other clauses in this Standard and the project specification. In particular, the supplier has the obligations to — (a)

satisfy the strength criteria;

(b)

deliver concrete with a truck life to ensure the ability to place and compact concrete within a practical time; and

(c)

guarantee the yield.

Suppliers state that these constraints have in the past and will continue to provide the protection needed to ensure the integrity of the concrete delivered, without breaching environmental regulations or incurring a substantial and unnecessary cost burden on the product to avoid the breaches.

Licensed to E.S.SURESH on 04 Jun 2002. Single licence only. Storage, distribution or use on network prohibited.

ixtures recently available appear to prolong the life of plastic concrete for long periods, such as overnight. Currently their use cannot be ed by sufficient data, but in future their use may be permitted by an amendment or a revised Standard if adequate documentation of their satisfactory performance becomes available. C4.2.3 Addition of water or ixtures to a mixed batch Paragraph C3.3.2 discusses the various stages of water addition in the process of production of concrete. For the purposes of this Clause, the batch is deemed to be ‘mixed’ after it leaves the plant, and the addition of water en route to the site or after arrival at the site is dealt with by this Clause. If water is added before the commencement of discharge either on site or en route to the site, the Standard requires adequate mixing prior to commencement of discharge. All such additions of water before commencement of discharge are deemed to be at the supplier’s initiative and under his sole control. The commencement of discharge is a contractually significant point in the process of supplying a batch of concrete. Water added after commencement of discharge may be at the initiative of the customer and negate the supplier’s warranty for the material. Such additions also carry greater potential risks dependent on the time between the contemplated addition and the original commencement of mixing. For these reasons the commencement of discharge is critical and needs some definition. On occasions a small amount of the batch may be discharged, say 0.2 m3, before it is apparent that the slump is too low. Addition of water at this stage is considered to be no different to the addition prior to any discharge. However, after discharge beyond this point commencement of discharge is said to have commenced and the Standard calls up additional assessment, testing and recording procedures. This distinction between additions prior to and after commencement of discharge seeks to regulate the irresponsible addition of water to facilitate handling, which will cause the consistency of the batch to exceed the specified tolerances. To this end the Standard calls for determination of slump when water is added after commencement of discharge. In some regions industrial custom and practice requires the presence of an industrially authorized tester on site to determine slump by the Standard method. Such an individual may not be readily available. It is also possible that timely availability of personnel or equipment preclude the determination of slump by the Standard method. In such cases an assessment is accepted in lieu of a measurement. In other circumstances a measurement would always be preferred.

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The quality assurance intended to be provided by the Standard is lost if samples are taken for the casting of test specimens, and then water added. Clause 4.2.3(c)(ii) precludes this practice. C4.2.4 Period for completion of discharge The Standard would be incomplete if no limit was placed on the period for completion of discharge, however the ‘correct’ limit is incapable of definition for all conditions. Temperature plays a great role in the speed of the chemical reactions between cement and water, as do the properties of the batch ingredients. The clause for these reasons does not prescribe 90 min as an inflexible limit. C4.3 C4.3.1

CONTINUOUS PRODUCTION Batching

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C4.3.1.1 Solid ingredients Periodic assessment of flow rates of solid ingredients from storage into the production stream is not considered to constitute measurement of materials by mass. A variety of acceptable procedures such as integrating belt weighers exist. Continuous measurement by mass is critical as the flow characteristics of solid ingredients change frequently. C4.3.1.3 Accuracy of batching rates for cementitious materials To avoid adverse combinations of tolerances for aggregates and cementitious materials, i.e. overbatching aggregates and underbatching cement, a ‘skewed’ tolerance has been adopted as for batch production to ensure maintenance of a tolerance of +0.2 to -0.4 in the overall aggregate to cementitious materials ratio. The discussion in Paragraph C4.2.1.1 on the need to sustain a conforming level of yield in batch production applies equally to continuous production. C4.3.1.4 Accuracy of batching rates for coarse, fine, and total aggregates The same provisions for accuracy of component ingredients is used as in the clauses on batch production. C4.4

DELIVERY

C4.4.2 Temperature at point of delivery Where the ambient temperature is below 10°C or above 30°C, measures such as heating or cooling the water or aggregates may need to be adopted in order to comply with the requirement that delivered concrete has a temperature at the acceptance point of not less than 5°C nor greater than 35°C.

