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Discussion in 'Hardscaping' started by JimLewis, Feb 1, 2007.

  1. JimLewis

    JimLewis LawnSite Fanatic
    Posts: 6,839

    We sub all of our concrete flatwork out. But I have a customer who had a bad experience with her previous concrete driveway (cracking, in particular). And so they are having us redo their entire driveway and front landscape. But this time the customer wants us to go the extra mile on everything. She requested we excavate not only the concrete, but whatever is under it for 10", install 10" of new gravel, compact the gravel, install lots of rebar, and then install the new concrete.

    So for the concrete itself, I have the option of using 3000 psi concrete, or 4000 psi. for only a little more. My question is this; will the 4000 psi resist cracking more than the 3000 psi will?

    There are no big trees around, no tree roots to cause lift. And the foundation underneath will be about as [overkill] perfect as you can get. So it's not going to crack from lack of sub-structure, that's for sure. The only thing I can see cracking it would be heavy delivery trucks, which is what caused the cracking the first time, according to the customer. So I was thinking if we use the higher psi concrete, this would help prevent that from happening. But I am not really an expert on concrete. For those who are ... your thoughts?
  2. Let-it-mow!

    Let-it-mow! LawnSite Member
    Posts: 91

    For a slab that's won't crack do this:

    (1) Pour it thicker. When loaded, a slab is just like any other beam supporting a load. Imaging how much a a 2x4 floor joist will flex when loaded compared to a 2x8 floor joist. Concrete can't flex much without cracking. The thicker it is, the stornger it will be.

    For commercial driveways and slabs, 6" is standard.

    (2) Use welded wire mesh. Don't rely on the fiiber concrete to prevent crackinhg. Use fiber reinforced concrete, the fibers help to control spalling from freeze/thaw but they don't work as well as re-wire for crack control. If you want bomb proof, use re-bar instead of mesh on 2' centers. Either way, make sure your rewire or rebar is floated into the center of the slab. It does nothing if it sits on the bottom.

    (3) provide control and expansion joints. A large slab ofconcrete will crack somewhere eventually. If you plan your slab like a bunch of smaller, independent slabs that can move a little, you will have a much better slab. Just make sure you have a stable base or your mini-slabs can heave and leave the drive uneven (but not cracked).
  3. McKeeLand

    McKeeLand LawnSite Senior Member
    Posts: 681

    Your better of going with at least 3500psi concrete and why not 4000# for the little bit extra. You also want crack control joints every 10' and expansion every 20' and against any fixed structure, such as the curb, the garage pad, or the house. You can joint the concrete while it is wet with a joint tool or saw cut it in after it cures, or with in 24 hrs of the pour. A good idea with driveways is to pour the edges a little thicker too. Hope this helps, we just did a 2500sf drive this fall with exposed aggregate concrete.
  4. ozd12005

    ozd12005 LawnSite Senior Member
    Posts: 342

    When ordering a six bag mix with a 3- 4 inch slump you will get 4000 psi
    also the more water you add you bring that (psi) number down. Also remember to use fiber board along any existing concrete (foundation wall, garage, sidewalks etc) Control joints should be spaced no more then 8' apart, Make sure your base is compacted and use wire mesh (pull up on the wire as you pour the concrete)
  5. pavespec

    pavespec LawnSite Member
    from nyc, ny
    Posts: 10

    All you have to do is contact the concrete supplier you plan on buying the concrete from. Ask for the sales dept. This will put in contact with a salesman. Explain to him what you are going to use the concrete for. The will use a mix design that will benefit your project. Please note that when the load leaves the plant it has the amount of water needed in that mixes design. Its at the jobsite that contractors add water that know changes the mix design. Your psi will fall from what you ordered if you add to much water.
  6. RockSet N' Grade

    RockSet N' Grade LawnSite Silver Member
    Posts: 2,454

    We've poured alot of concrete. Tilt ups, interior industrial building floors within 1/8th inch plus/minus, parking lots, sidewalks, approaches........the whole enchilada. By nature concrete wants to crack, so you have to work with its nature....concrete loves to crack at corners, so beware of corners and saw cut or control joint off your corners.
    Compact your subbase uniformly. Use #3 rebar instead of any type of wire mesh ( I have yet to see a mesh job where the mesh stays in the concrete and doesn't end up in the subbase at least in some substantial areas). Bullnose all your edges and ends. Have a uniform finish grade set up and pour an even 5" plus slab (no dips and humps). Make sure your concrete supplier will give you back to back trucks so there is no down time between trucks ( significant time between trucks creates cold joints and I guarantee a crack at that spot). Have enough finishers on hand to do the job correctly from screeding to floating to finishing and saw cut your slab at the proper time or use control joints in the finish. Just some thoughts.....
  7. kootoomootoo

    kootoomootoo LawnSite Platinum Member
    Posts: 4,369

    All of the above and don't guarantee against cracking.

    The only guarantee you give is it will crack.
  8. ozd12005

    ozd12005 LawnSite Senior Member
    Posts: 342

    I like to call it my tail light guarantee
  9. Dirty Water

    Dirty Water LawnSite Fanatic
    Posts: 6,799

    I used to do concrete work, mostly foundations, but I did a good share of flatwork.

    We were oldschool though, and still used zipstrips instead of sawcutting.

    I believe that the cracks a zipstrip creates is superior than a sawcut for controlling cracking of the slab, however they aren't as pretty.

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