concrete over asphalt for machine shop - Page 2
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  1. #21
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    A couple things about your question stick out, so I'm going to separate:

    - you use the word "reinforced" in the same sentence with "fibers" when describing the concrete. Fiber is NOT reinforcing, unless you are using carbon fiber grid, which is very unlikely. the small fibers they put in concrete increase the tensile strength quite a bit, but it still sucks. a 50% increase of almost nothing is still almost nothing. For example, the tensile strength of 4000 psi concrete is in the neighborhood of 300 psi, which sucks. Fiber is used to improve the tensile strength of concrete to "reduce" unwanted cracking due to the shrinkage that concrete experiences as it cures during the first few weeks. I say "reduce" because there is no way to avoid the cracking without going to extreme measures. As concrete cures, it consumes the water in the hydration reaction, and since the water disappears, there is a reduction in volume, so the concrete shrinks. because it is sitting on the subgrade or whatever else, there is friction preventing the slab from shrinking, so it cracks. if you poured a 10 mile by 10 mile slab floating in space, it wouldn't crack, it would just shrink to 9.something miles by 9.something miles and look fantastic. adding the fibers increases the tensile strength to reduce the frequency of cracks. the only "reinforcing" in concrete is steel rebar or prestressed/post-tensioning strands (which are steel rebar essentially).

    -18,000 lbs isnt a lot of load when it comes to concrete and slabs and foundations. a 42"x42" concrete pad is all that is needed to support a column with 18,000 lbs of load, and that is based on a conservatively low number for allowable bearing capacity of the soil. Loads are not the issue. your machine isnt going to get swallowed up. The most important thing to prevent is movement, or deflection. building design is based on some allowable movement. machines do not have as much flexibility as buildings. Are you talking about pouring a slab over the entire floor, or just individual concrete pads for each machine?

    - concrete over asphalt is better than concrete over subgrade. If you pour concrete on top of the asphalt, you probably end up with concrete on top of asphalt on top of subgrade on top of soil. While that sounds like breakfast at IHOP, it shouldnt be an issue. Asphalt works just like subgrade - it doesnt "span" or distribute load like a "beam" as concrete does - the mechanism is called "aggregate interlock" and it distributes the load at a 45 degree (or so) angle outward through the depth of the asphalt. Concrete on top of asphalt is a fantastic pavement for roadways, but is prohibitively expensive. Its safe to say that asphalt performs better than subgrade.

    final thoughts
    - Everything depends on the quality of the asphalt and the subgrade below it. I would find out how the space has been used in the past, i.e. weight of equipment, forklifts, etc. how does that compare to your use?
    - inspect the existing floor carefully, especially where the high traffic areas are (assuming forklifts were used there) and look for settlement, depressions, low spots. This will give you a good indication of the quality of the existing floor and some kind of indication of what your use will result in. not exact, but a rough idea.
    - removing all the asphalt and placing concrete is pricey. pouring concrete on top of asphalt will raise the floor which will add "steps" which may not meet code for egress, but more importantly could create a trip hazard.
    - pouring pads on top of the asphalt for each machine is a good solution, but will raise the machine height for the operators, which could suck.
    - removing the asphalt at each machine location and pouring individual pads may be the best option, assuming you don't need to "restore" the asphalt if you ever leave. If you go this route you should make sure each machine has a single monolithic pad - don't pour separate pads for each end of a lathe or long piece of equipment - each pad can move independent of each other. also, if you go this route you need to think about the pad size for each machine - lathes and mills are pretty straightforward but large cantilever machines like some of the offensive radial arm drills that can swing way out to each side can cause unbalanced loads on the concrete base which may result in problems - the base for those type machines needs to be large enough to keep the machine stable at all positions (no tipping, please) but also keep the bearing pressures low enough so as not to result in movement.

    This kind of got away from me, but i hope some of it is helpful. let me know if you have any other questions.

    Cheers,

    Daniel

  2. #22
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    I have 6" of concrete with wire over 4" of compacted 304, over 8" of ground asphalt fill that we let sit for about 8 months before starting construction.

    I specifically asked about the 18k lb weight for a machine, because the mazak I was considering at the time weighed about that much. Engineer said as long as the foot print was larger than 10x10, it was not a problem.

    HE WAS concerned about the tow motor carrying the 18k lb machine...

  3. #23
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    Quote Originally Posted by Fal Grunt View Post
    ....

    HE WAS concerned about the tow motor carrying the 18k lb machine...
    Perhaps missed sometimes is the ability to support the machine might be different than supporting it plus the lift truck bringing it in.
    In these cases you need to skate which is just a pain.
    Bob

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    Is this normal to do major changes (permanent ones at that) to a leased building ?

  5. #25
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    Quote Originally Posted by CarbideBob View Post
    Perhaps missed sometimes is the ability to support the machine might be different than supporting it plus the lift truck bringing it in.
    In these cases you need to skate which is just a pain.
    Bob
    Short term loads such as moving machinery into place are much less challenging than sustained/long term loads. Soil bearing capacity is usually driven by the need to prevent settlement, which isn't a concern with short term loads. a loaded dump truck driving on a construction site can apply pressures to the soil that are 10x higher than what we are permitted to apply when we design foundations for buildings, and if the soil is dry the trucks will not cause ruts or displace the soil. With that said, if that truck were left to sit for months it would inevitably sink in.

    slabs and other structural elements typically respond better to quick short term loads than they do for long term loads, and that includes asphalt. A good example of this is when asphalt parking lots develop depressions at wheel locations in the parking spaces before any other issues.

  6. #26
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    most lease agreement have a clause that allows the owner to require the space to be returned to its original condition upon termination of the lease. You can negotiate exceptions to that clause if you know what you plan to do before you sign on the line, but without a specific exception in the lease agreement, you are on the hook to undo whatever you do. in our last lease we provided drawings for our renovation plan and had those improvements added as exceptions and the drawings were part of the lease agreement.

    if you are already in a lease and want to make an improvement that is a benefit to the building, you may be able to work out an addendum to the lease to add an exception, assuming the improvement benefits the owner.

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