More than 25 laboratories affiliated with quarrying operations have recently participated in the first round of a fine aggregates 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 first Fine Aggregates Proficiency Testing Program. The program was approved by International Accreditation New Zealand and organised with the technical assistance of Boral Resources’ Materials Technical Services Laboratory in Sydney, New South Wales and Soil Engineering Services, in Townsville, Queensland.
Twenty-eight participating laboratories were provided ~5kg of fine aggregate (sand) for Round 1 of the program in September 2020. All but two of the laboratories were in Australia, with the overseas participants in New Zealand and Malaysia. Only one of the laboratories did not submit its results by the due date. Each laboratory was allocated a unique code number for the program to ensure confidentiality of results.
The program was designed to cover the methods outlined in:
- Australian Standard (AS) 1141.5 Particle density and water absorption of fine aggregate.
- AS 1141.24 Aggregate soundness test by exposure to sodium sulphate.
- AS 1141.25.3 Degradation factor – Fine aggregate.
- AS 1141.33 Clay and fine silt – settling method.
- AS 1141.34 Organic impurities other than sugar.
|Test||No. of results||Median||Normalised IQR|
|Particle density and water absorption of fine aggregate|
|Apparent particle density (nearest 0.01 t/m3)||24||2.620||0.041|
|Particle density on a dry basis (nearest 0.01 t/m3)||25||2.560||0.044|
|Particle density on a saturated surface dry basis (nearest 0.01 t/m3)||25||2.580||0.044|
|Water absorption (nearest 0.1%)||25||1.30||0.59|
|Aggregate Soundness Test by Exposure to Sodium Sulphate Solution|
|Total weighted loss (nearest 0.1%)||16||0.85||0.41|
|Fraction 4.75mm to 2.36mm (nearest 0.1%)||3||N/A||N/A|
|Fraction 2.36mm to 1.18mm (nearest 0.1%)||16||0.55||0.56|
|Fraction 1.18mm to 0.600mm (nearest 0.1%)||16||0.40||0.35|
|Fraction 0.600mm to 0.300mm (nearest 0.1%)||16||0.55||0.28|
|Degradation factor (nearest 0.1%)||6||94.0||2.8|
|Clay and fine silt – C ratio (nearest 1% or ‘indeterminate’)||19||3.0||1.1|
|Organic impurities other than sugar (lighter or darker than standard)||17||N/A||N/A|
Table A: Summary statistics. Note: N/A = ‘not applicable’.
Laboratories were requested to perform the tests as per PTA instructions and record their findings on a PTA-supplied results sheet. Before sample distribution, ten randomly selected samples were analysed for homogeneity, which was established for the tests.
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).
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 (above) summarises the results submitted by the program participants.
LABORATORY PERFORMANCE AND TECHNICAL COMMENTS
There were 15 outliers (or 7.32 per cent) from the 205 results analysed across the 28 laboratories. All laboratories with outliers or an absolute z-score between 2.0 and 3.0 have been encouraged to review their procedures.
The results for Particle Density and Water Absorption of Fine Aggregate, Degradation Factor and Clay and Fine Silt results showed a good agreement between participating laboratories, with only seven outliers identified.
Homogeneity testing for Organic Impurities Other Than Sugar showed two results that appeared to be the same colour as the colour reference with the remainder lighter. Results equal to the reference colour are reported as “Lighter than Standard” as per AS 1141.34:2018.
Of the 17 participants that performed the Organic Impurities Other Than Sugar testing, 12 produced a result of “Lighter than Standard” and the remainder “Darker than Standard”. Being a visual assessment process, some variation can be expected, especially when the resultant solution colour is borderline.
The homogeneity results for the Aggregate Soundness Test by Exposure to Sodium Sulphate Solution displayed higher variations. Two of the three fractions had significant outliers which resulted in higher CVs (between 31.9 per cent and 40 per cent), possibly due to non-representative portions. When disregarding these outliers, it was ruled the overall results indicated a homogenous material.
There was a wide range of results in the Aggregates Soundness Test between participants with notable outliers, implying numerous participants did not follow the test method procedure and calculations as specified. However, most laboratories performed the testing to the indicated standards.
ADVICE TO PARTICIPANTS
As this was the first round of the Fine Aggregates Proficiency Testing Program, the infrequent test results may serve as an indicator to participants on their performance and provide areas for improvement. PTA urges all participants (not just those with outliers) to review their procedures and precisely follow the test methods. All samples need to be split and quartered accordingly to provide a representative sample portion and equipment needs to be maintained and calibrated as per requirements. All test and reference solutions should be prepared regularly as per the method and stored appropriately.•
The final program report for Round 1 of the Pilot Fines Aggregates Program (Aggregates 23) can be found on the PTA website: pta.asn.au
Registration for participation in Round 2 (Aggregates 26) of the Fine Aggregates Proficiency Testing Program will open in August 2021. For more information about the program, contact the PTA via email: firstname.lastname@example.org
This article appears in the July edition of Quarry Magazine.