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Rocks put to the test: A proficiency testing program

 

Nearly 30 laboratories affiliated with quarrying operations have recently participated in the third round of a rocks proficiency testing program to assess the strengths and purities of construction materials. Proficiency Testing Australia co-ordinated the program and analysed the results.

Proficiency testing is a means of determining the performance of individual laboratories for specific tests or measurement and an external audit of their quality control procedures. A standard proficiency testing program compares the measuring results of different laboratories on identical samples.

Proficiency Testing Australia (PTA), one of Australia’s largest proficiency testing providers, can service the construction materials industry by offering proficiency testing programs for aggregate, soils, concrete, rocks, cement, asphalt and bitumen.

PTA recently announced the completion of its third Rocks Proficiency Testing Program. The program was approved by International Accreditation New Zealand and organised with the technical assistance and sample supply of Gayani Samaradiwakara of Golder Associates’ Melbourne Laboratory. 

The aim of the program was to assess laboratories’ abilities to competently perform the prescribed analyses.

Twenty-nine participating laboratories participated in this program in April 2021. All but two of the laboratories were in Australia, with the overseas participants in Bulgaria and Peru. 

All of the laboratories submitted their results by the due date. Each laboratory was allocated a unique code number for the program to ensure confidentiality of results.

All participants performed the required testing by either Australian Standards or ASTM methods, eg: 

  • AS 4133.4.1 Determination of point load strength index – (clause 3.3 Axial test).
  • AS 4133.4.2.2/1.1.1 Determination of Uniaxial Compressive Strength/ Determination of the moisture content.
  • AS 4133.4.3.2 Determination of deformability of rock in uniaxial compression.
  • ASTM D3967 Splitting tensile strength on intact rock specimen.

Laboratories were requested to perform the tests as per PTA instructions and record their findings on a PTA-supplied results sheet. Two or four cylinders (nominally 50mm diameter x 150mm length) were supplied to each laboratory. Cylinders had the identification A or B, depending on the tests performed by each laboratory. 

Prior to sample distribution, a number of randomly selected samples were analysed for homogeneity. Based on the results of this testing, the homogeneity of the samples was established.

STATISTICAL ANALYSIS OF PARTICIPANTS’ RESULTS

PTA used a robust statistical approach to assess the laboratories’ testing performance. 

A list of summary statistics appears at the bottom of each of the tables of

results and consists of:

  • The number of results for that test/sample (No. of Results).
  • The median of these results, ie the middle value (Median).
  • The uncertainty of the median; a robust estimate of the standard deviation of the Median.
  • The normalised interquartile range of the results (Normalised IQR).
  • The robust co-efficient of variation, expressed as a percentage (Robust CV), ie 100 x Normalised IQR / Median.
  • The minimum and maximum laboratory results.
  • The range (Maximum – Minimum).
The results by lab of the Rocks Deformability – Density test as per AS 4133.4.3.2.

Z-scores measure how far a result is from the median consensus value and create a “score” for each result relative to the others in the group.

A z-score close to zero indicates the result is in agreeance with those from other laboratories. A z-score with an absolute value greater than or equal to 3.0 is considered an outlier and is marked by the symbol §.

On the Ordered Z-Score Charts, each laboratory’s robust z-score was shown in order of magnitude and marked with its code number. From these charts, each laboratory can compare its performance relative to its peers.

Table A below summarises the results that were submitted by the program participants. 

LABORATORY PERFORMANCE AND TECHNICAL COMMENTS

There were 17 outliers (or 8.85 per cent) from the 192 results analysed across the 29 laboratories. All laboratories with outliers or an absolute z-score between 2.0 and 3.0 have been encouraged to review their procedures.

Table A: Summary statistics. Note: N/C = ‘not calculated.

In this round, the results for Uniaxial Compressive Strength Moisture Content, Rock Moisture Content and Rock Deformability Moisture Content were not analysed. About 34 of the 44 submitted results were under 40 per cent. 

The pre-testing of the samples showed that the Moisture Content of PTA Sample B was between 41.1 per cent and 43.2 per cent. However, the correct moisture percentage was obtained only after drying the samples for four days. 

As the rock samples used in this round were artificial and a four-day drying of natural rocks is uncommon, a decision was made to not analyse the Moisture results in this round. 

However, as this is the third round of the Rocks program (with 50 to 58 per cent Moisture content obtained in previous rounds), the incorrect results are considered outliers and it is suggested that the multi-day drying of samples be considered in future rounds. 

As stated in Australian Standard AS 4133.1.1.1 Methods of testing rocks for engineering purposes Rock moisture content tests – Determination of the moisture content of rock – Oven drying method (standard method), the drying process should continue until the loss in the mass of the rock between successive drying periods is less than 0.1 per cent of the initial mass of the wet rock.

Thus, participants have been recommended to review their Rock Moisture determination processes.

A few laboratory results were excluded from the statistical analyses and identified as non-statistical outliers. Laboratories were encouraged to investigate their procedures for these tests. 

The areas for improvement included Uniaxial Compressive Strength (UCS), UCS Density, Poisson’s Ratio, and Rock Deformability Density.

Other possible sources of error that might have affected the outcome of the proficiency testing program included but were not limited to:

  • Interpretation of test methods and procedures.
  • Calibration/condition of equipment.
  • Incorrect calculations.
  • Transcription errors.
  • Technician competency.
  • Precision in calculations and reporting.

HOMOGENEITY AND STABILITY TESTING

The samples utilised in this program were supplied by Golder Associates’ Melbourne Laboratory. For this program, samples were cast on two separate days: Samples A on Day 1 and Samples B on Day 2. 

Eight random samples from each day were selected and tested for homogeneity. Statistical analysis showed that the samples were sufficiently homogeneous so that any results identified as outliers could not be attributed to sample variability.

The homogeneity acceptance criteria was: three per cent CV for the Density test and five per cent CV for the Uniaxial Compression Strength test.

As testing was performed on the same day by all participants, stability testing was not considered necessary.

Determination of deformability of rock in uniaxial compression as per AS 4133.4.3.2.
Determination of the uniaxial compressive strength/determination of the moisture content, as per AS 4133.4.2.2/1.1.1.

 

ADVICE TO PARTICIPANTS

The Rocks Proficiency Testing Program was developed to offer laboratories the opportunity to evaluate their testing capabilities in comparison to other laboratories, and to provide an opportunity for participants to improve their performance after the identification of outlier results.

 Participants with outliers in Rounds 1 and/or 2 were encouraged to investigate their performance in the current round to see if any improvement had been made.

A comparison of laboratory performance in Rounds 1 and 2 with Round 3 showed that five participants that obtained outlier results in Rounds 1 and 2 improved their performance and reported correct results in Round 3. 

One laboratory did not improve its performance. Some participants from the earlier rounds did not participate in Round 3, and were therefore excluded from analysis. 

The final program report for Round 3 of the Rocks Proficiency Testing Program can be found on the PTA website: pta.asn.au

Round 4 of the Rocks Proficiency Testing Program is scheduled for March 2022. 

For more information about the program and to register, contact the PTA, via email
ptaenquiry@pta.asn.au or visit https://www.pta.asn.au/index.php/programs/request-form

This article appears in the September issue of Quarry Magazine.

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