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Discussion Forum

concrete cold joint

mackllm | Posted in General Discussion on March 1, 2006 09:23am

I am thinking of pouring a basement wall for a 12 X22 addition.  I have been thinking that I can pour it in two lifts of 4 feet each.  I will have a cold joint at the 4 ft mark but I figure if I put a key way in the bottom pour and rebar the joint I should have a strong connection.  Also most of the wall will be above grade or part of a stem wall.  One 12′ section will have about 6 feet of soil behind it so the joint would only have about two feet of soil above it.

I was also thinking of making the wall 12″.  Won’t really cost that much more because of the minimum delivery.

Any thoughts on whether this is doable?  Also looking for ideas on the size and spacing of the rebar.

Thanks

Allen 

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Replies

  1. Catskinner | Mar 02, 2006 12:47am | #1

    No reason why not.

    I would run rebar top to bottom rather than just at the joint. An old engineering principle that translates surprisingly well into the rest of our lives; A localized strength can produce a generalized weakness.

    If you are in seismic zone 2A or less, #4 bar 2' oc should be more than enough.

    Clean the joint real well if you need to (air, pressure wash, hose, whatever) before the second pour. Mix up a dash coat of half Anti-Hydro and half water, mix in enough straight Portland cement to make a sloppy paste, and dash it on to the joint right before the second lift.

    http://www.anti-hydro.com/brochure.htm

    The stuff really is amazing. Works well on grout, too.

    Shouldn't have a care in the world then.

    Power always thinks it has a great soul and vast views beyond the
    comprehension of the weak; and that it is doing God's service when it is
    violating all his laws. -John Adams, 2nd US president (1735-1826)

  2. justaformguy | Mar 02, 2006 01:30am | #2

    For the record, a key way provides no structural support. So I wouldn't count on that being approved by your BI.

    1. Piffin | Mar 02, 2006 01:44am | #7

      Depends what kind of structural strrength you nean. It sure helps with lateral shear 

       

      Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

      1. mackllm | Mar 02, 2006 01:52am | #11

        Lateral shear is something to consider.  The soil is clay (would backfill with good gravel).  Years ago I built a block wall where the foundation would now go.  The block wall has multiple cracks and pushed inward a couple of inches.

  3. Piffin | Mar 02, 2006 01:34am | #3

    Why not pour it all at once? It would be so much easier and better.

    To do it this way...

    Pause a minute - you haven't filled out your profile so I don't know where this is happening, but if this joit is at the same level as the worst of the frost lens in the ground, the weakest part of the wall will be forced to resist the greatest pressure exerted against it - if you have any freeze in the ground

    ...you will need extra rebar placed so as to resist the inward pressures - lateral loading of the soils

     

     

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    Taunton University of
    Knowledge FHB Campus at Breaktime.
     where ...
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    1. mackllm | Mar 02, 2006 01:43am | #6

      When you say frost lens I am not sure what you mean.  I live in MA (did update my profile) where we need to have 4' frost wall. 

      I would like to pour the concrete myself and an 8' wall is a bit much for me.  I have done a four foot with no problem. 

      Getting a foundation guy is not that easy.  Maybe getting easier with a slowing housing market.

      Extra rebar is not a problem as it is fairly cheap.

