h a l f b a k e r yFlaky rehab
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When pouring concrete to a reinforced concrete structure, it is usually vibrated using a hand held "air poker" (see link). This does the job of removing the entrained air, allowing the concrete to consolidate and also to bond with the steel. One problem with this is that it is often difficult to be thorough
with the air poker, which is a cumbersome device and it is easy to miss bits, resulting in a poor finish and/or areas of structural weakness.
The idea is to use a sonic generating device which could be clipped to the steel reinforcement itself causing the entire cage to vibrate, thus removing the need for the air poker. I'm not sure if it would work, but I can't see why not. I also believe that this would be more thorough, and would take less time to do.
Not stupop's link. He may be referring to something else.
http://www.mytoolst...akita/mak1a-21.html
The type of concrete vibrator that I've seen. [half, Oct 04 2004]
Concrete vibrator in use on dam construction
http://www.dur.ac.u...ams/cons/conss3.htm
2nd photo down. It's hard to find photos of these things, but the guys are holding long tubes with air pokers on the end. They are so unwieldy, it's like wrestling an anaconda. [stupop, Oct 04 2004]
(?) concrete vibrators
http://www.michigan...crete_vibrators.pdf
[stupop, Oct 04 2004]
Rebar shaker
http://www.southern...c_rebar_shaker.html
Hmmm. Found this, but it looks like it is only suitable for shaking one bar at a time, and perhaps only suitable for thin walls. [stupop, Oct 04 2004]
Industry Standard Concrete Vibrator
http://construction...ower-Unit-15HP.html
Picture Shown without the whip (the part you dip in the concrete) [renegademonk, Oct 04 2004]
Northrock Rebar Concrete Vibrator
http://www.contract...rock-Rebar-Vibrator
Powerful rebar tool allows you to vibrate the inserted piece of rebar as you fill the cell [swed419, Jan 11 2008]
[link]
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As the steel is tied together, I would envisage the entire cage vibrating at once. The frequency could be changed within a specified range (concrete needs to be vibrated above 6000 Hz I believe although I'm not sure what the upper limit is) until a harmonic of the natural frequency is reached. I have no doubt that this would require a lot of energy, but I imagine it would take a lot less time than the conventional method. |
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is Mac the Knife in town? |
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Excellent. Has the backing of science and vibratonics! + |
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Thats some dam fine logic there, [stupop] ... well done. |
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I like the thinking. There might be
a need for the formwork to be
modified to allow free (or freeer)
transmission of the vibrations
from the outside generator of
vibes to the interior. On the upside
this is, as i understand it, run as
you pour. |
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On the down side the inter-ties
between bars looks to be a poor
way to conduct vibrations. As a
result the bar layout would need to
be optimized for transmission. The
benefits might not be worth the
extra effort but I like the thinking
alot. |
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good-Good-GOOD,
Good vibrations. |
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I've never lost a game of Air Poker. |
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I speak with the experience of having placed and finished thousands of cubic meters of concrete and I say this would never work. The obvious reason would be the lack of control of which sections get vibrated. With a concrete vibrator (also a sonic device) quite often a quick dip is all that is needed for an area of 1 square meter @ 150-200mm (Roughly 1 sq yard @ 6-8inches for you Yankees). How long would it take to attach a device to the steel? By the time you have it hooked up the device will be covered in concrete and you will wish you hadn't touched it because concrete doesn't feel too good on the skin. |
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If your idea was to vibrate the entire steel reinforcement then it still wouldn't work. As deadmanwalking and humanbean have pointed out some portions of the steel wouldn't vibrate. I won't get into how some portions would be overvibrated. I am also sure that even the bravest guy would be queezy with the whole deck vibrating 20 stories above the earth. |
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Pouring concrete requires alot of finnesse, strength, speed, and a level eye. So the next time you see some hung over-smelly-dirty-cigarette smokin-long haired cement finisher you can say "glad I am not doing that for a living" |
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PS... dam fine logic? - let's not build a fort and have a beer instead, yes - a beer with your fishbone. :) |
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[renegademonk] I agree with you that vibrating each bar separately would be too time consuming and awkward, but the idea here is to vibrate the whole reinforcement. The device could be clipped on to the cage at various points (a minimum spacing would need to be worked out). It would be possible to vibrate all the bars because you would vibrate through a range of frequencies so while your 16mm bars might vibrate at, say 8000Hz, your 25mm bars might not vibrate until 12000Hz. Over vibration may be a problem, but this could be counteracted by only turning the device on for a few seconds at a time.
The most valid objection here I think is the ability of the vibration to be passed along the ties between the bars. Maybe some sort of special clip would be required instead of using tie-wire. |
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Or use Magneto from the Xmen? |
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Sorry Stupop, but your idea is way too sophisticated to be anywhere near practical and hence it will never see the light of day. Special clips? - sounds expensive and ALL contractors have to be cheap in order to make any cash (Tie-wire cheap - me use that says the contractor). |
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Ok, now I am going to bring the overvibration to the forefront of the equation... overvibration is very bad - all the aggregate (which provides most of the strength in concrete) falls to the bottom of the mix and when any inspector sees it he will say
"hmmmm how long will it take you to rip the entire slab out of here and repour it?" I have heard this bankrupts men. This reminds me of my very first slab I did for my parents - they took it easy on me and are living with some ugly concrete. |
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The bottom line is relying on the steel to do a good job of vibrating the concrete is risky, expensive, and equally as labor intensive as using a concrete vibrator. You are living in the distant future - why not just teleport the whole thing into place or ask Magneto very nicely to do this for you. |
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Luddite. Mind you, the prospect of trying to explain this to a general foreman does make me shudder. |
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Please pardon my ignorance in this subject, I have no construction experience... :-) |
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Do you think it would be possible to place the setting concrete under a partial vaccum to supplement the vibrations in remving the air? |
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Stupop, would you be shuddering because deep in your heart you realize this is a bad idea or would you be standing on the steel? Ha ha. |
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Lud ud ud ud ud ud ddddd iiittttttttt eeeeee |
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I think that one of the benefits of a portable vibrator is that the energy required to adequately remove pockets of air from concrete is too great to be done on a large scale, and/or at a long distance. Without taking actual measurements, I would venture a guess and say that wet cement has excellent vibration damping qualities.
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If you've ever seen a crew shoot concrete from overhead, they move pretty fast. |
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"Like shit off a shovel" I believe is the expression. |
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I think what this idea illustrates is
that some technologies are really
surprisingly well developed.
Concrete is one of those things. |
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I wonder if there might be a niche
application for this. Like mass
production of jersey barriers or
planters. Where you could
amortize the required studies and
R&D. |
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But Like someone said the existing
way really works and is not so very
much trouble and is flexible (I'll
bet different batches of concrete
and different delivery methods
would require different amounts of
vibing). Also, looks like you'd
need a super computer to
dynamically alter the inputs as the
pour continues. Otherwise you
have the same guy operating alot
more gear with more setup time
and complexity. |
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Yes, of course it works, especially on 8-10' walls. There are both benefits and limitations. Unlike the traditional vibrator, you don't have to guess where it is, pull it up and drip mud all over, and try to ram it back down through tied rebar. You simply leave out some of the verticals (every 4 feet), drop them almost all the way down (leave yourself >12"), vibrate the rod 5-7 seconds and send it home. Trying to vibrate a wired-together rebar mesh wouldn't work. |
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Didn't Tesla almost collapse a building in New Jersey this way? |
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