Asphalt mix types field guide: surface, binder, and base course

Why a pavement uses a different mix in every layer

A pavement is not one slab of asphalt. It is a stack of separate lifts, each placed and compacted on its own, and each one doing a different job. The lift on top has to ride smooth, shed water, and grip a tire in the rain. The lifts underneath have to carry the load down to the soil. Those are different jobs, so they get different mixes, and the size of the stone is what mostly sets them apart.

The pattern is simple once you see it. Small stone rides smooth and seals tight but is expensive and weak per inch, so it goes on top in a thin wearing course. Big stone is cheap and strong in the structure but rough and open, so it goes deep in the base where nobody drives on it directly. The binder course splits the difference in the middle. The thickness-design guide covers how the engineer sizes that whole stack to the traffic and the subgrade; this guide is about which mix goes in each layer and why.

Get the layers straight and a lot of field arguments end before they start. A coarse base mix laid as a surface course will ravel and leak. A fine surface mix laid as a base will cost a fortune and add little structure. The mix matches the lift, the lift matches the job, and the job is set by where the layer sits in the section.

What is nominal maximum aggregate size, and the lift thickness rule?

Nominal maximum aggregate size, the NMAS, is the size that names a mix and sorts it into a layer. It is one sieve larger than the first sieve that retains more than 10 percent of the aggregate, so a 12.5 mm mix and a 25 mm mix are built around different top-size stone. Surface mixes run small, commonly 9.5 or 12.5 mm. Binder mixes run coarser, often 19 mm. Base mixes carry the biggest stone, 25 mm and up. The Superpave mix-design guide covers how the NMAS is fixed from the gradation; here it is the number that decides how thick the lift has to be.

The rule that ties them together is the lift thickness rule, and it is the one a new foreman gets wrong first. The compacted lift has to be at least three to four times the NMAS so the stones can shift and lock down under the roller instead of bridging against each other. The Asphalt Institute commonly puts the floor at four times the NMAS for dense-graded mixes and three times for finer ones. Run a 19 mm mix in a lift that is too thin and the big stones cannot rotate into place, the mat tears under the screed, and you cannot get the density no matter how many passes you make.

So the NMAS and the lift drive each other. A thin top course needs small stone. A thick structural lift can take big stone and gets it for the economy. When you pick a mix, you are also picking a lift range, and when you set a lift thickness, you are ruling some mixes in and others out. Confirm the actual multiplier against the project specification, since agencies word the minimum-lift rule a little differently.

The surface course: the wearing layer

The surface course, also called the wearing course or the top, is the lift the tire actually touches. It takes everything the pavement faces directly: the abrasion of traffic, the freeze and thaw, the sun, the studded tire and the snowplow, and the water that has to run off instead of soaking in. Because it does all of that and shows every flaw, it is built from the finest, densest, highest-quality mix in the section, usually a 9.5 or 12.5 mm dense-graded mix on a binder graded for the local climate.

Three things the surface has to deliver. It has to ride smooth, because ride quality is what the public feels and what a profilograph measures for pay. It has to grip, so the aggregate carries a polish-resistance requirement to keep the surface from going slick under traffic. And it has to seal, because a dense, well-compacted surface keeps water and air out of everything below it, and water in the section is what kills pavements early.

The surface is also the layer you renew. On a mill-and-overlay you grind off the worn surface and lay a new one while the structure underneath stays put, which is why agencies spend the aggregate quality and the binder grade here that they would not spend deeper. The surface wears so the structure does not have to. Skimp on the surface mix and you are repaving sooner, not saving money.

The binder course: the workhorse middle lift

The binder course, the intermediate course, sits between the surface and the base, and it is the workhorse of the asphalt section. The name confuses people: this is not the asphalt cement, the liquid binder. It is a structural lift of asphalt mix, usually a 19 mm dense-graded mix, coarser and cheaper than the surface and finer than the base. Its job is to carry load and to level the coarse base into a tight, true plane for the surface to ride on.

Because nobody drives on it directly, the binder course does not need the polish resistance or the fine texture of the surface, so the mix runs coarser and costs less per ton while still carrying real structure. On a thick section you put most of your structural asphalt inches here, in a mix that does the load-spreading work without paying surface-course money for it. That is the economy of the layered approach in one lift.

The binder also takes the abuse of construction. It carries the haul trucks and the paver during the surface lift, and it gives the surface a sound, compacted base to bond to. A binder course left low on density or rough on grade telegraphs straight up into the surface, so the lift that nobody sees in the finished road still decides how the finished road rides.

