Why Do Concrete Roads Need Expansion Joints While Asphalt Seems Endless?

Most of the joints in a concrete road are not expansion joints at all. They are contraction joints, sawn into the fresh slab to fix where the concrete will crack as it dries out and cools, because a curing slab shrinks more from losing water than from any summer heat. Asphalt skips them because it creeps: warm asphalt flows a tiny amount and lets the pulling force drain away, though in a hard freeze it stiffens up and cracks on its own anyway.

Drive an older stretch of interstate and your tires keep score for you. Thunk. Thunk. Thunk. A steady beat, one every couple of seconds, as the car crosses gap after gap in a concrete highway.

Then the road switches to asphalt and the noise stops. No gaps. No beat. Just black surface running to the horizon like someone poured it out of a jug.

The usual explanation goes like this. Concrete expands in the heat, so engineers leave it room. Asphalt does not expand, so it needs none. It sounds right. It is wrong twice over. Asphalt moves more per degree than concrete does, and most of those gaps have almost nothing to do with summer.

What Is Thermal Expansion, And How Much Does A Concrete Road Move?

Put your hand on a road surface in August. It is hot enough to bend light into a fake puddle up ahead. It is also bigger than it was at dawn.

Heat one end of a metal spoon and the whole spoon gets longer. Every solid does this. The atoms jiggle harder when warm, so they sit a hair further apart. A billion of those hairs add up to something you can measure. Water is the famous oddball that expands when it freezes instead. Roads play by the normal rules.

Engineers give each material a number for how much it grows per degree. The Federal Highway Administration sorts concrete by the stone in the mix. Limestone concrete grows about 6.8 millionths of its length per °C. Quartz concrete reaches 11.9.

Millionths sound like nothing. Take a standard 4.6 m (15 ft) slab of gravel concrete and cool it by 30 °C (54 °F). It shrinks by about 1.5 mm. Roughly two credit cards, stacked.

Hold on to that number. A road slab moves, and it does not move far.

Why Does Concrete Crack Instead Of Stretching?

Now stop the slab from moving. Bury it in friction, wedge it against its neighbors. It still wants to shrink, and it cannot.

That trapped wanting has a name: tensile stress. The slab is being pulled apart from the inside. Pulling is the one thing concrete is terrible at.

Under squeezing, concrete is a champion. Standard mixes take 20 to 40 MPa (3,000 to 6,000 psi) in compression. Pull on the same block and it gives up at roughly a tenth of that. It is full of microscopic flaws, and stretching opens every one into a crack.

The concrete industry's own guidance sets out the chain. When shrinkage is restrained, "internal tensile stresses develop." And concrete "is weak in tension." Push past that strength and "concrete will crack."

So a restrained slab has no choice about cracking. Only about where.

Are Those Really Expansion Joints In A Concrete Road?

Here is where the popular story falls apart. The gaps your tires are counting are mostly not expansion joints. They are contraction joints.

A contraction joint is "the most common type of joint" in a concrete pavement. It is a groove sawn into the young slab to make a weak line, so the crack forms there instead of somewhere ugly. A true expansion joint is a "special-use joint," used at bridges and structures. The FHWA warns that "the overuse of expansion joints should be avoided."

Now read what a contraction joint controls. The FHWA's own order gives the game away. "Moisture-related concrete shrinkage, thermal contraction, temperature curling and moisture warping." Heat is second.

Fresh concrete is wet, and it does not stay wet. As the water leaves, the paste pulls in on itself. That is drying shrinkage, the biggest movement a slab will ever make. The FHWA's joint-movement equation carries a term for it, right beside the temperature term. Typical values run from 200 to 800 millionths.

Run the numbers for our 4.6 m slab. Cooling it by 30 °C buys 324 millionths. Drying it out buys 450. Shrinkage wins.

How Far Apart Should Expansion Joints Be In Concrete?

If the slab will crack no matter what, the design question becomes: how far apart do you let it happen?

The FHWA's rule is a thickness rule, not a distance rule. Panel length in feet should stay under 1.5 to 2 times the slab thickness in inches. Drop the units and that is 18 to 24 times the thickness. A 200 mm (8 in) highway slab lands at 3.7 to 4.9 m (12 to 16 ft). Many states cap it at 4.6 m (15 ft), which is why the thunking has such a steady rhythm.

Panels also stay squarish, no more than 1.5 times as long as they are wide. A long skinny panel cracks across its own middle. The same logic scales down to a driveway, which is why a home slab gets cut into squares. The concrete does not know it is a driveway. It only knows it is drying.

Do not over-cut, either. Panels hand load across the rough crack faces under each groove, which grind together like interlaced fingers. That grip fades fast as the joint opens. The FHWA puts the limit at 0.03 inches (0.76 mm).

How Do You Cut A Joint Before The Concrete Cracks On Its Own?

Sawing a joint is a race, and the window is narrow at both ends.

Cut too early and the concrete is still soft. The blade tears the edge into gravel instead of leaving a groove. Cut too late and you are not deciding anything, because the slab has cracked where it liked.

The FHWA calls the gap between those failures the sawing window. The saw must arrive before the trapped stress can "exceed the concrete strength." It cannot arrive until the slab can hold a clean edge.

There is no standard test for when that window opens. The advisory notes that crews often scratch the surface with a tool and judge it by feel. A decision worth millions, made with a nail and a thumbnail.

Miss the window and the road tells on you for thirty years.

Does Asphalt Expand Less Than Concrete?

So asphalt must sit still. That would explain everything.

It does not sit still. It moves more.

Researchers measured the thermal movement of asphalt in 22 real pavement sections, run between 0 and −25 °C (32 and −13 °F). A Minnesota DOT review of low-temperature cracking collected the results. The values landed between 13.3 and 29.7 millionths of its length per °C.

