Presentation on theme: "Workshop on Specifications and Producer QC – Ken Day I shall start by spending a few minutes describing some provisions of the Australian Code before inviting."— Presentation transcript:
Workshop on Specifications and Producer QC – Ken Day I shall start by spending a few minutes describing some provisions of the Australian Code before inviting your participation. I will then make provocative statements about the effects of prescription and performance specifications, plant and project control, achievable variability, quality of testing, the analysis of data and financial benefits I enjoy arguments, PLEASE OBLIGE!
Workshop on Specifications and Producer QC Australian Code Requirements AS1379 / DR05253 www.standards.com.au
Specification of Concrete Concrete shall be specified as either: Normal class or Special class and by strength grade ‘or other readily verifiable parameter’ Standard grades 20, 25, 32, 40, 50,/ 65, 80, 100 MPa (20=2,900, 50=7,250 / 100 =14,500psi) 32MPa (4,640psi) is the minimum strength allowed to be specified for external reinf. concrete Also max agg size, slump, method of placement, air entrainment are to be specified.
Other Standard Requirements (wide basic requirements for all concrete) Density 2100-2800kg/m (130-175lb/cu ft) Acid soluble chloride and sulphate limits Shrinkage (max 1000) 7 day strength 50% of grade strength (up to N50) Cement complying with AS3972 alone or plus ‘one or more supplementary cementitious materials’
Special Class Concrete Still preferably uses strength grade can be compressive, flexural or indirect tensile One of 3 exposure classifications - which imposes limits on aggregate durability and class and type of cementitious material >50MPa (7,250psi) has to be special class Can specify other requirements ‘in consultation with the supplier’
Special Class Concrete It is certainly allowed to specify tighter limits on shrinkage, low permeability, the use of fly ash, silica fume, or ggbfs etc. This will usually restrict the range of producers prepared to supply to those having arrangements with particular material suppliers and suitable storage arrangements (extra bin?) Other special class items could include self- compacting concrete, lightweight, very high early strengths, colour control, wear resistance, heat generation, shotcrete, underwater etc.
Assessment for Compliance Tests at least 1 per 100cu m (133cu yd). Assess at selected ‘production interval’ from 2 weeks to 3months (1 month usual) One grade to be selected as ‘control grade’ and to have at least 10 results per production interval (most have many more) Variability of production to be assessed on basis of this grade
Compliance Requirements The ‘Characteristic Strength’ is that exceeded by 95% of results so: = Mean –1.65SD (90% so -1.28SD in USA) However it is recognised that there is an error in assessment so k in mean-kSD is varied according to number of samples in control grade from 3.2 for 4 samples to 1.5 for 10samples and 1.25 for 15 or more samples (1.25 gives all error margin to producer, I do not agree with this)
Compliance Requirements For ‘associated grades’ the SD is scaled from that of the control grade using relative factors: <20(2,900psi)-0.9, 20-1.0, 25-1.1, 32-1.2, 40-1.3, 50 (7,250psi) -1.4 Note that these are far from assuming the same coefficient of variation applies, eg 40 @ 1.3 not 2x20 @1.0
Dissemination of Information A long list of information is to be prepared at the end of each ‘production interval’ and ‘kept readily available for inspection’ Customers can require relevant data to be submitted to them within 15 days The supplier shall notify the relevant customers, within 2 working days, if a particular quantity of concrete is likely to be below the specified strength
Project Assessment At least 1 sample per 50cu m Moving average of 3 to exceed f’c Not much different to US, less likely to find any problem than the plant control
The Purpose of Testing Concrete In 1958 I wrote: ‘The only rational objective for any but 100% testing is not to discover and reject faulty products but to ascertain the minimum quality level of the production’ -Some ideas take a while to sink in!
The Purpose of Testing Concrete More recently I have added a second objective of quality control: ‘To detect at the earliest possible moment any change in the quality of concrete being supplied’ The factors involved in such a detection are: the frequency of testing, the basic variability of the concrete, the analysis system in use.
Plant v Project Control 1.It is far more efficient to ensure that no defective concrete is produced at a plant than to ensure that no defective concrete is delivered to a project because: 2.More data is available at less cost per cubic yard 3.Problems can be detected and identified earlier 4.Producers can react to a limited amount of early age test data but project control usually requires a significant number of 28 day results to demand action on marginally defective concrete. 5.A limited number of standard mixes can be accurately maintained with data also on shrinkage, durability, fresh properties (slump, pumpability, bleeding) etc
DISCUSSION 1.Prescription specifications provide no incentive for producers to know or care anything about designing or controlling concrete. 2.As a consequence such specifications have to provide a very large safety margin to cover high variability and inefficient design and material selection. 3.Since there is no incentive for the producer to employ competent staff, purchase good materials, and have good production facilities, it is necessary for all such matters to be specified in detail and to employ supervision to ensure compliance. 4.Therefore prescription specification concrete is inevitably more expensive than producer-controlled performance concrete.
