Use of concretes that can do without vibrators?Any warning? 7

[IJR]

Dear Pals

I am in a situation where I have to use fresh concrete without using having to use vibrators because of too much reinforcement in the design

A dealer has suggested a special mix of concrete already certified for this use.

Being a new product to me, I have to ask you if you know of any bad side of this product

respects
ijr

 

[Ussuri]

I have never used self compacting concrete for structural purposes. But I have used it for concrete road construction. They basically add a superplasticiser which reduces the mix to a near water consistency.

If it is the same type of thing you are proposing to use I would be concerned whether my shuttering was watertight enough to retain the water cement mixture and not just leave aggregate in the shutter. I also remember this stuff reached a very high compressive strength when cured. The link below might also be of some interest.

http://www.concretecentre.com/main.asp?page=609

 

[StephenA]

Depending on the speed of casting, you might get higher pressures and thus increased loads, so check the formwork design

 

[GeoPaveTraffic]

I have seen self consolidating concrete that pretty much has a 12" slump. It works, but is costly. It used frequently used in pre-cast yards. Don't know of any "got ya's".

 

[DRC1]

A. If your reinforcement is so tight (we refer to it as mouse proof when we see it) then sombody should look at the design.
B. smaller "pencil virbrators" are available for rent. theyare smaller than standard vibrators. I have never used them, but external vibrators may be an option. They attach to and bang on the sides of the forms. They bang the living bejeepers (that's a technical term) out of the forms, so they have to beconsiderablly heavier than internally vibrated forms.
C. Since the bars are tight I would try to find some means of vibrating the concrete. Otherwise I would worry that even with high slump, the concrete will bridge between the bars in places

 

[BigH]

Actually self-compacting (self-consolidating) concrete is now well grounded - they even are using it in India. (no offense) I attended a seminar put on by the Indian Concrete Institute and there seemed to be various opinions to my question on the tightness of formwork joints. One guy says it is okay to have a bit of space - others say not. I would go with the later. Joint tape is a must. I'll try to post some references later on this.

 

[Ussuri]

I have used pencil vibrators before (both 1" and 2") and they work fine. The only downside is because of their size they don't have a particularly large zone of influence so if it's a big pour you have you might be at it for a while.

 

[boffintech]

I recently observed a concrete placement for an elevated reinforced concrete deck with post tensioning. Upon arrival of the concrete mix to the job I noticed that no one had yet retrieved the vibrators out of the tool shed. I asked the concrete sub and the general about this and both announced that no vibrators would be needed as they had decided to go with so-called "self-consolidating concrete".

The rebar sub then announced that self-consolidating concrete or not his contract was for placing the rebar one time and not for re-doing it after demo. Obviously he was making an early prediction on the self-consolidating part of the term "self-consolidating concrete".

So the stuff had a high cement content and had a high-range water reducer to make it very flowable. Slumps were 10 to 12" and not totally verifiable. Breaks at 3 days were 3000+ average. Upon removal of the edge forms in preparation for PT stressing we got our first clue as to the magnitude of the honeycombing yet to come. It took 4 guys 3 days to bush out and patch behind the anchors enough to get the PT stressing accomplished. The rest of the slab took 3 more days to get patched properly.

 

[IJR]

Boffintech

Your post seems to reflect an experience. But I havent properly understood, apart from the big lumps and high strengths.

Can you put it more clearer?

Nothing can replace fellow engineers experience.

respects
IJr

 

[cvg]

I think the key understanding is that vibrating is a necessary part of the process. If the Contractor insists, and is allowed to omit this step, be prepared for some big challenges. Also be aware that the end product may not meet the high quality that you expect.

For road construction and other flat work, it appears that vibrating is not so critical.

For columns, walls and other tall structures, it may result in an unacceptable product.

Aggregate size probably should be reduced if the steel is "tight". This will allow better consolidation around the rebar. Note that this should be done whether the concrete is normal or high slump.

Rodding and tamping, external or pencil vibrators should all be used if possible. Don't let the Contractor tell you otherwise.

Make the Contractor responsible for quality - inspect after stripping, take core samples and be prepared for tear out and reconstruct. If the contractor is aware of your desire for high quality, he may decide to put a little more effort into it.

 

[GeoPaveTraffic]

While I understand the problems that have been reported, and I have seen many of them in normal concrete operations, there is such a thing as "Self-Consolidating Concrete" that does not need vibrating. It is relatively new and it is NOT just high slump concrete. It is heavily dosed with admixtures and is not for use just anywhere. ASTM C9.47, C1611, and ACI 237 may be usefull. You may also want to contact a local admixture company for more information of Degussa Admixtures.

 

[dicksewerrat]

Has the concrete mix design taken into account the spacing of the bars and the aggregate sizes?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[LCruiser]

GeoPaveTraffic knows what he's talking about.

http://tinyurl.com/py2yv

 

[DRC1]

I have not used self consolidating concrete, but if you have so much reinforcing that you can not vibrate, you really should go back and look at the design.

 

[IJR]

Yes DRC1

But do not ignore real life situations, where teams involved in the project force you to use no more than the available space allocated to you(You sometimes do the same to them)

I find such situations to be what makes engineering worth practicing.

respects
IJR

 

[DRC1]

My point is that there are maximum reiforcement raios for a reason. Lack of strain compatibility can lead to sudden failures in beams and columns. I have seen a pie haed fail due to excessive reinforcement and incorrect assuptions regarding load transfer. Many structures today appear to be excessively reinforced. Fo example I recently completed a 5 foot high wall with 2 mats of #9's at 6"c.c.
Although this structure will probably never be loaded anywhere near failure, structures that are overly reinforced and loaded near failure do not behave like under reinforced sections designed in accordance with ACI. The bigest problem is that these failures may not be ductile but rather be a sudden failure.
The problem with project teams is that everyone else assumes that some has checked a part of the design. Also no one on the team wants to stand up a say we have a problem because everything is on a schedule. Sometimes the best thing to do is just stop and reevalute what you are doing. Most dramatic engineering failures were due to flaws that were rationalized away on several levels of review because a.) no one wanted to stop the schedule or b.) someone had assumed someone else must have reviewed the detail and it was ok.

If you can not place the concrete, this is an indication you are trying to force a solution to a problem. I think this may be a wake up call to reevaluate.

 

[IJR]

DRC1

Thank you for sharing your experience. We truly are against modification of original designs, but there are so many cases we have to, trying to do so within technical limits.

Respects
IJR