Mix Optimization

A formula for greenbacks and green roads

To avoid fracturing aggregates when compacting stone matrix asphalt (SMA) or open-graded mixes, it may be necessary to use two rollers set in low amplitude during the breakdown phase.To avoid fracturing aggregates when compacting stone matrix asphalt (SMA) or open-graded mixes, it may be necessary to use two rollers set in low amplitude during the breakdown phase.

When it comes to mix-design optimization, two goals stand out: dollars to be saved and how sustainable a road you can build. This means doing your homework and understanding calculations in mix design, explains Shane Buchanan, asphalt performance manager with Oldcastle Materials Group’s Asphalt Performance Team. 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,” he says. It also means, he adds, taking the time to ensure you are up to the current specifications in your area.

“It goes back to being too busy,” Buchanan points out. “You may not be aware of special provisions or specification changes. These could include changes from Departments of Transportation, a city or municipality, or even the Federal Aviation Administration. “Changes can open up the door to make a more economical mix,” Buchanan says. “If you are not aware of that, you are losing at the beginning of the process.”

Specification changes aren’t communicated as methodically as in the past when it resulted in a printed description in a specs book. “But with advancement of technology, specifications can change quite frequently,” Buchanan says. “It’s not always in printed form. This means you must constantly monitor an agency’s website.”

Buchanan also recommends that operations get involved with their local asphalt or stone association. “A lot of time, they help work with the agency to change or create new specifications,” he says. “It’s very important that you stayed tuned into the process.”

For example, the use of recycled asphalt pavement (RAP) and recycled asphalt shingles (RAS) in mixes is growing and changing very quickly. “It’s very dynamic,” Buchanan says. “You need to stay on top of what the state is allowing. If your understanding is that 15 percent can be RAP, but it was changed to a 25-percent allowance, you are leaving a lot of opportunity on the table.

“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. Over 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.”

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Knowing the correct way is absolutely essential with volumetrics in mix design. “You need to understand the impact of aggregate grading on volumetrics, of asphalt grading on volumetrics,” Buchanan says. “The most critical volumetric property is VMA (voids in mineral aggregate).”

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 phase 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 at the National Center for Asphalt Technology (NCAT), through the National Asphalt Pavement Association (NAPA) or even a state asphalt association, Buchanan says.

When you design the mix, Buchanan says, you need the minimum VMA. It can be equated to the extent of the mix. One of these components is liquid asphalt content.

Too much force and excessive rolling caused severe damage to aggregates and the surface of this asphalt layer.Too much force and excessive rolling caused severe damage to aggregates and the surface of this asphalt layer.

Typically, during production, VMA is lost. “If you are designing with a VMA of 14.5, typically you are going to lose 0.5 to 0.7 percent of VMA during production,” Buchanan says. This is referred to as ‘VMA collapse.’ voids in mineral aggregate

“Because of VMA collapse with a minimum VMA requirement of 14.0, 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 may drive you in a different direction with regards to your design VMA.”

One key with VMA is paying attention to aggregate gravities and making sure they are accurate. Typically, accurate specific gravities are more easily obtained with coarse aggregates Buchanan says. With fine aggregate, you can have issue of getting the correct bulk specific gravity (Gsb) determined.”

The test method is such that materials with a lot of fines, such as minus-200, can give a misleading result. This can cause you to calculate a bulk gravity that is too low and an absorption that is too high. Relating that back to VMA, the bulk specific gravity of aggregate blend is a critical factor.

“If you are getting incorrect bulk specific gravity — if the gravity (is) too low — the result will be a slightly lower VMA calculated and you are directly penalizing yourself,” Buchanan says. “A 0.01 change in your bulk specific gravity is a 0.3 percent change in your VMA.”

When calculating aggregate gravities, Buchanan says, very close attention should be paid to aggregate absorption. “If at all possible, use the absolute lowest absorption possible,” Buchanan explains. “The higher the absorption, the more liquid asphalt will be absorbed in the stone so you are just wasting money. A good rule of thumb, if no other information is known, is that asphalt absorption will be about 50 percent of water absorption.”

