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Thread: Concrete Engineering Question

  1. #1
    Rob Will Guest

    Concrete Engineering Question

    I am in the process of pouring an 8" thick 36" high concrete retaining wall for my new WW shop. This wall is 80' long and returns 30' across one end. The wall sits on a 24" footer with plenty of steel. Every 4' or so there is a #4 rebar cast into the footer and extending up into the wall forms.

    The building will be post frame construction and one wall will sit directly on the 8" concrete retaining wall. Every 8' we have positioned a 4" steel channel to be cast into the concrete in a vertical position. The purpose of these channels is to extend above the wall about 2' and hold the 16' wood posts (using bolts). (The posts for the other two sides of the building will be installed in the earth as in typical post frame construction). Each of the steel channels has 5 holes cut with a torch and #4 rebar goes through each of these holes. The 5 rebar extend for the entire length of the wall and are each tied to the rebar cast into the footer.

    Because this wall is critical and supports one wall of my building, I want to do the best possible job and want to specify the optimum concrete mix. We want something that will flow well around all of this steel and have excellent strength and finish. We may not actually rub the wall out but we will definitely use a vibrator. We are set to pour this wall on Friday.

    So WOODWORKERS, there must be some engineers out there in WW land that can tell me what concrete mix to use for this wall. Example: 4000 psi, 5000 psi, pea gravel, 1" aggregate ....etc, etc. Thanks for any concrete expertise you care to offer.

    Rob
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  2. #2
    Join Date
    Nov 2003
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    Well, a long time ago i used to do concrete inspections for a testing service. I would recommend a 4,000 psi mix, keep the slump up (keep the mixture dry and vibrate to consolidate it. Make sure the forms are strong as the vibrator can cause them to blow. A good 4,000 mix poured on the dry side will yield over 5,000 on the test breaks. Keep it from freezing and you will do fine.

    CPeter

  3. #3
    Join Date
    Jan 2006
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    Corpus Christi, Texas
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    I have a good friend that is a concrete engineer down here in Texas. He is always helping me with several jobs. From what he has told me I would go with 5000 psi and I would also call someone out to do a slump test on the concrete before it is poured. Alot of guys love to pour very wet concrete because it is easier to work with but the more water that is added the weaker the concrete will be. Also make sure that you vibrate the concrete or you have honeycombing all over the place and that is ugly!
    Last edited by Keith Webster; 02-09-2006 at 1:09 AM.
    The Beach Bum Woodworker

  4. #4
    My dad's a civil materials engineer, and I grew up as a slump puppy in his testing lab through high school. In my adult life, I've done a lot of on-site concrete inspection for testing labs and engineering firms, and have a some background in structural design and inspection. Offhand, I don't recall seeing any structure like this using 4000 or 5000 psi concrete...usually I've seen 3000 or 3500 psi mixes specified unless it was it was for paving or big commercial or public utility structures. I've seen a lot of commercial structures placed on 3000 psi footings and stem walls.

    That said, unless the cost is prohibitive, go with a stronger mix. 4000 psi would be more than sufficient, but 5000 psi would be overkill IMHO. (Although if it's within the budget, why not? Can't hurt.) I also heartily endorse the other suggestions to make sure the contractor keeps the mix relatively dry (low slump). If you're concerned about flowability, consider using a superplasticizer instead of adding water. (Your concrete supplier can make recommendations about this and any other admixtures, but they need to know about it before the mixer truck leaves the plant.) It's good to see you're already making sure it's properly vibrated. Not only is the honeycombing ugly, it's potential trouble down the road. I also second the recommendation to prevent the wall from freezing for as long as possible.

    HTH -

    - Vaughn

  5. #5
    no formal training here what-so-ever just lotsa years in the trades......4 bag mix is plenty, 5 bag is overkill.......02 tod
    TO WHOM IT MAY CONCERN; I ACCEPT FULL LEGAL RESPONSIBILITY FOR MY POSTS ON THIS FORUM, ALL POSTS ARE MADE IN GOOD FAITH CONTAINING FACTUAL INFORMATION AS I KNOW IT.

  6. #6
    Rob Will Guest

    Weather

    I thought I could pour this Friday but now the weather forecast is for snow Friday. Not sure if I can finish with the setup today but I had best get out there and start slinging a few forms. Thanks for the suggestions.

