Asphalt doesn't fail for one reason. Rough, gravelly surfaces, fatigue cracks, potholes and gray, brittle pavement each point to a different cause, and the right repair depends on finding it. These are the five we see most often.
1. Material failure
If stripping or raveling occurs, substandard material could be the cause. If the asphalt mix doesn't have the proper ratio of large aggregate to fines, or the asphalt binder isn't adhering to the aggregate, the larger aggregate will strip away or unravel, and the asphalt will fail. An indicator of this type of failure is a very rough, gravelly surface.
2. Overloading
Unlike concrete, which is made for its rigid strength, asphalt pavement is made to flex with movement. On a hot day asphalt can actually soften enough to take a shoe print. The sub-grade and base material, which are higher in compressive strength, are what actually support the weight of the loads above. Depending on the purpose of the pavement, the asphalt layer and the base layer need the proper thickness and compaction to support the loads they will bear.
The asphalt layer also bears load. At the proper thickness, it distributes the weight of a load to the base layer beneath. If the asphalt is too thin, it can't distribute the weight, which puts too much compressive stress at the load's point of contact. That causes fatigue cracks beneath the surface, which eventually migrate upward and weaken the surrounding asphalt.
3. Base failure
Because asphalt lacks the compressive strength of concrete, the sub-grade and base layers make up the difference, so their stability has to stay intact. Water is the number one culprit in compromising it.
In cold climates, the freeze-thaw cycle makes the pavement heave and split. Southern California doesn't have that problem. Our problem is the opposite: heavy sun exposure burns away the asphalt binder, creating deep cracks that let rain and run-off penetrate the sub-grade. California's seismic activity can crack pavement too.
Other causes work from the ground up: tree roots, leaky water mains, punctured plumbing and faulty irrigation. In each case, the source of the problem must be repaired, or the failures will continue. Broken pipes and leaks must be fixed before new asphalt is applied. Tree roots must be cut and removed at least 12 inches deep. If dead roots and other vegetation stay in the sub-grade, they rot and leave voids that compromise its stability.
4. Environmental: UV, oxidation and pollutants
Oxidation is happening all around us, all the time. It's what makes metal rust, a cut apple turn brown, and asphalt turn gray and brittle. When oxygen contacts the binder in your asphalt, a reaction breaks the surface molecules away. Ultraviolet rays and pollution also contribute to pavement deterioration.
There's no cure for oxidation, but you can slow it and prolong the life of your pavement:
- Sealcoat acts as a protective layer, like the skin of an apple protecting the fruit.
- Skin patching repairs areas that deteriorate faster, such as spots with high pollution, direct sunlight and low traffic, without unnecessary work on the rest of the pavement.
- Crack filling keeps environmental elements from reaching the inner layers of the pavement through the walls of the cracks.
5. Water damage
Water is a powerful force. Picture the Grand Canyon. On a much smaller scale, its effects show up on roads and parking lots: overspray from watering systems, leaky sprinklers and inadequate run-off repeatedly batter pavement, causing potholes, puddling, cracking, water stains and eventually sub-grade failure. Any time water penetrates to the sub-grade, the maintenance needed increases dramatically.
Watering systems should be checked regularly for overspray and leaks. Reducing overspray onto pavement conserves water and preserves the pavement. The best prevention is a properly designed run-off system: proper sloping and flowlines let water run off before it causes damage, and a good run-off system can save you a lot in future maintenance.
