What causes transverse cracks in asphalt?

Transverse cracks run across the pavement, perpendicular to traffic. They are thermal. Asphalt shrinks as it cools, and when it cannot shrink any further it splits across its width. Boise Air Terminal records about 82 nights a year that drop below freezing and warm back above it, which is what drives them.

Why do transverse cracks form?

Asphalt contracts when it gets cold. It is a flexible material with a binder that stiffens as temperature drops, and a large surface that cools overnight wants to pull itself inward from every direction. The surface is fixed at its edges and bonded to the ground, so it cannot actually move. The stress builds until the pavement relieves it by splitting, and the split runs across the shorter dimension.

That is why transverse cracks tend to appear at fairly regular intervals down a driveway or a drive aisle. Each one relieves the stress in the stretch around it.

It is also why they show up on pavement that barely gets used. A driveway that sees two cars a day still goes through every temperature swing the weather delivers.

How does the Treasure Valley climate drive transverse cracking?

Through the number of cycles rather than the depth of the cold. This is not a place with brutal winters. It is a place with a very large number of nights that drop below freezing and days that climb back above it.

Boise Air Terminal records 100.9 nights a year below freezing, and working through the normals gives about 82 nights that freeze and then thaw again. Nampa is harsher, with roughly 130 freezing nights and a mean daily high that never falls below freezing in any month, which means nearly every one of those nights is followed by a thaw.

Each cycle is one contraction and one expansion. Eighty two of them a year, year after year, is the mechanism.

One note on local data. Meridian has no active weather station, so any normal published for Meridian comes from Boise Air Terminal, about ten miles east. We would rather say that than publish a number we made up for the city we work in.

What makes transverse cracks worse over winter?

Water, arriving in the same months as the cold. At Boise Air Terminal 55.6 percent of annual precipitation falls between November and March, about 6.52 inches across those five months. July and August together deliver 0.57 inches. So the cracks are at their widest exactly when there is the most water around to fill them.

So the cracks open widest in the months when there is the most water available to fill them. Water goes in, freezes, expands, and levers the crack a little wider. By spring the crack that was hairline in October takes a coin.

That timing is the argument for sealing cracks before winter rather than after it. Gideon Duarte, Co-Owner of Select Coatings, recommends fall work for the same reason, because temperatures are moderate and there is less rain than in spring.

When is it too cold to repair a transverse crack?

When the material cannot cure. Crack sealant and sealcoat both need the surface to stay warm enough for long enough to bond, and a cold night partway through curing undoes the work. Our working threshold is above about 40°F for roughly 95 percent of the day, with around 80°F being ideal.

The transition months are the ones to watch. October averages 3.3 freezing nights at Boise Air Terminal and April averages 4.5. A repair made on a mild afternoon in one of those months can freeze before it has fully cured overnight, and it will not bond properly.

How are transverse cracks repaired?

The same way as any repairable crack. The type, width and depth get assessed, the inside of the crack is cleaned out so the sealant bonds to pavement rather than to debris, and hot pour sealant is worked in until the void is full and the seal is watertight.

Then the surface gets sealcoated, once the filler has cured. Crack filling and sealcoating together is the combination that keeps new cracks from opening. Sealing without filling does not close the cracks that are already there, and it traps moisture underneath them.

Transverse cracks in particular will reappear in roughly the same places, because the thermal stress that created them has not gone anywhere. That is normal and it is not a failed repair. What the filler does is keep water out of the base during the years in between.