The base course: the structural bottom lift

The asphalt base course is the bottom bound lift, the foundation of the asphalt section, and it carries the largest stone, 25 mm and frequently 37.5 mm. Its job is structure, not surface: it spreads the wheel load over a wide enough footprint that the soil and aggregate below it never see more pressure than they can take. Built from the coarsest, cheapest mix in the stack, it buys the most structural strength per dollar of any asphalt lift.

There are two kinds of base, and the words get used loosely, so read the section the project calls for. An asphalt base, a bound lift of coarse mix, makes a full-depth or deep-strength section that resists water well and performs over weak soil. An unbound aggregate base, crushed stone with no asphalt, is the cheaper foundation under a thinner asphalt section. The thickness-design guide covers the trade between carrying structure in asphalt versus in stone; the point here is that the asphalt base, when it is used, is its own coarse mix with its own thick lift.

Because the base carries big stone, it carries the thickest lifts, 3 to 5 in or more, to satisfy the three-to-four-times-NMAS rule. That is fine, because it is deep in the section where lift thickness is not constrained by grade or ride. The base is where you lay the cheap, strong inches and let the layers above worry about smoothness.

Dense-graded HMA: the standard mix

Dense-graded hot mix asphalt is the standard, the mix that makes up most of the asphalt laid anywhere. It is a well-graded mix, meaning the aggregate runs continuously from the top size down through the sand and the dust, so the small particles fill the gaps between the large ones and the mix packs tight. That tight packing is what makes it strong and watertight, and it is why the same family of mix serves the surface, the binder, and the base, just at different stone sizes.

Because the gradation is continuous and fills its own voids, a well-compacted dense-graded mat seals against water and air. That is the property that protects the binder from oxidizing and the section from stripping, and it is why dense-graded is the default for anything that needs to keep water out, which is most pavement. When somebody says HMA without a qualifier, this is what they mean.

Everything else in this guide is a deliberate departure from dense-graded for a specific reason. You break from it to fight rutting under heavy slow load, to drain water through the surface, to repair a hole without a plant, or to lay cooler. If none of those reasons applies, the answer is dense-graded, and the only real choices are the NMAS for the layer and the binder grade for the climate and traffic.

Dense, gap, and open: how gradation changes the mix

Gradation is how the aggregate sizes are distributed, and it sorts the mix families more than anything else. There are three patterns, and each one behaves the way its name says. Dense-graded runs continuous from top size to dust and packs tight. Gap-graded leaves a deliberate gap in the middle sizes so the coarse stone touches stone to stone. Open-graded leaves out most of the fines on purpose so the mix is full of connected voids that water runs through.

The pattern decides the behavior. Fill the voids and you get strength and a seal, which is dense-graded. Take out the middle sizes and the big stones carry load directly against each other, which is the rut resistance of stone matrix asphalt. Take out the fines and you get a sponge that drains, which is the open-graded friction course. Same aggregate, same binder family, completely different pavement, all from where you put the stone on the gradation curve.

The Superpave guide covers how the gradation is plotted on the 0.45 power chart and held to control points, and that is where the detail lives. For the field, the takeaway is that gradation is the lever that separates the standard mix from the two specialty families, and you choose the family by the problem you are solving on that surface.

What is stone matrix asphalt?

Stone matrix asphalt, SMA, is a gap-graded surface mix built around stone-on-stone contact, designed to resist rutting under heavy, slow, concentrated traffic. It runs heavy on coarse aggregate, commonly 70 to 80 percent, with a deliberate gap in the middle sizes so the large stones bear directly against each other instead of floating in sand. That stone skeleton carries the load through direct contact, which is what shrugs off the shoving and rutting that pushes a dense-graded mix into ruts at an intersection or a port apron.

The gap between the stones is filled with a rich mastic of asphalt binder and mineral filler, and that mastic carries a high binder content, around 6 to 7 percent. Pour that much binder into an open stone skeleton and it wants to drain off the stone in the truck and on the road, so SMA carries a fiber, usually cellulose or mineral, at around 0.3 percent to hold the binder in place. Skip the fiber and you get binder drain-down, fat spots, and a ruined mat.

