Value of testing lies in the sampling

Value of testing lies in the sampling

By Eamonn Ryan | Photos by Eamonn Ryan

Many tests are conducted to check the quality of aggregates, the core components of concrete and road building, therefore aggregate quality really matters.

The nature of mining is that there is no guarantee as to the quality of material throughout production. It’s a changing landscape (no pun intended).

Stones don’t always break in an exact square according to a formula. SANS 1083 permits a certain range of results, provided it falls within the specification envelope. This often leads to unhappiness between the customer and the supplier. Where aggregate is marginally out-of-spec but still usable, it is simply sent back at enormous cost to the mine. Worse, if it has already been used to make a concrete floor which develops problems – subsequent problems may be attributed to the quality of the aggregate.

This leads to a costly dispute between quarry and customer – and only if the quarry has evidence of a consistent testing history will the odds be in their favour.

Afrimat Glen Douglas, a dolomite quarry in Henley-on-Klip which commenced operations in the 1950s, has a dedicated on-site laboratory to regularly test its aggregate. Implementing a Quality Management System minimises such disputes.

Saartjie3Saartjie Duvenhage, chairperson of Aspasa’s technical committee on quality management, at Afrimat Glen Douglas in Henley-on-Klip.
Image credit: Eamonn Ryan

“The moment a customer sees a quarry is doing something about its quality, he or she will be much slower to point the finger of blame at the quarry,” says Saartjie Duvenhage, chairperson of Aspasa’s technical committee on quality management.

The Glen Douglas laboratory is not the biggest in the Afrimat group, which has a state-of-the-art facility in the Western Cape capable of doing the full battery of tests. Yet its on-site laboratory is still at the top end of the scale by South African standards. This is an expense which many independent quarries are unwilling to make, says Duvenhage, “because they don’t understand the value to the business of a laboratory and in fact often think it’s a waste of time and money.”

Glen Douglas mitigates the risk of customer disputes by sending out regular reports of its aggregate quality to its customers, which engenders confidence.

Duvenhage says there is a misconception in the industry that quarries have to have a facility equivalent to that of Gen Douglas, or none at all.

“Though it is more efficient and practical to test aggregate with appropriate equipment, the reality is that gradings can be done manually for the cost of a sequence of sieves and a good balance. The major challenge is to persuade quarries that they can still test in the absence of an expensive laboratory on site. Without one, they rather don’t do any testing.”

There is no statutory requirement to have a laboratory, or to do testing, but its absence creates a problem when there are quality disputes with clients. In addition, major customers like SANRAL have higher than average specifications and an internal requirement for rigorous testing.

“This leads to a costly dispute between quarry and customer – and only if the quarry has a consistent testing history will the odds be in its favour.”

Boitumelo Mothobi, laboratory analyst at Afrimat Glen Douglas, says if he needs to do more detailed tests than his laboratory is specified for, he sends a sample to one of the local ISO 17025 accredited-labs, as Afrimat’s own Western Cape facility is too far. Mothobi says they also regularly send out a couple of samples a year for external testing.

Successful QA is founded on communication

“Successful quality control is really founded on communication,” says Duvenhage. The most basic test that needs to be performed on-site is a grading analysis. This tells the mine that it is producing the aggregate size that it says it is selling. The sample is put through a series of sieves of decreasing mesh sizes, which can be done by hand. The only thing that requires electricity for this test is the balance.

“We look at the percentage of material that is retained on each sieve and calculate the cumulative percentage retained and the cumulative percentage passing. This is plotted on a graph and has to match the requirement of the customer, and also the agreed specifications of standard SANS 1083, which is the requirement for concrete aggregates,” says Mothobi. SANS 1083 describes the lower and upper envelope limit of each aggregate size. If a client requires 20mm stone, then SANS 1083 sets the parameters for that. In line with SANS 1083, a quarry and a customer may agree on a product that falls outside of the SANS 1083 envelope.

Sam3Boitumelo Mothobi, laboratory analyst at Afrimat Glen Douglas. 
Image credit: Eamonn Ryan

“A quarry cannot consistently claim it is producing a certain size, unless it is doing tests. This is why it is important to have a laboratory – even if that lab is under a tree.” There are many other tests to measure characteristics such as flakiness, water absorption, strength, abrasion resistance – but the basic test is for size. Duvenhage says the ideal is one test for every 500t, but sometimes this is impractical and could be highly disruptive to manpower (on some mines this may require four or five tests a day).

She says the way to get around this demanding requirement is to establish a history of consistency by regular testing and by supplementing that testing with highly experienced staff in whom there is confidence they would visually pick up any variation in quality. Glen Douglas has employees who have been on staff for 20 years or more – one has been there 45 years – and has official testing records going back many years. They have the experience to recognise problems. If a problem has been identified visually, it is always followed up by a laboratory analysis.

Mothobi points out that just as important as testing is the amount of attention paid to the quality and extent of communication between operations and the laboratory. “When anything changes at the plant, such as a change in sieve sizes, the mine manager and supervisors will notify us immediately to test a sample to verify we are within the spec.”

This regular communication did not occur overnight and took years to evolve, he says. The fact that Mothobi has the authority to stop operations is also unusual, as laboratory staff typically do not have such authority (one reason is that their reports were historically habitually ignored).

A weakness of testing throughout the industry has been that while the laboratory tests and produces reports, operations personnel do not take notice of them. When laboratory analysts have the authority to stop production – testing is taken a lot more seriously. The analyst will only order such an unusual action if the sample was out-of-spec.

