concrete over asphalt for machine shop
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  1. #1
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    Default concrete over asphalt for machine shop

    I am getting ready to move my machines to a larger leased building but the shop floor is currently very mature asphalt.

    Our larger machines are up to 18,000 pounds including mills, lathes and grinding machines.

    The question is whether a mature asphalt needs to be removed prior to pouring concrete. The concrete will be reinforced and contain
    fiberglass strands. Each machine will be on a 12 x 12 or larger segment isolated from the other segments by expansion joints.

    Roads are built all over the country with concrete on asphalt and they last for years under vehicle traffic. I am being advised
    by the riggers that the asphalt will not be a stable base. Other opinions have suggested that the asphalt would be as good as sand.

    Does anyone have experience with machine tools on a concrete floor poured over asphalt?

    Thanks

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    not unusual for floor to settle in spots often over a inch and somebody puts patch concrete or asphalt on top to level floor flatter again
    .
    you putting in heavy equipment normally you remove floor and old foundation and put new one in, often foundations on new machines are 18 to 36" thick steel reinforced concrete

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    Concrete roads are not built on top of asphalt roads. The do grind up the asphalt and use the it for a base material but that is very different than pouring a new concrete floor over existing asphalt.

    You mentioned that the asphalt is very mature. Does that mean in poor condition? Are you sure that the existing floor is asphalt and not seal coat?

    Seal coat is just stone chips topped with asphalt oil and rolled with a rubber tire roller. not anywhere near as strong as true asphalt construction.

    This sounds like a very low budget approach to putting a proper floor in and in my opinion, a waste of money. Without removing the existing floor, you will not have a good handle on how good the base material under it is. Asphalt also tends not to be as level as needed to pour concrete over it. You will either end up with thick or thin spots depending on what elevation you set the floor at.

    I strong recommend to rethink this approach.

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    Thanks I plan to remove the asphalt but wanted to be sure it would be necessary. The contractor is recommending 6” only.

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    concrete on dirt can settle so much there are companies that will jack concrete slabs up and spray expanding polyurethane foam underneath to fill the void. think uneven concrete driveways
    .
    most companies tear out old floor and foundation and put in new one made for that particular machine

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    I think pouring concrete over pre-existing asphault would be a very bad idea. Given that you are committing to serious money doing the concrete and will have machines for which you want a stable concrete pad, I would strongly advise that you do this properly by digging down and preparing proper compressed base rock (minimum 12+ inches) and also install moisture control membrane.

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    Remove the asphalt but note any sunken areas first (bad sub grade) . Stabilize the noted bad sub grade areas when prepping for concrete.

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    I'd have a chat with a engineer. The load carrying ability of a concrete floor heavily depends on the base under it. When you're building an industrial building, you don't just specify the slab, you specify how many inches of compacted crushed gravel you have underneath it

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    What kind of work did they do in the building with an asphalt floor? I have never seen asphalt used indoors. I doubt if the asphalt was very thick or properly compacted/rolled. Doubtful if the base was done correctly either.
    Bill D

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    Our building is also asphalt floors. What a crappy thing. The original owner had it done, in the belief that it would be nicer to the operators standing up all day.
    Now it means we have to place our machines on big steel plates, to avoid the pressure on the leveling feet being to high and stop them from sinking in.
    We have a 10-12 inches layer, directly laid on pure sand. We need to glue everything we want to mount in the floor. Anchorbolts do not function, they just expand the holes..

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    Good concrete starts with proper subgrade preparation. While I'm sure it exists somewhere, I have never seen asphalt have proper subgrade prep for concrete. In fact, zero subgrade prep seems to be far more frequent with ssphalt.

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    If the asphalt is over a good base I would be fine with it, but if it isn't or you don't know I would tear it up.

    You don't need much of a floor for 18K machines though. That's not much of a load for a 6" floor. If you have forklifts whizzing around all day that weigh 12K+ lbs and picking up 3K+ pallets all the time that's way more stress on the floor than some 9 ton chunks of iron just sitting there.

    I think most machine shops are fine with 6" slabs over a good base. A warehouse for forklift traffic needs a foot thick slab over a very good packed and aged base (packed and let sit for awhile).

    Do your research on concrete too. I hate mesh in concrete. Bitch to work with and little fuzzy shit forever! Fiberglass fibers always getting stuck in your hands.

    If I were speccing a slab I would want 6-8" of low strength finish grade concrete with large aggregate. Basically, you don't want the 4000 PSI at 7 days hot mix because it's brittle and will often times pull and curl from all the stress in it. You want 3000 or 3500 which is the same thing with either less cement or fly ash added as a retarder. Put whatever rebar in it that makes you feel good.

    The strength number they give you for concrete is the theoretical minimum strength at 7 days. The actually cured strength for "high strength fast set" after a decade or so is going to be LOWER than the lower PSI slow set mix. But, the slow set mix will have a lot more toughness to it. It will bend a bit before breaking. The fast cure mix will just snap.

