Asphalt mix design field guide: Superpave, PG binder, volumetrics

What the mix design is, and who owns it

Asphalt mix design is the recipe: the proportions of aggregate and asphalt binder that let a pavement carry its traffic in its climate without rutting in the heat or cracking in the cold. Superpave is the system most agencies use to arrive at that recipe. It is not a field activity. The lab and the producer design the mix and submit it. The field's job is to build what the design promised.

Get this division straight or the finger-pointing never ends. A mix can be designed well and laid badly, or designed wrong and laid by a crew that did everything right. The design sets the target. Compaction on the road is how you reach it, and a mix that never gets its air voids down in place will rut or ravel no matter how clean the lab sheet looked. That handoff, from the design to the mat, is where most disputes actually live. The compaction window guide covers the field side of it.

What the mix design controls is narrow and specific: which aggregates, in what gradation, with which binder grade, at what binder content, checked against a set of volumetric targets. Everything downstream, the plant settings, the haul, the rolling pattern, exists to deliver that design to the road unchanged.

Superpave versus Marshall and Hveem

Superpave replaced two older methods, and knowing what it fixed tells you why it looks the way it does. The Marshall method compacted samples by dropping a hammer a set number of blows and read stability and flow off a press. Hveem used a kneading compactor and a stabilometer. Both worked for decades, and a lot of good road went down under them.

The complaint that drove the change in the 1990s was that the impact hammer did not match how a roller actually densifies a mat, and neither method tied the binder to the climate in a measured way. Superpave came out of the Strategic Highway Research Program with two moves: a gyratory compactor that kneads the sample under a fixed angle and pressure, closer to field action, and a performance-graded binder picked from the project's pavement temperatures. The design leans hard on volumetrics, the air-void and voids math, rather than a single stability number.

You still hear Marshall numbers from older techs and on some local and airfield work, where the method persists. For most state DOT highway paving today the design is Superpave under AASHTO M323 and R35. When someone quotes you a blow count, they are speaking Marshall. The gyration count is the Superpave version of the same question.

What is a PG binder grade?

A PG binder grade is the performance grade of the asphalt cement, written as the high grade minus the low grade, like PG 64-22. The first number is the high-temperature grade in degrees Celsius, the second the low-temperature grade. PG 64-22 is built to stay stiff enough to resist rutting up to a 64°C pavement and flexible enough to resist cracking down to -22°C.

The grades step in 6°C increments: highs of 58, 64, 70, 76, and 82, and lows of -16, -22, -28, and -34. The high number is tied to the seven-day average maximum pavement temperature at the site, the low number to the coldest one-day pavement temperature, both at a reliability the agency picks. You do not guess the grade. The climate sets the base grade, and the agency map or an LTPPBind run names it.

Then you bump for traffic. Slow or heavy loading, intersections, bus lanes, ports, lets the binder behave softer than its grade implies, so you step the high grade up one or two notches, usually with a polymer-modified binder, to hold against rutting. A PG 70-22 or PG 76-22 on a hot, heavily loaded road is the same low grade with a stiffer high end bought with polymer. A spread of 90°C or more between the high and the low almost always means a modified binder.

The aggregate skeleton and the consensus properties

Aggregate is most of the mix by weight, north of 90 percent, so its shape and cleanliness drive how the pavement holds up under load. Superpave puts hard limits on the aggregate that the lab cannot design around. These are the consensus properties, agreed across the SHRP work, and they tighten as the traffic level climbs.

Coarse aggregate angularity is the count of crushed faces. Stone with fractured faces locks together and resists rutting; rounded gravel rolls and shoves. Fine aggregate angularity measures the same idea in the sand sizes, through the uncompacted void content of the fine fraction. Flat and elongated particles are limited because a sliver of stone snaps under the roller and under traffic, changing the gradation after the fact. Clay content is capped through the sand equivalent test, because clay coatings keep the binder from sticking to the stone and feed stripping.

The nominal maximum aggregate size, the NMAS, names the mix. It is one sieve larger than the first sieve that retains more than 10 percent of the aggregate, and the maximum size is one sieve above that. A 12.5 mm surface mix and a 19 mm or 25 mm base mix are different animals. The NMAS also sets the minimum lift thickness, commonly three to four times the NMAS, so the stones can lie down and compact instead of bridging and tearing.

