Role of Specification, Mix Design, and QC in Developing Durable, Economical and Sustainable Concrete Construction

1. Role of Specification, Mix Design, and QC in Developing Durable, Economical and Sustainable Concrete Construction Ken W Day FACI, FICT, Hon CIA

2. WHAT NOT TO DO! • Specify concrete by strength supplemented by a generous minimum cement content or a maximum W/C ratio “for durability” • Require a trial mix to be approved and not departed from without a further trial • View cement replacement materials, if permitted at all, as diluting cement content.

3. WHY? • It has been clearly established that materials such as fly ash, slag, silica fume, rice hull ash, superfine silica etc are far more effective than a low W/C ratio in reducing permeability and therefore improving durability. • Specifying a minimum cement content destroys any incentive for a concrete producer to become competent – it explains why many US producers are more than 20 years behind Australian producers in developing good quality control.

4. WHAT ELSE IS IMPORTANT? • The best Quality Control is that which is able to detect any change in mix performance at the earliest possible moment and quickly compensate for it. • Any low 28day strength should have been predicted and compensated for WEEKS before it occurs • It is ridiculous to wait for enough low 28day results to statistically confirm a downturn!

5. WHAT TO SPECIFY • It is desirable that you should NOT specify a unique set of requirements unnecessarily • If a suitable mix is already in production, advantage can be taken of history to date. • At a given strength level there are still many • properties to consider: • Shrinkage, Pumpability, Heat generation, • Early strength for demoulding or prestressing, • Low bleeding for good (formed) surface finish, • A short-term test for long-term durability is urgently needed (RCP?)

6. MIX DESIGN • Mix design has recently changed significantly • W/C ratio is no longer useful in design • The important aspect of Grading is no longer the sand % and grading curve but the packing density of the fine materials • Some producers may need assistance in arriving at a suitable design • But ultimately the producer must “own” the design in the sense of being responsible for fine tuning of the mix in production

7. COMPETITIVE MIX DESIGN • It is very easy to design a mix over the telephone if what is needed is good “ordinary” concrete and there is no competition • It gets hard when you are competing with other producers and a large variety of materials is available • In time, producers will have experimented with many mixes and materials and must be encouraged and allowed to use the materials and proportions found best economically when they can demonstrate satisfactory results

8. “SUPER” CONCRETE • High strength? 100MPa fairly easy, 200 limit? • Pumpability: limit about 500m? • Durability??? Permeability the criterion? • For all these attention is needed to the grading/packing density of cement with finer materials, slag, pfa, silica fume, RHA, superfine calcium carbonate, nano silica • I do not have a mix design system to give a definitive answer for this and experimentation is necessary

9. QUALITY CONTROL • A margin has to be provided between the required properties and their average • Strength may not be what matters most, but it is the best means of detecting change • The financial benefit of reducing variability is easily calculated and can be substantial • Other benefits such as smoother working and better appearance are less calculable but can be even greater

10. PROBLEM DETECTION • A major benefit of good control is earlier detection of problems • To put a number on it, a downturn of half a standard deviation is detectable, a wide range since SD can be anywhere from 2 to 6MPa • But consider the effect on pumpability, appearance of formed surfaces, variable finishing times, wear resistance and surface tolerance of floors................

11. HOW LONG TO DETECT DOWNTURN? • Specifications the world over deal with analysing a number of 28day results • Action really needs to be taken BEFORE there are ANY low 28day results!! • A downturn in mean strength can usually be detected in 3 or 4 7day (or 3day) results using cumulative sum graphing • The cause may be detected even earlier than this using cusum graphs of 24hr density/unit wt, slump and temperature, batch plant error or sand grading

12. WHO SHOULD INITIATE CONTROL ACTION? • It is obvious that a consulting engineer cannot demand action on the slender grounds proposed • The concrete producer should be the one initiating mix revision • He may do this to avoid future trouble, but I think a clear (small) cash penalty is desirable --- the one thing that almost no-one in the world agrees with me!!

