I am looking at a large building with an asphalt floor, it is in excellent cond. , EXCEPT it's too rough for what I intend to use it for.

It has been used to store large boats , RV's , dump trucks, ect. It is VERY solid.

If I were to pour a cement cap over it, would 2" be sufficient.

It will not have any heavy equipment on it (probably not more weight than a pickup) .

It will have ALOT of light wt. traffic , about 800 -1000# vehicle

I would rather not tear up the solid asphalt base if not necessary.

Give me so reasons why/why not .

The knowledge here is both  amazingly good and shockingly bad . Just hope I can determine which is which on this one

Link?

I will start with the bad....





You probably want to take up the asphalt first.



Also there are various coatings that you can put on top of the asphalt that may be the best of all worlds.

3.5" min.

Ok , why , would the asphalt base be too solid?

What are these coatings?  It would need to be "filled" as much as 1.5" in some places if you put a 20 'straight edge on it,the asphalt is fairly smooth just not as "flat" as I would need it . Basically kinda wavey

i've never heard of putting concrete over asphalt, but i don't know why you couldn't,especially given the parameters you've stated. that being said, i'd pour 6"
with rebar,with minimum slump, and be prepared to watch it set forever before you could tool it. 2" is a nogo on any install,unless you're a big fan of the crack, and 3.5" is asking for trouble as well.
if height is an important consideration, you might  think about putting a couple of inches of new asphalt down to smooth it out.

I know you want to saw money, but, and its a big one, you can't pour over asphalt. DONT DO IT!!!!!!!!

If you want to keep it, pave 2" ROLLED (most guys lay down 2" with the box and with the compaction of top you end up with 1 1/4 -1 1/2) 2" is nice thickness for top. If its in budget mill 1/2 or remove top layer if has stabilized base. And lay down your top.

Asphalt is soft. Trust me. It will cause your concrete floor to crack like crazy. You have to remove the asphalt and assuming there is 3/4- 1 1/2clean stone, qp or recycled concrete under it pour 4" over that. Minimum for concrete floor is 4"of 5000psi concrete over 4" compacted base. Personally for vehicle traffic I would want 6" of concrete with #6 bar or wire mesh over 4" of recycled concrete(similar to QP)  

Spend once cry once. Ive seen alot of dumb stuff done to save money. Every year it gets worse and I have to do things that previously would be frowned upon. Do it right once.

I wouldn't put concrete on an asphalt base, mainly because I would question the compaction of whats below the asphalt. However there are companies that do this type of work especially resurfacing runways and road surfaces. It's not something I'd trust to your average concrete crew. Either way it's not gonna be cheap.

So true.

First of all the people claiming you can't do it are wrong.  It can be done, its called "white topping".





I've never designed such a pavement, because we don't use them up here (could be a clue as to cost effectiveness).  I've never really read much about them but let me go look through some of my pavement design textbooks and see if I can come up with anything.





As far as thickness I wouldn't hazard a guess without running any numbers.





And no, the asphalt is not "too hard" to place concrete on.  The harder the subgrade the better it is for concrete.

 

my thoughts for recommending 6" minimum. at what  thickness point does the slab become monolithic? once the asphalt is compressed over a base,it shouldn't compress further,i'd think. it would help to know why the asphalt is uneven in the first place, whether it's due to displacement by vehicular traffic,or due to an improper base. the 4" you recommend i think is insufficient. from what i understand, cracks are usually caused by excessive moisture/ insufficient thickness.perhaps,given that it is to be inside a building, a higher pozzolan ratio may be called for to give greater workability? OP, i think you should go for it; think of the knowledge you could share with us!

Remove the asphalt.  There should be a crushed gravel base under it, this will serve as a good substrate if it is at least 2" thick.  If not, add crushed gravel (recycled concrete).  Compact gravel with a plate compacter which you can rent.  Pour a minimum of 4" of concrete with wire or rebar in it.  It will last for years.

Gaaaah...this is frustrating.  I thought for sure I remember seeing an example of designing a white topped pavement in one of my text books but I cannot find it.
So, I resorted to a Google search.





How thick is the existing asphalt?


Is it in good condition? (IE is there cracking or raveling?)





If I were you, I'd find a company in your area that does white topping and ask them for their opinion.  Tell them up front the dimensions of your warehouse so they know you just aren't Joe Blow with a 30'x10' driveway that probably won't be worth their time or yours for them to do.  Have them give you an estimate.  