The 0.45 power gradation chart and control points

Superpave plots the gradation on a 0.45 power chart, where the sieve sizes on the horizontal axis are raised to the 0.45 power. On that scale the line of maximum density, the gradation that packs the tightest, plots as a straight line from the origin to the maximum aggregate size. The plot is how you see at a glance whether a blend runs fine, coarse, or close to maximum density.

Control points are the master ranges the gradation has to thread through. They sit on the nominal maximum sieve, an intermediate sieve at 2.36 mm, and the 0.075 mm dust sieve. Pass through the control points and the gradation is in bounds. Miss one and the blend is out, usually too heavy on the dust or off on the intermediate.

There used to be a restricted zone, a hump along the maximum density line through the sand sizes that gradations were told to avoid, meant to keep out rounded natural sand that ruts. Research through the late 1990s found the restricted zone did not predict performance well, and it was dropped from the specification around 2002. You will still see it on old plots and hear older techs mention it. It is no longer a requirement. Build the gradation to the control points and the consensus properties, not to the ghost of the restricted zone.

How many gyrations does Superpave use?

Superpave compacts its design samples in a gyratory compactor, which presses the mix under a fixed vertical pressure while gyrating the mold at a set angle. That kneading densifies the sample much closer to how a roller works than the old Marshall drop hammer. The number of gyrations stands in for traffic.

Three counts matter. Ndesign is the design number, set so the sample reaches the density the road is expected to hit under its traffic over a 20-year life, and the mix is designed to 4 percent air voids at Ndesign. Ninitial, a low count, catches tender mixes that build density too fast and will shove under the roller and under traffic. Nmax, a high count, is the density the pavement should never exceed in service, the check against a mix that will densify down to too few air voids and bleed or rut.

Ndesign rises with traffic, counted in equivalent single axle loads, ESALs, over the design life. The common values run 50 gyrations for the lightest traffic up to 100 for heavy, with 75 the workhorse for medium roads. Confirm the count against the agency's adopted Ndesign table. The values were revised down from the original SHRP numbers after research in the early 2000s, and not every spec sits in the same place.

The volumetrics, the heart of the mix

The volumetrics are the heart of Superpave. The whole design comes down to four numbers computed from the specific gravities of the compacted sample, and they decide whether the mix lives or fails on the road.

Air voids, Va, is the volume of air left in the compacted mix, designed to 4 percent at Ndesign. That 4 percent is not arbitrary. It leaves room for the mix to densify a little more under traffic without closing up tight, while staying low enough to keep water and air out. VMA, the voids in the mineral aggregate, is the space between the aggregate particles, the room that holds both the air and the effective binder. There is a minimum VMA by aggregate size, because too little VMA means too thin a binder film and a brittle, short-lived mix. VFA, the voids filled with asphalt, is the share of the VMA that the effective binder fills, held in a band by traffic level. The dust-to-binder ratio, the percent passing the 0.075 mm sieve divided by the effective binder content, controls how stiff the mastic is.

These four are linked. Push the binder up and air voids fall, VFA rises, and VMA bottoms out before it climbs again. The design problem is finding the one binder content where all four land in spec at once. Miss VMA and you cannot fix it by adding binder. You have to change the gradation, which is the move that sends a mix back to the drawing board most often.

What are air voids in asphalt and why 4 percent?

Air voids are the small pockets of air left between the coated aggregate particles in a compacted asphalt mix, expressed as a percent of total volume. Superpave designs to 4 percent air voids at the design gyration count. You compute it from two specific gravities: the maximum theoretical, Gmm, the Rice gravity with no air at all, and the bulk gravity of the compacted sample, Gmb. Air voids equal one minus Gmb over Gmm, times 100.

Four percent is the balance point between two failures. Too few air voids, from too much binder or over-compaction, and the mix has nowhere to go when traffic densifies it further. It bleeds binder to the surface and shoves into ruts. Too many air voids, from too little binder or poor compaction, and water and air get in, the binder oxidizes and strips, and the mat ravels and cracks early. The design sits at 4 percent so the in-place mat, which starts higher and densifies under traffic, settles toward 4 without dropping under.

This is where the lab and the field meet. The design air voids are at Ndesign in the lab. In-place air voids on the road are controlled by compaction, usually accepted as a percent of Gmm, and a mat left at 8 percent in-place voids will not last whatever the design said. The compaction window guide covers how the field actually reaches that density before the mat goes cold.

