Regulating Concrete Quality

Ken Day, ConsultantMelbourne, Australia

The Objectives

To achieve suitable regulation it is first necessary to:

A) Realise what you are trying to achieve

B) Realise what you are trying to prevent

Historically:

Specification was related to an individual batch of concrete

Batch quantities were the subject of the regulation

Full time inspection was affordable

Strength as a Criterion

Strength was then recognised as the only workable basis

An absolute minimum strength was specified

Inevitable Variability recognised

Strengths of successive deliveries of supposedly identical concrete were seen to vary by up to +/- 15MPa, rarely less than +/- 5MPa

Grouping Results

Small groups of 3, 4 or 6 results were tried by various countries

Even groups of 6 did not provide an accurate mean strength and variability

Even groups of 3 represented too much concrete to reject as a unit

“Percentage Defective” A “Normal Distribution” was found

to be applicable so that results could be analysed for mean strength, standard deviation, and % below any given strength

About 30 results were needed to give good accuracy

“Percentage Defective”

Percentage defectives of 1, 5 and 10% have been used, multiplying the SD by 2.33, 1.645 and 1.28 respectively

Decision based on “what is a reasonable margin”

I would suggest it should be based on the value placed on low variability

What are You Trying to Stop? A low mean strength?

A high variability?

Occasional gross errors?

ALL OF THE ABOVE!

Gross Errors Even testing alternate trucks (at

excessive expense) would give only a 50% chance of detection

You are reliant on the producer’s equipment and QC system so these need maximum encouragement/reward

Penalisation Marginal underperformance cannot

be fairly dealt with any other way than financial penalisation (marginal is grey, not black or white!)

Failure to penalise underperformers places good producers at a disadvantage

Downturn detection Even with appropriate financial

compensation, purchaser (and producer!) will be keen to avoid defective concrete. This raises two questions:

How to predict eventual strength from early result?

How to get enough results quickly at acceptable cost?

Speeding downturn detectionTwo techniques make a huge difference:

Base control on plant rather than project

Use multigrade basis, i.e. combine results from possibly hundreds of grades of concrete in an analysis of situation

Speeding downturn detectionThe combination of these techniques

can increase a hundredfold the number of results available and drastically reduce time to detection of a downturn

A downturn in a particular grade at a particular project may be detected before any results are available on that project, or even on that grade

Speeding downturn detectionFurther improvement in detection time

possible using advanced analysis system

Cusum analysis has been shown to be approximately three times as effective as Shewhart charting – which is still better than normal graphing

Speeding downturn detection

Better Prediction:Early results not usually % of later

results, adding average gain better Needs continuous feedback of true

gain which can change abruptly

Speeding downturn detection

Cusum graphs of many items – density, slump, temperature, cement tests, sand specific surface etc etc can give instant explanation of strength changes

Cusums are Cumulative Sums of difference between current value and previous mean – can include LW and dense on same density graph, high and low strength grades on strength graph

Multivariable Analysis

Speeding downturn detection

The purchaser is not in as good a position as the producer to detect downturns early

If a later penalty is inevitable, the producer will be just as keen as the purchaser to detect and rectify downturns early

Conclusion

What is needed is a type of regulation that will encourage producers to expend every effort to establish a system and physical facilities that will:

Produce low variability concrete Correctly target mean strength React quickly to any downturn

Regulation in UK and Europe Recent new standard EN206

Requirements rather than control system

QSRMC is real control system in UK

QSRMCQuality Scheme for Ready Mixed Concrete Established by the industry, big

advance on world scale

First to introduce Cusum (dev by RMC)

Multigrade technique uses transposition of results to a single grade for analysis

USA Strangely resistant to innovation Perhaps partly due to fragmented

industry but prime example of specification-driven barrier to progress

Prescription mixes still common Mix adjustment actually prohibited Producer designs abused if permitted

Australia (AS1379)

Regulations are by Aust. Standards Assn. Production mainly by few large producers Producers required to undertake own

testing and report monthly to purchasers Not perfect, but best example of suitable

regulation leading to good control – could be better early reporting, penalties

Draft of Desirable RegulationsThe concrete producer shall have in

operation an effective QC system with at least the following features:

1) Plant to produce, preserve, and link to QC system, complete record of actual and intended batch quantities of every batch

Draft of Desirable Regulations2) Batch records to be analysed to

show any systematic trend to error or any significant individual error and any such to be reported to purchasers

3) Mixes may be collected into multigrade groups and each such group shall have a minimum rate of testing each month

Draft of Desirable Regulations4) All data shall be entered in control

system within 24hrs of obtaining and analysed daily to detect change using graphical, multigrade, cusum analysis or proven equally effective alternative

5) All purchasers of concrete PREDICTED to be sub-standard shall be immediately informed

Draft of Desirable Regulations6) A monthly report detailing for each

mix in production, at least: number of results, early age and predicted and actual mean strength, standard deviationminimum strength, No & % of results below specified strength

Draft of Desirable Regulations Note emphasis on early detection

of any problem and ready availability of data to establish cause

A usually trivial cost penalty of twice the cost of the amount of cement that would have raised the month’s mean strength to the required would be sufficient to ensure fair competition

Quality Implications W/C ratio basic factor and directly

related to strength – at a given strength the mix with the LOWEST cement content is the best (lower water)

Pozzolanic materials reduce cost, improve durability and environment

More uniform concrete likely to be easier to place, better appearance

Quality Implications Important to understand that this

paper does not pass any judgement on desirable strength margins in structural design, or for durability considerations

Author believes extra cost of higher margin often worthwhile but should not be by requiring higher mean regardless

Cost Implications

Difficult to quantify savings by proposals

Avoiding costs of further testing, negotiations, rejections, due to poor control (or poor testing!)?

Better mix design, wider material choice?

Reduced expenditure on control testing?

Reduced mean strength due lower SD!

Conclusions

Paper is concerned with best way to ensure a selected strength obtained with max certainty and min cost

A key factor is that regulations must not inhibit progress and must provide a fair basis for competition

Conclusions

A comparison of practice in different countries illustrates that failure to apply these principles inhibits development of improved technology

Conclusions It may never be possible to

completely eliminate problems but if they can be largely foreseen and the rest detected and resolved in minutes or hours instead of days or weeks, the economic benefits could be substantial

The main losers are likely to be the legal profession and the physical investigators of defective concrete!