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concrete vibrator

Yersmay | Posted in Construction Techniques on March 16, 2008 09:42am

I now have a trench dug that is 40 inches deep, not including the 4 inches of slab that dowels into an existing slab. On top of this is some form work that defines a curb – 5 3/4 wide and approximately 4 1/2 inches high (above the slab). The width of the trench is 16 inches. There are four horizontal #4 rebars running down the center of the trench with vertical #4’s every 16 inches O.C. I have seven anchor bolts already positioned. It’s just about time to order the cement truck (assuming it passes inspection tomorrow) and now it’s time to admit – I’m a newbie at this. Yikes. Here are some questions.

1. Unless the pump hose can fit between rebar and formwork, the concrete will fall about 40 inches. Is that okay? If not, what to do?

2. I’m planning on placing the concrete in 1 1/2 foot to 2 foot lifts… In other words, fill the trench about a third of the way before going back for another layer. Good practice? As I go, I will have a helper poke the concrete with a shovel to try to break up voids. If anyone can amplify on strategy of how this trench should be filled with a pump mix, please do.

3. I’ve been advised that a concrete vibrator would be a good thing. I’ve also read some cautionary posts about this tool. My concrete textbook described a bit about making sure the tool penetrates the previous lift for a good mix. In my case, is the vibrator used for each lift or only after the entire trench is filled? It also said to look for visual cues such as aggregate sinking out of view and mortar puddling on the top surface and along the forms. If anyone can describe a bit more about how to use this tool, it would be greatly appreciated. Mainly, how much vibrating is enough? What is the distance between the points at which you vibrate? Do you vibrate along the sides of the trench or only down the center? Do you angle the vibrator across the width of the trench or does it dip straight down?

Thank you in advance for any and all advice!

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Replies

  1. davidmeiland | Mar 16, 2008 10:07pm | #1

    1. Rule of thumb is to avoid dropping concrete more than 4 feet. If you can rig up some sort of chute... a piece of 12" drain pipe split in half, or similar... and slide the mud into place that's a little better.

    2. Place the concrete in 12" lifts if you can. If you are using a pump this should be fairly easy. If it's all being tailgated and it means moving the truck repeatedly, forget it.

    3. A vibrator is essential. Rent one (actually, around here, rent two, because the first one will break). Go around the pour and vibrate each lift. Then go around again after the next lift, running the vibrator down into the first one. Stick it in fairly quickly and remove it over a period of maybe two seconds. It is possible but not easy to over-vibrate concrete. Keep moving and don't go answer the phone with the vibrator left in the pour.

    I am a big fan of water reducers, additives that make the concrete flow MUCH better with a lot less water. We typically get concrete batched for a 3-4" slump and then add water reducer to get to a 6-7" slump. Makes it a lot easier to move the concrete around. A good stiff mix with no WR will pile up like dogsh!t and need a lot of rodding. With WR it flows out. Talk to your batch plant about this. I would NOT use WR if I were doing a pour where 'crete might be able to flow out of the forms, such as a sloped walkway. Your trench sounds perfect for it.

  2. rez | Mar 16, 2008 10:15pm | #2

    bump

     

     

    Peach full,
    easy feelin'.

    1. brownbagg | Mar 16, 2008 10:45pm | #3

      waiting on me huh....1) rule of thumb is four feet, but it has been proven fall doesnt matter, it can fall as much as twenty feet with no problem.2) vibrator yes3) I would use a super plastizer so number 2 lift height doesnt matter. just dont blow outRemember do not over vibrate, just stick it in and pull out.

  3. Riversong | Mar 16, 2008 10:56pm | #4

    You're making this much more complicated than it needs to be.

    With a 16" wide by 40" deep trench, just fill to top from the transit mixer shute.  Drop is not a problem - 8' high forms are filled from the top and in one lift.  By doing such a small pour in lifts you're risking poor bonding and requiring a vibrator which would be otherwiser unnecessary.

    With that wide a trench and no forms to blow out, a little extra water won't hurt if there is not good truck access to each corner or not enough shute and it helps in moving the mix around (the earth is going to absorb some of the water anyway).

    Forget the vibrator.  Don't use a shovel, use a piece of 2x4 and rod the concrete (tamp it a bit) to fill voids.  You're not looking for a pretty wall since it's underground, just filling the trench fully.

    This should be a piece of cake.  Stop worrying.