Finding the optimum binder content

The optimum binder content is the asphalt content where the mix hits 4 percent air voids at Ndesign with the other volumetrics in spec. The lab does not pick it by feel. It batches the trial blend at several binder contents, usually four points spanning roughly a percent and a half, compacts each to Ndesign, and reads the air voids, VMA, VFA, and dust ratio at each. The binder content that lands on 4 percent air voids is the design number, then checked that VMA, VFA, and dust all pass there.

For most dense-graded surface mixes the optimum lands somewhere around 5 to 6 percent binder by weight of total mix, but that is a habit, not a spec. Aggregate absorption, gradation, and NMAS all move it. A finer mix with more surface area to coat carries more binder; a coarse base mix less.

The error on both sides is expensive. Too rich, above optimum, and the mix runs low on air voids, shoves and bleeds in hot weather, and ruts under load. Too lean, below optimum, and the binder film is thin, the mix is dry and brittle, and it ravels and cracks. A half percent of binder is the difference between a mix that lasts and one that fails early, which is why binder content is held to a tight production tolerance.

Moisture damage, TSR, and anti-strip

Moisture damage is the binder letting go of the aggregate in the presence of water, called stripping, and it fails a pavement from the bottom up where you cannot see it coming. A mix that looks fine on top can be coming apart underneath because the bond between the binder and the stone never held against water.

Superpave checks this with the tensile strength ratio, TSR, under AASHTO T283. You make two sets of compacted samples, condition one set wet through a soak and often a freeze-thaw cycle, leave the other dry, and break both in indirect tension. The TSR is the wet strength divided by the dry strength. Most agencies want a minimum of 0.80, meaning the conditioned mix keeps at least 80 percent of its dry strength. Below that the mix is moisture-susceptible and needs help.

The help is an anti-strip additive. Hydrated lime, added at around 1 to 1.5 percent, is the common one and does double duty as a mineral filler. Liquid amine anti-strips are the other route. Some aggregates, certain siliceous gravels especially, strip badly without it. Skip the moisture check or the additive on a stripping-prone aggregate and you own a pavement that delaminates in a few wet seasons no matter how good the volumetrics looked.

RAP and recycled binder

Reclaimed asphalt pavement, RAP, is milled-up old asphalt fed back into the new mix, and nearly every mix run today carries some. It is good economics and good practice, because the aggregate and the aged binder in RAP are not waste. The catch is that the binder in RAP is old and stiff, oxidized hard over its first life, and it counts toward the binder in the new mix.

At low RAP fractions, often up to 15 or 20 percent, the spec usually lets you ignore the grade effect and run the same virgin binder. Above that, the aged RAP binder stiffens the blend enough that you step the virgin binder grade softer to compensate, dropping the high and the low one notch so the blended binder lands back near the target grade. Push RAP high without softening the virgin binder and the mix ends up too stiff, which shows as cracking in the cold.

A recycling agent or rejuvenator can restore some of the aged binder's properties and let a mix carry more RAP. The design still has to make the volumetric targets with the RAP in it, and the RAP has to be characterized, its gradation and binder content known, not just shoveled in. The mill-and-overlay guide covers where that milled material comes from.

Mix types by layer: surface, binder, and base

Asphalt goes down in layers, and each layer gets a mix built for its job. The surface or wearing course takes the tire, the weather, and the friction demand, so it uses a smaller NMAS, usually 9.5 or 12.5 mm, for a tight, smooth, skid-resistant surface. The intermediate or binder course underneath carries load and levels, often a 19 mm mix. The base course at the bottom spreads load to the subgrade and uses the largest stone, 25 mm or more, where economy and structure matter more than surface texture.

The pattern follows from the stone size. Big stone is cheaper per ton and stronger in the structure but rough and open on top. Small stone rides smooth and sheds water but costs more and carries less structure per inch. So you put the coarse, cheap, strong mix deep and the fine, tight, durable mix on top.

Most of these are dense-graded mixes, the continuous gradation that fills its own voids and seals the surface. Two specialty families break that pattern for specific reasons, the gap-graded stone matrix asphalt and the open-graded friction course, covered next.

SMA and open-graded friction courses

Stone matrix asphalt, SMA, is a gap-graded mix built around stone-on-stone contact. It runs heavy on coarse aggregate with a gap in the middle sizes, so the big stones carry the load directly against each other instead of floating in sand and binder. That makes it rut-resistant on heavy, slow, channelized traffic, the kind of loading that shoves a dense-graded mix. SMA carries a high binder content held up by a fiber or filler to keep it from draining off the stone, and it costs more, so it earns its place on interstates, ports, and intersections, not on a subdivision street.

