I’d like to hear some expert advice and opinions on vibrating concrete as it is placed in forms. I just finished a pour of a 2′ stem wall with someone who has more experience than me. From my limited experience with concrete, I have always been under the impression that it is easy to over vibrate – thus pushing most of the aggregate to the bottom and/or forcing air out of the concrete. There were a few hard to reach spots where we used the vibrator to move the concrete laterally. Isn’t this a no no?
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I'm no expert but you don't need an expert on this one.
>>"forcing air out of the concrete"
A problem for air entrained concrete.
>>"There were a few hard to reach spots where we used the vibrator to move the concrete laterally. Isn't this a no no?"
Yes. Lateral movement by vibrator or otherwise could segregate the concrete.
[edit -- deleted high school locker room level of humor -- I think I had had a few drinks when I posted this]
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
Edited 3/11/2006 12:23 pm ET by philarenewal
A 2' stemwall sounds pretty typical of pours I have done. The majority of the effort goes into getting the concrete into place before vibrating. Depending on the thickness of the wall, I'll use length of 2x4 or maybe steel pipe to 'rod' the concrete into place right after it comes out of the hose. This is basically a vertical motion, shoving the 2x down into the concrete to get it to spread out, and then pulling back up. This should make the surface of the mud 'oscillate' up and down. This is especially important near corners, steps, etc. Use something that's not so big that you're repeatedly hitting the horizontal bars.
If you're pouring a 2' wall it's probably only one lift. If you are pouring more than one lift, go around with the vibrator after the first lift is done. Stick the vibrator in vertically, fairly quickly, and then pull it out a little slower. Move a foot or two and do it again. It does not take much, and don't leave it in there. If you do more than one lift, vibrate after each, and extend the vibrator down into the lift below.
Concrete in residential construction is usually fairly workable, wet enough so that a major amount of rodding and vibrating is not needed. In some commercial work apparently they use much stiffer mixes that take a lot more work and vibration. One thing that I think it very helpful is using a pump, preferably a boom, to pour walls. It makes it very easy to get the mud in the right place. Shoveling concrete around inside forms as it comes out of a chute is harder.
The American Concrete Institute puts out a booklet you'd do well to read called "Cast in Place Walls". In it, they note that "undervibration is more common than overvibration". And that "Vibrators should be kept moving up and down, never allowed to remain in one position in the concrete, and they should not be dragged or allowed to touch the forms or reinforcement."
Large emphasis is placed on the "area of influence" of the particular vibrator head. They show the problem of a too-small head that leaves areas of unvibrated wall where the areas of influence don't overlap. Overlapping areas of influence being critical. Again, undervibration is commonplace.
When I was first pouring concrete I was told all sorts of nonsense, including from concrete salesmen who warned me that using a vibrator would get me into lots of trouble. In fact, the only pour where I did really get into trouble was when my vibrator shaft broke and I didn't have a back-up. We rodded, and I regretted it.
I got every booklet I could from both ACI and the Portland Cement Association to learn what I could, and avoid a lot of misconceptions common in the industry. The above will help you avoid poor advice, which I found prevalent here. Not an expert, I've only supervised a little over a thousand yards of relatively small pours, but when you strip the forms it's all real clear.
PAHS Designer/Builder- Bury it!
The concrete that we get here comes at a 4-5" slump with no water added at the site. I have found it easy to get it into place in footing and stemwall forms, mostly by rodding, and have no honeycomb when I strip forms. A little vibration sometimes helps, but residential crews routinely pour without a vibrator at all. I mostly use one when doing two or more lifts in a form.
Since your whole house is concrete your experience may be quite different. A basement or any other tall wall is a rarity here.
Since your whole house is concrete your experience may be quite different
David, not so much the house as my approach. I knew that I was pretty ignorant on the subject and didn't want my 240 ton roof coming down on me. So I read a lot and hired engineering. The trade groups only want the user to have a good experience with concrete, no other agenda. Engineers clearly are cya.
Slump is always specified, rarely measured. Do you actually measure that 4-5" you're getting?
Around here the guys doing the vast majority of the work would never use the maximum 4" slump I insist on. My assumption is that the engineers I've hired have a reason for that stiff a mix. Super-plasticizer makes the flow pretty easy and I still get the specified strength. When I talk to the drivers, it seems I'm the only residential guy using what they always supply to the large projects. Not vibrating my mud will result in honeycomb. The reason the large projects are so fussy? Engineers. I'm not going to second guess them.
