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rebar problem on footings i've just cast

Discussion in 'Building' started by tvrbeaver, 8 Oct 2014.

  1. tvrbeaver

    tvrbeaver

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    Just become aware of a problem with my cast footings.
    I’ve just cast a 3.5 x 3.5 meter L shape onto the house to take my new extension.. its a cast strip footing 600mm wide and 10 to 15 inch deep depending where in the L you measure it. The trench has been dug out and is on real solid clay / stone bed so very good base.
    However, one area where the existing drains ran has been cut deeper initially and was a tad soft. The planning engineer said put a bit of mesh over this area so basically we put rebar spaced off the floor on that leg. Our builder said we could put the left over rebar in below the surface once poured to add additional strength... all was going well and the trench was poured... but then I had to take a call on my daughters car. My ‘Mate’ who was helping for some reason picked a bit of mesh up and placed it into the concrete.. he pushed it down 2-3’’ as we’d done on the other leg....
    He told me what he’d done when I came back and I thought nothing more about it until today when I noticed which bit he’d put in... basically I’d been using double squares (16'') by 3.2 meters long (8’’ mesh).. so I’d cut the strips from both sides of a sheet (these went in the base).. this left a strip with 8’’ squares in the middle but with 7.5’’ bits sticking out either side to give a total width of 23” so 584mm... (you see where this is going??....)
    So I’ve basically got cast into one of the legs at 2-3’’ deep a piece of rebar where the ends could be close to the side of the concrete... min recommended distance 2’’ !
    What to do?.. do I just leave it?...
    Or do i dig down the side of the new concrete once fully set (on the outside leg of the L).. and either cast some more in... or put some other protection in there like tar or something to stop any potential of the rebar ends getting wet and rusting into the structure...
    If i just left it and it did rust over time.. how long would it take?... 8mm dia bar BTW.. and if it did crack up the footings would it matter as it can’t actually go anywhere and it’s 95% on very solid ground....
    Any advice appreciated on this self inflicted issue 
     
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  3. tony1851

    tony1851

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    FFS find something serious to worry about! :LOL: Nothing's going to happen to it.
     
  4. tvrbeaver

    tvrbeaver

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    Won't it rust over time and crack the concrete? And enable water to get into the footing. Not sure how long it takes to rust out but have seen the result of rebar cracking on old concrete structures? Or is it seriously nothing even to think about? Cheers
     
  5. Andyc84

    Andyc84

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    Its a problem on bridges and the likes because the strength of the structure is in the steel, in your case, the strength is in the concrete, it is in compression not tension, and as you say, the ground it is on is good so the rebar was only to help with the weak spot.

    In the worst case you are likely to find cracks around the ends of the bar closest to the surface where the bar is, it is unlikely to crack the foundation through and you won't have loss of concrete section as the lot will be burried so nowhere for the concrete to go...
     
  6. tvrbeaver

    tvrbeaver

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    Thanks for your replies... TBH that's what I was thinking... even worst case if it all cracked up into 8'' square blocks, where would it go?.. so the structure above ground should show no signs of anything below.. I'll mention it to the builder as there may be some kind of foundation block he can use to minimise any longer term effect.... I did think about soaking the area with concrete sealer to stop any ingress?.... (although ingress would tend to be from the side i guess)
    any idea how long 6-8mm rebar takes to rust (obviously depends on the conditions it sees)...

    I'm a aerospace engineer to tend to be OTT on things... important if you're on board, but may be not so critical from a foundation point of view LOL

    ;)
     
  7. jeds

    jeds

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    50mm cover is a specification for bridges and road structures to counteract excessive chloride build up from de-icing salts. (it's chlorides that rust re-bars - not the water) 25mm cover is standard for domestic structures. Even at 25mm cover you won't be around to deal with the consequences.
     
  8. tvrbeaver

    tvrbeaver

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    but if its 10mm?.. or even no cover??.. what happens and how long? :rolleyes:
     
  9. JohnD

    JohnD

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    think of a concrete fence post. The steel is less than an inch from the surface, but generally will not rust and spall until, on the top, the weather has eroded the concrete away and exposed it. Or when someone has driven a car into the post and cracked the concrete.

    Underground concrete usually stays wet. As well as improving its strength and hardness, this prevents air reaching the steel, which is needed for rust.
     
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  11. tvrbeaver

    tvrbeaver

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    so keep it wet and it wont rust?.... Mmmm if i put a metal bar in my pond it rusts .... are you saying there is no Oxygen in water below ground??...

    The problem is that the ends 'could' be sticking out the sides of the concrete... :)
     
  12. JohnD

    JohnD

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    there is some oxygen in groundwater, but well-consolidated dense concrete is not very porous, and will greatly slow the diffusion of air through the concrete towards the surface of the embedded bar.

    Packed clay is waterproof.

    Air-entrained mortar with additives is much more porous
     
  13. tvrbeaver

    tvrbeaver

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    so it all depends on the interface between the concrete and the clay... and at 2'' deep from the top of the cast (worst case).. it should have found its way to be tight against it... let's hope so... sounds like clay is my best option for keeping it oxygen free anyway....

    it looks like 'doing something' with it would actually turn out to be more detrimental than just letting it be...

    its put my mind at rest anyway... thanks for all your replies :)
     
  14. tvrbeaver

    tvrbeaver

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    I've just looked down the hole and the clay does not start until further down.. so if the rebar is stuck out its onto a softer clay / sand area... so will get water going past it at it will sink and hit the clay level and stop... probably right where the rebar is... great !!
     
  15. JohnD

    JohnD

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    I suppose you could paint it.
     
  16. tvrbeaver

    tvrbeaver

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    Paint it ! So you think it does need attention ? Paint it with what as it would need to last many years ? POR15 is very good ? Although it will be quite a rough side cast if exposed.
     
  17. freddiemercurystwin

    freddiemercurystwin

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