Open-graded mixes go the other way on purpose. An open-graded friction course, OGFC, is built with little fine aggregate so the mix is full of connected voids that water drains through. On a wet highway that cuts splash, spray, and hydroplaning and quiets the tire. The same open structure that drains water also lets air in, so the binder, usually polymer-modified, ages faster and the course has a shorter life than dense-graded. Permeable mixes for parking and low-speed work follow the same drainage idea at lower strength.

Pick the family by the problem. Rutting under heavy slow load points to SMA. Wet-weather safety on a high-speed road points to OGFC. General paving stays dense-graded, which is most of what gets laid.

What is a job mix formula?

A job mix formula, the JMF, is the approved recipe for a specific project: the exact aggregate blend, gradation, binder grade, binder content, and the volumetric targets, plus the tolerances production is allowed to vary within. The mix design produces the JMF. The JMF is what the plant runs and what the agency holds the producer to.

The path is set. The producer designs the mix and submits it. The agency reviews and approves the JMF, sometimes after verifying it in an independent lab or a trial production run. Once approved, the JMF is the contract for the mix, and changing it, a new aggregate source or a gradation shift, means a new submittal, not a quiet adjustment at the plant.

Production never hits the target dead on every load, so the JMF carries tolerances. Gradation runs roughly plus or minus 5 to 7 percent on the coarse sieves, tightening toward plus or minus 2 percent on the 0.075 mm dust. Binder content holds to a few tenths of a percent, often around plus or minus 0.3. Air voids carry their own production band. The exact tolerances vary by agency and come from the adopted specification. The point is constant: the field result has to track the JMF, and a load that runs outside tolerance is a load the agency can reject.

From the lab to the mat: how the field protects the design

The mix design is a promise made in the lab. The field is where it gets kept or broken, and the line between them is sharper than most arguments admit. The plant has to make the JMF mix, load after load, at the right temperature, gradation, and binder content. The crew has to haul it without segregating or cooling it, lay it at the right thickness, and compact it to density before the mat goes cold.

Compaction is where the design lives or dies on the road. The lab designed 4 percent air voids at Ndesign. The field has to get the in-place voids down to the accepted range, usually expressed as a percent of Gmm, while the mix is still hot enough to move. Miss the compaction window and the mat sits high on air voids no matter how clean the design was, and it ravels, strips, and cracks years early. The compaction window guide covers the temperatures and the rolling pattern that get there.

The PG binder ties the two ends together. The grade was picked for the climate and bumped for the traffic, and the field has to lay it in a temperature window that matches that binder. A stiff polymer-modified binder needs more heat and more roller to compact, and a crew rolling it like a soft mix will leave it high on voids. The design assumed the field would protect it. The cores at the end tell whether the field did.

What does the agency test to accept the asphalt?

Agencies accept asphalt against the JMF on a short list of properties, sampled from production and from the road. The plant-side checks are gradation, binder content, and the volumetrics, air voids and VMA, run on samples taken behind the plant or off the truck and compacted to Ndesign. These confirm the producer is making the mix that was approved, load to load.

The road-side check is in-place density, cored or measured with a nuclear or non-nuclear gauge and reported as a percent of Gmm. A common acceptance floor sits around 92 percent of Gmm, meaning no more than about 8 percent in-place air voids, but the number and the method are the agency's to set. Smoothness, measured by profilograph or inertial profiler and reported as an IRI or a ride number, often carries its own pay adjustment.

Most of this rides on statistical acceptance with pay factors. The lot is sampled, the results are run against the spec, and the producer is paid full price, a bonus, or a penalty based on how the numbers fall, with the agency's verification testing checking the producer's own results. Know whether a given property is QC, the producer's control, or QA, the agency's acceptance, because the one that adjusts the pay is the one that controls.

Matching the mix to the use: heavy-duty and value engineering

The right mix is the one matched to the use, and the two ways to get it wrong cost money in opposite directions. Under-build a heavy-duty pavement and it ruts and fails early. Over-build a light-duty one and you paid for performance the traffic will never use.

Heavy, slow, concentrated loading is its own design problem. Port aprons, intersection approaches, bus pads, truck terminals, and the equipment yards and generator pads around a data center see loads that sit and grind rather than roll past. That loading bumps the binder grade up, often to a polymer-modified PG 76 or higher, pushes Ndesign to the high traffic level, and frequently points to SMA for the stone-on-stone rut resistance. A standard highway surface mix laid in a container yard will shove into ruts in a season.