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O F

GENERAL

C5.1.2 Grouping of plants For some purposes of the Standard, different plants are allowed to be grouped and considered as one plant. For compliance with the statistical compressive strength criteria of Section 6, relatively wide grouping is allowed. This grouping only prevails for the purpose of calculating a standard deviation to apply to each of the plants in the group, where a minimum of 30 results is needed to obtain a value of standard deviation in which a fair degree of confidence can be held. Standard deviations for small groups would usually be less than for larger groups. Since a higher standard deviation increases the values to be achieved to comply with the acceptance criteria, a supplier s decision on grouping of plants will have to take of the disadvantages of large groups.

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A second permissible grouping is for chemical contents and shrinkage compliance. In this case grouped plants are required to use similar materials to give confidence that the measurement obtained from one plant will be those which would be obtained from others in the group. C5.2

SLUMP

C5.2.1 Frequency of testing Slump is almost always a parameter specified in the order. Further, as discussed elsewhere in this commentary, assessment of slump is usually the primary means of controlling the amount of water in concrete, an assessment being made shortly after batching to fine tune the adjustment to water content. For these reasons, slump needs to be assessed for every batch of concrete produced. However, the measurement of slump in accordance with AS 1012.3 is not performed on every batch, nor can it be without an expensive restructuring of manning levels in concrete plants. Experienced operators such as batchers or operators of truck-mounted mixers develop skill in estimating slump from the appearance of the mix and the mechanical behaviour of mixers to a point where satisfactory control is achieved without the need to measure the slump of each batch. Alternatively, where the calibration of a slump meter is possible this device may be used to complement the slump cone measurements and provide information on each batch. Experience and skill is also vital when slump is determined in accordance with AS 1012.3, as the test result can be corrupted by seemingly minor deviations from the standard procedure. It is uncommon for skill in estimation or measurement of slump to be developed by people not continuously involved in observing and working with concrete. When strength tests are to be taken from a batch containing superplasticizers slump assessment before and measurement after the addition of superplasticizers is strongly recommended. C5.2.2 Determination of slump For some batches slump meters provide an alternative method for slump assessment. Some slump meters are attached to the hydraulic system of the truck-mounted agitator and measure the energy required to rotate the drum at mixing speed. The slump meter reading is unique to the truck, batch size and mix proportions. The slump meter may be useful for quality control rather than compliance testing and should be supplemented by normal testing.

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C5.3 STRENGTH The parameter upon which to assess compliance for quality in respect to strength is predominantly the 28-day characteristic strength, according to the detailed criteria specified in Section 6. When flexural or indirect tensile strengths are specified, rules are expressed to determine an equivalent mean compressive strength by which the grade can be monitored. The regime for monitoring characteristic compressive strength is so well established and understood there is obvious merit in adopting a reliable conversion to enable flexural or indirect tensile strengths to be monitored the same way. Alternatively, when a strength other than 28-day characteristic strength is required as the principal quality parameter, the specifier is free to develop a monitoring regime under rules stipulated to achieve the same statistical operating characteristic as that upon which Section 6 is based. C5.3.4 Action to be taken on noncompliance for strength As a matter of policy contractual issues are not included in Australian Standards. None the less Australian Standards are frequently referred to or annexed in one way or another to contract documents.

Licensed to E.S.SURESH on 04 Jun 2002. Single licence only. Storage, distribution or use on network prohibited.