SMA is a premium surface that earns its place by the load, not by habit. It developed in Germany in the 1960s and proved itself on interstates, airfields, and heavy urban routes. It costs more than dense-graded, in aggregate quality, in binder, and in the fiber, so it belongs where the traffic is heavy and slow enough to rut a standard mix: interstate lanes, intersections, bus pads, truck terminals, port aprons. On a subdivision street it is overkill, and the money is wasted.

What is an open-graded friction course?

An open-graded friction course, OGFC, is a thin surface mix built to drain water through itself instead of shedding it off the top. It is made with mostly coarse aggregate and almost no fines, so the compacted mat holds a high interconnected void content, commonly around 18 to 22 percent, and water runs down through the mat and out the side rather than sheeting across the surface under a tire. It is the permeable wearing layer, laid on top of a dense, watertight structural surface that actually keeps the water out of the section.

The reason to use it is wet-weather safety on a fast road. With the water draining through the mat, the tire stays in contact with the stone, so splash and spray drop, hydroplaning risk falls, and visibility behind a truck in the rain improves. The open texture also quiets the tire, on the order of a few decibels, which is why agencies use it through noise-sensitive corridors.

The same openness that drains water is also the weakness. Air moves freely through the mat, so the binder oxidizes and ages faster than it would in a sealed dense-graded surface, and OGFC has a shorter service life as a result. To fight that, the binder is almost always polymer-modified and the mix carries fibers, both to resist the raveling that an aging open mix is prone to. And the voids only work if they stay open. On a low-speed road where traffic does not keep them flushed, or where sand and debris pack in, the voids clog and the drainage is gone, so OGFC belongs on high-speed routes, not on slow urban streets or parking lots.

Porous asphalt for stormwater

Porous asphalt is the full-depth cousin of OGFC, built to manage stormwater rather than just to drain a driving surface. Where OGFC is a thin open layer over a sealed road, a porous pavement is open all the way down: an open-graded surface over an open-graded base over a thick stone reservoir, so rain falls through the pavement, into the stone bed, and infiltrates into the soil or releases slowly instead of running off. It turns the parking lot into part of the drainage system.

The use case is sites where stormwater rules limit runoff, parking lots, low-speed lots, and overflow areas, where the pavement doubles as detention and cuts the size of the ponds and pipes the site would otherwise need. The structural strength is lower than a dense section because the voids that pass water are voids that do not carry load, so porous asphalt is for light-duty traffic, not truck routes.

The thing that kills a porous pavement is clogging. Fines, sediment, and debris work into the voids and seal them, and once it stops draining it is a weak pavement with no upside. Keeping it working means keeping sediment off it and vacuum-sweeping it on a schedule, which is a maintenance commitment the owner has to accept before it goes in. A porous lot nobody maintains becomes a clogged lot in a few seasons.

What is warm mix asphalt?

Warm mix asphalt, WMA, is not a separate mix type but a way of producing and placing the same mixes at a lower temperature. Through an additive, a foaming process, or a chemical, the asphalt is made workable at production and compaction temperatures commonly 50 to 100°F below conventional hot mix, roughly 30 to 55°C. You can run a dense-graded, an SMA, or an OGFC as warm mix; the gradation and the binder grade do not change, the temperature does.

The reasons are practical, not just environmental. Lower temperature means less fuel burned and fewer fumes at the plant and behind the paver, which is the sustainability case and a real improvement in the working air for the crew. It also buys time. Because the mix does not have to be as hot to compact, the cooling clock runs longer, which opens up longer hauls, cooler-weather paving, and night work that a hot mix would lose to the cold before the rollers got density.

The catch is that lower production temperature can leave aggregate moisture in the mix and can change how the binder coats and ages, so a WMA still has to meet the same volumetric and moisture-damage criteria as the hot version, and some additives carry their own checks. Warm mix has moved from a novelty to routine practice across much of the industry, but it is a production method laid over the mix types in this guide, not a replacement for them.

RAP and recycled content in the mix

Reclaimed asphalt pavement, RAP, is milled-up old asphalt fed back into a new mix, and nearly every mix run today carries some. It is not waste. The aggregate and the aged binder in RAP both have value, and reusing them is good economics and standard practice. National averages for RAP content in new mixes have run around 20 to 22 percent in recent years, with many agencies allowing more, and asphalt is among the most-recycled materials in the country by tonnage.

The catch is the binder. The asphalt in RAP is old and oxidized, stiffer than fresh binder, and it counts toward the binder in the new mix. At low fractions, often up to about 15 to 20 percent, the spec usually lets you run the same virgin binder and ignore the grade effect. Push RAP higher and the aged binder stiffens the blend enough that you step the virgin grade softer to bring the blended binder back to target, or the mix ends up too stiff and cracks in the cold. The Superpave guide covers how that binder blending is handled in the mix design.