Variability of Concrete (COMMENTS?) 1.Under good quality control, the SD of concrete strength should not exceed 450psi and can be as low as 300psi (achieved on concrete of 14,000psi mean strength on Petronas Towers) 2.Under the UK QSRMC system a figure of 600psi is regarded as normal 3.Without formal QC, 800 to 1000psi would not be surprising (what is yours?) 4.Even under the relaxed US criterion of 10% defective, each extra 100psi of SD requires a mean strength increase of 128psi and so at least 20lb/cu yd additional cement
Testing of Concrete 1.In a paper ‘Bad Concrete or Bad Testing’ to ACI San Diego in 1989 I showed that individual tests can be inaccurate to the extent of over 1,000psi but that an analysed pattern of results was very reliable. 2.A single test result is not an invariably accurate assessment of the quality of concrete in a single truck 3.ALSO we should show as much concern for those trucks we did not test as for those we did
Testing of Concrete 1.Best criterion of testing quality is average pair difference of 28day results. 2.75psi is best attainable, 150psi just OK, 200psi is POOR 3.Average strength of a pair is depressed by at least half the pair difference (more likely twice this) 4.So sub-standard testing costs you money even without considering rejections and penalties (Prof Juran’s ‘Gold in the Mine’)
Testing of Concrete 1.Who does the best job of testing? 2.In Australia a lab is required to be NATA registered. An independent lab must achieve registration but derived no benefit from additional quality since the lab was chosen by the main contractor in the 1970s 3.Since higher testing quality meant a higher mean and lower variability, supplier’s labs had more incentive to achieve a higher standard 4.So, as an independent analyst and a NATA assessor, I found that supplier’s labs generally achieved at least a slightly higher standard
Testing of Concrete: Duplicate Testing 1.When concrete producers began doing the main testing, specifiers were initially keen to have a proportion of check tests by independent labs 2.It proved very inconclusive to test different trucks 3.Duplicating tests by two lab teams on the same truck was more enlightening, but expensive 4.Requiring producer’s sampling personnel to take a proportion of double samples and deliver half to an independent lab worked well 5.Now, after years of experience of such checks, very few purchasers require independent checks
Conclusion on Producer Testing 1.After 20+ years of experience it is clear that it is worthwhile for any substantial producer to establish a high standard, officially certified, laboratory 2.The laboratory and its test data and analysis records must be open to inspection by customer’s representatives at all times 3.In these circumstances, plant control with producer testing is substantially more economical and reliable than project control
Physical and Financial Benefits 1.It is clear that producer operated plant control results in better controlled, lower variability concrete at a lower real cost of production. So there is scope for everyone to benefit 2.The question is how to get started – specifiers probably happy to use plant control once it is seen to exist but producers have to provide it first?
Physical and Financial Benefits 3) I suggest specifiers include an alternative clause waiving requirements such as minimum cement content and aggregate gradings ‘where an approved system of plant control is in operation’. They could initially be generous as to what is required for approval. 4) Some US producers are already using plant control and will immediately benefit. Competition will ensure others follow.
Physical and Financial Benefits 1.The initial benefit will be to those producers who first achieve low variability and so are more competitive 2.As other producers catch up, the benefit is to the customers as competition reduces price 3.There may be some tendency for takeovers as some small producers are unable to compete 4.However small producers headed by technically competent persons can compete and may be able to react quicker than larger producers and gain an initial advantage (eg Don Bain at Maricopa)
Analysis of Data 1.The efficient analysis of data is of as much significance as quality of testing 2.It is to be judged by the rapidity of corrective action in terms of number of results needed to reach a decision 3.Multigrade, Multivariable, Cusum analysis is 3 to 10 times as fast as normal Shewhart graphing 4.A semi-automatic mix correction system is also required for rapid response to give low variability
Analysis of Data 1.Such analysis can be undertaken using the free programs on my website: www.kenday.id.au 2.There are far more elaborate systems (such as ConAd) giving greater insight into the causes of problems and more precise mix correction 3.Such programs may also be of substantial value in production engineering matters, locating batching plant and constituent material problems and also monitoring the performance of individual testing personnel and truck drivers