    Rob

  7. #7
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    It's the temperature that matters. Generally if it's below 40, you need to take some cold weather steps which usually involve keeping the mix above 50 and not allowing the forms/material to freeze. This also involves keeping it heated while it cures, but may not be necessary in residential applications.

    Make sure the contractor doesn't give you left over concrete from another job. That's a classic sleazy-contractor trick.

  8. #8
    Join Date
    Apr 2003
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    Oakdale, CT
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    The 3000 PSI mix should be all that you need for this application, but as mentioned if the 4000 is not a cost breaker go for it. As an engineer my concern 2 fold. With the temps low keep it dry and vibrate along with the appropriate cold weather additive to keep from freezing. You can place concreat blankets and hay around the forms. The second concern is that you will have the structure fixed to this wall and the rest of the structure to posts in the ground. You should strongle consider pouring footings for these to the ground level and supporting the whole structure this way. If not you risk settlement over time and all the issues related to that. Good luck, rent what you need and get as many friends to help as you can. This work goes quick.
    Bob Nazro
    A Connecticut Yankee
    I've measured this three times and each time I cut it, it's still to short...

  9. #9
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    I believe in overkill. Go with the strongest mix within the budget...Build it strong. Its the foundation for all.
    Jerry

  10. #10
    Join Date
    Feb 2003
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    Harrisville, PA
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    Hi Rob,

    Make sure that you have excellent drainage behind that wall. I don't care how strong the wall is hydraulic force and time will move the wall if it doesn't have proper drainage. Did you have a soil test done? Anytime you cut the toe of a slope you are creating potential for movement.
    Chuck

    When all else fails increase hammer size!
    "You can know what other people know. You can do what other people can do."-Dave Gingery

  11. #11
    Rob,

    I think you are potentially creating problems with those "I" beams. They look like they may create a natural break point in the wall and potentially make the concrete wall weaker,settle and crack. Have you thought about creating separate footings for these?. Or hae you thought about making pilasters out-bound of the main wall section and put anchors and a mounting plate for the steel posts to sit on and bolt to the foundation? My recommendation is to consult a certified architect or engineer before pouring.
    Last edited by David LaRue; 02-09-2006 at 9:20 PM.

  12. #12
    Rob Will Guest
    Quote Originally Posted by David LaRue
    Rob,

    I think you are potentially creating problems with those "I" beams. They look like they may create a natural break point in the wall and potentially make the concrete wall weaker,settle and crack. Have you thought about creating separate footings for these?. Or hae you thought about making pilasters out-bound of the main wall section and put anchors and a mounting plate for the steel posts to sit on and bolt to the foundation? My recommendation is to consult a certified architect or engineer before pouring.
    David, I have been worried about the same thing. Unfortunately, if weather allows we have to pour tomorrow. Otherwise the trench will collapse and fill the forms full of mud. I think if I had it to do over, I would simply cast some anchor bolts into the wall and attach an angle plate later. We do have 5 rebars in the wall but I'll bet you are correct.....the wall will crack at the steel channels. Lets hope that this will be a hairline crack and not turn into a real problem . Keep your fingers crossed for me.

    Rob

  13. #13
    Join Date
    Jan 2004
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    Lewiston, Idaho
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    Rob....Good luck with your pour! I'll be thinking about you.
    Ken

    So much to learn, so little time.....

  14. #14
    Rob Will Guest
    Quote Originally Posted by Matt Day
    It's the temperature that matters. Generally if it's below 40, you need to take some cold weather steps which usually involve keeping the mix above 50 and not allowing the forms/material to freeze. This also involves keeping it heated while it cures, but may not be necessary in residential applications.

    Make sure the contractor doesn't give you left over concrete from another job. That's a classic sleazy-contractor trick.
    Matt, temps will be good for pouring tomorrow but we have blankets for the next few nights. No contractor here except us farmers.

    Thanks,
    Rob

  15. #15
    Rob Will Guest
    Quote Originally Posted by Charles McKinley
    Hi Rob,

    Make sure that you have excellent drainage behind that wall. I don't care how strong the wall is hydraulic force and time will move the wall if it doesn't have proper drainage. Did you have a soil test done? Anytime you cut the toe of a slope you are creating potential for movement.
    Chuck, good point......actually the footprint is mostly an old building built in 1956. The wall eliminates the rip rap embankment where groundhogs like to live. Tore the whole building down.......that'll show em'.

    Here's the drainage plow.....well maybe we won't use it in the yard. We'll take your advice on this one.

    Rob
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