Why do I see entire parking lots get a coat of sealant, but never really see streets or highways get a large coat of sealant?
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For example: [https://youtu.be/Lmd1r0P0GWE?t=192](https://youtu.be/Lmd1r0P0GWE?t=192)
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It’s because it would cost too much and the value that the coating gives is miniscule at best even for a driveway or parking lot.
The only real benefit that asphalt coatings provide is a crisp, uniform, black look. This look will only last a year or two under light use as the UV in sunlight breaks it down and the top aggregate is exposed again. Under heavy use the look would last weeks at best before the preferred grooves are worn down again.
If an asphalt surface is never sealed it will UV degrade only a finite amount before it goes steady state. The seal coat doesn’t prevent cracks and cracks are what destroy the surface which is why you’ll see the road services putting a tar/rubber caulking into the cracks on the roadway if it’s in a climate zone that experiences frequent freeze/thaw cycles.
There are a couple of reasons. Regular roads have better drainage, both by design and because cars go over it regularly at speed which tends to move the water off of them. A parking lot will potentially have a lot of standing water. Plus, regular roads are maintained by cities and they have a schedule for a grind up and re-asphalt, a parking lot will potentially not have the same level of attention and a higher bar to have the lot chewed up and re-asphalted.
I’ve heard that the only real advantage of coating a driveway is that the darker surface will absorb more energy from the sun and melt off snow easier.
Is that not what a Chip and Seal is? We do those all the time in Northern Ohio
They literally did this very thing to all the streets in my neighborhood just a few weeks ago. So at least some streets get it done.
Because the heavy wear and traffic volumes will wear the sealant away quickly.
My city put a seal coat on a major roadway and within 6 weeks it had noticeable wear patterns under the vehicle tires. Within 4 months, it had worn thru to the underlying base road and looked horrible, plus it had all the original cracks, potholes and defects of the original road.
Seal coats are mostly liquid asphalt emulsion, so it does a good job of making the existing pavement smooth, uniform in color, and extends the life of the pavement by a few years. The problem is that most seal coats do not offer much grip. You can add some fine sand to the mix, but it doesn’t provide the level of skid resistance that you need to give vehicles traction when driving in wet conditions. Seal coats being all liquid also do not last very long, and depending on the amount of traffic using the street might only last a year or two before it would need to be applied again. Parking lots get the seal coat treatment because of the lower speeds and smooth, dark surface that a seal coat provides.
For streets, cities and counties can go with a slurry seal or chip seal treatment, which is asphalt emulsion, water, and small aggregate. This provides a rougher surface (increased skid resistance) and a more durable surface. These treatments can last over 5 years before they have to be reapplied. Slurry seal takes a few hours to try and can be inconvenient to the people who live or work on the street that is getting treated, so the longer life span means they won’t be inconvenienced frequently.
An additional reason not yet mentioned is intended use of a street vs a parking lot. Tires moving at high speed will peel the seal coat off the asphalt. That’s why most seal coat applications are done in parking lots or low speed limit streets. 30mph and up will damage the seal coat.
I need an ELI5: Why does my town throw down that loose gravel stuff which results in us having to drive 10 mph over it for a few weeks until it flattens out into new “pavement”? They do it here every few years and I swear the roads don’t benefit from it. They remain rough and do not feel like driving on fresh tar at all.
edit: fwiw, I live in a small beach town with higher than average taxes. We aren’t poor but seem to have the poor mans way to repave our small town roads. Please correct me if I am wrong.
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