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Infill concrete slab

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  1. #1
    mxh
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    Default Infill concrete slab

    Iíve got a small area (approx. 4 m2) that I want to Ďinfillí in my house slab.

    Itís currently just a dirt area next to the walkway between the porch and the garage. Itís under the roofline, so doesnít get any rain, and has just ended up being a bit of a junk area. So it might as well be a junk area with a nice floor!

    In the diagram, you can see that a lot of the new slab will butt up against the existing slab, and also against brick walls. The brick walls (in red) are built on footings which are attached to the slab, but the footing is lower, so the new slab will actually be butting up to brick at these points.

    I intend to drill rebar into the existing slab and use rebar throughout the new slab.

    Some questions

    1. Are there any issue doing what Iím suggesting?
    2. How many bits of rebar do I need to drill into the existing slab (ie at what spacing?)
    3. Should I rebar into the brick walls? Or if not, attach in any other way?
    4. What base should I use Ė will the (very dry and hardpacked) dirt be OK, especially as Iíll be using plenty of rebar, or should I dig down and put in some gravel base?
    5. Do I need any waterproof membrane (again, reiterating that this is under the roofline and is always very dry)
    6. If I do need to use a waterproof membrane, how should it end (ie should it come up the sides between the old and new slabs?)
    7. I would eventually like to tile over the whole area, including the walkway (so tiles will span the join). Are there any extra precautions I can take to ensure that the two slabs donít move apart and crack the tiles?
    8. What cement / sand mix would work best in this environment?
    9. Any other advice / ideas etc that would be useful.


    Thanks in advance!

    slab.jpg
    Attached Thumbnails Attached Thumbnails slab.jpg  

  2. #2
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    Photo/s will clarify the drawing for improved feedback and to minimise a swag of additional questions

  3. #3
    mxh
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    Quote Originally Posted by Bart1080 View Post
    Photo/s will clarify the drawing for improved feedback and to minimise a swag of additional questions
    Here's a couple - but not sure what specifically you want the photos to show. Let me know if any details are required.

    One shows the existing walkway - I'll knock the single layer of bricks off, and the existing tiles, so that the new slab is level with the existing one. The corner it shows is the top left corner in the diagram.

    And the other shows the bottom left corner - you can see the slab under the bricks on one wall.
    corner1.jpg
    corner2.jpg

  4. #4
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    tkx, Below response is based on what Ive done before as DIY.

    struggling to make sense of your drawing, is it the l shape your concreting or the entire rectangle area and was hoping to relate the area to your photo's particularly with the walls jutting out. Photos supplied provide no perspective.




    1. Are there any issue doing what Iím suggesting? See first comment above
    2. How many bits of rebar do I need to drill into the existing slab (ie at what spacing?) F72 mesh on plastic reo stands or 8mm rebar tied together in a grid. If you tie the new slab to the old, 500 to 600mm spacings with 12mm rebar, min 50mm below the top of the slab and 100mm deep
    3. Should I rebar into the brick walls? Or if not, attach in any other way? NO, buy a roll of expansion foam to go right around the edges
    4. What base should I use Ė will the (very dry and hardpacked) dirt be OK, especially as Iíll be using plenty of rebar, or should I dig down and put in some gravel base? I'd always do a crushed rock grade 3 road base compacted down with a mechanical compactor
    5. Do I need any waterproof membrane (again, reiterating that this is under the roofline and is always very dry) a See first comment however if youve access to a small sheet of builders plastic, lay it down. Can assist in slowing down the curing rather then the ground sucking the moisture out too quickly....but not nessessary
    6. If I do need to use a waterproof membrane, how should it end (ie should it come up the sides between the old and new slabs?) See first comment, but just on the ground only
    7. I would eventually like to tile over the whole area, including the walkway (so tiles will span the join). Are there any extra precautions I can take to ensure that the two slabs donít move apart and crack the tiles? - the rebar joining the existing slab to the new is all thats required
    8. What cement / sand mix would work best in this environment? Just a 25mpa mix should be fine for a path
    9. Any other advice / ideas etc that would be useful. Make sure rebar is 50 to 60mm from the finished edges/top or bottom of the slab which is minimium recommended distance in some of the various concreting guides. Its to stop the rebar rusting (encased in concrete rather than close to the edge or exposed.. Use reo stands rather than lifting the reo into place after the pour to get the best results

  5. #5
    mxh
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    Quote Originally Posted by Bart1080 View Post
    tkx, Below response is based on what Ive done before as DIY.

    struggling to make sense of your drawing, is it the l shape your concreting or the entire rectangle area and was hoping to relate the area to your photo's particularly with the walls jutting out. Photos supplied provide no perspective.
    Thanks for the detailed response. I'm a bit confused as to what's the issue with the drawing - but understand that what I think is obvious might not be to someone else!