      1. Piffin | Mar 02, 2006 02:07am | #12

        The concrete pours in at the top of the form. So it is no harder for one man to let to run eight feet deep than four feet. What am I missing there?A frost lens is, briefly, the layer of soil where most freezing water cllects and does most damage to surronding structure.
        Say - the water starts to freeze at 18" down below the surface. Once the soil there is frozen, it is harder for any surface water to penetrate it, so even if there is a warm day thawing or a warm rain, when the surface water hits the frozen strata, it freezes thicker every time more water is added to it.
        Generally it is worst in late winter/early spring, which is why you see the frost heaves in the road then. Water cannot go down any more, so it heaves the surface up instead. In the case of a foundation, it pushes laterally at the joint you propose.So for your purposes, if you do have to make a cold joint, you do not want to simply "add in some rebar"
        It needs to be placed in the right portion of the wall to offset teh stresses that will develope on the joint. Whether that is in the inner third of the wall or the outer third of the wal will depend on the level of the soils relative to the joint and the amount of anticipated laoding.In pure model, if the worst of the load( I have to assume for illustration here) is puching inward at the joint, then you need to place the rebar nearer teh inner surfacce of the wall, so it acts in tension to resist the thrust. Placing it on the exterior side or the middle will render it nearly useless, except the resist shear. It will merely act as a hinge pin when the wall flexes. 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

        1. mackllm | Mar 02, 2006 02:12am | #13

          The problem with an 8 ft wall is the stress at the bottom.  According to articles I have read the pressure at the bottom of a 8ft  wall is 1200lbs vs. 600 for a 4 ft wall.  My feeling is that I have a better chance of not having a  blowout with a four ft wall.

          I am also trying to find an economical solution to getting the foundation done.  I would need much more lumber for a 8 ft wall. 

          1. Piffin | Mar 02, 2006 02:39am | #14

            all that is true enough, and a 12" thick wall is going to develope more pressure than an 8" wall will - to each his own, but I would sure hate to have to reset forms that way myself. Twice the work more or less.Have you considered ICF blocks to do this? You get the insuilation value built in, they are very easy to set up, and they would use only about 55% of te same amt of crete as a 12" wall. That would help offset the cost of the ICF blocks 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          2. mackllm | Mar 02, 2006 02:43am | #15

            I have though of ICF may concern again is that a 8 foot wall held in place by foam would blow out.

          3. Piffin | Mar 02, 2006 04:29am | #16

            You pour it slow and keep moving around to get the first four foot lift , then have the second truck start at the beginnings and finish before the joint is cold, but the base is settled enough to be safe. I have poured a few 8' walls with ICF and never a blow-out. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          4. User avater
            trout | Mar 02, 2006 05:56am | #17

            I have though of ICF may concern again is that a 8 foot wall held in place by foam would blow out.

            As Piffin pointed out, pouring the walls in "lifts" limits the amount of pressure on the wall to basically the hight of the lift.  ICF walls should be vibrated and at the juncture of the two lifts you only vibrate deep enough to mix the two lifts.  By carefully watching the wall during vibration it's possible to get a feel for how much pressure is at any one area.  Using a low powered vibrator also helps, and the corners aren't directly vibrated at all.

            What you might consider is buying ICF block, setting the block up yourself and hiring an experienced crew for a couple hours to do the actual pour.  We've done this with clients and it worked out very well.  We would typically send out one person to go over the specifics of putting the block together and work with you for a few hours so you get the hang of it.  Then, our guy would visit to see how things are going and point out potential problem areas.  Finally, the four of us would do the actual pour and some final aligning.

             

  4. User avater
    Soultrain | Mar 02, 2006 01:38am | #4

    I don't see why that wouldn't work (especially if you have rebar throughout the entire wall ).

    It would have to be at least as strong as a grouted block wall...

    1. Piffin | Mar 02, 2006 01:48am | #9

      "It would have to be at least as strong as a grouted block wall..."but that is not very strong! The reason for poured crete must be because he wants more, I'd have to assume. 

       

      Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

    2. mackllm | Mar 02, 2006 01:49am | #10

      I was also thinking of a 12" block wall grouted an re-barred.  From what I have read this does not seem to be a good option.

  5. sungod | Mar 02, 2006 01:38am | #5

    Why 2 pours? I hope your not hand mixing. Cold joints a few minutes old can be vibrated. A bad cold joint (gaps and air bubbles) will leak and also rust the rebar.

    1. mackllm | Mar 02, 2006 01:47am | #8

      No hand pouring.  I have done that enough to know better.  I would get about 5 yards ready mix delivered for first pour then strip the form and set or the top half.

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