Asphalt class, lift thickness and use of prime and tack coats

[boathead]

I'm finalizing asphalt pavement design for site in Connecticut that will be used for storage of dumpsters - so there is pretty heavy vehicle loading as well as impact loads from dumpsters being dropped. I have geotech recommendations for 10" subbase and 2 courses of asphalt to equal 5". My question is, what class asphalt to use for the two courses, what lift thickness to use, and whether to use prime and tack coats.

I've heard that lifts should not exceed 2", but I see other guidance that suggests otherwise.

Thanks much!

 

[Ron]

I would not use asphalt for a dumpster pad. Too much impact, gouging from dragging the dumpster and shoving from the truck slamming on brakes ahead of the dumpster.

Use concrete.

 

[CarlB]

I agree concrete would hold up better, but I see asphalt used all over and it usually performs adequately.

Whether 5" pavement & 10" subbase is enough depends on the subgrade. Due to frost concerns in our locale, subbase is usually thicker (12" to 48")and asphalt thinner, often 2" asphalt & 2" base course. So 5" sounds like a lot to me. There have been discussions here whether prime coat & tack coat are worthwhile, I think not worth it unless there are delays & exposure to weather, etc during construction.

 

[boathead]

Thank you both for the feedback. I agree that concrete would be better from an impact and gouging standpoint. But given the area involved (over 1 acre) and the planned use for long-term storage of EMPTY containers (not regular use of full dumpsters), we're thinking that asphalt will be adequate and less costly, and the client agrees. Sorry I was not more specific on the use.

The 10" and 5" were my geotech's recommendations based on the pretty compact subgrade and heavy duty use of the area, and I gotta go with them. Any concerns about using a 3" coarse asphalt lift as base (compaction issues?) and a 2" surface asphalt lift?

There does seem to be an ongoing debate about tack coats. From my former life as a boat builder, when I always pre-wet materials to be bonded, I'm leaning toward using the tack coat to fill the surface of the substrate and lock the new material to it.

Thanks again.

 

[oldestguy]

Two courses make it easier to do the job and end up with a "straighter" surface.

For the type of mix, check with your state DOT for what they would use for heavy traffic to avoid rutting. Then, you need someone to test that you get that mix and the density the state would require. Typically a materials testing firm would do the job of the inspection.

Density of asphalt mixes makes a heck of a difference in how they behave.

On the design thicknesses, I too think it is over designed.

 

[Ron]

A 3-inch coarse mix should be OK as the first lift of asphalt. Shouldn't have a compaction problem as long as the laydown temperature is maintained at a reasonable level.

As for tack coat, I would recommend it. You will have a lot of twisting, stopping, turning movements on this pavement, so good bond with the base is essential to keeping the shoving problem in check.

 

[boathead]

Excellent insights. Thanks to all!

Regards
Bill

 

[jgailla]

I hate to disagree with Ron, who knows way more than me about pavement, but if I had a 5" pavement, I'd specify three lifts (2", 1.5", 1.5"), even if a coarse mix is used.

If the contractor has any trouble getting compaction, they'll point the finger at you for specifying the lifts too thick.

Tack coat should be used for the reasons Ron said. Prime coat is generally unnecessary unless the base course has been sitting for a while.

 

[BigH]

I'd add cement by pug mill to the 10 inches of granular to give a cement treated agg base. Say 4% by weight. I would trust that you have a "decent" subgrade. Depending on need for "smoothness" you could top with a chip seal - or a layer of asphalt. I'd suggest 2 inches or so due to the improved agg layer is all that would be needed. Likely much cheaper than 5 inches of asphalt overall.

 

[Ron]

jgailla...you're right; however, most DOT's will allow 3-3 1/2 inch thicknesses for course mixes (nominal coarse aggregate size of 3/4" (19mm)). Since most pavements are placed by contractors who also do DOT work, it's tough to argue against an accepted DOT specification. The key is, as I previously noted, the laydown temperature. If they screw that up, they'll have a tough time getting compaction in any of the lifts.

Using 3 lifts instead of 2 lifts also increases the cost of the laydown by 50%. Owners tend to respond to that sort of stuff and will overrule you based on the costs. Since the contractor can point to an established "standard" (that being the DOT), you have little basis to refute the process.