I'd go in telling them the square footage, the existing asphalt thickness, and the clearances in your building (IE how big the door is, how tall the ceiling is, etc.)  It probably would be a good idea to give them a sketch of the building and take some pictures.





The worst things that could happen are:


1) They say they can't do it.


2) They give you a price that is higher close to or higher than what it would be to do full depth replacement.





ETA::I just realized you a LOOKING at this building...not that you already own it.
 

If the asphalt is good, why get rid of it?



 

First , Thanks for the replies , they give me a good base(no pun intended)to work from

I really don't know why it's so uneven except to say it was laid that way ,kinda like MODOT does our highways , dump a load , then grade it out with a road grader, then roll it.

There are NO CRACKS from being displaced by vehicle traffic , no cracks of any kind I can see anywhere.

As far as I know it was laid about 20 - 25 years ago , building was used for trade shows for a while , they do regularlly use a telehandler(about 20,000# )inside it.

Wouldn't the asphalt be MORE solid than a gravel base?

Around here , MO. about 95% of home driveways are 4" (2x4 for form) so 3.5" they seem to holdup fine to moderate vehicle traffic

A lot of variables in play.

I'm a building contractor and I don't like to put new over old on any job. Just the way i came up through the trade.

If you remove the asphalt you can make sure the base is correct, that the reinforcing steel is correct and most importantly, that any waterproofing can be addressed and installed correctly.

Bonus, you can add any plumbing at that time for any future additions, shower, floor drains, etc.

Yes it would.





Couple more thoughts:



If they have been running heavy vehicles on it for 20 years and there are no cracks, you can bet your ass that the asphalt is thick.



Ripping that out and replacing it with aggregate and concrete (as others are suggesting) will not be cheap.



Because of this id guess it would be cost effective to white top it instead.



Again...try to find someone who does whitetopping in the area (maybe call MODOT?)...I WOULD NOT just get some concrete and go to town placing it.



Thickness is going to depend on how the concrete is bonded to the underlying pavement.  



As for the blue...thats a pavement engineer's nighmare!  My god...the segregation!





 

I would NOT lay 2" of concrete - you don't want to do anything less than 3.5" generally. You can lay concrete directly on top of the asphalt, but the quality of the finished product with greatly depend on how well that asphalt was laid. It might be good to bring someone in to take a look at it. You might as well spend a little extra now and have it done right.

here's a little treatise on the subject of whitetopping:

http://www.dot.state.il.us/materials/research/pdf/ptam4.pdf

indicates polypropylene fibres at a rate of 2-3 lb/yd. it doesn't look like you'd be reinventing the wheel if you were to do an overlay. what part of mo. are you in? how big is the building?

The pavement is 20 years old with no cracks...I'd say the base is correct.



 

Looks like a lot of work.


 

Talk to a paving contractor.  If you really want to leave the existing AC in place, run a heavy roller over the whole floor to find the soft places and deal with those first.  If you really want a concrete floor, check your grades and make sure you have enough clearance for the new driveway, etc.  The best situation is to remove everything, compact the sub grade, and start building from there.  Ask the contractor if he can grind up the existing AC and compact it.

If you do go with white topping, make sure you post pics here.  I'd love to see that!

Excellent link!
The money shot:



ETA::the part in red sounds like a bitch
CONSTRUCTION



Whitetopping utilizes existing pavement



construction equipment and practices;



however, extra care is required for



surface preparation prior to placement.



Milling or scarifying the existing HMA



surface for improved bonding is very



important. After milling or scarifying, the



pavement to be overlaid should be



cleaned thoroughly and covered until the



concrete is ready to be placed. When



ambient temperatures are greater than



90°F, water fogging of the prepared



surface immediately before placing the



whitetopping is recommended. No standing



water is allowed.
Placement of the whitetopping overlay is



similar to new concrete construction, with



continuous placement of the concrete



recommended throughout each pour.



