Slump test after adding Super P? 7

[Mr168]

Please forgive my ignorance on the subject, as I am a welding guy trying to help a fellow QC guy find an answer.

Is it required or outlined in any code as to when slump tests should be performed when Super P plasticizer is used? The concrete had all tests run before adding the Super P, air tested ok, slump was routinely low. These results were documented, the Super P was added, and no additional testing was performed before it was poured.

This is foundation work for a fossil power plant.

We're going under the assumption that the addtion of the Super P would pretty much invalidate any slump test anyways, but we're looking for some evidence or section of code that confirms or refutes this process of running no further tests after the plasticizer is added (break tests were all coming back good).

This is pretty much a CYA scenario in case of a QA audit or something like that. Thanks for the help.

 

[DRC1]

Often the best super -P is water. I have seen many problems with concrete where w/c is kept very tight and and a tight tolarence on the slump. bumping the water at the site wil improve workability for a moderate sacrafice in strength. I have not seen loss of strength below design strength. I have seen very stiff concrete go into rebar congested forms and not be able to encapsulte the bars.
Many applications especially flat work (except decks)concrete strength is not a major factor. Increase the slump with water instead of super P will give a much better finsh, especially in hot weather.
My one point is that, with in reason, and with consideration, raising the w/c at the job is not a cardnial sin.
My other comment is although good documentation is important, it is increasingly difficult to deal with field conditions in the field.

 

[Ron]

DRC1...not sure where you practice, but I've seen many examples of adding water at the site compromise the design compressive strength. More than I care to recall.

That practice should not be allowed....it compromises the mix design which is the basis of f'c...the design.

 

[cvg]

Every time I have seen issues with stiff concrete, it was because of a)trucks took too long to deliver b)improper equipment or procedures at the job site to handle the stiff mix or c)poor quality mix provided by readymix company. If super p is necessary, let the contractor request it ahead of time, let the engineer approve it ahead of time and don't rely on the truck driver who only gets paid by the hour to add in the proper amount of admixture and properly mix it in. I can almost guarantee that if that happens and there is a problem, the ready mix company will not warranty that concrete. Extra water should never be allowed. Maybe this is common procedure on residential work, but not anywhere else.

 

[civilmate09]

Gents, I also agree that adding water to the mix at site is not really acceptable.

However, Is it possible between concrete supplier and the QC personell in charge to the mix to have an arrangement for some allowances to put extra water or plasticer to site due site condition and or type of structures to be poured without compromising the mix design itself?

I think this case is acceptable to some degree.

 

[Ron]

dik...that was exactly my point. I've dealt with this issue for many years and once someone other than the supplier adds water at the site, the supplier says "you bought it".

 

[hokie66]

Adding water not only reduces strength, it adversely affects durability, causes more shrinkage, and is generally not an acceptable practice. If the concrete can't be placed and consolidated without additional water, something has gone wrong in the mixing or transport. Time in truck and temperature of concrete are the most likely suspects.

 

[bb29510]

you can tell by those that work directly by the book and those that actually do the work in the field. I agree with DRC, bump the slump up to a 4, you loose about 50psi in strength, and then add the super P you have something you can really work with.

 

[cvg]

those of us who work by the book have very little to gain by doing otherwise. and there is no excuse for not being able to place per the specs.

 

[DRC1]

AS Brownbag pointed out, it is easy to specify a particular slump, placing it can be more problematic. My point was that by keeping a low w/c you may end up with higher concrete strength, but a lower capacity of your reinforced concrete structure. There is a difference between producing a quality product and complying with a specification.

 

[hokie66]

With respect, DRC1 and brownbagg, you know not of what you speak. May be best to leave the comments about placing concrete to those who have experience in building high quality reinforced concrete structures.

 

[bb29510]

But with a four slump, most time you are still within the w/c ratio. Super P just does not like a dry slump of around 3. we are not talking 100"s of psi loss. Most mixes are design at a four, with a little safety built into them. A typical 4000 here poured a 4 slump with super p added is breaking 6200 so 50 psi is not a big loss. It still within spec.

 

[Ron]

Well said hokie66.

Concrete can be properly designed for numerous applications and construction techniques. It's not that difficult to match the two and they don't have to be incompatible. The problem comes when you have contractors and subcontractors who know NOTHING about concrete technology indiscriminately adulterating a mix for their purposes. That causes concrete failures..both strength and durability for the sake of placement ease.

Compressive strength is only one parameter of quality concrete, and by far the easiest to meet. Placeability and durability can be achieved consistent with strength.

Don't screw it up in the field. Don't bring a residential concrete mentality to a structural concrete necessity.

 

[concretedoc]

With respect, Hokie66 and Ron, you know not of what you speak. Have you read ASTM C 94, which allows one addition of water at the jobsite? Have you tried to pump or finish concrete with very little water added at the plant, but enough superplasticizer to still produce the specified slump? The concrete pump knows the difference between a water slump and an admixture slump. So do finishers. Your condescending comments about other posters and about contractors reveal an arrogance that should be removed from this web site. "May be best to leave the comments about placing concrete to those who have experience in building high quality reinforced concrete structures." In other words, anyone who disagrees with you has no experience with high quality concrete. I do have experience with high quality concrete. Your post, Hokie, should have been red flagged. And Ron's too. I'd guess that not many contractors even use this site because of the know-it-all arrogance of some regular posters. Many contractors know more about concrete technology and building with concrete than you seem to know based on your previous comments. For instance, when someone disagrees with Hokie—and supplies references--as with the discussion of clear cover for flexural reinforcement—you give a response that “There are more ideas about how to do it than Carter has little pills, and everybody thinks his way is best.” Well you obviously think your way is best, Hokie, because you didn’t have the data or references to dispute someone else’s opinion that was backed by a reasoned and well thought out response. So sour grapes had to do.

