Pouring Concrete over blacktop

[Steve-Maine]

I thinking of adding an addition to my shop. It will be on a blacktopped area. Can you pour a concrete slab over blacktop?
What would be min. thickness of the concrete? The blacktop area has a base of about 18" of gravel Thanks

 

[RobMD]

How old is the blacktopped area? Why not use the blacktopped area for storage space and use the concrete part of your shop for the work?

Of course you can pour concrete directly over it. I have asphalt millings under my concrete slab anyway. Your asphalt area is probably comapcted pretty darn good! I would not go any less than 3.5 inches, and just use wire mesh in your concrete.

EIther way you go, your base is already there, no problem WHATSOEVER with pouring concrete over asphalt.

 

[Jeff in TX]

The main problem is the concrete will not stick to the blacktop. Make sure you anchor the rebar to the blacktop very well. If not given the right conditions the concrete will move some. Down here in TX, the interstates are poured on top of asphalt. Makes for a very good base for the concrete if joined together right.

 

[Sid]

Don"t they make sealers that you can treat the black top with and use it.

 

[Billy NY]

If the sub base is stable (18" of gravel) and well compacted, drained, should not be a problem. This is a cold joint I suppose you could dowel in and tie your slab reinforcing in to those dowels, but they probably won't do much being in asphalt paving. Being both materials are rigid, hard to say how each would expand or contract differently when temperatures change, not sure if there would be any reactions to worry about. Might be possible to use an epoxy to bond them together like what is used when placing fresh concrete onto existing concrete. I would think 4" minimum, and you want to prevent water from getting between the layers if you are in a cold climate, that could cause some problems freeze/thaw. Sounds like a no brainer, with some minor details to consider.

 

[Puddles]

You might consider pouring a monolithic slab on grade, to keep the concrete from moving. Or a few keyway / shear keys dug into the asphalt slab.

 

[Phil in Pa]

Rob is correct. Relatively flat blacktop is an ideal sub base for a concrete slab.
I personally would not want a bond. We prepare our sub bases so the slab can perform as a "slipsheet" on top of the base.

General thickness rules I go by are
4" homeowner stuff
5" heavy homeowner/shop
6" and up farm / industrial

 

[rockyhawaii]

Wouldn't adding four inches of concrete over the blacktop cause clearance problems?

 

[PJH]

The state of Illinois has built roads on a 4" asphalt sub-base for many years. The soil under the asphalt is well compacted, and the concrete on top of the asphalt is 10 inches thick. The asphalt mix used is porous to allow moisture to drain out from under the concrete. It's designed for 20 years of no maintenance under heavy traffic. There is no intended bond between the asphalt and the concrete.

After saying all that, it's hard to tell what will work in your particular situation without looking at it.

Some things to consider - if you have any cracks in your existing asphalt, you can expect them to project up through your new concrete. Wire mesh in the new concrete might prevent it, but sometimes it doesn't. The thinner the concrete, the more risk of crack projection.

As the other poster mentioned, head clearance might be an issue, but you'll know that when you set your grade.

Personally, I don't like floor concrete less than 5 inches thick for light loads like a car or pickup.

I think I'd act like the asphalt wasn't there, and pour five inches on top of it, assuming it isn't all broken up already, and assuming it's for light loads.

Pretty easy for me to say when I'm talking about your money, and not mine.

I worked on an experimental project where we poured 3 1/2 inches of concrete on top of an existing asphalt roadway. It was designed to prevent wheel rutting in the roadway which is so common in asphalt surfaced intersections. The odd thing that we did was: we had to saw cut the new concrete into approximately 3 1/2 foot squares both directions, and it had to be sawed the same day it was poured. This was to control the cracking that was expected to be projected from the lower asphalt surface. I thought it would rattle when a truck drove over it, but it actually worked as it was intended.

Sorry for the LONG post.

Good luck with your project,
Paul