This Standard is often directly referenced in the suppliers standard quotation forms and hence becomes an element in the simplest contracts, such as those created by a letter of offer and an expressed or implied acceptance. Further it is caught by references to it, indirectly via AS 3600, in the Building Code of Australia (BCA). Very little construction is not caught by the force of law attaching to the BCA. As a result the majority of formal construction contracts will directly or indirectly require compliance with this Standard. The statistical process of production assessment in this Standard does not readily translate into appropriate procedures in the event of noncompliance. While more sophisticated contracts may specifically deal with procedures to apply in the event of noncompliance, it cannot be expected that all contracts, expressed and implied, would be so carefully drawn. It is desirable then that this Standard has some provisions that permit a linkage with the contractual consequences of noncompliance while still maintaining the exclusion of specifically contractual issues from the Standard intrinsically in compliance with policy of Standards Australia. In the event of noncompliance, the 1991 edition of the Standard was potentially very onerous in its application. The principles of Section 6 represent a statistical process for monitoring the quality of production on all projects within a particular production interval. It could be construed that ALL concrete produced in the production interval of the grade which did not comply should be rejected. This has the potential to span over a period from one to several months and cover thousands of cubic metres of concrete. It could not in any practical sense be enforced. In fact no instance is recorded of the matter arising, although it is certain that in the 5 years of the Standard s application concrete has failed to conform. The problem to be addressed with this Standard was seen to be one in which that compliance is determined for a production ‘lot’ of concrete of one grade produced in a production interval of one or more months, without any relation to the projects on which the concrete was used. The potential interpretation of the 1991 edition that all concrete in a production interval was to be rejected, or even made liable to rejection was completely unacceptable to the supply industry. This Clause addresses the problem by providing the supplier with procedures to be implemented to restrict, where appropriate, the potential for rejection of concrete to a subset(s) of the production of all the grade in the production interval.

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The burden of implementing these procedures rests on the supplier. None the less, if calculations are necessary to determine if the noncomplying concrete is of sufficient quality so as not to prejudice the durability or structural adequacy of the structure, the designer will logically be expected to undertake such calculations. C5.5 CHLORIDE AND SULFATE CONTENT There is a mismatch in the units used to express chloride and sulfate contents in the Standard test procedures and those generally used to express the accepted safe levels of each in concrete. It is hoped that more compatible units may be adopted in the Standard test procedure in due course. Meanwhile this Clause expresses the means by which the test results should be converted to those units used to express permissible limits in this Standard. C5.6

DRYING SHRINKAGE

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5.6.1 Drying shrinkage of normal-class concrete The test specimens for drying shrinkage are easily damaged and care needs to be exercised when field sampling is undertaken. The Standards for the test methods, AS 1012.13, specifically address all the necessary issues in technique. Because of the particular sensitivity of this test procedure it is especially important that the Standard is assiduously applied. Remote areas are particularly prone to bad practices if experienced testing staff is in short supply. The supply industry expresses a preference for control shrinkage sampling to be done in accredited laboratories on laboratory mixes simulating the field mix, to reduce the potential variation in results arising from sampling and handling procedures.

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S E C T I O N C 6 S A M P L I N G , T E S T I N G A N D A S S E S S M E N T F O R C O M P L I A N C E O F C O N C R E T E S P E C I F I E D B Y C O M P R E S S I V E S T R E N G T H

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INTRODUCTION (a)

Summary The assessment provisions in AS 3600 (1988) and AS 1379 (1991) were strongly related to the concept of plant-based production control using a controlled grade for which reasonable numbers of test data would be available. A complex variety of provisions were given for other grades at the plant and for smaller plants along with appropriate project control rules. Although technically sound there have been practical problems. These provisions have been greatly simplified by combining all the production control systems into one procedure that can deal with a wide range of available sample numbers. This overcomes problems previously encountered with very small production runs and production runs separated by long periods of time. An important change is that the production interval length is defined only for the controlled grade and the assessment of this grade and all the other associated grades at the plant are made once only and at the same time — at the end of that production interval.

(b)

Normal distribution In order to understand quality assurance testing and to evaluate some of the alternative strategies, it is necessary to have some appreciation of statistical concepts. The characteristic strength, fc′, is defined as the strength that would be exceeded by 95 percent of the test results. Therefore, this definition accepts that for satisfactory concrete 5 percent of results will be less than fc′. It has been found that the long-term distribution of concrete test strengths approximates a normal distribution. From the normal distribution, the definition of fc′ requires that the target (average) strength be fc′ +1.65σ where σ is the standard deviation of the concrete population. In of proportion of defectives the use of a normal distribution may be a little severe as industry experience is that errors tend to be more often on the high side rather than low. The control method adopted compares the mean fcm of a group of results with a threshold level in the form fc′ +kcs. The definition of the threshold level has to consider the variability from determining the mean of the population using a small sample and from the need to estimate the standard deviation of the entire population, σ, using the standard deviation of the sample, s. These uncertainties result in the acceptance threshold level fc′ +kcs being dependent upon the number of tests.