 

[dik]

I was going to wade in, but decided not too... could be more fun to watch from the sidelines...

Dik

 

[hokie66]

My disagreement with DRC1 was his comment that "Often the best super -P is water." I believe that is clearly incorrect. The recommendation of Brownbagg's to "bump the slump up to 4 and then add Super P" is also inappropriate as a general statement in my opinion. These decisions are best left to the individual project, where the material and climatic variables can be taken into account.

Concretedoc, I admit my comment on rereading does sound arrogant. I apologise. Feel free to red flag any of my comments which you consider inappropriate. That is what the red flag function is for.

I agree that many contractors know a lot about concrete, but there are also a lot who don't, and only want to know what benefits them.

As to the other quote of a comment made by me, it does sound a bit flippant, but I don't think it was sour grapes. I will try to find the discussion and see what it was about.

 

[bb29510]

Brownbagg's to "bump the slump up to 4 and then add Super P" is also inappropriate as a general statement in my opinion.

How would it be inappropiate, according to aci I am allow one inch tolerant at 3 inch slump. so basically I,m legal within the slump range.i just remark that super p does not like dry slumps. The slump mostly is base on concrete before the super p added.

 

[DRC1]

First, thanks to Concretedoc for the support.
I have been building bridges and foundations for too many years now. I have been sworn at by too many inspectors becuase I have taken 3 inch slump to 6 inch slump. Everbody thinks that the spec is what needs to be followed when a.) the engineer never considered the congestion of the rebar in the design of the mix (Often I do not think the engineer has any clue as to how congested the rebar designs have become) B. Uses a spec that has been used on 100 prevoius jobs with out any thought that pouring in April is any different than pouring in July. C. The concrete now has so much chemistry in it, the suppliers will tell you "Sure we can produce it for a steep surcharge, but we have no idea if it will do what you want it to do". D. The engineer assumes anything that water can do super P can do which is not always the case. For example, mix designs assume the concrete is consistent through out the load, which it is not. As you move through the load, the consistancy of the mix will change. Sometinmes, especially on a dry load, a little water can improve the last few yards. E. All concrete does not need the same perfomance criteria. A four foot high retaining wall does not need to meet the same requirements as a 50 ft long cast in place self supproting deck. Yet with today's Q/C proceedures, documentation is required to show that all the parameters were met. All this ends up costing the owner considerable money yet does not result in a better structure.
Quality of Design and Quality of construction needs to be better enforced for the industry as a whole to improve.

 

[conceng]

Kinda late to jump in here. It seems there is still a huge amount of misinformation regarding superplasticizers or high-range water reducers.

I'll start with the original question. So far as I know, there is no code guidance regarding testing the properties of the concrete before or after adding the super. Best practices would dictate that one should verify the initial slump before super to assure the mix is properly proportioned as delivered. Then the concrete should again be tested after the super for plastic properties and compressive strength.

I typically recommend not exceeding an 8" slump unless the concrete is designed to meet self-consolidating stability requirements or there may be risk of segregation.

Testing the properties of the concrete after the super is an absolute must as some super's add entrained air while others will cause the air content to drop. In most cases, the super will provide additional strength due to better cement particle dispersion. I've seen increases as high as 1500 psi.

So far as adding at the site or the plant, it all depends on the haul time plus unload time. In most cases I see it added at the site to provide the longest possible working time. Someone at the site, either QA or QC should be assigned the task of monitoring the final mix time.

What the slump should be before the super depends on the mix requirements. In most cases it is easier to control all the properties if the initial slump is 3-4", but I have designed mixes with basically no-slump prior to the super in order to meet project requirements.

Superplasticizer technology has come a long long way in my 20-years of seeing it develop. We are to the point where we will choose between multiple super's based on the performance requirements of the plastic or hardened concrete.

 

[bridgebuster]

On NYSDOT projects, central mix and transit mix concrete contains 90% of the design water when it arrives on site (at least it's supposed to). The remaining water is added to achieve the desired slump. If admixtures are used, the slump and air is measured after not before.

NYSDOT uses a somewhat idiot proof system. The specs provide the contractor with the mix design and there are enough safeguards to ensure that 3000psi is consistently achieved. What is required of the contractor is to meet the air and slump requirements for the specified mix. Of course, from my days as an inspector, there were some concrete truck drivers who could turn a 4" slump into 12" slump.

 

[GavL]

The Slump Tests primary objective is to determine the workability of the concrete for field use!

I would have the slump taken at batch plant, and on site after all additional mixtures are added, remember to keep the concrete trucks drum rolling at the correct rotation.

It is essential to take your cylinder samples to prove that the concrete is still at designed strength after the additives are mixed in with the concrete.

Maybe run a 3 day compressive break as well as a 7 day and a 28 day break. This should cover you from a QA/QC point of view. (If I am not mistaken)