Reclaimed asphalt shingles, RAS, are a related recycled stream, a source of stiff binder and fine aggregate used in smaller fractions and handled carefully because shingle binder is very hard. Recycling agents or rejuvenators can restore some of the aged binder's properties and let a mix carry more RAP. The recycling and mill-and-overlay work is its own topic; the point here is that recycled content is in almost every mix you lay, and it changes the binder behavior, so the design has to account for it rather than just shovel it in.

Polymer-modified binder mixes

Polymer-modified mixes are not a different gradation; they are a standard mix carrying a binder that has been stiffened and toughened with a polymer to hold up under conditions plain binder cannot. The polymer, often an SBS, widens the temperature range the binder works across, so it stays stiff enough to resist rutting in summer heat and flexible enough to resist cracking in winter cold, and it resists the raveling that ages an open mix. You see it written as a bumped PG grade, a PG 76-22 where the climate alone would have called for a PG 64-22.

The reasons to bump to polymer track the hard cases: heavy traffic, slow and standing loads, hot climates, and the open or gap-graded specialty mixes. The Superpave guide covers how the grade gets selected and bumped, so the detail lives there. What matters at the mix-type level is that SMA and OGFC almost always run a polymer-modified binder, that heavy-duty and intersection mixes usually do, and that the polymer is bought for a reason, not as a default.

Polymer changes how you lay it. A stiff modified binder needs more heat and more roller to reach density, and a crew that rolls it like a soft dense-graded mix will leave it high on voids and short on life. The binder grade and the rolling pattern go together, which is the compaction window guide's territory, but it starts with knowing the mix has a modified binder before the first load shows up.

Which surface mix do I use: dense-graded, SMA, or OGFC?

For the wearing course you are usually choosing among three, and the choice follows the problem on that surface. Dense-graded is the default and the right answer for most surfaces: it rides smooth, seals tight, and costs the least. You move off it only when the traffic or the weather gives you a specific reason, and then you move toward SMA or OGFC depending on which problem you have.

Rutting under heavy, slow, channelized load points to SMA. If the surface is an intersection, a bus pad, a port apron, or an interstate truck lane where a dense mix would shove into ruts, the stone-on-stone skeleton of SMA earns its premium. Wet-weather safety on a high-speed road points to OGFC. If the problem is spray, hydroplaning, and noise on a fast highway, the draining open mix solves it, with the understanding that it ages faster and needs the speed to stay unclogged. Both are surface-only specialty mixes laid over a sound structural section, not structural layers themselves.

The mistake is reaching for the premium mix where the default would do. SMA on a parking lot or a light street is money spent on rut resistance the traffic will never demand. OGFC on a slow urban road clogs and turns into a short-lived liability. Match the surface mix to the surface's actual problem, and most of the time that problem is ordinary, so the answer is dense-graded.

Matching the mix to the traffic and use

The right mix is the one matched to the load, and the load varies more across a single site than people expect. A passenger-car parking lot, a collector road, and a truck terminal are three different problems even on the same soil, and the mix and the binder grade should track that. The thickness-design guide covers sizing the section to the traffic; the mix-type version of the same idea is that the binder grade, the mix family, and the surface quality follow the load on each part of the job.

Light-duty work, parking fields, residential streets, light commercial drives, is dense-graded with a standard binder grade, sized for a sound surface and constructability rather than for heavy structure. Ordinary roads and collectors carry a steady diet of trucks and buses and get a real dense-graded section with the binder graded for the climate. Heavy and slow loading, intersections, bus pads, port and terminal aprons, intermodal yards, is where the binder bumps to polymer and the surface often goes to SMA, because that loading shoves a standard mix.

The expensive errors run in both directions. Specify a high-traffic mix and a modified binder on a subdivision street and you paid for performance the cars will never use. Lay a standard highway surface mix in a container yard or a bus pad and it ruts in a season. The mix is a set of levers, the gradation family, the NMAS by layer, the binder grade, and the value is in pulling the right ones for the load in front of you instead of defaulting high or low.

Cold mix and patching: the utility-cut mix

Cold mix is asphalt made without heating, an aggregate blended with an asphalt emulsion or a cutback asphalt that stays workable at ambient temperature and out of a bag or a stockpile. It does not need a plant, a hot truck, or a tight cooling window, which is exactly why it exists. It cures slowly as the water in the emulsion or the solvent in the cutback leaves, gaining strength over days to weeks rather than setting up as it cools the way hot mix does.