    I can't see anything on my drawing that I'd class as an 'I' shape. To clarify further, the bit I want to concrete is the 'L' shaped area that I've labelled 'Proposed Slab - currently dirt'. The black lines bordering it are where it meets the existing slab, the red lines are where it meets the brick walls and the thin blue lines are the where I will take the slab out to.

    I've added another photo, taken from the position marked 'door' on the diagram, which shows the ends of the two walls. The slab will run between these ends as shown by the thin blue line in the diagram - hopefully that'll help make sense of it.

    Quote Originally Posted by Bart1080 View Post

    3. Should I rebar into the brick walls? Or if not, attach in any other way? NO, buy a roll of expansion foam to go right around the edges
    So are you saying I should use expansion foam between the new slab and the old slab as well as between the new slab and the bricks?

    Quote Originally Posted by Bart1080 View Post

    4.What base should I use Ė will the (very dry and hardpacked) dirt be OK, especially as Iíll be using plenty of rebar, or should I dig down and put in some gravel base? I'd always do a crushed rock grade 3 road base compacted down with a mechanical compactor
    How deep should the crushed rock layer be? And how deep should the concrete layer be?

    Quote Originally Posted by Bart1080 View Post

    6.If I do need to use a waterproof membrane, how should it end (ie should it come up the sides between the old and new slabs?) See first comment, but just on the ground only
    I'm assuming that's between the gravel and the concrete, not below the gravel?

    And regarding the easiest / most cost effective way of doing this - I reckon I'll need approx 0.33 m3 (assuming a 100mm slab depth). Access is easy enough, so could get it delivered (quoted $460 online - not sure if that's a headline figure and I'd end up paying twice as much for 'extra's!). Would I actually save much by knocking it up myself after buying materials, hiring a mixer for the day etc?

    walls.jpg

  6. #6
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    That photo helps.

    3. Definitely the brick walls to allow for natural expansion/contraction and possibly the ends.
    4. Crushed rock - As deep as required to ensure you have a solid base with a 90-100mm concrete slab on top. Hard to judge from a photo, possibly 50 to 100mm, depending on the underlying soil. You will want to remove the top soil as a min.
    Make sure there is some fall to the outside area and that it doesn't slop back towards the ends/back. 10mm in ever meter fall in the min requirement
    6. correct

    And regarding the easiest / most cost effective way of doing this - I reckon I'll need approx 0.33 m3 (assuming a 100mm slab depth). Access is easy enough, so could get it delivered (quoted $460 online - not sure if that's a headline figure and I'd end up paying twice as much for 'extra's!). Would I actually save much by knocking it up myself after buying materials, hiring a mixer for the day etc? - It will be cheaper but is for you to judge and the variables required:
    Delivery of mix:
    - Have you got easy access to barrow in the concrete from the delivery truck? There is usually a max time allow and if you go over, then its additional cost. Its only a small amount but would be advisable to have 2 people on 2 good wheelbarrows.
    - Make sure your formwork/levels are all in place so nothing goes wrong
    - A chalk stringline on the ends and brick wall can assist with where the finished levels are required

    Hand mix:
    - suggest to have a electric cement mixer to make the operation a little smoother
    - You can get either separate sand/stone mix delivered, by the cement to mix up in your cement mixture will be the cheapest but time consuming and need somewhere to dump more than required gravel and sand
    - the other alt is you can get "premix" sand/stone delivered and buy the cement to mix in you cement mixer ...guessing $200, similar to above option
    - or but the premix bags of cement but at 33 bags is ~$300 see Blocklayer calculator link https://www.blocklayer.com/concrete/premix

    + you have some materials: 90x35 timber for formwork, rebar either for pining to existing slap and/or making your own reo grid, F72mesh, plastic reo stands, plastic membrane (if required), tie wire Estimated cost $200 (the steel is the unknown here as its increased a lot in the last 2 years)
    + tools depending on option taken - screed bar (or straight timber length), edging tool, hand screed tool, cement mixer, plate compactor Hire usually ~$60 per day, a couple of wheelbarrows Estimated cost???

    Get a few quotes over the phone, you should allow additional concrete and it will be abysmal if you run short! How much all depends on your site prep finish results ie: is it a consistent 90 to 100mm thickness or are there variables in depth over the proposed slab height. A small area shouldn't be too much extra but something to keep in mind.

    Is it worth all the additional trouble for a ~$150 to 250 saving, only you can be the judge of that

  7. #7
    mxh
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    Thanks, there's some useful pointers there. As with most things, I'm sure good prep is the key to doing a good job, so hopefully putting all this into practice it'll work out OK!

  8. #8
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    for pest/termite prevention, the slab of the building should not be covered up by concrete else termites/ants can work their way in unnoticed and do a lot of damage.
    any pathways etc.. should not be higher than the building slab.

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