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Concrete Deck Footers

rock | Posted in Construction Techniques on November 10, 2003 01:30am

I am a homeowner building a freestanding deck in my backyard. I have dug 13 holes 42 inches deep and 12 inches in diameter. I would like to pour concrete footers approximately 8 inches thick and place 6 X 6 posts on top of these footers and backfill to stabalize the posts.

My problem is that after many months of large amounts of rainfall the water table is much higher than usual. Four of the holes continue to have standing ground water.  When the water is pumped out by hand, it returns within a day. At times the water has been as deep as 30 inches.

I have had recommendations such as waiting for a dropping of the water table (we have already been waiting 5 months,) another suggestion was to pump out the water, immdeiately pour the footers and forget the problem existed, lastly it was suggested that I use hydrolyic concrete……but I do not know of a source for it????

Does anyone have any experience with this type of problem. We live in Bucks County Pennyslvania.

 

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  1. Piffin | Nov 10, 2003 03:55am | #1

    pump and pour immediately.

    Use PT for the posts.

    Do you get frost? If so, wrap the subterranean part of the post with plastic so the frozen ice lense does not get a grip on the wood to heave it out.

    .

    Excellence is its own reward!

  2. BobKovacs | Nov 10, 2003 05:34am | #2

    Dump dry concrete mix into the holes, and fill them to the top- don't set the posts below grade- the changing water table will rot them, even if they're pressure-treated (that'll only prolong the rot time). 

    Some will say that the uncontrolled amount of water will weaken the concrete's strength.  Bear in mind that you'll never get the footing to fail in compression- you're just trying to spread the load of the post to the soil below.  The natural moisture in the soil will cure the concrete just fine, and you'll save having to mix all that concrete.

    I built a 26x24 deck with around 24 posts about 10 years ago using this method (and many more since), and none of them have moved.

    Bob

  3. TLJ | Nov 10, 2003 06:38am | #3

    To add to Piffin's question/comment about frost heaving: flare the bottom of the holes into what I've heard called an "elephant's foot." This flared bottom resists frost heaving of the concrete out of the hole.

  4. csnow | Nov 11, 2003 12:39am | #4

    Pump & pour.

    The groundwater will only make the concrete stronger.  A "slow wet set" is considered ideal.  If you are worried about heaving, use cardboard tubes to bring your pour slightly above grade.  Smooth sides prevent heaving.

    1. numbfinger | Nov 15, 2003 02:10pm | #6

      Quick question...

      I was recently reading that concrete requires the absolute minimum amount of water for maximum strength (concrete counter top book) - you indicate that a "slow - wet set" is ideal?

      Can you clear up my confusion?

      1. PurpleThumb | Nov 15, 2003 07:24pm | #7

        Take a look at the plastic deck footings described in the Oct/Nov issue of Fine Homebuilding, p. 36.  Should avoid the problem of concrete completely.

      2. csnow | Nov 16, 2003 04:53am | #10

        "Quick question...

        I was recently reading that concrete requires the absolute minimum amount of water for maximum strength (concrete counter top book) - you indicate that a "slow - wet set" is ideal?

        Can you clear up my confusion?"

        Sure. They are right as well.  That is confusing!

        A dryer mix makes a stronger concrete.  A slow wet set makes a stronger concrete.

        The difference is that the groundwater will not be part of the mix.  It will come along later, and keeps the set concrete wet while it cures. Curing reaction can take months to completely finish. If it drys, the curing stops.  Most portland products are rated based upon a 28 day cure.

        A wet mix is weaker because water makes the concrete less dense.  Anyplace there was water becomes a void after the concrete sets. Water is essential for the reaction, but does not contribute structurally. The strongest possible concrete mix would be too dry to work with.

        1. Buglehead | Nov 16, 2003 10:28am | #11

          csnow - Thank you for writing such a clear, succint answer to the gentleman's question.  Good Job.

      3. slykarma | Nov 16, 2003 12:00pm | #12

        Yes it is true that low water:cement ratio gives stronger compressive strength values. What the poster termed 'slow wet set' I think refers to moist curing. Curing allows the concrete to fully hydrate - and therefore achieve maximum design stregnth - but in a controlled manner. In many cases surface water evaporation prevents the cement in the upper layer from getting the water it needs for full strength. This is why slabs laid in hot weather are often sprinklered or covered in wet burlap after finishing is complete.

        Wally

  5. mike4244 | Nov 11, 2003 02:29am | #5

    Buy a sump pump, there not exspensive( about $55.00 or so). Pump water out pour 3" slump and don't worry about the water. As a previous poster mentioned the water around tyhe outside of the piers will make the concrete stronger.

    mike

  6. User avater
    CapnMac | Nov 15, 2003 10:34pm | #8

    You can pour concrete into water, as the concrete will displace water.  What you may not be able to do is get a water-filled hole ok'ed by an inspector.  Also, sonotube forms do not like to be immersed in water.  No real problem, just set them just before you pour.

    As previously stated a low slump (3-4") concrete around 2800-3000 psi will cure better in damp conditions.  As it goes into the holw, it will displace any standing water--so be ready when the water comes out.

    Get the footers up above grade, then use a post anchor to set the wooden posts on those.  That keeps the wood out of ground (or concrete) contact, which is good for the posts & your deck.

    Occupational hazard of my occupation not being around (sorry Bubba)
  7. MojoMan | Nov 16, 2003 01:16am | #9

    I had to deal with this this summer. I used a submersible pump to remove the water. Then, I put some gravel in the bottom of the holes to make a firm base.  I cut the bottoms out of some plastic 5 gal. buckets and put these on the gravel to make forms for the footings. Since most of the water was coming in from the sides of the holes, I wanted to protect the concrete until it set up a bit. As soon as the footing concrete was firm enough to support the cardboard tubes, these went on top of the footings, the holes were back-filled and the rest of the concrete was poured to above grade.

    Al Mollitor, Sharon MA

  8. User avater
    hammer1 | Nov 16, 2003 12:27pm | #13

    Don't worry about the water. We pour under water all the time in bridge and dock work. For deck tubes I place the sona tubes in their holes, run a string to line up, backfill carefully. leave the tubes just above finished grade 1"-2" and fill with concrete. Place an anchor bolt in the wet concrete to later use for the post base. As someone else said, posts should not contact ground or concrete. There is no guarantee, mother nature may decide to push a tube up and out but you have done your best. Frost in the back yard seldom goes deeper than three feet here in Maine and we get cold.

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