(c)

Supplier’s and customer’s risk When a decision on the acceptability of a unit or lot of concrete is made, based on limited data, there is considerable uncertainty and the concept of who receives the benefit of the ensuing doubt becomes important. The customer would like to receive cast iron assurances that the concrete is not less than the specified strength. The supplier would expect that if the target strength is just satisfactory then there would be no chance of noncompliance or rejection from test results. Unfortunately, since testing is expensive and samples are limited, both parties have to accept a compromise situation. However, since public safety is allied with the customer’s interest, that compromise must take this into . In the production control methods in Section 6, the benefit is technically given to the producer. However, basically the clauses are set up primarily to reject concrete that is significantly defective, protecting the consumer and safety.

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The customer’s risk is defined in the alternative assessment procedures in Clause 6.6(b) which are the basis of Section 6. This states that ‘concrete with a proportion defective of 0.30 shall have a probability of rejection of at least 98%. Ideally the customer would prefer a 100% chance that any concrete with more than the specified 0.05 proportion defective should be rejected, but uncertainty prevents such absolutes. Defective in this sense means test values below the specified grade value of f ′c. It may be thought that a value of strength so low that the proportion defective is as high as 0.30 would be unsafe but a quick calculation using the normal distribution would imply that such concrete would have a target of fc′+ 0.53σ compared with the desired value of fc′ +1.65σ so the difference is only 1.12σ or, with a standard deviation of say 3 MPa, only 3.4 MPa. Although by no means insignificant, such a low strength is unlikely to seriously affect the safety of the structure. On the other hand, there is little evidence to suggest that this is too severe on suppliers.

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The suppliers risk is also implied in Clause 6.6(a) as ‘concrete with a proportion defective of 0.05 should have a probability of acceptance of at least 50% . This means that the producer has to accept that if the concrete is up to the correct grade, i.e. 0.05 proportion below the specified value, then there is still as much as a 50% chance of noncompliance. What is worse is that with low sampling rates the chance of noncompliance may be higher and barely good concrete will more often than not be rejected. Small samples create uncertainty and that uncertainty will lead to risks for both suppliers and customers. To avoid these high risks some adjustment in the target strength is necessary and suppliers will need to adapt to this trade-off, particularly where low sampling rates are involved. On the other hand this requirement infers that concrete that is just below grade still has a 50% chance of acceptance. Other structural materials impose much more onerous statistical conditions on suppliers. As can be seen from the above discussion the distribution of risk is even-handed with the consumer accepting the risk that the concrete could be a little below grade but confident that concrete significantly below grade will be rejected, while the producer has to accept the risk that even satisfactory concrete may be rejected, particularly if sampling rates are low. (d)

Basic principles of assessment clauses The basic principles of production assessment for a particular grade are that where a large number of samples of a grade are obtained during a production interval and their strengths determined, the mean grade strength (fcm) should be not less than the specified characteristic strength (f ′c) plus 1.65 times the standard deviation (s) of the sample strengths determined for that interval. Where a limited number of samples are obtained during a production interval, it is not possible to directly confirm that the mean grade strength and an alternative is taken of assuming it is correct unless the tests on available data are not consistent with this hypothesis. Specifically if the strength is so significantly low that the defective quantity could reach 30% then the concrete should almost certainly be rejected. Thus concrete represented by the tests is deemed not to comply if the mean grade strength for the relevant production interval falls below a threshold value equal to the specified characteristic strength (f ′c) plus the standard deviation of the sample strengths (s) multiplied by a factor (kc) which is dependent on the number of samples obtained during the production interval. Hence compliance requires that — fcm ≥ f ′c + kc s

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Since there are many mixes supplied by a plant the above requirement is applied mainly to a frequently tested mix designated as the ‘controlled grade’ with the other ‘associated grades’ assumed to be subject to similar mix design and control practices. It must be appreciated that satisfactory concrete can fail this test and it may sometimes be necessary for the average strength of the concrete to be even greater than the minimum required to reduce the occurrence of random failures. (e)

Production assessment vs Project assessment Production assessment using the relatively large number of test data available to the supplier is the cornerstone of the requirements of the Standard. Project assessment is also included, but the provisions are less statistically rigorous, as such assessment is intended only as a supplementary check rather than an assurance of quality. Engineers are strongly recommended to rely generally on production assessment with project assessment only used as an extra check for critical elements or special-class concrete.