The job for cold mix is repair and utility work: filling potholes, patching a utility cut, holding a trench until the permanent restoration, and small fixes where firing up a hot-mix operation makes no sense. A crew can throw a bag of cold patch in a hole in February and tamp it down, and it will hold traffic. That convenience is the whole value.

Treat cold patch as temporary, because that is what it is. It does not bond and seal like a hot-mix repair, and a cold-patched pothole or utility cut typically lasts months, not years, before it needs a proper hot-mix restoration. The failure mode is using it as a permanent fix. The patch holds long enough that nobody comes back, it ravels and sinks under traffic, and the same hole reopens a season later. Use it to buy time, then schedule the permanent repair.

The mix, the lift, and the compaction window

The mix you pick, the lift you place, and the window you have to compact it are one connected decision, and they fail together when they are treated separately. The NMAS sets the minimum lift so the stones can lock down. The lift thickness and the mix temperature set how much time the rollers have before the mat goes cold and stops moving. A thin lift of a stiff mix loses heat fast and gives a short window; a thick base lift holds heat and gives a long one.

Density is where the mix design gets kept or broken on the road. The Superpave guide explains that a mix is designed to a target air void content, and the field has to compact the in-place mat down to its accepted density before it cools out of the workable range. Miss the window and the mat sits high on voids no matter how clean the design was, which lets water and air in and ravels and cracks the surface years early. The compaction window guide covers the temperatures and the rolling pattern that get there.

The mix type changes the window. A polymer-modified SMA or a stiff base mix needs more heat and more roller than a soft dense-graded surface, and an open mix cools fast through its voids. So the rolling pattern is not one pattern. It follows the mix, and a crew rolling a modified or open mix like a standard one will leave it short on density. Pick the mix, set the lift to the NMAS, and match the rolling to both.

Recognizing a mix in the field

You can read most mixes off the surface texture once you know what you are looking at, which is useful when the paperwork is thin and you are trying to confirm what went down. A dense-graded surface looks tight and closed, a fairly smooth face with the fines and small stone filling in around the larger aggregate. That closed look is the seal that keeps water out, and it is what most finished pavement looks like.

The specialty mixes look different on purpose. An SMA surface looks coarse and stony, with the large aggregate prominent at the surface and a rich black mastic between the stones, because the gap-graded skeleton puts the big stone right at the top. An OGFC or porous surface looks open and honeycombed, visibly full of holes, and the tell is the water: rain disappears into it instead of beading and sheeting on top. A base mix exposed during construction looks the coarsest of all, big stone and an open, rough face, because it was never meant to be a finished surface.

Cold patch is the easy one to spot. It looks loose, dark, and granular, sitting in a hole rather than bonded into a continuous mat, and it is usually a different texture and color than the pavement around it. When you see that in a road that is otherwise sound, you are looking at a temporary repair that someone still owes a permanent fix.

Data center pads, industrial yards, and heavy-duty mixes

Heavy-duty industrial pavement is its own mix problem because the loads sit and grind instead of rolling past. Container terminals, laydown and crane yards, truck terminals, transformer-haul routes, and the equipment pads around a data center see slow, concentrated, sometimes static loads that shove a standard dense-graded surface into ruts. The mix answer follows the loading: a binder bumped to polymer, often a PG 76 or higher, frequently an SMA surface for the stone-on-stone rut resistance, and the structural inches sized by the thickness-design guide.

Slow and standing load is the hard part. Asphalt creeps under sustained heavy load in a way it does not under a fast wheel, so a crane outrigger or a parked loaded trailer is brutal on a surface that would handle highway trucks fine. That is why the heaviest, slowest, most concentrated spots, dock aprons, crane pads, container stacks, often leave asphalt entirely for concrete, with asphalt kept to the drives and the lighter areas. The thickness-design guide covers that flexible-versus-rigid call.

On a data center or critical-facility site the rigor goes up because differential settlement next to sensitive equipment is a real problem, not a cosmetic one. These jobs lean on the geotech and an engineered design, run the polymer-modified and SMA mixes where the loads demand them, and hold the compaction tight. Design and specify these to the engineered loads on the drawings, not to a mix borrowed from a commercial parking lot.