     
    Riversong HouseWright
    Design *  * Build *  * Renovate *  * Consult
    Solar & Super-Insulated Healthy Homes



    Edited 3/16/2008 3:57 pm ET by Riversong

    1. Yersmay | Mar 16, 2008 11:45pm | #5

      Thank you all for your responses. There actually is an opportunity for a blowout. I'll try to describe the situation and even better I will try to post a picture. One end of the trench is open. One one side of the open is house foundation which is parallel to the direction of the trench. On the other side of the open end is the edge of the existing slab and that edge is perpendicular to the trench. Since it is the edge of the slab, there is footing thickness under the slab. I used a Ramset to apply a vertical ledger to the house foundation and a vertical ledger to the slab and underlying footing. It's kind of hard to explain but the 2 X 6 that blocks the opening is locked in place, tucked behind one ledger and lag bolted to the other ledger. I am, however, depending on the strength of the Ramset nails to hold. I used four or five 2 1/2 inch ramset nails, set with a red shot, for each ledger. This, of course, keeps me up at night. Given this added piece of information, is the use of a water reducer now a bad idea?

      1. Riversong | Mar 17, 2008 12:15am | #6

        That blocking doesn't look like it's going anywhere.

        Keep the mix to a 4" slump and you'll be fine.

          

        Riversong HouseWright

        Design *  * Build *  * Renovate *  * ConsultSolar & Super-Insulated Healthy Homes

      2. Henley | Mar 17, 2008 12:46am | #7

        I agree your bolts look solid. That said when I'm loosing sleep,
        I'll back fill against the weak spot.
        Just pile up some weight against the form and dig it out after. Not necessary but "Just cause your paranoid, doesn't mean there not out to get you".

        1. brownbagg | Mar 17, 2008 12:50am | #8

          thats an easy pour, 3 inch slump. add a super p, pour away, tap the wood with a hammer, no vibrator needed.

  4. bobbys | Mar 17, 2008 02:23am | #9

    i dont think you really need one but as a side note i worked on bridges and the inspectors stood by me and told me how much i could vibrate, We had so much rebar and the mix was so dry we needed it to even get the concrete in the forms, When they were not looking i {insert bad vibrator joke here} really cranked that puppy up, OK theres no speeds on it it just sounds good.

    1. Yersmay | Mar 21, 2008 04:31am | #10

      I thought I'd resurrect this thread to let you all know we placed the concrete today! It's really quite something when the concrete truck pulls up. It went fairly well. It wasn't a nightmare but it wasn't easy. I was very lucky to have an excellent pump operator and he really led the way during the pour. I used a vibrator minimally but it seemed that jamming a two by four into the concrete pushed everything into place just fine. My main problem arose when it was time to remove the forms. The day before I had oiled the forms with linseed oil and I didn't expect to have the concrete stick... but it did. In some places it pulled away chunks that left voids. Instead of beating fresh concrete into the side of the fragile curb, I decided that I would repair this another day. Now, the question is -- how? Would it be a good idea if I applied concrete glue and then floated mortar mix? I've posted some pics of the foundation and curb and a pic of the worst of the voids.

      1. brownbagg | Mar 21, 2008 05:11am | #11

        concrete was too wet and you pulled your forms too quick

      2. Henley | Mar 21, 2008 02:57pm | #12

        Looks like its just cosmetic damage.
        Are you going to see it in the end? If so parge it smooth. You might want to parge the whole surface
        to match, if it really matters.

        1. Yersmay | Mar 21, 2008 05:13pm | #13

          Yes, I believe it is only cosmetic in nature. There's plenty of concrete behind that void and I think the curb is still sound. But I'm wondering what material and/or method would be the best choice to make the repair. Would an application of concrete glue with a couple of 1/2 inch layers of mortar mix be a good idea? Would I be better off using non shrink grout? Although the area is visible it will be tucked between the weep screed of a new stucco wall and the slab below it. If the repair is really ugly I'm guessing there are materials with which I can parge the surface of the curb for better uniformity. Thank you in advance!

          1. Henley | Mar 22, 2008 01:16am | #14

            you will probably get some more technical answers
            soon, but my two cents- Common mortar will bond very well to green concrete.
            Yes fill it over a couple of layers instead of one,
            and some bonding agent is always a good idea. This isn't a serious problem, just make it look nice. PS

            I always seal any mortar that close to the ground.

