What Is Asphalt?
Asphalt is a black, sticky binder refined from crude oil. The word also means the pavement made by mixing that binder with crushed stone, sand and gravel. Most people mean the finished pavement on roads, driveways and parking lots. By weight, that pavement is about 95% stone, sand and gravel and 5% binder. Those figures come from the National Asphalt Pavement Association's overview of asphalt pavement.
Asphalt pavement is rock held together by a petroleum glue that stays slightly flexible. That flexibility helps it ride out the freeze-thaw winters around Pittsburgh and the surrounding counties. If you're asking what is asphalt because you're planning a driveway, start with this. The stone base under it matters as much as the asphalt on top.
What Is Asphalt Made Of?
Every asphalt pavement has two main ingredients. Some mixes add a third.
Ingredient | Share of the mix by weight | What it does |
Aggregate (crushed stone, sand, gravel) | About 95% | Carries the weight of cars and trucks |
Asphalt binder (also called bitumen or asphalt cement) | About 5% | Glues the stone together and seals out water |
Additives (polymers, anti-stripping agents) | A small fraction, when used | Add flexibility or help the binder grip the stone |
The aggregate is graded by size. Larger stones lock together for strength, and smaller stones and sand fill the gaps. The binder coats every piece and holds the whole mat together once it cools.
Where Does Asphalt Come From?
Almost all asphalt binder used today comes from refining crude oil. At a refinery, crude oil is heated in a tall distillation tower. Lighter products like gasoline and diesel boil off and rise. The heaviest, thickest part stays at the bottom, and that residue is processed into asphalt binder.
Asphalt also occurs in nature. Pitch Lake in Trinidad and the La Brea Tar Pits in Los Angeles are two well-known natural deposits.
What Is the Difference Between Asphalt, Bitumen, Blacktop and Tarmac?
These words get mixed up all the time. Here's how they line up in the United States:
Asphalt binder or bitumen is the black glue by itself. In the U.S., "asphalt" can mean just the binder, so engineers say "asphalt binder" or "asphalt cement" to be clear.
Asphalt concrete is the engineering name for the finished pavement of binder and stone.
Blacktop is an everyday American name for the same finished pavement.
Tarmac is the British name. It comes from tar macadam, an older paving method that used coal tar. Americans often call airport aprons "the tarmac" even though they're usually asphalt or concrete.
Asphalt and coal tar are different materials. Asphalt binder comes from petroleum. Coal tar comes from processing coal.
How Is Asphalt Made Into Pavement?
The pavement you drive on is mixed at an asphalt plant and trucked to the job. Here's the process for hot mix asphalt, the type used on most driveways and roads:
The plant dries and heats the aggregate to drive out moisture.
Hot liquid asphalt binder is added, and the two are mixed until every stone is coated.
The mix leaves the plant at around 300 degrees Fahrenheit and is hauled to the job in dump trucks.
A paving machine spreads the mix to an even thickness.
Rollers compact it while it's still hot. Compaction is what makes the pavement dense and tight.
Timing matters at every step. Hot mix has to be placed and rolled before it cools. That's why crews watch the air temperature, the ground and the forecast closely late in the season.
What Are the Main Types of Asphalt?
Asphalt comes in a few forms, and each one has a job:
Hot mix asphalt is the standard for driveways, roads and parking lots. It's mixed hot at the plant and laid hot.
Warm mix asphalt uses additives so it can be mixed and laid at lower temperatures. That saves fuel at the plant.
Cold mix asphalt stays workable at outdoor temperatures. It's sold in bags and buckets for patching potholes, mostly as a temporary fix.
Porous asphalt has gaps built into the mix so rainwater drains through the pavement into a stone bed below. It's used for stormwater control in some parking lots.
Asphalt emulsions are binder mixed with water. They're used for tack coats that help new asphalt bond to old asphalt, and in many driveway sealers.
Chip seal (tar and chip) is a sprayed layer of liquid asphalt with crushed stone rolled into it. You can see how it looks on our tar and chip driveway page.
How Is Asphalt Binder Graded?
Binder comes in grades. In the U.S., it's graded by the pavement temperatures it can handle, using a system called performance grading. A common grade is PG 64-22. The first number means the binder resists rutting at pavement temperatures up to 64°C (147°F). The second means it resists cracking down to -22°C (-8°F).
Grades are chosen to match the local climate. Around Pittsburgh, the binder has to stay firm on a hot August afternoon and stay flexible on a January night. Polymers are sometimes added to widen that range for busy roads and lots.
A Short History of Asphalt Roads
People have used natural asphalt for thousands of years to waterproof boats and buildings. Around 625 BC, a processional street in ancient Babylon was built with burnt brick set in asphalt.
Modern asphalt paving took off in the 1800s. Asphalt footpaths and sidewalks appeared in Paris, London and Philadelphia in the 1830s. The first asphalt road on record ran from Paris to Perpignan, France, in 1852. In 1870, Belgian-American chemist Edward de Smedt patented a sand-asphalt paving method he developed at Columbia University. That same year, he laid the first true asphalt pavement in the U.S. in Newark, New Jersey. Refined petroleum asphalt then replaced natural deposits as cars spread in the 1900s.