What to document

Pull a core from a lift that failed early and the first question is which mix was specified for that layer and whether it actually went in, and without a record no one can answer it. The record that settles the argument is which mix went in which layer, at what size, for what job, and where, so that a core or a failure can be traced back to whether the right mix was in the right place. Capture it layer by layer as the job goes down.

For each lift, record the mix type and family, the NMAS, the binder grade, the layer it served and its job, where on the site it was placed, and the lift thickness against the NMAS rule. Note the specialty mixes and why they were used, the SMA at the intersection, the OGFC on the highway lanes, the cold patch that is still owed a permanent repair. Tie the mix record to the placement and density records so the file shows both that the right mix was chosen and that it was laid and compacted right. The table below is the spine of that record.

Common mistakes

• Laying a coarse base mix as a surface course, or a fine surface mix as a base, so the layer cannot do its job.
• Placing a lift too thin for the NMAS, below three to four times the top stone size, so it will not compact.
• Putting OGFC or porous asphalt on a slow road or a lot where it clogs and loses its drainage.
• Specifying SMA where the traffic is light, paying for rut resistance the load will never demand.
• Ignoring the binder grade for the traffic, running a plain grade where slow heavy load needs a polymer bump.
• Treating cold patch as a permanent fix instead of buying time for a hot-mix restoration.
• Rolling a polymer-modified, SMA, or open mix with the pattern meant for a soft dense-graded surface.
• Leaving an open or porous surface over a structure that does not actually carry the water away below it.

Field checklist

Standards and references

The mix itself is designed and specified under the Superpave system in most United States practice, with the volumetric requirements in AASHTO M323 and the procedure in AASHTO R35, and the Asphalt Institute SP-2 manual as the working bench reference. Those documents set the NMAS, the gradation, the binder, and the volumetrics; the Superpave mix-design guide covers them in detail. The specialty mixes have their own references: SMA and OGFC mix design and practice are covered in National Asphalt Pavement Association and FHWA guidance, and the Asphalt Institute manuals cover dense-graded design and construction.

The lift thickness rule, commonly three to four times the NMAS, traces to Asphalt Institute and agency construction guidance, and the exact multiplier and minimum lift are written into the project specification, so confirm it there rather than quoting a single number onto a submittal. Binder grading is under AASHTO M320, with the multiple-stress creep recovery work in M332 for the polymer-modified grades the older grading did not capture well. Warm-mix and recycled-content practice sit in FHWA and NAPA guidance and in the agency's own provisions.

Above all of it is the agency. The state DOT or owner specification adopts these standards, names the mix types by designation, sets the acceptance limits and the lift rules, and amends them locally. Cite the standard that governs the point, but the adopted specification and its edition control the actual mix designations, the lift minimums, and the binder grades. Do not carry a mix designation or a lift number from one agency's job onto another's without checking the spec it is built under.

Units, terms, and conversions

Asphalt mix carries a mix of metric and US units, because the gradation came in metric through Superpave while the field still talks tons, inches, and degrees Fahrenheit. The same mix can read differently across the mix design, the spec, and the plant ticket.

Aggregate sizes and the NMAS are in millimeters, even on US jobs: a 12.5 mm surface mix, a 25 mm base. Lift thickness is in inches in the field and millimeters on metric drawings. Binder grades are in degrees Celsius, like PG 64-22. Production and compaction temperatures run in degrees Fahrenheit on most US plant tickets. Binder content, air voids, and RAP content are percentages. The terms below are the ones that sort the mixes.

NMAS

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

Surface / wearing course

The top lift the tires ride on; fine, dense, watertight, skid-resistant

Binder / intermediate course

The structural mix lift between surface and base; coarser, carries load and levels

Base course

The bottom asphalt lift; largest stone, the structural foundation of the bound layers

Dense-graded HMA

Well-graded mix that packs tight and seals; the standard for most layers

SMA

Stone matrix asphalt, a gap-graded mix with a stone-on-stone skeleton for rut resistance

OGFC

Open-graded friction course, a porous surface mix that drains water and cuts spray and noise

Porous / permeable asphalt

Full-depth open pavement over a stone reservoir for stormwater infiltration

WMA

Warm mix asphalt, the same mixes produced and placed at a lower temperature

RAP / RAS

Reclaimed asphalt pavement and reclaimed asphalt shingles, recycled binder and aggregate reused in the mix

Polymer-modified binder

Asphalt cement stiffened with polymer to widen the working temperature range; a bumped PG grade

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