    The way they rate it, the higher the PSI number does NOT mean it is stronger. There is a ton of misunderstanding out there in concrete. You want just enough cement in it to get the finish your floor needs. Other than that cement is a bad thing. It makes it brittle. The toughness comes from the aggregate.

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    Get an engineer involved who understands soil mechanics and pavement design. Anything else is just speculation.

    18k is not much load to spread. I suspect that for the loads involved anything over 4" of concrete over the existing blacktop would wotk. I admit I an guessing the asphalt is over a stable base. If the asphalt floor has sustained truck traffic or forklift traffic without serious damage it will carry your equipment loads with a 4" overlay.

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    Quote Originally Posted by gbent View Post
    Good concrete starts with proper subgrade preparation. While I'm sure it exists somewhere, I have never seen asphalt have proper subgrade prep for concrete. In fact, zero subgrade prep seems to be far more frequent with ssphalt.
    Agreed.

    Have to say that I've rarely seen proper subgrade prep for concrete, either. 1-1/2" crushed stone compacted with a plate compactor is about as good as it gets around here.

    The typical concrete contractor's attitude seems to be "Hey, it's concrete, it's gonna crack now or crack later." Convenient attitude for someone that wants to do it quick and get out. He'll never have to deal with it again, but the customer will.

    Flood it till it ponds after preparing/compacting the sub base, then let it dry out. Only way to get no slab cracks in Vermont. IM not so HO. This is the land of frost heaves, after all.

    Fiberglass is to prevent surface cracking, nothing else. And if the finishers are careless, you end up with fiberglass strands exposed.

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    Quote Originally Posted by neilho View Post

    Have to say that I've rarely seen proper subgrade prep for concrete, either. 1-1/2" crushed stone compacted with a plate compactor is about as good as it gets around here.
    .
    engineering scenarios or bubba in the backyard? If engineered, it must be that soil bearing qualities only require that much crushed stone.

    The floor pour is the most important part of industrial construction. I've been involved in lots of industrial design builds. It's all about whats under the concrete, really the concrete is just in compression and distributes a load over the base. For example the point loads with skid racking in a 30' clear warehouse can be very high, its still usually a 6"- 8" floor.....but whether it works or not is dependent-on the load carrying ability of what is underneath. If not done carefully you can get very large variances in thickness, but in being involved in millions of sq ft of them, they still seem to stand up the the if the base done properly
    Last edited by Mcgyver; 06-12-2019 at 03:09 PM.

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    Yup, concrete is just the 'cover' for the base, same as asphalt.

    Standard blue foam under concrete stands up fine. Worked in my garage floor.

    Not saying I would put giant machines over foam, I did pour footings for my lift.....

    If the asphalt was put over a sold base, it will be fine, but do you know?

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    Have a soil boring test done. Then they will tell you what site preparation needs to be done to support your required load.

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    Quote Originally Posted by slnielsen View Post
    Our building is also asphalt floors. What a crappy thing. The original owner had it done, in the belief that it would be nicer to the operators standing up all day.
    Now it means we have to place our machines on big steel plates, to avoid the pressure on the leveling feet being to high and stop them from sinking in.
    We have a 10-12 inches layer, directly laid on pure sand. We need to glue everything we want to mount in the floor. Anchorbolts do not function, they just expand the holes..
    10/10 for thinking of the operators BUT...!!!

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    Quote Originally Posted by Garwood View Post
    If I were speccing a slab I would want 6-8" of low strength finish grade concrete with large aggregate. Basically, you don't want the 4000 PSI at 7 days hot mix because it's brittle and will often times pull and curl from all the stress in it. You want 3000 or 3500 which is the same thing with either less cement or fly ash added as a retarder. Put whatever rebar in it that makes you feel good.

    I'm going to disagree with that. I have an older 3000 psi shop, and a newer with a 5000 psi floor. The durability of the 5000# floor is much better. Smaller impacts that would spall the weaker floor don't leave a mark. Larger impacts will leave a mark, but not nearly as large.

    There aren't many areas where I disagree with Garwood, but his take on concrete leaves me scratching my head.

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    Quote Originally Posted by Mcgyver View Post
    engineering scenarios or bubba in the backyard? If engineered, it must be that soil bearing qualities only require that much crushed stone.
    Both. There are excellent contractors around, but not many. The crushed stone is partly for good drainage too.

    The floor pour is the most important part of industrial construction. I've been involved in lots of industrial design builds. It's all about whats under the concrete, really the concrete is just in compression and distributes a load over the base.....
    Yes, all in compression if there are no voids in the base. And if water gets into parts of the base and freezes there'll be more force in those places.

    1-1/2" crushed stone is used because it settles less than smaller stone. Plate compactors are sometimes used around here, the pond method, almost never.


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