Pennies DO matter in an aggregates operation.

In a recent conversation with Shane Buchanan, asphalt performance manager with Oldcastle Materials Group’s Asphalt Performance Team, about mix-design optimization, he told me how watching your pennies can save your operation millions.

But it’s not about cutting corners to save money. It’s about doing your homework and understanding calculations in mix design. He points out that when designing a mix, it’s important to conduct a quick review of mix volumetrics to make sure you know what you think you know.

“A lot of times when people are new to the industry, they are trained by someone else who was trained by someone else,” Buchanan says. Throughout time, training can get somewhat diluted. “People take leeway on concepts. If three steps removed from truth, you may not know exactly the correct way.”

This makes it important, he says, for a person to go back to the beginning and not just use the information on which he or she has been trained by another person or through internal training.

“You can really get into trouble by not understanding volumetrics,” Buchanan explains. “You need to understand impact of aggregate grading on volumetrics, of asphalt grading on volumetrics. The most critical volumetric property is VMA [Voids in the Mineral Aggregate]. VMA is one of those targets that is sometimes very difficult to achieve. How do you achieve it? That is the million dollar question. The whole question dovetails into other concepts.”

Before trying to just achieve volumetrics, Buchanan says, you need to understand what it means. “Do not get caught in the trap of using formulas blindly,” he points out. “At least go back and work a volumetric phase diagram from start to finish so you can understand what all the components mean when tackling volumetrics. It is all related to mass and volume, so there is just one way to do it correctly.”

This information is easily found online, in a National Center for Asphalt Technology (NCAT) textbook, through the National Asphalt Pavement Association (NAPA), or even a state asphalt association, Buchanan says.

“Never be too proud to review the basics,” he says. “Pride has cost a lot of companies a lot of money. These days, that’s the last thing you can afford. Very likely, it is costing you a lot of money.”

When you design the mix, Buchanan says, you need the minimum VMA — but you don’t need too much. It can be equated to the extent of the mix. One of these components is your liquid asphalt content.

Typically, during production, you will lose VMA, Buchanan says. If you are designing with a VMA of 14.5, typically, you are going to 1/2 to 7/10 of VMA during production. This is referred to as “VMA collapse” and is due to excess volume.

“Knowing that you can design your mixes with that cushion, if minimum VMA is 14, you may choose to design 14.5 to 14.7 based on your history or experience,” Buchanan notes. “Some states will pay on VMA during production, some will not. That also goes into the equation on the mix design side. If you’re not specifically paid on VMA during production, it will drive you in a different direction.”

This means paying attention to your aggregate gravities. Make sure you gravities are very accurate.

Typically, coarse aggregates make it easier to get the correct gravity, Buchanan says. “With fine aggregate, you can have the issue of getting the correct GSP determined. That is the cause of the test method.”

The test method is such that materials with a lot of fines, such as -200, can give a misleading result. This can cause you to undershoot your bulk gravity, and have bulk gravity and absorption that is too high. Relating that back to VMA, a  critical factor in VMA is bulk gravity of aggregate blend.