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C6.1 GENERAL REQUIREMENTS The general requirement is that all concrete of strength likely to be structurally important (grade 20 MPa or more) shall be assessed by the supplier by a system of production assessment which invariably will involve Clause 6.3. When project assessment is specified then additional assessment by Clause 6.5 will apply. Clause 6.6 allows for a rarely used alternative means of production and project control. Where project assessment is specified, then the testing and assessment is normally carried out by the prime contractor or a subcontractor. The concrete supplier may be used for project assessment if there is an agreement between the supplier and the party responsible for project assessment. C6.2

SAMPLING AND TESTING

C6.2.2 Sampling The point at which samples are taken is common to production and project assessment. It is not intended that samples for production assessment be taken prior to the normal point of discharge from the mixer. C6.2.5 Test strength of samples Provision has been made for the rejection of test specimens which differ significantly from results expected when good testing practice has been followed. The two rejection criteria for individual test specimens are based on — (a)

cylinder strengths which are significantly different from the expected average, i.e. deviating by greater than ±3s: and

(b)

cylinder strengths from one sample which show excessive difference.

The first of these is a new requirement and effectively removes a result which has an extremely low probability of belonging to the sample group, from being included in the statistical analysis. Recent test data should be used to calculate the expected average strength based on the equation, fcm = f ′c + 1.65s. The rejection criteria for the range of cylinder strengths within a sample is similar in principle to that of AS 1480 except that the range has been halved, reflecting improved testing practices. The reduced acceptable difference between test specimens was evaluated from more that 3000 sets of test results, taken over a period of 3 years during the construction of the new Parliament House. The data came from 3 separate testing laboratories, testing 5 grades of concrete supplied to the project by 3 different manufacturers. Evaluation of the difference between the 28-day strengths of companion specimens showed that, for a maximum difference of 2 MPa, the probability of obtaining acceptable pairs of specimens is in the range of 97% to 99%. COPYRIGHT

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When no cause can be determined for excessive range, the lowest cylinder strength is now to be rejected. These modifications are designed to minimize dispute over which test strengths are to be deleted from analysis of test data. C6.3

PRODUCTION ASSESSMENT

C6.3.1 C6.3.1.1

Basic requirements of production control General requirement

This Clause is mainly a directory to the other clauses.

C6.3.1.2 Designation of grades The system of control is based on the concept that only one grade of a plant’s production is a controlled grade. This grade should generally be the most commonly tested grade over 6 months but since this information is not known in advance this Clause permits some latitude. Certainly it will be in the supplier’s interest to adopt the most frequently tested grade. The controlled grade can vary from one production interval to the next. For this controlled grade, 10 samples per production interval which can be up to three months, should be taken; but there is no actual minimum. However, the assessment is structured so that there is an implied penalty for lower sampling rates.

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All other grades are associated grades. C6.3.1.3 Frequency of assessment An important change is that all grades both the controlled grade and the associated grades are assessed on the basis of the mean strength at the end of the production interval of the controlled grade. This has the disadvantage that some small sample sizes may result for the associated grades but the advantage of a single clear-cut assessment period with no problems of multiple assessment of the one lot of concrete. C6.3.1.4 Assessment factor The Clause for compliance of a controlled grade is derived from some complex statistics due to Manton-Hall using the recommendation for compliance of Clause 6.6(b) discussed in the Introduction. The compliance test is based on the mean grade strength determined each month from the tests over the production interval being greater than a threshold value of f ′c + kcsc. The value of kc is given in Table 7 where it can be seen to vary from 3.2 for 4 samples to 1.25 for 15 or more. (It effectively combines the earlier (AS 1379 1991) production assessment criteria with that for non-standard strength grades.) The use of a compliance criteria based on a defined probability of noncompliance of a 0.30 defective concrete using the mean and standard deviation determined from a limited sample demands that the variability of the sample standard deviation must be taken into . Usually this can be done using the t-distribution which includes an allowance for the variability of the standard deviation on the value of the mean of a sample. However, as the criteria in of the 0.30 defective level also includes an estimate of the standard deviation, a more complex non-central t-distribution is needed. Computations using a program developed by Manton-Hall produced the numbers in Table 8 which were included in AS 3600 (1988), but in the relatively obscure Clause 20.7.3. Certainly it is not possible to these numbers using simple normal distribution, although it is possible to check the results using simulation. The same assessment factor is used for the associated grades as well as for the controlled grade. This is based on the assumption that all grades at a plant are subject to a similar process of control so that the standard deviations are related and any adjustments to the controlled grade should be made to all grades. Because the numbers involved in the associated grades are lower the risk that substandard concrete will be accepted is higher than desired while the risk to the producer of satisfactory concrete being rejected is also higher. This is not ideal but the alternative of spreading the assessment over a long period in order to obtain sufficient data also presents problems. COPYRIGHT