Once the newly placed concrete can



support a lightweight saw and operator,



partial-depth saw cuts are made at



locations of underlying joints, patch



edges, and working cracks. This operation



results in large panels of varied



dimension. Additional partial-depth cuts



are made to create smaller, nearly



square panels. Final panel dimensions



on each side should measure (in feet)



about 1 to 1.5 times the thickness of the



overlay (in inches). After curing, the initial



partial-depth saw cuts at patches,



cracks, and joints are sawed full-depth



and sealed.
SPECIAL CONSIDERATIONS



Whitetopping overlays are not suitable



for every location. Sections that result in



"hybrid” overlays [thin whitetopping adjoining



a thin bonded concrete overlay



(see PTA-M3)] are strongly discouraged.
Other considerations include the thickness



and condition of the existing HMA



surface. Whitetopping should be placed



on a minimum HMA thickness of



4 inches to provide a sufficient base for



the overlay. If milling is performed, at



least 4 inches of HMA should remain in



place, or new material placed to meet the



4-inch requirement. Severely dis-tressed



areas of the existing pavement should be



repaired prior to whitetopping. Some



areas may require removal and



replacement.
Plastic shrinkage cracking, resulting from



rapid water evaporation in the fresh



concrete, can occur within hours after



placement. Ultra-thin whitetopping is



especially vulnerable. Some measures



can be taken to minimize plastic



shrinkage cracking, including: water



fogging of the fresh concrete surface



immediately after finishing; applying



curing compound as soon as possible;



and covering the new concrete with wet



burlap and plastic immediately after initial



saw cutting. Adding polypropylene fibers



at a rate of 2 to 3 pounds per cubic yard



of concrete can also be effective.
 

Remember, Asphalt tends to 'run' when it gets hot, that could be why you aren't seeing a flat surface.  Here in Az, road crews have to go grind down corners, etc, where the Asphalt flows to and gets thicker.  And you can really see the ruts left by cars driving over it.  Concrete doesn't do that.

I would think that would be a concern in laying the Concrete over the top.  You're putting it on a 'movable' surface, moreso than gravel, and there's probably design considerations on keeping it in place, and not trying to move when the Asphalt flows.

I doubt that the warehouse AC floor was placed to DOT standards.  Meaning, white topping the warehouse floor might not turn out as well as doing a highway.  Or maybe it will.  Try it.

As compared to what?  Ripping out an entire asphalt pavement and constructing a new concrete one.



Mill, broom, place concrete, saw cut, cure.  Sounds pretty straight forward to me.



They wouldn't have to do all the concrete testing on the OP's job.



 

Thanks , for the info , many times getting the right terminology for a search makes ALL the differance.
I'm in central mo.
building is over 100,000 sq ft. looking to "fix" 60,000 . OUCH!!!

I don't understand what you mean by "movable surface, more-so than gravel".  Do you mean a visco-elastic material?

No, this won't be a problem.  



If this whitetopped floor is going to have problems, they'll be reflection of existing cracks or joints and shrinkage cracking.  



We don't have the problem up north.
 

Live pavements are subjected to years of heavy loads, de-icing salt, heat, cold, water, sunlight.  The pavement in a building isn't, so its probably in better shape than "real" pavements that get whitetopped.  So I doubt the pavement materials itself will be a problem, even if they are a little "out of spec".
However, you do bring up a good point: smoothness might be an issue.  I'm not talking about texture, I'm talking about the "levelness" of the floor.
On a regular highway the pavement is placed with a paver and smoothness is generally monitored.
On this floor, there could be dips and bumps that would complicate things.  If they aren't too bad, slight milling might take them out.
 

first, i am with the others that say your base is probably very good if it has seen 20 years of use with little cracking.  part of this is the nature of the asphalt.  asphalt is considered a flexible paving, and that helps to to some degree in crack control.  the other thing that has helped with the condition of the  asphalt is the fact that it is protected from the weather.  freeze/thaw cycles, chemicals, salts and vehicular traffic at high way speeds are very tough on any surface.  your surface has eliminated 3 of the big four as far as wear factors.

i think it could probably be done, but i'd go about it a little bit differently. i would consider the asphalt as "lean slab" or mud mat.  i would also attempt to avoid any bond between the concrete and asphalt.  if they're bonded any problems that develop with the asphalt or underlying base will telegraph through your new finish.  I'd place a 10 mil vapor retarder on the asphalt, seal tape on all the joints, and cap it with 4" of clean, washed sand.  then, i'd pour a 4",  4000 psi slab on top and make sure it was cured with an appropriate surface hardener to give you some abrasion resistance.  you'll need some wire in it...fiber is a secondary reinforcing, and is only intended to control random surface cracking due to shrinkage as the concrete cures.  you'll need joints...lots of 'em if you go by the latest ACI guidelines.  (~10' on center each way for a 4" slab).

Well, from what I have read (due to this thread) you are talking about an unbonded concrete overlay over asphalt.  These are generally thicker than bonded concrete overlays on asphalt.  You are correct that it would significantly reduce potential for reflective cracking.  Vertical clearance might become an issue...thought I have no idea without seeing the site.





