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Thread: Expert help pls re cement slab

  1. #1
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    Default Expert help pls re cement slab

    Hi all,

    I'm preparing a 6x2x2 tank and not sure if my house will support it, we have a small extension of our house which has a thin cement slab floor which the stand will be located on, the cement slab looks to be around 10-15mm thick only, I'm concern this might not be strong enough, I'm thinking to fill another 50mm cement with reinforced mesh on top, what do you think? Advice please.

    Regards
    George

  2. #2
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    10 to 15mm thick slab????
    That is almost impossible to do !!

    We do blinding to temp support formwork on sand at work and thats 50mm thick!!
    A typical concrete slab on sand is 100mm in Perth

    Graeme
    Read this very helpful thread .
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  3. #3
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    It probs won't hold the tank for to long before cracking, I'd get it re cemented

  4. #4
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    Sorry Graeme, my bad. Might be 30mm, definitely not the standard thickness, there used to be a wardrobe on top of it, so this could be a DIY from the previous owner for this small area. So do you think this will support the tank? If not, how thick should I make it?

    Thanks

  5. #5
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    any other advise please? the thought of re concrete the area gives me backache. 2.2m x 0.64 x 0.1 (around 28 bags of pre-mix concrete). geezzz and I need to remove the existing one as well. ohh my god, what am I getting into.

  6. #6
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    How are you measuring the thickness of the slab - does not sound anywgere near right.......... Pictures will help!
    officer cadet of the bogun brigade

  7. #7
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    Hi Gingerbear, I just drilled a few holes to measure the thickness. below are the pictures of the site. it's about 50mm lower than the tile floor which might explain why it's not the standard 100mm concrete slab. but it looks like the concrete is set on sandy ground.



  8. #8
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    I am not an engineer but would suggest that if you can, forget about redoing the slab, get a piece of good quality waterproof plywood and place tank ontop of it.
    The ply will evenly distribute the load of the tank over a large surface area rather than point load the slab like a standard stand would.

    Graeme
    Last edited by Graeme; 29-01-13 at 07:35 PM. Reason: spelling
    Read this very helpful thread .
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  9. #9
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    LxWxD and if you found out the MPA of that concrete then you would then surely find some sort of calculation to work out the standard amount of pressure a slab of concrete could hold at those dimensions i should know this seriously i learnt it at tafe and was in my final duck shows you how good i am

  10. #10
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    thank you all for the input. a good quality waterproof plywood would cost a fortune, worse thing is the slab is not even. so will bite the bullet and fill in more concrete to the tile level, hopefully the frame form by the wall and the existing tile floor will hold everything in place.

    cheers

  11. #11
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    That concrete looks a little strange... I can't see any aggregate in it, (stones) and it looks like it might be crumbling a bit. If the existing concrete is not looking strong I personally would pull it out and replace it. Also concrete should never be laid less than 50mm thick as an absolute minimum and just because the old stuff is 50mm in one point that could vary across the slab especially if it is a dodgy DIY job. Putting more good concrete onto a bad foundation just makes more bad concrete. I would recommend ripping out the old and starting again, you might be surprised how easy it is to get out once the initial hold is a bit bigger.

    I think somewhere you might have go your calculations a bit off as to how much pre-mix you might need 28 bags sounds like a hell of a lot for an area that small

    Check out this link http://www.boral.com.au/cementdrymix...y-reckoner.asp

    I calculated about 18 bags for 100mm concrete.

    I would also use some steel rio mesh to keep it all together. Even if you dont pull out the old concrete I would still put in some mesh. If you do pull everything out and start again make sure the sand underneath is nice and compact. Remember once you get the tank and stand all your gear and a whole lot of water your gona be looking at over 3/4 of a ton of weight. Not worth skimping on IMHO.

    Cheers

    Darcy

  12. #12
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    This is very strange for a house. With no reinforcement the slab is likely to crack. Leveling it may be OK to walk on, but not to put a tank on. Sand is just leveling sand.

    Marine ply is named as the plys are continuous as needed to build boats, nothing to do with being better, just more boaty. Get a H3 ply and it will do the same job.

    Unfortunately IMO the ply will however do nothing. It increases the bearing area very slightly.

    I have question marks over the concrete, as concrete continues to harden. When a house is built chippies like to stand the frame as soon as possible as it is easier to put in the concrete nails. Try putting a nail into 10 year old concrete.

    I would get in a builder. How long ago was the extension done? I would not DIY a solution and put a tank on it.
    officer cadet of the bogun brigade

  13. #13
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    Man this is dodge ey!! I'm not expert in concrete but I am doing a house reno atm personally and if I seen that I'd rip it up straight away just cuz its not right, its looks very sandy too which is bad. Pain in the butt ripping put concrete but it can be done without hiring equipment, my driveway came up with a hammer n chisel. Yes a whole 3.5m drieway lol.
    If you do pull up and get new concrete throw extra reo in there beforehand so not only are you fixing it your making it stronger. Again no expert but that's what I'd do in my house.

    Lol @ ginger with putting nails into old concrete as this exact problem had me stumped for a few hours
    last year. Had a nail bout 100 mil from where I was putting a new nail so I figured she'd be right only to curse a dozen times n throw the hammer baha
    Good luck with whatever you do man

  14. #14
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    Anychance the extension is filling in the pergola? If so you may have come across a old garden bed, may explain good slab either side of the offending area. If it is I'd dig it up, check if the pergola slab is a add on to the house slab. If so your gonna have to join you new slab with the pergola slab using reo etc. Bit of work but can be done, just ask my partner haha

  15. #15
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    Thank you all, lots of food for thought.
    Yep, this is just an extension, main house is raised above ground, not on slab. Phewwww,,,

    Regards

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