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One important aspect is that the higher thresholds for the lower sampling rates implies that unless the producers are to be plagued by noncompliance they should use higher target strengths. C6.3.1.5 Assessment of grade The assessment is based on the mean fcm of the test results for the entire production interval. This assessment is carried out on all grades at the same time — the end of the production interval for the controlled grade. Moreover the threshold value given is based on the controlled grade and the number of tests available in that grade. C6.3.2 Grade being assessed The assessment procedure is based on the concept of there being one population of concrete represented by a single mix. For practical reasons some flexibility is needed in interpreting this ideal situation. The strength grade being assessed has at least the common characteristic of the same required characteristic strength. However, the clause permits considerable flexibility as to what constitutes a strength grade. It can include variations in mix type aggregate size and cement types and subject to Clause 6.3.6, can even come from different plants if operated by a single supplier. It can include special-class concrete as well as normal-class and of course, concrete with various slumps.

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It need not include all concrete of that grade as subsets are permitted. Note that for sampling, compliance and all the other requirements of the Section, such subsets are treated as a separate strength grade. Ideally the assessment should be based on a single plant and a single mix. The extension beyond this assumes certain common practices of adjusting and controlling the strength. From the supplier’s point of view, widening the definition of strength grade will possibly increase the standard deviation, although there are considerable advantages in threshold levels for increasing the number of samples. Obviously the supplier is the appropriate party to determine which mixes are to be included in the grade. C6.3.3 Sampling frequency A sample consists of two or three cylinders. The cost of sampling and testing is not insignificant and the rate is therefore a compromise between cost and statistical accuracy while minimizing the volume of concrete at risk from failure to comply. A concession is offered for 20 MPa grade concrete where production is higher and sampling rate is correspondingly high. However the reduction in sampling rate after 15 samples per month are achieved should still recognize the need for the sampling to be random or at least uniform through the month. C6.3.4 Production interval The choice of the production interval is a compromise between the volume of concrete at risk, the need for frequent reports and the influence of small sample sizes on rejection rates. If at least 10 samples are available the producer can choose the production interval within wide limits of two weeks to three months. It must be recalled that the production interval is set for the controlled grade but applies to all associated grades as well. Where the production is low so that 10 samples of the controlled grade are not expected in the three months then the production interval is set at three months even though a lower number of samples will result. The minimum sampling rate will influence the number available. Where the production interval exceeds one month the producer may choose to issue interim assessment of the concrete based on the samples available over a period equal to a production interval. Such reports would be advisory and could not lead to a situation of noncompliance.

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C6.3.5.2 Sample size for the standard deviation of a controlled grade Clause 6.3.5.2 includes only minor modifications and provides a practical method of determining the standard deviation. The basic calculation is on the controlled grade. If the number of samples in the production interval is 30 or more then the calculation is based on the current production interval. If there are less than 30 results then past production interval results can be included up to a total of 30 results but not back further than 6 months. This is a compromise between the need for large samples and the requirement that the samples should represent the concrete being assessed. If there are only five or fewer samples available they shall be used but in this case the standard deviation shall be taken as not less than 3 MPa. The use of standard deviation over a period longer than the current production interval is somewhat inconsistent with the statistical basis of the compliance, but limited simulation studies indicate that the results would be more conservative. This means unsatisfactory concrete would have an even higher chance of rejection. The sample standard deviation (sc), is determined from the statistical expression with ‘n − 1’ as the denominator. This is of course only an estimate of the true standard deviation σ. The difference is significant when compliance is being considered. The formula is as follows:

