By Mat Herron
Properly managing aggregate stockpiles means overcoming challenges both old and new, though drone technology is playing a greater role in solving those problems, experts say.
Among the current obstacles faced by aggregate managers, especially those who run smaller plants, is the growing number of mixes being developed and requested by clients, which means more piles on site.
“As a result you can have increased potential for cross-pile segregation and mixing up of stockpiles,” says Dr. Shane Buchanan, asphalt performance manager at Oldcastle Materials in Birmingham, Alabama.
Moisture management, is another big issue, Buchanan said. Proper covering, grading, (and) paving under stockpiles, especially for fine aggregate, can help reduce dampness.
“The importance of this must not be understated,” he said. All things being equal, “a one percent reduction in aggregate composite moisture will reduce the drying energy by 11 percent and increase plant production by 11 percent.”
Jeremy Goad, general manager at Bluegrass Materials Company in Bowling Green, Kentucky, said the number of products his company manufactures for state departments of transportation, federal agencies and commercial architects is also growing. “The more products you have, the more it exacerbates stockpile issues,” he said.
Lauren Elmore, president of Firmatek, LLC, in San Antonio, Texas, says another trend her company has noticed is the simplification of stockpile measurements as more companies “without knowledge of the industry come in and measure stockpiles,” she said. “Specifically, they are measuring the piles against flat plane bases and not removing things like equipment and vegetation properly.”
These emerging challenges appear alongside continual problems commonly called the “–ations”: degradation, contamination and segregation.
Degradation
With degradation, time is not aggregate’s friend. For example, if a product specifically designed for roadwork sits for longer than it should, that product could degrade to the point it can’t be used. Firmatek measures many piles that have been added to over the years and grown, Elmore said.
“We know with the aggregates that the density can increase six to eight pounds a year simply by sitting,” she said. “That can have a large financial impact.”
Goad said degradation is the hardest issue to deal with. “You have your best practices in place and try to produce the material with a certain amount of degradation built in,” he said. “The less you move material, the less expense is in it.”
Degradation arises with marginal aggregates that break down after stockpiling, Buchanan said. “This is typically not a huge problem, as most aggregates are properly tested to ensure durability prior to purchase (or) delivery,” he said.
To lessen degradation, aggregate managers must understand materials and crushing operations to limit storage time, implement strict inventory control, minimize traffic to the stockpile and avoid reworking a stockpile before shipment, Buchanan said.
Contamination
Contamination can occur when oversized aggregate find their way into finer-sized aggregate, either via the delivery truck or loader buckets that have previously hauled other materials like wet clay or sand, Buchanan said.
Keeping out-of-place material from the stockpile, and ensuring trucks and loader buckets are clean of other products is key to avoiding contamination, he added.
Elmore encouraged plants to keep maintenance of their piles as simple as possible. “When you consolidate one material into as few piles as possible, you are making things easier on yourself in terms of maintaining your inventory,” she said. Goad advised keeping stockpiles an appropriate distance apart and to adopt best practices with loaders and quality control staff.
Segregation
Segregation — what Buchanan calls “the undesired separation of material by particle size” — is the biggest issue, he said. This can occur when material is initially stockpiled during load-out and while being discharged from conveyor head pulleys.
“When material is coming off the head pulley, finer rock will settle on the bottom of the belt, and the big rock wants to shoot out farther than the finer rock,” Buchanan said. Minimizing the drop height from discharge can alleviate this problem.
“You know segregation is going to occur.” Goad said. To prevent it, his plant loads out of the stockpile. Many Bluegrass plants have paddlewheels at the end of conveyors to blend material as it is stockpiled, he said. One piece of machinery, the telescoping conveyor, “has done more to reduce segregation and degradation than anything else that has come on to the market in a long while.”
Numerous financial and non-financial costs are associated with these negative effects. Goad said that regulating DOT agencies can levy penalties, and re-screening or re-crushing material in order for it to pass specifications adds additional cost. With segregation, the result can be an aggregate blend grading, which doesn’t match the job mix formula, Buchanan said. “This may have negative implications with regards to asphalt mix volumetrics achievement and consistency.”
Producing inconsistent mixes hurts a business’s reputation with its clients. “No one likes to have an inconsistent mix supplied to them,” Buchanan added.
Here come the drones
To ensure consistency across the board, plant managers are using new technologies and processes to ensure their materials meet spec and their inventory is managed correctly. Managers deploy automated, strong-featured drones by companies like Kespry to take quick, accurate measurements of inventory.
“Every piece of technology Firmatek has deployed over the years from total stations to mobile lidar to drones has made stockpile measuring more accurate and generally made things quicker and safer,” Elmore said.
Goad said drones are a “game-changer” for the industry. “Five years ago, it was a tedious, multi-day process to figure out how much material is in a stockpile,” he said. “Now, if you have access to a drone, you can know how much material is in a stockpile in a matter of a couple of hours.”
Using drones presents a new problem, though: ensuring the data it reports is correct. “We have always used lasers to do the measuring of our piles, and lasers could be off slightly, but easily correctable. Generally, we would not see scale issues or data warping or other issues,” Elmore said.
With drones, photogrammetry has many factors that go into the accuracy and inaccuracy of the data. “Due to the large amounts of data coming in, understanding and fixing data can be a very time-consuming process,” she said, and the company won’t know if the data is useful until many days later.
“Companies that just make drones don’t necessarily have the knowledge or experience to deal with the issues facing those managing the inventory, but can just provide the technical knowledge of a drone or software system,” Elmore said.
Aside from the proliferation of drones, Buchanan said he sees more companies emphasizing a demand forecast model, which can tell them how much material they will need for a client and adjust that forecast accordingly. Demand forecast helps ensure a company knows which aggregates it will need for a given time period, and that it can obtain them efficiently.
Best practices
Proper management of aggregate stockpiles requires many steps: appropriate environmental regulation compliance, inventory control, as well as accurate stockpiling and load-out practices, Buchanan said.
Managers must track inventory more frequently, and also know the product coming through their plants, Elmore said, and the plant that handles its material least handles it best. “Material handling is not only expensive, it also multiplies error in your inventory,” she said. “What we like to call good housekeeping — keeping piles clean and consolidating the same type of material into as few piles as possible — is also key to maintaining accurate inventory.”
Goad said that reducing interim stockpiles will prevent the need to handle material multiple times. He emphasized creating a training program not only for a plant’s staff, but also the end user. “All the best practices in the world are not going to help you, if the asphalt or concrete plant you are selling the material to is not also using these best practices.”