 

Concrete over blacktop is a well known process in the major road construction biz. If the base under the blacktop was installed correctly and the blacktop is in suitable condition to accept concrete over top, it's a legitimate practice to follow. Doing it in your building though, I would seriously have doubts if the base and blacktop are suitable for such. If you are seriously considering it, get someone over there to do a core sample test.

ACI 360 agrees that unbounded are typically thicker.  given his description of proposed use, it sounded like a light duty concrete paving would suffice...therefore just ignore the asphalt (consider it a REALLY well compacted subgrade) and pour a slab as if it were new construction.  every slab-on-slab pour i have ever seen inside a building has turned into a cluster^#($ because the preparation always gets done by the low bidder.  

this. you have to know what the subgrade is. what ever is under the asphalt fails so does the concrete

Probably more like 4"+ with reinforcement mesh to prevent cracking



 

.
he's probably looking at 2k yds,if my math is correct for a free-floating slab vs. 12k yds plus removal of existing asphalt and remediation of base for a new 6" slab. a core sample is warranted in any case. given the scale of the project, i don't blame the op for looking for alternatives. op, that white topping looks like it would bear investigating. i would love to see the end result. i wonder if a latex-modified mix is indicated,or just what the formula is.

Ah,here it is:

http://www.concreteconstruction.net/Images/W-H-I-T-E-T-O-P-P-I-N-G%20Spells%20Relief%20in%20Iowa_tcm45-342676.pdf

according to this, simply placing the concrete at a minimum of 4-5 in thickness over the highest part of the asphalt is sufficient. at 60k sq ft, i'd think that would be the most cost-efficient solution, given the op's original requirement parameters.

excellent discussion folks.

A couple of years ago I put a US Open surface on top of four outdoor asphalt tennis courts and then put an inflatable bubble over then.



There were parts of the old court that were as much as 4 inches low.  A company came in a filled all of the low spots with a sand resin mixture until level and then proceeded in putting a number of latex coatings on top of that for the playing surface.



You probably do not need anything that elaborate or specialized but it wasn't hard and the surface is really good.  

no. asphalt is a liquid.

you put all that weight on it and the concrete will crack.  you need to pour concrete over an approved base.  get a milling machine in there and respread the millings, you might be ok.

First, find out what kind of base is under the HMAC by taking core samples.  No sense in doing anything fancy without know what is under to a depth below your frost line.  Then look at drainage around the building.  Water flow under the building through the sub base needs to be stopped.



Next, you need to consider what the building's use will be.  HMAC isn't too good for service, jacks and stands and the like but for storage of vehicles, it is fine.  A good, quality top coat should be fine for any pneumatic tire vehicle indoors.  Solid forklift tires can tear it up.



If the base and existing top coat are in good shape without major cracks and the base is deep enough, milling the high spots and a tack coat plus a high quality machine laid top coat will do wonders and last a long while.  Without sun and rain exposure, it will last forever.  Asphalt costs have dropped where portland cement is only getting more expensive.

Mesh and rebar do not prevent cracking and that is not the purpose of either. The purpose is to hold the pieces together after it cracks.
Crack control is placing cuts where you want it to crack to avoid widespread, spiderweb, willy nilly cracking. Even tho you do your best to control the cracking sometimes you get unwanted cracking, especially like up here in MN where you have to deal with frost. I usually try to keep my cuts under 10ft. I get a lot of shit about from the guys who help me, but I can't ever remember having to explain to a customer why his slab is cracked. I've had customers tell me they are suprised that the slab didn'tt crack. I take them to a cut, clean out the dirt and show em the crack

You guys ROCK!!!!  You've probablly saved several days of consulting
If this project goes  I'll keep you appraised on the outcome . there'll be something in it for you guys.

This is definitely the safe move.





Us guys on the internet can say "it sounds like from your description the existing HMA will be good candidate for whitetopping", but to be sure core samples are the way to go.  If there are underlying problems, you'll stand a much better chance of finding them.




This is another option...how smooth does said floor have to be?  I guess when I envisioned the OP's request I was thinking of "smooth as glass" concrete, not a fine graded aggregate HMA overlay.




You bring up a good point about the type of loading that the pavement will experience, especially with regards to solid tire vehicles.





Is there a topic that Keith J doesn't know about?
 

according to what i've read on the interwebs and depending on your point of view,maybe not so much about this one.ymmv