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Concrete slab

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  1. #1
    Senior Member Bluegum's Avatar
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    Default Concrete slab

    I am going to put down a slab at the back of the shed as part of it being extended into my new work shop. The area will be 6x4m and the slab 100mm thick giving me well needed workshop area. I have never had to look into concreting before so I am looking for advice and perhaps even a few rough ideas on costings. The area is level and wont take much to box up so any suggestions will be greatly appreciated.
    Dave,
    hug the tree before you start the chainsaw.

  2. #2
    Diamond Member Terrian's Avatar
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    mark out the area where you are going to put your slab. Dig out the dirt that is there to a depth of 4" or there about, more if the ground is soft. and top it up with 20mm crushed rock, level the area out as best you can,
    Box it up with 4" x 1" timber,(above the crushed rock) put in as many stakes as you think you need, then double the number I usually use 450mm star stakes, bang one in every 500mm and belt it down to the level of the top of where you want the slab to be, the length of the stakes will depend on how stable (hard) the ground is, you don't want the boxing to blow out from the weight of the concrete.

    Get hold of 3 dozen bar chairs (25/40 is about the height you want, use the 40mm size) and 2 sheets of F62 or F72 reo (I prefer F72) put the first sheet in and use 1/2 the bar chair to support it, cut the second sheet to size to fill the rest of the boxed area, use remaining bar chair to support it, try and keep the reo 20mm or so in from the boxing.

    You are going to need a screed (4.5m length of straight sturdy timber will do) a 18" wooden float, and edger would be nice, a square mouth shovel and 2 decent wheel barrows (if the truck can't get right to where the slab is going) (4.5m will allow you to reach from one side to the other, one person each side, see saw it from side to side while moving forward to screed the concrete)

    when the truck arrives you are going to want at least 2 people to help you, there is wheel barrow work and spreading (with the shovel) that will need to be done. Bear in mind that most will charge waiting time after about 20 minutes, so you want to unload the truck as quickly as you can. Order the concrete at about 75 slump, at 20mpa or 25mpa, slump refers to how sloppy the concrete will be, mpa refers to the strength of the concrete

    After the concrete has been roughly spread, screed it off to get it all level with the top of your boxing, use the float to smooth it out as you go, last job is going around the edge with the edger to get that curved finish.

    It all actually sounds harder than what it really is, one big 'secret' with concrete is only work the surface as much as you really really really have to to get it reasonably smooth.
    Do not keep troweling the concrete trying to get that perfect finish!
    Do not pour the slab on a hot day (don't want the concrete going off before you finish the job!)

    Having said that, a 6m x 4m slab is pretty large for a first slab, I would be looking for a mate who has some concreting experience to help / lead the job.(slabs I do by myself at home are usually 6m x 2.4, biggest in one hit was 9 x 7 for a friend though)

    Rough costs, concrete (at $160 per m3) x 2.4m3 = $384, reo, about $150, bar chairs, about $20, timber for boxing, no idea. (would guess that $600 would cover the whole job)

    2.4m5 is what you need, order 2.6m3, take it all, if you have any left over either find a use for it, or spread it out thinly (and clean it up after it drives)

    After clean up, have a nice cool ale or two, you will have earned it!

  3. #3
    Owner Builder
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    That was a great post Terrian, thanks for that, I have benefited enormously from your generous advice here

  4. #4
    House Husband - 1K Club Member
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    Terrain
    do you have any reccommendations for local concreters? Someone who can handle a shed 6x8m and house extension 8x8m slabs? unfortunately I need a line-pumper too (its all downhill from the road). I'm after a quality job.

  5. #5
    Senior Member Bluegum's Avatar
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    Terrain,

    Thanks mate there is a swag good of info in there it at least gives me a starting point to work with.
    Dave,
    hug the tree before you start the chainsaw.

  6. #6
    Diamond Member Terrian's Avatar
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    Quote Originally Posted by zacnelson View Post
    That was a great post Terrian, thanks for that, I have benefited enormously from your generous advice here
    awwww

    Quote Originally Posted by sundancewfs View Post
    Terrain
    do you have any reccommendations for local concreters? Someone who can handle a shed 6x8m and house extension 8x8m slabs? unfortunately I need a line-pumper too (its all downhill from the road). I'm after a quality job.
    no, sorry, best I can offer is ask at your local concrete yard (eastlink in fern tree gully rd, or perhaps pronto in mt evelyn, call in person, not over the phone though.

    Quote Originally Posted by Bluegum View Post
    Terrain,

    Thanks mate there is a swag good of info in there it at least gives me a starting point to work with.
    no worries, concrete use to scare me, now it scares me for a different reason (my aching back!!)

    a snap from the last concrete job the wife & I did here (have pics of each step if anyone wants to see them)

    dsc00282.jpg note, 1.5m3 of concrete, all poured using 1/2 filled 20lt drums!

    Wall is 6m long, about 900mm high, and thick (but no where near as thick as the second part of the wall!)

  7. #7
    Owner Builder
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    I would really like to see all the progress pics you mentioned if you want to post them here or somewhere

  8. #8
    Diamond Member Terrian's Avatar
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    Quote Originally Posted by zacnelson View Post
    I would really like to see all the progress pics you mentioned if you want to post them here or somewhere
    just made a thread here

  9. #9
    Golden Member nev25's Avatar
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    I'm wondering when and where would and why you use plastic sheeting
    And How often do you put groves in the finish (Crack control)

    In the slab plan I got for my shed it states saw cuts???

  10. #10
    Diamond Member Terrian's Avatar
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    Quote Originally Posted by nev25 View Post
    I'm wondering when and where would and why you use plastic sheeting
    I wouldn't, but then again, I am never likely to lay a house slab...

    And How often do you put groves in the finish (Crack control)
    a sheet of reo is 6m long & 2.4m wide, I usually put a grove it at the 3m mark.
    (keep in mind that I am an amature!)

    In the slab plan I got for my shed it states saw cuts???
    same as the grove, for (surface) crack control, the cut is only shallow, not all the way through the slab, you let the slab dry, et a diamond blade for your angle grinder and cut some straight lines in the slab (dusty job)

  11. #11
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    Quote Originally Posted by Terrian View Post
    I wouldn't, but then again, I am never likely to lay a house slab...
    Is waterproof membrane (plastic sheeting) usually required for a garage slab ?

  12. #12
    Diamond Member Terrian's Avatar
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    Quote Originally Posted by blak View Post
    Is waterproof membrane (plastic sheeting) usually required for a garage slab ?
    I use to manage a garden supplies yard, we also did mini mix concrete (accounted for near on 45% of the business!), over the years we supplied the concrete for over a hundred garage slabs, I could count on one hand the number of times plastic was used, the vast majority were being done by professional concretors, not home handy people.

    Have the rules & regulations changed in the last few years, I have no idea, it may well be that some pencil pusher located in the deep dark bowels of some government building has decided that it was time to change the rules and earn his/her keep... if in doubt, check with your local council...

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