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sc =

(x − x )2 n − 1

where sc = standard deviation of a controlled grade x = sample strength x = mean strength n = number of samples C6.3.5.3 Standard deviation for an associated grade For an associated grade, the standard deviation, sa, is computed from the results if there are 30 in the current production interval. Otherwise the standard deviation is computed using relative factors. For example, if 25 MPa was the controlled grade for which a standard deviation of 3.0 MPa was found, then the value for 40 MPa grade would be given by: s40 = 3.0 × 1.3/1.1 = 3.5 MPa where 1.1 and 1.3 are the relative factors given in Table 8 for 25 and 40 grade respectively. C6.3.6 Grouping of plants to determine nc and s The need for large sample sizes is in conflict with the requirement that the sample should represent one population of concrete. In some circumstances a single supplier may elect to pool results from different plants and this is permitted. Ideally such pooling should only be used when the materials and control processes at the plants are similar. C6.3.7 Action on noncompliance Where the concrete does not comply with Clause 6.3.1 or even Clause 6.5, then it is important to realise that no further testing or assessment can change that condition of noncompliance. However, the action to be taken should recognize two aspects. Firstly, satisfactory concrete can often fail to comply, and secondly even if the concrete is not entirely satisfactory it may still be suitable for its intended purpose. Consumers should take a long term view on the reliability of concrete from a producer. C6.3.8 Early age assessment of controlled grades The industry practice of early age testing resulted in this Clause which is intended to provided additional assurance to the consumers that the strength is being maintained and that any major errors will result in early action. COPYRIGHT

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APPENDIX CB

GUIDE TO THE SPECIFICATION OF SPECIAL-CLASS CONCRETE CB1 GENERAL The specification options listed in Table B1 of Appendix B are generally limited by practical considerations of available materials and plant capabilities. These limitations are in some cases specific to a region or a plant configuration. The ranges of practically available options are discussed below. Additional costs may be incurred complying with special-class concrete parameters compared with those incurred in complying with normal-class concrete parameters. Attention is directed to Clause 5.8 which calls for the method of production control and the criteria for compliance which shall be specified for ‘other parameters’. This applies to some of the parameters discussed below. NOTE: Values other than shown may be negotiated between the supplier and the customer.

TABLE

CB1

ALTERNATIVE VALUES FOR CONSIDERATION

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Parameter, Clause, normal-class default value Mass per unit volume of hardened concrete Clause 1.6.3.1(a) 2100-2800 kg/m3

Alternative values for consideration Lightweight aggregates can be used to reduce the deadweight of the . The aggregates available and the weight reduction achievable with them are region specific and should be identified with the concrete and aggregate suppliers. Dense concrete has occasionally been required, e.g. for nuclear shielding. Region specific comment applies as above.

Chloride content Clause 1.6.3.1(b) 0.8 kg/m3

The level of chloride present in the regionally available materials, notably water, will set achievable limits.

Sulfate content by weight of cement Clause 1.6.3.1(b) 50 g/kg

As for chlorides, achievable sulfate limits are region specific.

Shrinkage strain Clause 1.6.3.1(c) 1000 × 10-6

Characteristics of available aggregates make shrinkage strain a region specific parameter. The subject has been discussed in some detail in Clause C1.6.4. The Commentary on AS 3600 1994 (AS 3600 Supplement 1 — 1994) includes [Table 6.1.7(A)] the typical values achieved at some capital cities, reproduced below. Data on typical shrinkage values for commercial structural concrete, suitable for pumping at a slump of approximately 80 mm. Location Brisbane Sydney Melbourne Adelaide Perth

Range of basic strain values 10-6 500 to 900 500 to 900 600 to 1000 700 to 1100 600 to 1200 (continued)

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TABLE Parameter, Clause, normal-class default value Cement type Clause 1.6.3.1(e) as required within Clause 2.2

CB1

(continued) Alternative values for consideration

Normal-class concrete may contain any combination of cements and supplementary cementitious materials, an implied control being achieved with compliance with the 7-day strength specification. Special-class concrete may specify a cement type to achieve results appropriate for the structure, e.g. low heat, sulfate resistance and the like.

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f ′c Clause 1.6.3.2(a) 20, 25, 32, 40 or 50 MPa

Strengths higher than 50 MPa can be supplied. The upper limit is both plant and region specific.