Compare that with concrete's 6.8 to 11.9. On a hot afternoon, asphalt out-expands the concrete beside it by a factor of two to four.

Which makes the road stranger than it looked. The jointless material is the one that moves most.

Why Doesn't Asphalt Need Expansion Joints?

The answer is not that asphalt stays put. It is that asphalt gives up.

Asphalt mixtures are viscoelastic: part springy solid, part slow-moving liquid. Squeeze one and it pushes back like rubber. Lean on it for an hour and it oozes. Researchers describe asphalt as showing "both viscous (fluid) and elastic (solid) behavior". How it acts depends on the temperature, and on how long the load sits there.

That oozing is the trick. A warm asphalt road cools, and the pulling force starts to build. Then the binder creeps a hair, and the force drains out before it nears breaking point. Engineers call this stress relaxation. You can call it flinching.

A 2024 study of asphalt under cold waves put a temperature on it. Above 5 °C (41 °F), the mixtures "exhibited excellent relaxation capabilities, resulting in no thermal stress."

None. Not a little. The road shrank all summer and never built a force worth counting. The concrete slab next door spent that summer loading itself toward a crack.

Does Asphalt Ever Crack From The Cold?

Asphalt's trick comes with an expiry temperature.

Cool the binder far enough and the molecules stop sliding. The same 2024 study found that below about −10 °C (14 °F), thermal stress starts to "accelerate significantly." Relaxation cannot keep pace. The force builds the way it does in concrete, and asphalt starts acting like the brittle material it was never meant to be.

How big does that force get? A 2024 review of cold-region cracking has a number. On the Tibetan Plateau, a 15 °C drop in one hour drove the thermal stress in the pavement to 3.33 MPa. That is a real pull, inside a material that has run out of ways to shed it.

The Minnesota DOT review is blunt. Low-temperature cracking is "the main distress in asphalt pavements built in northern U.S. and Canada." It shows up as "a set of parallel surface-initiated transverse cracks," repeating across the lane.

So in a cold enough winter, asphalt builds its own joints. It does a worse job of it, in a worse place, with no sealant. Water gets in and weakens the base underneath, the same slow water damage that wrecks a building nobody maintains.

This is why binders get graded for cold. Labs bend a beam of bitumen in a bending beam rheometer and check two things. Has it gone too stiff, and can it still relax? One study put a common 70/100 bitumen at −17 °C (1 °F) for stiffness and −15 °C (5 °F) for relaxation. Below that, the road is on its own.

Do You Need An Expansion Joint Between Concrete And Asphalt?

Yes, and the FHWA has a drawing for it.

Where concrete meets asphalt, the advisory calls for a specialty joint. Its whole job is "transitions between concrete and asphalt pavements." Crews often thicken the concrete side. Neither material carries the other's load across the seam. Others taper the concrete and run the asphalt over it, to stop the hump that grows there.

True expansion joints turn up where movement has somewhere expensive to go. The FHWA lists them for "bridge decks and approach panels." A jammed joint there would drive a squeezing force into a structure with its own load paths to worry about.

That failure has a name: a blowup. Grit works into a joint over the years and stops it closing. The slab warms, tries to expand, finds the gap full, and buckles up out of the road. The joint full of dirt is the joint that fails.

Railways hit the same problem and answer it differently, by floating the track on beds of loose stone that shift instead of cracking.

Does A Concrete Road Last Longer Than Asphalt?

Concrete is the stiffer, longer-lived surface. Its joints are the price of admission, and its weak point. The FHWA notes that many concrete pavement problems "either develop at the joints" or come from poor joint design. The list runs long: faulting, pumping, spalling, corner breaks, blowups, mid-panel cracking. A concrete road is a chain of good slabs held together by its worst parts, which are also the parts you can hear.

Asphalt trades that away. It goes down fast, runs quiet, has nothing to seal, and can be resurfaced a lane at a time. In exchange it ruts under trucks in summer and cracks in deep winter. Most of the time that is the better deal. The same review puts asphalt at 92 to 94% of all pavement in the United States.

Engineers have also built a concrete road that dodges joints altogether. Continuously reinforced concrete pavement runs steel down the whole slab, then lets it crack where it likes. The FHWA reports that it "has no active transverse joints, except at its ends." The steel holds those cracks under 0.5 mm (0.02 in), spaced every 0.6 to 1.8 m (2 to 6 ft).

Read that again. The jointless concrete road cracks about ten times more often than the jointed one. It holds them shut so well that you never notice.

So, Why Do Concrete Roads Need Joints While Asphalt Doesn't?

Strip away the folk explanation and the real difference is not about expanding. Both materials expand. Asphalt expands more.

The difference is what each does with a force it cannot escape. Concrete stores it. Asphalt sheds it. Stop a slab from shrinking and it holds the whole pulling force, right up until that force passes its limit. Then it cracks, because cracking is the only move it has. Warm asphalt under the same sky creeps a fraction of a millimeter and lets the force drain away.

Concrete also carries a second load asphalt never signed up for. It arrives full of water and spends its first months getting smaller. That drying shrinkage outweighs a 30 °C swing. Which is why the gaps are contraction joints, not expansion joints. Those slabs are not making room to grow. They are being handed permission to shrink.

So the thunk under your tires is not a road coping with summer. It is the sound of a decision. Somebody worked out in advance where every crack in that highway would go, then went out with a saw and put them there. The rhythm is the rhythm of being right.

The asphalt that follows is quiet for the opposite reason. It has the same problem. It is just flowing out from under it, a few millionths at a time, every day, for as long as the weather stays warm.