Duvenhage says she lobbies for such authority to become more widespread. “On some quarries the material is loaded directly onto a train. It is easy to load, but very hard to unload if it later becomes evident that the batch is out-of-spec. On many quarries, only the mine manager can halt production, but he has many responsibilities and may not be readily available, and so the mine will just continue wastefully producing out-of-spec material and worse, even incurring the cost of transporting it. Hence, I believe the laboratory manager should similarly have this authority.”  

Aspasa is currently involved with Barry Pearce from Learning Matters etc together with SANRAL, NLA-SA and other aggregate users, in developing a training programme for laboratory analysts to standardise qualifications and thereafter to strengthen their authority. At the moment there is no formal qualification for analysts. The proposed qualification is at NQF level 4, equivalent to Grade 12, with laboratory manager at NQF5, which allows them to register with ECSA. It is envisaged that this will be undertaken using the international standard SIO 17024 for the deeming of personnel competence for a particular vocational operation. 

A quarry may test twice a day, and after accumulating a year’s history of testing it becomes apparent that the product is entirely consistent. “If the trend of the graphs is a consistent flat line, there’s no point in continuing to test so frequently. The testing frequency can be reduced and supported by constant awareness and visual review as to source changes and alterations in operations. That is something which comes from experience and excellent communication – it cannot be taught. This does not mean sampling and testing can be done away with: it must continue to be tested.”

But testing is only viable as an activity to the extent it is cost effective. If testing is to be done on a quarry ‘by the book’, says Duvenhage, it might require as many as 18 samples to be taken per shift, which requires employing extra people.

Rules of sampling

Duvenhage says that one of the main problems in disputes is the sampling procedure.

“We are looking for a sample that is representative of the load that was delivered. If the sample is not representative, the result will not be accurate even though the test method may be correct. For example, lighter fines tend to gravitate to the bottom of a heap and is not representative of the entire batch. There is a specific methodology to sampling, and the entire testing process becomes tainted if this is not adhered to.

“Testing requires a sample which tells us what is in the stockpile. The customer does not want to know, for instance, what is on the conveyor belt at a particular moment in time (although this may be important for the production manager). Sometimes the operations on a quarry push out finer and sometimes coarser material, so we have to take enough samples of finer and coarser material to get a comprehensive idea of what is in the stockpile. This is best achieved by a mechanical sampler, but it is an expensive proposition. It can also be done manually, provided it is done according to rules to ensure a representative sample:

  • a sample cannot be taken closer than a metre to the ground, or there might be some contamination
  • you cannot take just one sample from a single location on the stockpile, as there is segregation of coarser and finer material in the stockpile

These samples are then mixed to prepare the laboratory sample.

The method for representative sampling is described in TMH5, which is also currently under review.

“We sometimes see sample problems where samples are taken directly from a truck. During transport, segregation can occur and by the time the truck arrives at the site all the fine material will have fallen to the bottom with all the course material at the top. If you take a sample from the bottom you will only get a disproportionately high percentage of fine material – similarly if you sample only from the top. Similar segregation occurs in a silo, where the fine material congregates at the centre and the coarse material to the sides,” says Duvenhage.

Due to the challenges with sampling, Glen Douglas is strict when it comes to allowing others to take a sample. Customers must make an appointment to take a sample and do it with a laboratory member present who either extracts the sample himself or monitors the customer’s sampling method. The sample is then split with Glen Douglas testing one half, and the customer’s chosen laboratory the other. “When we do that, our product is never out-of-spec,” says Mothobi. The results are generally close – and once that has been done once or twice, customers will rarely question our results again, he adds.

Industrial applications also require regular chemical analysis of materials. Drill samples are then taken before blasting.

“Regular testing which produces consistent results, mitigates the risk of customer disputes when combined with sending out regular reports of its aggregate quality to its customers, which engenders confidence.”

Mothobi explains that the bench is drilled and samples taken for testing before commencing blasting, so they know beforehand the silica content before proceeding. Different sections of the quarry have different silica content and there can be contamination in between – hence the need for testing before proceeding with blasting. The laboratory has a dust-free room for the XRF analysis, to ensure the most optimal working environment.

Testing is not just for the use of customers, but often gives early warning of operational issues such as a hole in a sieve, as evidenced by bigger aggregates than expected coming through. It will become quickly evident from the graphical plot of the grading.

The material grading is as much dependent on human and machine activity as on the raw material, and therefore grading analysis has to be done regularly to pick up errors.

Lab equipmentAn on-site laboratory is more efficient, but the basic tests can nonetheless be done manually.
Image credit: Eamonn Ryan

Future training / tech needs

Barry Pearce of Learning Matters etc says, “There is currently a big drive to implement an official qualification for the testers who produce the results in the civil engineering laboratories be they commercial laboratories, primary (quarries) or secondary (concrete or asphalt) material producers.  The main aim is to credit them with some form of official competence in testing as well as to place value towards the vital function they provide in ensuring that quality product conforming to the specifications is supplied to the clients’ base.”

New technology has been developing in tandem with skills: Xcentric Crusher, for instance, has managed to achieve profitable production levels for improved crusher buckets, says Pieter van der Merwe, MD of Xcentric Ripper SA. “The increase in production per hour, with long maintenance intervals (lubrication 200 hours), and a powerful heart that provides the necessary reliability to work with it constantly, makes out of the Xcentric Crusher a machine that will turn profitable your stone crushing or recycling materials processes.

“This is possible thanks to the patent-pending technology that applies a high inertia power train, circular jaw movement, as well as a new and easy antiestagnation plate, which prevents large shredding pieces from getting stuck in the bucket´s mouth, not allowing them to enter the crushing jaws. The granulometry adjustment is easy and intuitive, which enables a quick variation,” says Van der Merwe.


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