Why Does Asphalt Crack?

Asphalt cracks for several reasons: thermal expansion and contraction, water intrusion, traffic loading, oxidation from UV exposure, base failure, poor construction, and vegetation. Every crack tells a story about what is happening to the pavement. Understanding the type of crack helps you determine whether it is a surface issue that crack sealing can address or a structural problem that requires deeper repair.

Thermal contraction

Asphalt expands in heat and contracts in cold. In Middle Tennessee, temperature swings between summer and winter can exceed 60 degrees. When the pavement contracts in cold weather, it pulls apart, and cracks form. These are called thermal cracks, and they typically run across the pavement (transverse) or along the length of the paving lane (longitudinal).

Thermal cracks are normal and expected. Every asphalt surface will develop them. The goal is not to prevent them but to seal them promptly so water cannot enter and cause more serious damage.

Water intrusion and freeze-thaw

Water is the most destructive force acting on asphalt. When water enters a crack, it reaches the base layer and softens it. In Middle Tennessee's freeze-thaw cycles, water inside a crack expands when it freezes, forcing the crack wider. A crack that was barely visible in October can be a quarter inch wide by March.

This is why crack sealing is the most important preventive maintenance you can do. Sealing cracks stops water from reaching the base. See our crack sealing guide for the full process.

Traffic loading and fatigue

Every time a vehicle drives over asphalt, the pavement flexes slightly. Over thousands of load cycles, this flexing fatigues the asphalt. The result is fatigue cracking, also called alligator cracking, which looks like the interconnected scales of a reptile. Fatigue cracking means the pavement is failing under load, which usually indicates the base is inadequate for the traffic it is carrying.

Fatigue cracking is not a crack-sealing problem. It is a structural problem that requires full-depth repair. See our alligator cracking guide for more.

Oxidation and aging

UV rays from the sun break down the asphalt binder over time. This process, called oxidation, turns the pavement from black to gray and makes the binder brittle. Brittle asphalt is less flexible and more prone to cracking under thermal stress and traffic loading.

Sealcoating slows oxidation by blocking UV rays. This is one of the primary reasons to sealcoat every two to three years. See our sealcoating guide for more.

Base failure

When the base layer beneath the asphalt fails, the surface above it cracks and settles. Base failure can be caused by:

• Water saturation: The base has been saturated through open cracks and has lost load-bearing capacity.
• Inadequate compaction: The base was not properly compacted during construction, leading to settlement.
• Insufficient base thickness: The base is not thick enough for the traffic loads it is carrying.
• Poor drainage: Water is not draining away from the pavement and is undermining the base.

Cracks caused by base failure tend to be wide, deep, and accompanied by settlement or alligator cracking. These areas need full-depth repair, not crack sealing.

Poor construction

Some cracks are the result of poor construction practices:

• Inadequate compaction: If the base or asphalt was not properly compacted, the pavement will settle and crack.
• Thin pavement: Asphalt that is too thin for the traffic load will fatigue and crack prematurely.
• Poor drainage design: If water is not directed away from the pavement, it will undermine the base.
• Cold joints: Joints between paving lanes that were not properly bonded will separate over time.

These are construction defects, not maintenance issues. If your pavement is cracking because it was poorly built, no amount of sealcoating or crack sealing will fix the underlying problem. The pavement will need to be evaluated for overlay or replacement.

Tree roots and vegetation

Tree roots growing under pavement can heave and crack the asphalt. This is common along the edges of parking lots where trees are planted close to the pavement. If you see cracks that follow a curved or wandering path along the edge of the lot near trees, root intrusion may be the cause.

Vegetation growing through cracks is also a sign that the crack is wide and deep enough for seeds and water to enter. Grass and weeds in cracks indicate that the crack has been open for a long time and the base may be compromised.

Types of cracks and what they mean

What to do about cracks

The right response depends on the type and severity of the crack:

• Sealable cracks (longitudinal, transverse, block, edge): Clean and seal with hot rubberized crack sealant. See our crack sealing guide.
• Alligator cracking: Do not seal. The base has failed. The area needs full-depth saw-cut repair. See our alligator cracking guide.
• Reflective cracking: Seal to slow water intrusion, but expect the cracks to return as the underlying movement continues.
• Wide, deep cracks with vegetation: The base may be compromised. Have a professional assess whether full-depth repair is needed.