“What you have to recognize is that sampling is critical to a plant’s quality control. It’s just as important as testing,” says Tim Tipton, quality control director for Hinkle Contracting, a Summit Materials company.
Tipton explains that production quality control has evolved over the years. Not only are producers more in control of testing their product quality than in the past, when the DOT agencies held all control, but the systems and methods for managing quality at aggregate facilities also have become more sophisticated.
The first step in testing material for quality is taking a sample of a product that will provide a true picture of the product’s gradation for a customer’s need. Tipton says valid concerns for the final product also include contamination and segregation, which can occur on conveyors and in stockpiles — both of which provide methods for sampling. Of course, there are pros and cons for each method.
According to Tipton, Hinkle prefers to use the stockpile sampling method, acquiring the sample with a loader, which is safer than using a shovel and bucket to pull samples directly from the pile. “The loader operator takes material from three to five locations in the stockpile, loading at a right angle to the flow of how the stockpile was built,” he says. These samples are used to create a “mini” stockpile from which another sample is taken for testing. The method helps to address concerns of contamination and segregation, as long as an experienced operator is taking the sample.
Conveyor belt cuts are another way to sample aggregate materials, and can be accurate for determining gradation. The traditional method for obtaining a belt cut sample requires the plant to stop production. A template that matches the curvature of the conveyor belt is used to manually cut and separate a cross section of the material. The worker takes three to five cuts, brushing the material into a bucket for testing. “This type of testing doesn’t give you the full picture of your material. It relies on the employee taking a complete sample, including fines; it can affect your production rates, and it also creates safety concerns,” Tipton says.
Adam Orner, global product manager for wet and dry sampling at McLanahan Corp., agrees that the traditional belt cut method has its limitations and concerns. “This is why we promote the use of cross belt samplers,” Orner says. Cross belt samplers are automated, enclosed machines mounted on the conveyor. They use a “bucket” with a counterweight to swing and scoop samples from the moving conveyor, delivering them to a container. “With cross belt samplers, you have no downtime, and it eliminates safety concerns,” he says.
Quality Control: A Cross Section of Sampling Practices
With 40 years of aggregates production experience, Tim Tipton is quality control director for Hinkle Contracting, based in Paris, Ky. Tipton began his career with Hinkle in 1976, working initially as part of the cleanup crew at Hinkle’s Natural Bridge Stone facility. He was promoted to the position of aggregate technician at the plant in 1977, gaining certification as a bituminous technician in 1979. Six years later, Tipton became asphalt plant superintendent at Hinkle’s Central Paving division before his promotion to quality control director in 1986. He serves on advisory committees for the Kentucky Plant Mix Association and the Kentucky Crushed Stone Association.
Adam Orner is the global product manager – dry and wet sampling for McLanahan Corp., and is based in Hollidaysburg, Pa. He began his career with McLanahan Corp. in 2001 as mechanical engineering design specialist and project manager where he was involved in the design, fabrication, implementation, and field service of a wide variety of mechanical sampling systems and equipment. In 2008, Orner was promoted into the role of director of engineering for the company’s sampling product line, a role that he maintained until accepting his current global product manager position in 2016.
Voices of Experience
“Proper sampling is critical to successful quality control,” says Tim Tipton, quality control director for Paris, Ky.-based Hinkle Contracting. “You must be uniformly consistent in how and when and where you pull your samples.”
Hinkle Contracting takes material sampling seriously. The company has a sampling qualification program for its own production foremen and employees, as well as for DOT personnel.
“We prefer to sample from stockpiles,” Tipton says. “If you build your stockpiles right, we feel this method best addresses concerns with gradation, contamination, and segregation. A loader and an operator experienced in sampling will get you a very accurate picture of what is going out the gate to your customers.”
Accuracy of the sample starts with stockpiling practices, Tipton explains. Variables in how a stockpile is built can affect its quality. “Do you stockpile by truck? Do you use a fixed or radial stacker? A Superstacker (telescoping stacker)?” he asks.
Stockpiles built by truck grow in layers, keeping the material from cascading down the slope of the stockpile — reducing segregation. A radial stacker that moves back and forth also builds stockpiles in layers. In building a stockpile with a conventional fixed stacker, the key is to keep the stacker as low to the pile as possible. “If you let the material fall from 50 feet in the air, you are going to have segregation in your stockpile,” Tipton says. “I like the Superstacker style,” he adds. They’re both telescopic and radial, building the pile in windrows, for the best shot at eliminating segregation.”
In obtaining the samples, Tipton recommends pulling three to five samples from the pile to create a miniature stockpile. “You want to get the bucket into the stockpile as deep as you can at a right angle,” he says.
Of course the greatest piece of advice Tipton has is to be consistent with the sampling time, method, and operator. “Training is important, and you have to figure out which method is best and do it the same way every day,” he notes. “It also helps to make sure your operators — and all employees — understand what the customer needs, what their applications are, and the importance of producing quality products.
“Always try to keep everyone in the loop, so someone isn’t trying to do it by himself. Quality control is a journey, not a destination,” Tipton concludes.
Today’s methods for sampling aggregates can vary greatly. They include traditional methods such as taking shovels-full of material by hand or loader from a stockpile, manually holding a container under a conveyor head pulley, and obtaining conveyor belt cuts by hand. They also include more automated methods such as cross belt samplers.
“We try to steer people away from manual sampling, particularly from stockpiles. We believe that the best methods remove human interaction — for safety and consistency,” says Adam Orner, global product manager for wet and dry sampling at McLanahan Corp., based in Hollidaysburg, Pa. “If you have a person taking discretionary samples from a stockpile, you may not be getting the best samples. If you’re taking traditional hand samples or stopped belt cuts, you are dealing not only with potential safety issues, but also with costly down time and labor associated with manual material handling.”
A cross belt sampler collects a full cross section of material from a moving belt without human interaction. The operator can take samples with the push of a button or automatically set the sampler to take samples at certain times of a shift. “Cross belt samplers are cost-effective, easy to install, and collect manageable sample increments,” Orner says.
The cross belt sampler is a fully enclosed machine, which contains a rotating, counterweighted cutter assembly. The cutter assembly rotates through a full 360-degree rotation to cut a material sample from the moving conveyor belt before decelerating to a stop back at its original parked position. The key to obtaining a complete and representative cut is to also install a Cut Zone multi segment idler and impact cradle system beneath the conveyor as part of the cross belt sampler. The Cut Zone system forces the conveyor belt to conform into the proper radius to obtain a full cut from the belt — including fine material.
“With the cross belt sampler, you want to make sure that the cutter bucket capacity is larger than the cross section of material on the conveyor belt,” Orner explains. “The cutter opening varies based on your product size. We recommend a bucket width of three times greater than your top size material.” A consistent cutter speed is also important; the cutter should not slow down as it travels through the material flow. A good rule of thumb is a cutter speed greater than one-and-a-half times the belt speed to help assure a quality cut and minimize spillage,” he says. The time interval between cuts really depends on the producer’s sampling plan.