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Over-excavation for garage floor

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  1. #1
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    Default Over-excavation for garage floor

    Not sure if Iím overthinking this. I am getting a 100 mm floor poured for my garage. Approx 3.1 m x 5.2 m. I am doing the prep and have a concreter coming to supply membrane, mesh and lay and finish the slab.
    just checking my levels and I have finished the excavation about 10 mm deep. Not sure if this matters on a small job like this, an extra 0.16 on a 1.6 m3 pour.
    i can add a bit of fill back, or do nothing and have a slightly thicker slab. Is the difference important or will the minimix load include a little extra on the day to cover errors like this?
    I am but an egg in the ways of concrete orders.......🙁

  2. #2
    3K Club Member johnc's Avatar
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    Don't rely on an extra grain of sand in the concrete, it sounds like you need 1.9m3 allowing a tiny amount up your sleeve. You don't order 1.6m3 when the basic measurement is 1.612 you go up a bit to around 1.7

  3. #3
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    From my experience the concreter orders the concrete as he can tell the plant what he wants as far as consistency, aggregate size, slump, additives and volume.

  4. #4
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    Can the slab sit 10mm lower, or not?
    Like Bros said, last time I had concrete poured, the concreter ordered volume etc. Is the concreter doing the formwork ?

  5. #5
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    Yes, concreter will do formwork. No, has to have a finished level and can’t be lower.
    Given a 100 mm slab will be strong enough, will putting 10 mm of fill bacK create any problem?

  6. #6
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    (surely) the concreter will assess when doing the formwork and adjust the order to suit the need

  7. #7
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    Ah, that helps r3nov8or! So the concreter will finalise the order on the day. This works for me. Happy to pay the incremental cost of a thicker slab, but didn’t understand the ordering process for the concrete on the day. Was concerned it would end up short.

  8. #8
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    Concreters usually seem to order just a little bit extra, if you have a need for any cyclone anchors in the future have a couple of plastic buckets/ tubs close by and some steel rof bent into a "W" with the centre arm longer and use any left overs. always handy to have a wind anchor or two around
    "A big boy did it and ran away"

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  9. #9
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    He will probably order volume the day before, giving MPA and aggregate size etc. Depends on the batching plant but usually you don't try and confuse things with an approximate order and then a final order. Its a small job the concretor is probably going to cost you more than the actual concrete, which is fine.

    You can back fill and make sure you just have your 100mm depth, check carefully that your depth is ok but it might only cost you $75 approx for the bit extra you will need, I wouldn't get that worried one way or the other

  10. #10
    1K Club Member Pulse's Avatar
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    Default Over-excavation for garage floor

    go the thicker floor, my builder aims for 120mm, that way the depth of 100mm is guaranteed


    Sent f

  11. #11
    4K Club Member Marc's Avatar
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    100 mm is almost guaranteed to get cracks. Go 120 and up, Make sure the mesh is laid properly on chairs and not "I'll pull it up when I pour" crap.
    For a few dollars more, get 40 MPa not the usual crackly 20 or 25.
    Here is a calculator for concrete
    https://www.hy-tec.com.au/concrete
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  12. #12
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    Definitely go for the stronger concrete, the price difference is marginal
    "A big boy did it and ran away"

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  13. #13
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    Thank you for all of the advice folks. Concreting done today and there was no issue at all. Slight over excavation but concreter not worried and ordered enough, not even any extra charge so it must have been within cooee. They were happy with the site prep and delivered an excellent finish.

  14. #14
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    if you don't mind me asking, was there much of a price difference between you doing the prep/forming compared to him doing it?
    Remember if you don't sin, then Jesus died for nothing

  15. #15
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    I just got him to quote on me prepping, but based on other quotes for 16 m2 I saved around $500. In my case I had good 60mm pavers over it that I wanted to reuse, then just had to remove 40mm of mainly sand after that. No massive level changes or filling.
    I wanted a good quote, so kept extras out of it, was clear about dimensions and finish, nice and simple and bargain hard. I was going to do it all myself, but once I costed everything as tight as I could, getting it all done was only a few hundred more. Worth it alone in watching and learning while an experienced tradie did it and produced an excellent finish! ����

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