Anonymous asked in Home & GardenDo It Yourself (DIY) · 7 years ago

Can I pour concrete on an existing slab?

I have a shed I want to replace with a slightly bigger shop. The current slab is 13 x 10 and about 6 inches thick. Our yard flooded this past winter and put the slab about 2 inches underwater so I need to raise it about 4" I think. Unless there is a better way to keep water out of the new 18 x 10 shop that I am planning on building...the extended 5 feet will still be dirt to help prevent fires from a forge I am putting in. Can I pour concrete over the old slab and connect it to a running 10" thick perimeter around the new addition? New directions are also welcome. Thanks for any help.

8 Answers

  • XTX
    Lv 7
    7 years ago
    Best Answer

    === seems to me that all this bonding to existing is not as important as raising the new slab up about 8 0r 10 inches ---- or ---- dig around the shed and just pour a curb to act as a water dam ...

  • Anonymous
    7 years ago

    Of course you can pour concrete on top of concrete as long as the overhang is laying on well compacted hardcore but you will end up with a 10in slab. Might it not be better to dig out a little bit deeper around the new rear perimeter and lay a new 5ft x 10ft slab to finish level with the existing. You can then build bearers (sleeper walls) for your new shed to sit on. You can even increase the height of the sleepers to totally eliminate flooding. If you go for this idea there are just a couple hints.

    Position the sleepers so as the shed overhangs the outside ones by about 1in. so as the rain will run straight off and onto the slab. If you are building it in situ you will need to anchor the floor down (wind) if it is installed assembled you will need to drive 4ft x 2in x 1/4in galvanised steel plates into the ground a each corner and bolt them to the shed. If you go for retaining the dirt area with a 10in concrete band as you first suggested (you can still use bearers) make sure the fresh band of concrete is on good well compacted harcore. You sound as though you are from the other side of the pond, if so, increase the anchors because some of the winds you have scare the sh*t out of us Brits. I hope this helps. Good luck. P.S. new concrete will stick like sh*t to old stuff, and what the hell difference would it make if it didn't. The building trade isn't rocket science, it's just common sense.

  • 3 years ago

    Pouring Concrete Over Concrete

  • 7 years ago

    Since you are pouring 4 inches there really is no need to worry about connecting to the old slab. The new slab will form perfectly to the texture of the old slab. Spraying down the old slab with bonding agent is worth your time, drilling in new bolts isn't.

    This really only becomes an issue if you are only pouring 1 to 3 inches of concrete. Then there is a risk of plain concrete mixes might break apart while curing. Adding bonding agent eliminates that issue.

    If the old cement is in good shape, flat and no major cracks or bulges, then there is no need to add reinforcements to the new layer on top of the old cement. However you still want to bridge the 4 inch pour to the new slab areas with a lot of rebar, or you will have significant cracking when your extended slab settles.

    Also since you are pouring on plain dirt, be aware of frost bulge issues. In my area we would dig down 4 feet along all edges and fill the trench with gravel. On a hill we would just build a way for water to escape from the gravel down the hill. On a flat land, add a sump pump well along the gravel.

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  • gould
    Lv 4
    3 years ago

    Concrete Over Concrete

  • Reg
    Lv 7
    7 years ago

    I think that you can do that... I did it over a shed slab once, but I used a masonry bit, and drilled several holes, and put lag bolts as anchors in so that the new slab had something else to hold it in case. It seemed to work for me.

    I also threw some old pipes and a bed spring onto it so that it acted like strengtheners in the pour.

    Source(s): 75 yrs on the planet
  • Ashley
    Lv 4
    4 years ago

    For the best answers, search on this site

    Should not see a problem however some excavation may need to be done in order to lay your bed of crushed stone or hardcore for the concrete to rest on you don't give the sq footage of the building that would give me an idea as to the amount you might need and as for the depth of the slab I would recommend 4" of re-inforced concrete and a 4" hardcore base depending on the size you might manage it yourself but hiring a contractor is also an option you should look into.

  • Anonymous
    7 years ago

    Yes, as Fat, Old, Bearded, Bald Guy suggested, you should drill some holes into the existing concrete and install some lag bolts or rebar to tie the old to the new. They also make a bonding agent that can be used to help the new concrete adhere to the old concrete. New concrete generally does not stick well to old concrete without the bonding agent. I would suggest looking in to it.

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