Liquid asphalt price drives the mix cost, Buchanan says, so any measure that can be taken to optimize the mix with the amount of virgin liquid in the mix will directly correlate to an operation’s ability to make money. Every 0.1 percent of liquid asphalt binder has an impact of 55 cents per mix ton — with the assumption that virgin liquid is about $550 per ton. “That means every 0.1 percent of liquid you can save has an impact of 55 cents per mix ton,” Buchanan says. “That is a great deal of money.”

Capped products have essentially been “written off the books,” but using these products can also potentially reduce the bottom line mix by reducing aggregate cost, Buchanan explains.

This is applicable for externally or internally supplied products – for example, screenings and manufactured sand.

However, testing must be done to ensure capped product quality is present. These capped products are especially good for 9.5 or 4.5 mm mixes with higher P200 content, however, their use must be balanced with desired RAP use. RAP replaces fines at a 90-percent level. Fine-fractioned RAP replaces fine at a 100-percent level. A 100-percent replacement means that 1 percent of RAP directly replaces 1 percent of fines.

Buchanan notes that caution must be used when integrating these surplus or capped products into mixes.

“Make sure the materials work in the design, and make sure sufficient quantities exist for the jobs,” he says. “Do not be ‘guilted’ into using capped products, make sure their use is beneficial.”

But how should you determine what mixes to target for optimization?

Start by conducting an analysis of all produced mixes for the past year, Buchanan says. Then list mixes from top to bottom in terms of production volume.

“Most production will likely come from a relatively small number of mixes,” he says. “You might find that 10 percent of mixes yield 70 percent or more of the production. Make sure you focus time and available resources on critical mixes.”

To evaluate, use these formulas:

(1) Potential = (Opportunity) x (Volume) and

(2) Opportunity = f (specifications, material availability, margin potential, etc.).

“Do not overlook ‘specialty’ mixes such as SMA [stone-matrix asphalt], OGFC [open-graded friction courses], etc.,” Buchanan says. “You can never rest. Always remember, your competition is working when you are not.”


Aggregates impact density

When working with compaction crews, the asphalt manager needs to ensure the mix meets all the requirements to match production and get the required density to keep the mat smooth.

Part of achieving required density is knowing what kind of aggregate is in a mix and how it performs, explains Terry Humphrey, training consultant for Caterpillar Paving. When designing a mix, Humphrey suggests that it’s extremely important to know how the aggregate being used has performed in an L.A. Abrasion test. “This determines its durability – and durable aggregates stand up to traffic volume and axle loads over time,” he says. “This is an immediate opportunity to get required density right behind the paver. The team designing the road is concerned about these types of characteristics so it knows how the road will support loads over time.”

If there is good aggregate that has a low number in the L.A. Abrasion test there is confidence that density can be achieved without creating more air voids, Humphrey explains. “There is more risk for a contractor on poor density than anything else,” he says. “Poor density costs a lot of money. Having good rock helps you get full pay or a bonus. Aggregate quality is part of the puzzle.”

For example, on a highway project with high-traffic volume, local aggregate sources might not be good enough to handle the average daily traffic and high-axle loads. “You might have to haul in aggregate from 100 miles away,” Humphrey explains. “But if it’s a parking lot or a street with low volume, you have less concern and can user lower-quality aggregate.”

Granite is oftentimes considered more durable aggregate. However, slag from steel might be more durable, Humphrey points out. “Sedimentary rock, such as limestone, tends to be less durable – but there is a wide range.”

If the designers know the aggregate is durable, it also helps the design team determine the best amplitude, Humphrey adds. “Amplitude on a vibratory compactor determines the impact force that will be delivered onto the asphalt later,” he says. “The goal is to select the amplitude that is the highest amplitude the rock will accept without breaking or causing the drum to bounce. You don’t want the drum to bounce while it is working.”

If the road design team knows that if the rock is durable, it can focus efforts on other factors such as how thick the layer is and what is the maximum size aggregate to be used. “You need to have at least a 3-to-1 ratio of layer thickness to aggregate size,” Humphrey says. “Once you can check off that box, you need to ask whether the oil used in the binder is modified with polymers that make it thick.”