Slump Clause 1.6.3.2(b) 20 to 120 mm in intervals of 10 mm

As required

Maximum nominal aggregate size Clause 1.6.3.2(c) 10, 14, or 20 mm

As required

Project assessment Clause 1.6.3.2(e) subject to specification

As required

Air entrainment Clause 1.6.3.2(f) ≤ 5%

As required

Flexural strengths Clause 1.6.4(b) nil

As required

Indirect-tensile strength Clause 1.6.4(c) nil

As required

Testing of constituents Clauses 2.2 to 2.4 nil

In normal-class concrete the onus is entirely on the supplier to maintain records to establish compliance with the Standard and this is commonly incorporated in the supplier’s contracts with the material supplier. If required, reports on the quality of constituents can be incorporated in a special-class concrete specification. The tests required (preferably by reference to a Standards Australia reference) and their frequency should be specified.

Records and additional information Clause 4.1.6 and Clause 1.8.3 (Identification Certificate)

Provision of batch information as required in addition to that specified in Clause 1.8.3 may be specified. Computerized plants, becoming more common in metropolitan areas and on large projects, can readily provide a wide range of information. Smaller, lowcapacity plants are more limited in what can be readily produced. The identification certificates are limited in size, and usually additional information has to be presented in a supplementary document, and if so specified, attached to the identification certificate where practicable. (continued)

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TABLE Parameter, Clause, normal-class default value Tolerances on batching Clause 4.2.1

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(continued) Alternative values for consideration

The batching tolerances specified in Clause 4.2.1 are consistent with the capabilities of plant used in the industry. The onus of achieving the minimum criteria specified by the customer is discharged by the supplier, by means of targeting mean performances sufficiently above those minima to satisfy the compliance criteria. Some customers have expressed a desire to control the range between minima and maxima values achieved for the performance parameters in addition to the specification of minimum values. Standard deviation is a traditional statistical measure of variability and is considered appropriate in this context.

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The level of control exercised in the operation of a plant together with the nature of the plant itself and the availability of the material determine the standard deviation achieved in production from that plant. The standard deviation also increases, as the strength grade increases, although not linearly. However a difficulty arises if a maximum standard deviation is specified. The number of samples used to calculate the value is critical. With six values the Standard deviation computed could range over ±2 MPa of the value computed from 1000 results. This is such a large percentage of any value likely to be specified as to render impractical the definition of an acceptance criterion for anything but a very large number of samples. A practical approach may be to establish by inspection of plant records the Standard deviation achieved over a long period for the most commonly produced mix and either rely on that value being sustained or specify that it be sustained. Indicative values for a variety of strength grades and levels of control are: Level of control

Strength grade 20 MPa 25 MPa

Laboratory High level commercial plant predominantly one mix Typical commercial plant Low production remote plant

w/c Clause 4.2.1.2(b) nil

2.0 2.6 3.0 4.0

2.2 2.9 3.3 4.4

32 MPa 2.4 3.1 3.6 4.8

40 MPa 50 M Pa 2.6 3.4 3.9 5.2

2.8 3.6 4.2 5.6

As required

Addition of water or ixtures to a mixed batch Clause 4.2.3

The provisions of Clause 4.2.3 are those universally achievable in practice. Any variation to those would generally involve additional cost.

Period for completion of discharge Clause 4.2.4

The 90 minutes specified in Clause 4.2.4 is qualified by reference to prevailing temperatures.

90 minutes, subject to qualifications

If project considerations indicate a need for more precision in the specification of allowable times they may be appropriately specified.

Temperature at point of discharge (t) Clause 4.4.2 5°C <35°C

Project considerations, e.g. size of , may dictate the specification of a lower maximum temperature.

Testing of concrete Section 5

If a frequency and type of concrete testing more extensive than that specified in Section 5 is required, it should be specified and if the supplier is required to carry out that additional testing, this should also be specified.

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REFERENCES MANTON-HALL A.W., Comparison of Operating Characteristic of Overlapping and non-overlapping ‘Means of N’ type specification Uniciv Report R-171 August 1977. MANTON-HALL A.W., Tables of the Operating Characteristic of Overlapping Means of N type specification Uniciv Report R-176 March/August 1978.

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STANDARDS AUSTRALIA Concrete Structures: AS 3600 — 1994, Sydney 1994.

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