What is under a road that lasts
Nearly every pavement that fails early fails for the same reason: there was not enough rock under it, or the dirt under the rock was never compacted. The asphalt gets blamed because it is the part you can see. Here is what each layer is actually doing.
A public street a city will accept for maintenance. Built to the standard plan, inspected at every lift.
Typical R-value design for a residential street. The soils report sets the real numbers.
Feeds it8″ of base over 1,000 sq ft is about 48 tons, or 2 transfer loads. Our trucks, our rock.
Three layers, three jobs
Subgrade is the native ground, shaped and compacted. It carries the load. Everything above it exists to spread a wheel out over enough subgrade that the subgrade can take it.
Aggregate base is angular crushed rock, usually Class 2 in California, placed in lifts and compacted until the stones lock into each other. It is the structural layer. It takes a concentrated wheel load and spreads it into a wide, gentle pressure on the subgrade.
Asphalt is the wearing course. It is waterproof, it is smooth, and it holds the surface together. What it is not is a structural substitute for base rock: an inch of asphalt spreads load about as well as two inches of base, and it costs several times as much.
Where the numbers come from
A designed pavement section comes out of two inputs. The R-value is a laboratory measure of how strong the subgrade soil is, taken from a soils report. Valley clay is weak, often an R-value in the single digits or teens; sandy or gravelly ground is much stronger. The traffic index is a measure of how many heavy axles the road will see over its design life, and it is dominated by trucks, not cars.
Weak soil or heavy trucks means a thicker section. That is the whole design in one sentence, and it is why a rural road with school buses and nut trucks on it needs more section than a suburban street with two hundred cars a day.
- A residential driveway: about 2 inches of asphalt over 4 to 6 of base
- A subdivision street: about 3 inches over 8
- A county road rebuild: 4 to 5 inches over 10, often with fabric
- A truck yard or shop apron: 4 to 5 inches over 12, with concrete where wheels turn in place
- An all-weather ranch road: 6 inches of base, crowned, no pavement at all
The two failures you will actually see
Alligator cracking in the wheel path is a structural failure. The section is flexing more than it can stand, either because the base is too thin or because the subgrade under it is wet or was never compacted. Paving over it puts a new surface on a road that is still moving, and the pattern comes back through in a season or two.
Cracking that is not in the wheel path, the long straight ones and the block pattern, is age. The binder has oxidized and shrunk. That road is structurally fine and is a candidate for a seal or an overlay, which is the cheap outcome.
Telling the two apart before you spend money is worth a core and an afternoon.
Water, again
Every road failure has water in it somewhere. Water gets into the base through a crack or an unsealed joint, saturates it, and the base loses the friction between the stones that made it structural in the first place. Then a truck goes over it.
That is why grade, cross slope, and ditches are not landscaping. A road that sheds water into a functioning ditch outlives an identical road that ponds, by years.
Questions we get
Structurally you can trade about two inches of base for one inch of asphalt, and financially it is a bad trade almost every time: asphalt is several times the price per inch. Rock is the cheap way to buy strength.
Stabilization fabric sits between the subgrade and the base and stops the two from mixing. On soft ground it keeps the base rock from pumping down into the mud, which preserves the thickness you paid for. On a bad subgrade it saves more in rock than it costs.
A properly designed and built section, twenty years to a rehabilitation, with a seal every seven to ten years to keep the binder from drying out. An underbuilt one, five to eight, and it fails from the bottom, so the repair is a rebuild rather than a resurface.
For a truck yard where wheels turn while stopped, at a dock or a gate or a dumpster pad, yes, and it is worth the cost for those spots specifically. For everything else in this valley, asphalt at a proper thickness is the better value and is far cheaper to repair.
Photos are stand-ins until our own job photos are up: public domain images from the U.S. Army Corps of Engineers and the National Park Service, plus freely licensed photos from Wikimedia Commons (CC0 and CC BY-SA 4.0).
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- (530) 555-0142
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- bids@northstateconstruction.com
- Where we work
- Butte, Glenn, Tehama, Colusa, Shasta, Sutter, and Yuba counties
- Yard at 4180 Midway, Chico, CA 95928
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