13. STARTING ON QC • Good QC has to start with accurate testing and accurate batching • It has to cover the variability of input materials, especially sand, cement and coarse aggregate also important but (hopefully) less likely to vary • When these items are under control, a large proportion of day-to-day variability is usually due to variations in water content

14. WATER CONTENT VARIATION • This is usually caused by one of: temperature, slump or sand grading • Increased water reduces density so it can be detected by weighing cubes/cylinders at 24hr • So if your slump or temperature shows an increase and 24hr density is down, you can already be sure that your 28day strength will be lower

15. TESTING ACCURACY(1) • Defective testing will obviously invalidate QC • It can be detected by a high pair difference between 28day specimens or a variable 7 to 28day strength gain but this is very late. • It can be due to variable compaction or curing, defective capping of cylinders, defective cube moulds or more than a dozen other causes • My experience is that the best testing is usually achieved by the concrete producer, who is the one who suffers financially if it is sub-standard

16. TESTING ACCURACY(2) • So we should be continually watching average pair differences and 7 (or3) to 28day gain since they can be automatically available, but, if detected, urgent action is needed • Average pair difference should not exceed 1MPa and individual differences over 2MPa should be investigated and the lower of the pair discarded.

17. CUSUM CONTROL GRAPHING • So now we know what items must be controlled in order to obtain low variability concrete • “Change points” must be distinguished from random variation and CUSUM (Cumulative Sum) graphs are the answer. • What is graphed is the cumulative sum of differences from a target value

18. MULTIGRADE, MULTIVARIABLE, CUSUM QUALITY CONTROL (MMCQC) • The breakthrough is to use the current average value of a variable as the target, this has two major benefits over a fixed target: • a) you don’t need to calculate and revise a target • b) several grades of concrete can appear on the same graph of each variable, each automatically having its own target, but differences being accumulated as if all from the same target • There is not time here to explain the technique in detail, some explanation is in the written paper, but for a full account you need to go to my website www.kenday.id.au

19. AVAILABLE MMCQC PROGRAMS • There are at least two versions of this program: • One is the very comprehensive one shown on the website as Conad but now owned and marketed by Command Alkon as Command QC. This copes with the dozens of variables which can affect concrete. • What I am presenting here is a small free program “KensQC” which covers only temperature, slump, density and strength at two ages (3 or 7 and 28 days) This little program will very promptly detect the existence of any problem but can only detect the cause if it is one of the monitored variables or sub-standard testing. It works well if batching error, and sand and cement quality variation, are already under good control.

20. USE AND EFFECT OF MMCQC PROGRAM • The small program can be operated by a non-technical clerical person who simply enters results each day and raises an alarm if the (automatic) graphs show a change of slope. • The essential user is the concrete producer who must react to change and identify cause • The program is also suitable for use by a consulting engineer who wants to assure himself, with very little effort, that QC is being well done. • In the initial stages control could be duplicated by an international consultant taking only a few minutes a day

21. CONCLUSION AND RECOMMENDATION • The objective of this presentation has been no less than to transform how concrete is specified, produced and controlled • It presents the view that the only satisfactory situation is for concrete producers to develop expertise in the design and control of concrete mixes • They will not do this unless they can see the prospect of profiting by acquiring the necessary expertise. • This requires that specifications be based on required properties, allowing freedom in proportioning and selection of materials. • But which comes first, the chicken or the egg?

22. ACHIEVING THE TRANSFORMATION • Under a suitable specification (perhaps initially providing an additional margin) some producers would very rapidly acquire initial expertise by engaging consultants. • The KensQC program could be used to enable both specifiers and producers to examine current performance at little cost prior to change. • A big question is QC field and lab staff and facilities. It is desirable that the concrete producer should at least be the employer of the field staff and perhaps establish labs– does India have the equivalent of the Australian NATA to assess their performance?