You previously mentioned only vibrating every few feet? Unless you have an immense head, you have a lot of unvibrated concrete in between your vibration points unless you're rodding there. I rodded the hell out of 8' walls (retaining, for a client) after my vibrator broke. I got honeycombing. Not structurally significant, but ugly. If you're not getting honeycomb, I'd question that you have need of vibration at all, that you're pouring a high enough slump that it doesn't matter. That's commonplace here. Usually with the mud flowing around the forms unaided by the crew. The OP apparently had much stiffer mud. They used the vibrator to move it around the form.
Vibrator mfgs state how large an area is affected by whichever head. It's not a very large area if you can get the head into (and out of) an 8" wall with rebar in there.
Not wanting to argue with you, we were writing at the same time. And perhaps I'm being overly cautious. My roof's still up there. My main point was that I was told a lot of wrong things by people in the business. At least wrong from the trade groups' and engineers' point of view. Which certainly included potential for over-vibration.PAHS Designer/Builder- Bury it!
I don't measure the slump, but I know from talking to the plant that they batch it exactly the same every time, unless told to do otherwise, and that it's a 5-inch slump. It does not flow around corners or level itself in the forms. I just did my first pour with plasticizer and will do more in the future. The pump operator told me that almost no one is using it here yet. He told me that a lot of people add a lot of water at the site. The way I do it results in absolutely minimal shrinkage cracks and fairly smooth walls, maybe a few bubbles of 1/4" or less, no honeycomb.
As far as placing goes, I rod all of the mud in there. I usually run the hose and I have a guy right on my elbow with a stick or a pipe, sluicing the stuff around. I think that does 90% of what is needed. Walls here are usually 8" and there's enough room to rod effectively, not to mention that the pump places the mud with some force. I usually go back and vibrate around corners, around the bolts, and maybe every foot or two in general. I know that it might be OK to vibrate more, say every 12" or less, with overlapping areas of visible agitation. We also do what carpenters do and tap on the forms with our hammers.
You've obviously studied the subject quite a bit more than I have. I'm a cabinetmaker and trim guy turned GC and have only had to learn this stuff in the last 5 years or so.
David,
I learned concrete on civil work, The vibrator technique I was taught/developed/ learned is to imagine a grid about 4 times the size of the vibrator head and drop the vibrator down each square. It's OK to make the grid smaller but never bigger than 4X.
Drop it fast, fast, fast, and pull it up slow, about 3 seconds a foot. You should see marble sized bubbles leaving the mud just as the head is seen. Don't worry about air entrained, you gotta vibrate about three MINUTES to reduce the entrained air 15%-20%, 3 seconds/foot ain't gonna herd it.
With lots of experience you can notice a change in sound as the bubbles leave the effect zone.
When pouring in lifts, drop the vibrator to the bottom of the previous lift.
One company I worked for had a sub build a 16' CMU wall. The engineer/architech specified setting 6' of block, pouring, repeat, then repeat the last 4'. The sub convinced my boss to let him do a mono pour. Fortunately for the sub, my boss put me as the controller for the pour and I made him vibrate per the above.
When the client found out about the monopour, they were pizzed. Xrayed the walls and couldn't find any problems so they cored it in about a dozen places. Still passed.
LOL
SamT
Sam, thanks as always for the good info. That's a whole lot more vibrating than I've ever done. If I'm already getting smooth walls does it still make sense to vibrate more? I'm wondering if it might improve the 'tightness' around the rebar.
David,
"If I'm already getting smooth walls does it still make sense to vibrate more? I'm wondering if it might improve the 'tightness' around the rebar."
What you see at the face is pretty much what you would see at any sectional slice.
Those bubbles. . .Dang, I lost the word. Ya-no that seems to happen more often every time I add a year to my age. . . Wonder what that means?
To get a better idea of what's up, pound the surface with a rubber mallet, 1 whack per spot, mind you, those bubbles. . .can you remember what they're called? For the life of me, I can't find the memory address where that data is stored.
Many times a thin film of cream will flow between the form and those damme no-name bubbles making it hard to see all of them. The rubber mallet trick will break that cream film.
Bubbles! Paugh!