The other direction is just as real. A subdivision street, a parking lot, a light commercial drive does not need a high-traffic gyration level or a modified binder, and specifying one wastes money. Match the binder grade, the gyration level, the mix type, and the lift design to the actual traffic and climate. The design choices are levers, not a single best mix, and the value is in pulling the right ones for the job in front of you instead of defaulting high or low.

What to document

A mix design nobody can produce later is a number on a shelf. The record that travels with the job is the JMF and the production data that shows the field tracked it, and it is what settles a dispute when a core comes back low or a mat fails early.

Capture the mix designation and NMAS, the PG binder grade and source, the design binder content, the design volumetrics at Ndesign, the gyration level, the aggregate sources and consensus properties, the TSR and any anti-strip, the RAP percentage, and the production results against the JMF tolerances. Tie each lot to its acceptance density. When a mix is adjusted, log the new JMF and the reason for it.

Common mistakes

• Specifying the wrong PG grade for the climate, or not bumping it for slow, heavy traffic.
• Running a gradation that misses the control points, then trying to fix it with binder instead of the blend.
• Designing to 4 percent air voids in the lab but accepting a mat left high on in-place voids.
• A mix too rich on binder that ruts and bleeds, or too lean that ravels and cracks.
• Skipping the TSR check or the anti-strip on a stripping-prone aggregate.
• Pushing RAP high without stepping the virgin binder grade softer to compensate.
• Letting production drift outside the JMF tolerances and accepting the load anyway.
• Laying a polymer-modified or SMA mix with a rolling pattern meant for a soft dense-graded mix.

Field checklist

Standards and references

Superpave mix design lives in two AASHTO documents working together. AASHTO M323 is the Superpave volumetric mix design specification, the requirements: the consensus aggregate properties, the gradation control points, the volumetric criteria, and the binder selection. AASHTO R35 is the standard practice, the procedure for getting there. The Asphalt Institute SP-2 manual is the working reference most labs keep on the bench, and it walks the method in practical terms.

The binder is graded under AASHTO M320, with the newer multiple-stress creep recovery work in M332 addressing polymer-modified binders the older grading did not capture well. The test methods are their own standards: the gyratory compaction under AASHTO T312, the maximum theoretical gravity, Rice, under T209, the bulk gravity of the compacted sample under T166 or T331, and the moisture and TSR work under T283. ASTM carries parallel methods for much of this.

Above all of it sits the agency. The state DOT or owner specification adopts these standards, sets the Ndesign table, the acceptance limits, the production tolerances, and the pay factors, and amends them locally. Cite the standard that governs the point, but the adopted specification and its edition control the actual numbers. Do not quote a section from memory onto a submittal. Confirm it against the spec the project is built under.

Units, terms, and conversions

Asphalt mix design carries a stack of acronyms and a mix of metric and US units, because the science came in metric through SHRP while the field still talks in tons and inches.

Aggregate sizes and the gradation are metric, in millimeters, even on US jobs: a 12.5 mm mix, the 0.075 mm dust sieve. Binder grades are in degrees Celsius. Binder content and the volumetrics are percentages. In-place density is a percent of Gmm. Smoothness is reported as IRI, the international roughness index, in inches per mile or meters per kilometer. The terms below are the ones that carry the design.

Superpave

Superior Performing Asphalt Pavements, the AASHTO mix design system from the SHRP program

PG grade

Performance grade of the binder, high minus low temperature in Celsius, like PG 64-22

NMAS

Nominal maximum aggregate size, one sieve above the first to retain over 10 percent; names the mix

Ndesign

The design gyration count, set by traffic, where the mix is held to 4 percent air voids

Va / air voids

Air left in the compacted mix as a percent of volume, designed to 4 percent

VMA

Voids in the mineral aggregate, the space holding air plus effective binder; minimum by NMAS

VFA

Voids filled with asphalt, the share of VMA filled by effective binder; banded by traffic

Gmm / Gmb

Maximum theoretical (Rice) and bulk specific gravity; air voids come from their ratio

JMF

Job mix formula, the approved project recipe with its production tolerances

TSR

Tensile strength ratio, wet over dry strength; moisture-damage check, often 0.80 minimum

RAP

Reclaimed asphalt pavement, milled old asphalt reused in a new mix

ESALs

Equivalent single axle loads, the traffic measure that sets Ndesign

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