          2. Yersmay | Mar 22, 2008 03:25am | #15

            Thank you for your response, Henley. I spoke with a builder friend today and learned quite a bit. I will offer this in the hope that other people can learn, too... or maybe just start a discussion about various strategies on how to remove form work. Apparently, it is a bit of an art to remove form work. My friend tells me the best way to keep from ripping up the sides is to break the bond by sliding the form work in a parallel motion to the foundation. Simply rocking the board and pulling is not such a good idea, I learned. In my case, this would have taken some time and thought when I was building the forms, designing things in such a way that I would have had a gap to allow the form board to move parallel to the foundation. But it could have been done had I known. Today, I used a quick setting mortar mix to patch the sides of the curb. I floated it out with a wooden float and it's fine texturally, but the color variation makes it painfully obvious. Later, I will research the best way to parge this surface for a uniform look. I think it would make a fascinating thread -- the subject of disassembling concrete form work. First, what are the visual and textural cues that inform when the concrete is ready to stand on its own? I'm sure the experts on this board have other great ideas and strategies of how to break the bond between wood and concrete without damaging the surface. As to form oil... I used linseed oil, but maybe there are better choices? All information would be greatly appreciated!

          3. mike4244 | Mar 22, 2008 03:41am | #16

            Removing formwork is not art form. I could teach an oraguantan to do that.First thing you did wrong was use linseed oil. Form oil is the correct oil, rolled ,sprayed or even mopped on. The concrete was not set up completely when you stripped the forms.

            Patch it , parge the area above grade and be done with it.

            mike

          4. davidmeiland | Mar 22, 2008 03:44am | #17

            Agreed. I didn't read the OPs followup but it sounds like maybe same day stripping. Hard to get away with...

          5. Yersmay | Mar 22, 2008 04:26am | #18

            Yes, it was same day stripping... but isn't that done all the time? I needed to finish the concrete so the forms had to go... Maybe I stripped them prematurely, but the concrete seemed pretty set up by then. I don't know how I could have troweled it out if I had waited more. I will never use linseed oil again. I'll spring for a jug of official form oil and hopefully that will help!

          6. brownbagg | Mar 22, 2008 04:29am | #19

            we dont strip till four days

          7. mike4244 | Mar 22, 2008 04:52am | #20

            The top has to turn white first,this only tells you the top has set up.Yes ,stripping the same day is done often. The concrete stuck to the formwork and spalled off. What did you use for plywood? Sheathing is rough, form plywood is used commercially.Form plywood has an exterior glue that is different from sheathing ply.

            Next time ,either use form oil, or even better where practical staple on 6 mil polyethelene.It will take a little longer for the concrete to set up with poly, the water in the concrete won't evaporate as fast. What did you have to trowel?The sidewalls do not need troweling.

            I just reread your original post and then looked at your pictures. Apparently you wanted to trowel the curb wall. Not needed, bevel the bottom of the formwork so you can get the trowel underneath the hanging form. Similar to a concrete stair riser form. I bevel the bottom at  30° , leave about an 1/8" unbeveled so there is no sharp point.A sharp point gets splinters and the top of the trowel catches on them.Now you can trowel the top of the wall below the curb.

            Because this is only 4" or so high you should be able to strip the curb section only after 2 hours in mild weather.The top should be white,then you can strip the curb only. Don't touch the rest of the walls for a couple of days.

            mike

            Edited 3/21/2008 10:08 pm ET by mike4244

          8. Yersmay | Mar 22, 2008 05:57am | #21

            Mike4244,Great idea about beveling the bottom of the form!! I didn't use plywood. I used 2 X 6 that I ripped at a slight angle to follow the slope of the slab while leaving a level top surface. I noticed that the wood was rather rough but I thought with the oil it wouldn't matter. The poly on the inside of the form sounds kind of labor intensive but I'm sure it pulls away from the concrete effortlessly and it leaves a nice surface. Thanks.

          9. Henley | Mar 22, 2008 03:48pm | #22

            It's certainly not "GREEN" but Kerosene is the old
            school form oil. I prefer to strip forms the next day. They pop right off,
            and every thing is still green (in case a touch up is needed).

            Besides, after the pour is usually "Beerthirty".

          10. Piffin | Mar 22, 2008 06:39pm | #23

            It is dicey any time you do an immediate strip like that.
            I was silent when I first read that you used linseed because I had never heard of that before, but knowing that linseed will bond with crete as a top coat finish made me wonder...
            i'm sure that it had just enough curing time to make it tacky before you poured and then pulled the forms, so that was a major part of this problem 

             

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

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