LOL
But I still can't remember what those air pocket voids in the surface are called.
Edit: If you're alrerady getting smooth surfaces, I wouldn't worry about more vibrating, unless it's a critical application.
SamT
Edited 3/11/2006 9:19 pm by SamT
Waitaminit, yer saying to pound the form with a rubber mallet as you're pouring, rodding, vibrating, topping off, etc...? I usually tap the forms with a steel hammer to jiggle the mud slightly, it seems to help cream flow out against the inside face of the form. Us residential carpenters do ignorant stuff like that.
Or are you saying something else?
David,
No, no, no. After stripping the forms, if you see an area where you suspect there may be a few voids near the surface, or where there already a few in a clump, whack the concrete. Doing so will give you a better idea of how effective your vibrating technique is. It's not required. Call it the cheap mans' xray, so to speak.
You're right about pounding on the sides of the forms. It just drives the air bubbles under/behind a thin layer of cream.
The "best practice" is to rap the forms next to the vibrator head as the dickman pulls it up thru the mud. That hammering, along with the liquifying action of the vibrator, will allow more bubbles to rise to the top.
I have two dedicated, and real old, sawzall types and have the watchouts use them on the sides of the forms in conjunction with the vibrator. BUT. . .I'm positioning my business as the "best."
A dozen or so small air pockets per cubic foot is not going to seriously effect the wall. Even a horizontal cold joint is no reason to demo. Just sponge float it with some cement/sand and sack the wall so it looks uniform. then you can whack yourself in the head with that rubber mallet to remind yourself to vibrate cold joints better.
If you're not using accelerator and the weather is not above 85* or 90*, you can vibrate out a cold joint for up to 3 or 4 hours after the old lift. The trick is to vibrate the old surface smooth before pouring new mud on top, then set the head right down on the set up lift for about 30-60 seconds and use a slower vibration.SamT
>>"But I still can't remember what those air pocket voids in the surface are called"
Bug holes.
"A job well done is its own reward. Now would you prefer to make the final payment by cash, check or Master Card?"
PR,
Of course.
I thank you.
The best I could get out of that black hole I laughingly call my mind was "Bee hole" and I knew that wuzzent gonna gettit.
Amazing what 1 bad byte of memory can do to a program.
SamTSamT
<I just did my first pour with plasticizer and will do more in the future.>Rookie question, but what does plasticizer do and does it addto the cost of the concrete?
plasticizer makes the mix flow as if you had added water, but without the water.. so your mix maintains the ultimate design strength and the finishing characteristics of a stiff mix
i tnink it usually costs about $3/ cy.. but i haven't checked prices latelyMike Smith Rhode Island : Design / Build / Repair / Restore
thanks, learn sumpthin every day in this joint
Hmmmm........ here it costs about $80 per 7 yard load to add plasticizer, or specifically Master Builders Polyheed 997, which is described as a midrange water reducer. There's a PDF file if you google for it. If I remember correctly, they batched for a 3" slump, added ~450 oz. of the plasticizer which took it to a 7" slump for about 2 hours. This particular material has a slight retarding effect as well as plasticizing, so you're troweling it later than normal unless you also add an accelerator.
We recently poured a slab with this stuff in the mix. It's like screeding yogurt. We wore Bermuda shorts and sandals, and barely got out of our lawn chairs it was so easy.
it's been awhile since i've used a plasticizer..but i can ask my supplier what the charge isMike Smith Rhode Island : Design / Build / Repair / Restore
We have one batch plant here. You want mud, you pay what they charge. There was a guy that had a 'SiteMix' truck, trying to crack the market. I think he could have done it, but he had some quality problems and didn't last. I don't see how you can adequately mix concrete in a 10-foot chute with an auger.
plasticizer which took it to a 7" slump for about 2 hours. around here its about $5 a yard and its batch on cement weight, a higher sack mix would get more super. But you are right it will increase a 3 to a 8 inch slump. But time period is about 45 minutes. Its added to the mix at the site. I got a couple gallons in my shed.. 2+3=7
not an expert ... but a big fan of and user of concrete... as everyone said yes you can over do it... as with anything... gravity will make the big stuff heavy fall to the bottom... when doing countertops and castings or even some walls that will be seen... i like to vibrate the form more so than the concrete itself if i want a really void free surface look... sometimes i don't...
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