Setting a High Standard
Because not all aggregate is equal, quality control is a key part of the production process
by: Matthew Bradford
There are no guarantees that what’s pulled from the ground is ready for use at building sites. That’s why Ontario’s aggregate producers and suppliers undergo a rigorous quality control process that confirms they’re supplying first-rate materials.
“You don’t get what you expect. You get when you inspect,” says John MacKay, vice president of Green Infrastructure Partners. “The quality control process is in place to make sure, from a physical standpoint, that the aggregate you are mining is durable enough, hard enough, and meets the physical requirements for whatever they’re going into, be that asphalt, ready mix, or even a granular.”
Since aggregates are typically produced for highly engineered use, each jurisdiction has a vested interest in verifying that the materials used for critical projects within their boundaries meet quality parameters that have been set through extensive engineering and testing.
As Alex Caruana, national manager, Canadian Aggregates, Polydeck, notes, “Those specifications are there for a variety of reasons, and the quality control process is there to ensure the material being processed meets the specifications put out by the municipality, province or region you’re operating in. At the end of the day, it’s the way for quarries and pits to ensure their customers are getting the product they ordered, and it is suited for their purpose.”
From a broader perspective, quality control can also be seen as a control for protecting one’s business. Failing to meet specs can result in hefty fines and put aggregate producers and suppliers at risk of being excluded from bidding on work moving forward. Moreover, says MacKay, “A lot of larger owners, like the Ministry of Transportation, pay you based on the end result of your product, and that’s largely based on quality. So, if your product doesn’t meet quality, you’re going to get rejected or penalized.”
These potential consequences aren’t common in the industry, but they do serve as incentives to keep quality control top of mind. “That’s why a lot of companies have a more onerous target over and above the specification, just to allow that extra factor of safety,” explains Caruana. “If something goes wrong on any given day, the logic is that material should still meet or exceed the specification that the end purpose requires.”
QC PROCESS
There are key sequential steps that make up the quality control process. It begins by extracting a sample of materials from the source that will be used for testing. Some aggregate plants use automated samplers to pick material off the live belt and transport this representative sample into nearby bins or pails, where the sample is held for inspection by quality control technicians. In most plants, however, a loader is used to take a representative pad of material from a pile and place it down for a technician to then arrive and remove a sample themselves.
“In both instances, you’re working with the law of averages,” says Caruana. “Due to gravity and inertia, when you put up a pile of aggregate, the core stone tends to go to the outer side of the pile and the fine stone tends to stay on the underside. So, if you just go in there with a loader and just take one bucket and put it down for your tech to grab a sample from, it’s not going to be representative of that entire pile. It’s either going to be coarse or fine, depending on which side your loader operator went to.”
For this reason, when a loader is used to extract the sample, it blends the material so that, according to the law of averages, it is a representative pile. Once that pad is down, the technician uses their own shovel to take the material out in a representative way so it can be transported to their lab for testing.
LAB TESTING
Once a material sample is obtained from an aggregate product’s plant, it is taken to a lab for testing. Here, the sample pile is sifted through screens and separated by its individual fractions, which are then measured, counted and plotted on a graph to produce a gradation curve. At this point, technicians assess that curve to see if it fits with the maximum and minimum of the specification and the producer’s target.
The process is slightly more involved for finer materials. In this case, the fines (e.g., natural sand particles) are washed off, dried in an oven, and the dust is weighed. “This is done because dust is undesirable in an instance like concrete stone because we want that cement paste to adhere to the stones and form a very competent concrete mix,” says Caruana. “It’s like when you’re a kid and you try to tape something to a dusty surface, and it comes off the next second. It’s the same principle.”
Material may undergo other testing at the lab. This includes flat and elongated, crushed count, and abrasion testing to see how competent the stone actually is and how hard its individual particles are.
The testing phase of the quality control process is often performed and completed on the same day. This is because aggregate plants are putting up hundreds of tonnes an hour and need to know fairly soon if their production is out of spec. If it is, there are some actions to take.
“First of all, you want to fix your problem,” says Caruana. “If it’s a blown screen or a crusher liner that’s worn out of spec, you want to find that problem and address it.”
NEXT STEP
The next step is determining what to do with the pile that has out-of-spec material. Here again, there are options. The material may still be able to sell for lower specific use cases, such as concrete stone. In this case, the pile may be cordoned off by the operator and saved for separate use.
“At this point, we’re calling that material something different so it can meet a different or easier spec because the operator may not wish to shut down their plant and hit a cold stop until the issue is fixed,” says Caruana. “Instead, they may choose to run the rest of the shift because the plant is running relatively well, and all the other products are in spec. Then, they’ll ship that lower spec material as ‘product number two’ instead of ‘product number one’ and then fix the issue on their maintenance time in the evening, weekend, or whenever the shutdown ends up being.”
REFEREE SAMPLES
Quality control doesn’t always end there. More often than not, municipal or provincial job contracts include provisions enabling project owners to sample the material themselves when it arrives at the jobsite. In some instances, they may even be able to take representative material from the producer’s site itself. “So, while the operator’s QC cycle may have ended after the lab, there’s also an end user quality control cycle and it behoves the producer to monitor and referee that,” advises Caruana.
Depending on the job, the project owner may send a third-party lab that might not be familiar with the site. They may also grab a sample of material that is completely different from what the material on-site is. In those instances, whether the third party is sampling at the end user site (e.g., side of the road, or at the bridge work) or coming straight to the quarry or pit, it’s in the producer’s best interest to help that third party out.
“It’s good practice to be there to open up a conversation with that third party to say, ‘Hey, we’re shipping from here, not over there. You want to grab your sample where we’re shipping from because we’re shipping material from here because this is the premium stuff,’” says Caruana.
Like any component of aggregate operations, the quality control process is only as accurate, reliable and effective as the people behind it. For this reason, MacKay insists, “A good quality controller is invaluable to your organization. You have to have someone who knows what to look for, and most times, a good quality control professional will be able to tell if the product is to spec or not, even before they start testing it.”
While it requires additional skills and resources, and may not always produce the hoped-for results, a strong quality control process is essential to all sand, stone and gravel operations.