What Is Asphalt Used For?
Paving is by far the biggest use. About 94% of paved U.S. roads are surfaced with asphalt. It also covers driveways, parking lots, airport runways, bike paths and running tracks.
The binder also shows up away from the road. Asphalt roofing shingles, roll roofing and many waterproofing products for foundations and flat roofs all rely on it.
How Long Does Asphalt Last?
A well-built asphalt driveway commonly lasts 20 years or more. How long yours lasts depends on three things:
The base. Most asphalt failures start underneath, in a stone base that was too thin, poorly compacted or built on soft soil.
Water. Water that gets under the pavement softens the base. In winter it freezes, expands and pushes the asphalt apart. Around Pittsburgh, winter temperatures cross the freezing mark dozens of times a year, so drainage matters more here than in milder places.
Upkeep. Sun and air slowly dry out the binder at the surface. Filling cracks and sealing every two to three years slows that down.
When the cracks connect into a web, low spots hold water or potholes keep coming back, the base has usually given out. Our post on when to replace an asphalt driveway walks through the signs.
Thinking about new asphalt for your driveway or lot? Book a free on-site estimate and we'll look at the base and drainage first.
What Are the Pros and Cons of Asphalt?
Asphalt is the most common pavement for driveways and parking lots. It still has trade-offs:
Pros | Cons |
Usually costs less up front than concrete | Needs sealing every two to three years |
Can take traffic within a few days | Can soften and rut in very hot weather |
Flexes slightly as the ground freezes and thaws | Fades from black to gray as the sun dries the binder |
Easy to patch and resurface in sections | Usually doesn't last as long as concrete |
Dark color hides oil stains | Unsupported edges can crumble over time |
Concrete uses cement that cures rigid, while asphalt's petroleum binder stays flexible. Concrete generally lasts longer between replacements. Asphalt is easier to repair and flexes with freeze-thaw movement. That's why it's the usual pick for driveways and lots around Pittsburgh.
How Is an Asphalt Driveway Built?
A new driveway is only as good as what's under it. A typical job goes like this:
Tear out the old surface and remove soft or wet soil.
Fix drainage so water runs away from the house and off the driveway.
Spread, grade and compact a stone base.
Spray a tack coat where new asphalt meets existing pavement.
Lay hot mix asphalt with a paver and roll it tight.
Finish the edges and let the surface cool and harden before driving on it.
A well-built driveway often goes down in two layers, called lifts. The base course uses larger stone, about 3/4 to 1 inch, for strength. The surface course on top uses smaller stone, about 3/8 inch, for a smooth, tight finish. Industry guidance for driveways puts the base course at 3 to 4 inches and the surface course at 1 to 1.5 inches. Soft soil or heavy trucks call for more.
A full tear-out and repave is usually at least a two-day job, weather permitting. You can see what that work includes on our residential paving page.
Frequently Asked Questions
What is asphalt made of?
Asphalt pavement is about 95% crushed stone, sand and gravel and about 5% asphalt binder by weight. The binder is a heavy, sticky product refined from crude oil.
Is asphalt the same as blacktop?
Yes. In the U.S., blacktop is an everyday name for asphalt pavement. Engineers call the same material asphalt concrete.
Is asphalt the same as concrete?
No. Both use crushed stone, but asphalt is bound by a flexible petroleum binder and concrete is bound by cement. Asphalt flexes slightly, while concrete cures hard and rigid.
Is asphalt a petroleum product?
The binder is. It's the heavy residue left after lighter fuels are refined out of crude oil. The stone and sand that make up most of the pavement are not.
Why is asphalt black?
The binder is naturally dark brown to black. New pavement looks deep black because the binder coats every stone. Over time, sun and weather wear the binder off the surface, and the driveway fades toward gray.
How hot is asphalt when it's laid?
Hot mix asphalt usually leaves the plant at around 300 degrees Fahrenheit. It has to be spread and rolled while it's still hot so it compacts properly.
Can asphalt be laid in cold weather?
It can, within limits. Hot mix loses heat faster on cold ground, so crews watch the air temperature, the ground and the forecast before paving. Thin layers cool fastest, which is why late-season jobs need careful timing.
Why is it called asphalt?
The word traces back through Latin to the ancient Greek "asphaltos." People in the ancient world used natural asphalt as a waterproof glue and mortar long before roads were paved with it.
Is asphalt the same as tar?
No. Asphalt binder is refined from petroleum. Tar is made from coal. People often say "tar" for any black pavement, but driveways and roads in the U.S. are paved with asphalt.
How thick is an asphalt driveway?
It depends on the soil, the base and what the driveway carries. A well-built residential driveway gets a compacted stone base topped with two lifts of asphalt. B&M Paving builds driveways thicker than two inches.
Talk to a Paving Crew That Works With Asphalt Every Day
Our crews have worked with hot mix for about 50 years, out of the family shop in Latrobe. We pave driveways and parking lots in and all around Pittsburgh, plus Westmoreland, Washington, Butler, Indiana and Fayette Counties.
Weighing new asphalt, a repave or a repair? We'll come out, check the base and drainage and explain what your pavement needs. The estimate is free.
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