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Dodgy concrete job?

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  1. #1
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    Default Dodgy concrete job?

    Long time lurker, first time poster…

    I got a licensed concreter to extend my shed slab, add a slab for a carport, and a pad for a 15,000L tank.
    They levelled the carport space and scraped dirt away for tank pad.

    The original shed slab was put in by my father in 1984 and is 30cm above ground and quite a bit below - no reo but coarse aggregate all carried by hand out the river. 3m was slabbed and the other 3m a brick surround with timber beams (no idea why). There isn’t a single crack in the old slab.
    The guy used some bricks and smashed rubble for fill before dumping the dirt from levelling - compacting was done by smacking it with the bucket in the little digger. The old slab and fill was covered in plastic and reo added.

    A couple of things I don’t like about the job but I’m not sure to be honest.

    • There are gaps under the concrete over the brick bit that was filled - you can push a stick at least 50cm in before touching dirt - dirt that doesn’t look compacted at all
    • the carport and tank slabs sit on the ground - they did put down some crushed/ting gravel but we are talking <1cm and wasn’t compacted
    • the concrete over the old slab is about 6-7cm thick - is this thick enough to properly cover reo?
    • the carport slab is mostly 9cm thick
    • the tank pad is only 5-7cm thick — this seems extremely thin considering 15tonne of water will be on it
    • you can poke a stick under the slabs (not my dads) since they just sit on soil
    • Since the tank pad is low to ground it will be very hard to stop water coming down slope and under slab


    Does anyone with knowledge in concrete share my concerns? Is this work okay or am I being paranoid?
    Attached Thumbnails Attached Thumbnails 7c0d3a31-59d1-40ca-bc2b-6e1f9b76dcfe.jpeg   7eb75217-87e0-49a9-a33b-a8938ef5f9e1.jpeg   1ff63b06-42e5-4825-b3ca-d101897135db.jpeg   ecf001c7-aeb4-4a85-8617-c973e7e138b2.jpeg   fe1b56e4-670a-45d8-a4c6-268ecd96216e.jpeg  

    3b75807c-1436-422c-aa7d-eee966b9d579.jpeg  
    Last edited by RogerWilco5000; 10th Nov 2021 at 06:48 PM. Reason: More info

  2. #2
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    ...not an expert and only comment on what I've seen and done over the years (prep and boxing up ready for the pour) before bringing in the pro's for the pour.

    Also note: Whats done is done.

    Carport slab is about right in terms of thickness.
    • Assume they used reo bars or sheets - F82 would be a minimum sheet gauge although some may use F72 which would likely be fine if all prep'd really well but in my view is really only suited to paths.


    • Would be interesting if they used the plastic reo chairs something I always insist on or use myself if I do the boxing. Quite a few operators simply say they "lift" the reo up into the middle and in my DIY view is all BS particularly when your spending your hard earned time and money to then skimp on $40 of plastic chairs to ensure the best possible result with the reo in the ~middle and therefor achieving maximum strength of the slab. Been burnt 20 years ago with this BS and never again. All my jobs, they either use the chairs or the job goes to someone that will.
    • Are you planning to bolt a car port to the slab? Was this specified where you requested quotes? If so, then there should have been footings dug where the posts are going to go. Depending on your soil type and construction a general starting point would be holes of 350mm square by 350mm deep.


    Hard to tell in the photo's, but i'd never just lay a slab directly on top soil, I'd always scrape off the top soil and fill with road base or compaction sand to the required depth followed by the concrete boxing/form work on top of that.

    The tank pad, whilst its an even weight across the entire area under the tank, given its concrete I would have built it to the same specs as a car slab - ie: ~100mm thick, excavated top soil replaced with crushed rock or compaction sand.
    What you've got will hopefully function fine.
    Keep in mind there are plenty of tank "slabs" that are simply leveled compacted "compaction sand" and/or crushed rock which work fine, is cheaper and would be my preferred method unless there was some sort of site considerations that required concrete such as sitting one of those slim line tanks up against the house wall or maybe fence to tie in with paving, easier maintenance with weeding etc

    The old slab - are you sure its only 60mm thick as looking at the form work in the first photo looks to be 4" (90 to 100mm) to me.

  3. #3
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    Thanks Bart1080. They did use the grey plastic chair things, but that doesn’t leave much room above/below reo.

    The tank is an Aquaplate metal tank. The old 20 year old plastic tank has sunk and is lopsided. The base was compacted so I thought a beefier base might be better.

    Yeah it varies a little but it is 60 down the side. The form work was adjusted after the photo was taken.

    The plan was to bolt and was discussed beforehand. They pulled up the old footings and chipped some off the bottom and used for fill (last photo).

    I know it is a bit late now since it’s done and dusted. I just don’t want to find it all cracking once the shed/carport is built and the tank full of water.

  4. #4
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    Well, I think this sums up the tank base…
    Attached Thumbnails Attached Thumbnails b25ba63d-25a6-4d1e-b099-405b120279cd.jpeg  

  5. #5
    Golden Member havabeer's Avatar
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    Quote Originally Posted by RogerWilco5000 View Post
    Well, I think this sums up the tank base…
    but did YOU give that spec to the concreters?
    Remember if you don't sin, then Jesus died for nothing

  6. #6
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    To be fair, no. I did ask for a reinforced tank pad to hold 15tonne thinking a professional would know what to do.

  7. #7
    Community Moderator phild01's Avatar
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    Quote Originally Posted by RogerWilco5000 View Post
    the concrete over the old slab is about 6-7cm thick - is this thick enough to properly cover reo?
    Concrete toppers can be as little as 50mm


    the carport slab is mostly 9cm thick
    somehow the old 4inch slab ends up as 90mm likely because the old days green 4x2 timber became seasoned 90x45, slab should be 100mm though but ends up with a 90mm visual edge.

    the tank pad is only 5-7cm thick — this seems extremely thin considering 15tonne of water will be on it
    How do you know, did you measure across the formed up areas or just looking at the edges.

    The best way to know what is going on is to be there and inspect the formed up area before the pour.

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