Asphalt Driveway Over Gravel: Why The Gravel ?


Hello,
Was just wondering about this.
It is pretty well accepted, apparently, that a new asphalt driveway for
a residence should be laid over about 4" to 6" of gravel.
Never over soil directly.
Why ?
The hot, liquid, asphalt I would pretty much think makes the gravel into
one, solid, "clump" underneath (the asphalt).
So, it's hard to see that any improved drainage results.
Increased "stability" perhaps ? If so, how ?
So what does the gravel add as a benefit over just laying the asphalt
on top of well compacted soil ?
Thanks,
Bob
Reply to
Bob
> > > > > > > Hello, > > > Was just wondering about this. > > > It is pretty well accepted, apparently, that a new asphalt driveway for > > a residence should be laid over about 4" to 6" of gravel. > > Never over soil directly. > > > Why ? > > > The hot, liquid, asphalt I would pretty much think makes the gravel int= o > > one, solid, "clump" underneath (the asphalt). > > So, it's hard to see that any improved drainage results. > > > Increased "stability" perhaps ? If so, how ? > > > So what does the gravel add as a benefit over just laying the asphalt > > on top of well compacted soil ? > > > Thanks, > > Bob > > A: asphalt pavement is not liquid > B: the gravel is for drainagek and stability.
>snip
Reply to
Joe
The liquid, sticky, or plastic part (asphalt cement) is only 5% of the total mix. The rest is aggregate.
Reply to
Frank
The folk responding about asphalt are right, so I won't duplicate their effort. However, you might also be recalling the old Macadam road technique with the modern version.
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In my home town, many of the streets were Macadam and they did very well. Occasionally, the street department would employ a tractor with wheel disk to turn up the streets, and then they'd be rolled smooth, with a thick layer of asphalt then applied. The final touch was a new coating of pea gravel to reduce tar pick-up by car tires. The streets held up beautifully and the technique most likely would be considered, "Green," in today's world.
Micajah
Reply to
Micajah
Joe wrote in news: snipped-for-privacy@q22g2000yqm.googlegroups.com:
Then there's "hardtop", which is common in my area on lightly-loaded roads: You lay down the usual packed-aggregate roadbed, then spray really runny tar on the surface. Then you spread pea-gravel on top of that, and just let passing traffic roll it in the rest of the way. Excess gravel gets pushed off to the shoulder over time. It's fairly durable if you keep heavy trucks off of it.
Reply to
Tegger
Where is your hometown? I noticed you used the word "Macadam", that is a rare word in the US. Named for a man named John McAdam. The first to use stone and roll it to lock the stones together to make better roads. Later he added oil/tar on top to help it last through heavy rains. That is were the word "Tarmac" became popular and is still used at airports. I come from a part of PA, USA where the outdated word "Macadam" was still used to describe asphalt/blacktop. If I use the word today not many people will know what it means.
Reply to
Tony
The hometown is a small farming community in central MO, but dates back well before the War of Northern Aggression. The streets there are nowadays a typical mixed bag of asphalt, concrete and Macadam, but as far as I know, the Macadam ones are still maintained by the occasional wheel disking, rolling, new tar and a coat of pea gravel. FWIW, most of the older curbs are cut granite.
Micajah

Reply to
Micajah
-snip-
I'm in upstate NY & grew up in Schenectady, Schoharie & Greene counties in the 50s-60s. I used the term last year and was amazed that none of the folks I talked to knew what it was. My wife grew up in Saratoga county- and the folks I was talking to were all from the NY city area. [and a bit younger]
I didn't think to look it up back then, but when Micajah used it I Wiki'ed it. I knew it was named for Mr. McAdam- but didn't know the specifics of *his* roads.
I've always used the term[improperly, it appears] to refer to any asphalt surface.
Jim
Reply to
Jim Elbrecht
Tegger wrote in news:Xns9DE4D0BC8D2D1tegger@208.90.168.18:
This is a terrible way to surface roads. It really plays havoc on your skates after a icestorm. DAMHIKT!
Reply to
Han
The asphalt material is slightly pliable. The surface underneath needs to be a hard compacted surface -- I think it is compacted modified stone (or something like that). That creates a solid surface for the asphalt material. The first layer of asphalt material needs to also be compacted with at least a 5,000 pound roller and is called a stabilizing layer. Then a second coat of asphalt goes on top of that and is compacted.
Reply to
RogerT
If you don't put something down first and compact it, you will have a roller coaster surface once traffic has run on it a year or so.
Steve
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Reply to
Steve B
That is how I grew up using the name. Macadam meant Asphalt. I used the word Macadam around some friends a few years ago and got strange looks and questions, that's when I looked it up.
Reply to
Tony
Huh. I'm late 50s in upstate NY and that's what EVERYone around here calls it, both younger and my age. Or, at least, I've never had any strange looks or questions. Probably because I'm in the boonies. I'll admit, though, "Red Asphalt" is a sexier title for a drivers' ed video than "Red Macadam".
Reply to
h
The enemy of a road, driveway or parking lot is water under the surface. The inevitable vertical movement of the surface, when a vehicle passes over it, creates what's called "Pumping" of the dirt under the pavement. It liquefies dirt, clay or other fine material, removing even more support for the surface layer with the inevitable cascading result of alligatoring and pot holes. Compacted and drained stone doesn't lose support for the surface when moved about by traffic and if the water can be turned away at the surface level, then the underlying dirt doesn't liquefy. Just as important as the water-turning ability of the road surface is the ability for the water that makes it through to escape. That's why there are ditches and stone underlayment. They're just as important in draining out water under the roadway as they are to carry off rainwater to prevent roadway flooding.
I know of at least one very long stretch of an Interstate highway where the designers failed to understand this. The cure was to trench deeply alongside the concrete roadway of both lanes, on both sides, install drain tile and then backfill with coarse stone to carry the undersurface water to ditches.
By the way, expansive soils are also an enemy of roads, just as they are an enemy of basements. One lady, who was an NCSU graduate engineer, didn't understand what expansive soils were and approved an entire highway interchange to be constructed using expansive soil for the ramps. It never even opened before the concrete began to crack and fail.
Micajah
Reply to
Micajah
"Micajah" wrote in news: snipped-for-privacy@giganews.com:
I didn't know that. We just call it "hardtop". That's to differentiate it from the softer gravel/sand mix which goes on roads that are lesser to hardtopped roads. The big advantage of hardtop is that it's really cheap to install compared to asphalt, but is a lot more durable (and cleaner and washboard-resistant) than gravel/sand.
Reply to
Tegger

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